0 0 FINAL REPORT SOUill BRANCH, UPPER CARMACK SUB-BAS IN MANAGE)(ENT STUDY PHASE I - r ... I r r r r r r r r r r r r r r r r r FINAL REPORT SOUlll BRANCH, UPPER CARMACK SUB-BASIN MANAGE1>IENT STUDY PHASE I Prepared by: mDBERT ~. ~AND, P.E. Drainage & Flood-Control Engineering 1 W. William Carey Street Tucson, Arizona 85747 (602) 762-5668 Prepared for: Pilla County Department of Transport at ion and Flood Control District 201 North Stone Avenue Tucson, Arizona 85701-1207 June 1, 1992 Revised July 24, 1992 00 I r r TABLE OF CONTENTS I. r r r r INTRODUCTION 1.1 1.2 1.3 II. 2. 1 r 3.3 3.4 IV. r r r r r r r 2 4 Hydrology Flood Plain Delineations Flood-Damage Assessment Hydrology . . . . . Flood Plain Analysis 4 5 6 9 9 13 3.2.1 Watercourse Description and Definition 3.2.2 South Branch Wash Flood Plain 3.2.3 Shadow Mountain Wash Flood Plain r' r 1 1 DISCUSSION OF APPROACH AND RESULTS 3. 1 3.2 r Project Location Background Objectives GENERAL METHODOLOGY 2.2 2.3 III. 1 Flood-Damage Assessment . . Observations by Local Residents 13 15 29 33 38 CONCEPT MITIGATION MEASURES . . . . . . 40 4.1 Economic Feasibility Analysis 40 4.2 Identified Problem Areas . . • 41 4.3 Site-Specific Mitigation Measures 41 4.3.1 4.3.2 4.3.3 4.3.4 4.3.5 4.3.6 41 43 45 46 Problem Problem Problem Problem Problem Problem Area Area Area Area Area Area A B C D E F 47 47 v. RECOMMENDATIONS REGARDING PHASE II 51 VI. REFERENCES 53 r i oo't r r r r r r r r LIST OF FIGURES Figure 1 Project Location Map 54 figure 2 Photographic Base Map of the Study Area 55 figure 3 Offsite Drainage Basin Map 56 figure 4 Onsite Drainage Basin Map 57 figure 5 Precipitation Depth versus Return Period 58 Figure 6 Soil Classification Map 59 Figure 7 Zoning Classification Map 60 Figure 8 Cross Section Location and 100-year Flood Plain Map 61 Figure 9 Flood-Prone Structures Map 62 Figure 10 Typical Floodwall Renderings 63 r r r r ,. I r r r r r ,.. ii r LIST OF TABLES r Table 2.3A Damages as a Percentage of Value (FEMA, 1989) I"' Table 3.1A Hydrologic Summary Table 10 Table 3.2.2A Hydraulic Summary Table for the SBW Analysis 17 Table 3.2.2B Hydraulic Summary Table for the 300-305 Analysis 19 Table 3.2.2C Hydraulic Summary Table for the 200-212 Analysis 20 Table 3.2.2D Hydraulic Summary Table for the SBW-US1 Analysis 23 Table 3.2.2E Hydraulic Summary Table for the SBW-US2 Analysis 26 Table 3.2.2F Hydraulic Summary Table for the 401-409 Analysis 27 Table 3.2.2G Uniform-Flow Parameters Related to the 401-409 Analysis 30 Table 3.2.3A Hydraulic Summary Table for the 101-120 Analysis 31 Table 3.3A Depth of Flooding Relative to Each Flood-Prone Structure 34 Table 3.JB Damages as a Percentage of the Market Value 36 Table 4.2A Damages Grouped by Problem Area 42 ' 8 J r r r r r r r r r r r i r . r ' I"' iii 0 r r r LIST OF APPENDICES Appendix A Scope-of-Work for this Study Appendix B Hydrologic Data Sheets for the 100-year Return Interval (individual data sheets for the more frequent return intervals are also provided in a compressed format on an accompanying 5 1/4" double-density diskette) Appendix C Input and Summary Output Listings for all HEC-2 Analyses (detailed output listings are also provided in a compressed format on the accompanying 5 1/4" double-density diskettes) Appendix D Input and Output from the FHWA Culvert Analysis (HY-8) of the South Branch Wash Box Culvert Beneath Oracle Road Appendix E Input and Output from the FHWA Culvert Analysis (HY-8) of the Shadow Mountain Wash Box Culvert Beneath Oracle Road r r r r r r r r i i r r r iv r r r ,. I r r r ,. l ,. ,. I' r r r r r ,. '' r "' \ I. INTRODUCTION This report presents the results of both a qualitative and quantitative analysis of the drainage problems associated with the South Branch, Upper Carmack watershed. Also included is an economic assessment of the damage potential associated with three distinct storm events. This report represents Phase I (the existing-conditions analysis) of the South Branch, Upper Carmack Sub-Basin Management Study. The need and recommended scope for the second and final phase of the study is also discussed. The study was funded by Pima County at the request of the Town of Oro Valley. 1.1 Project Location The majority of the study area, which encompasses approximately 0.5 square miles, is located within the Town of Oro Valley. The approximate boundaries of the study area are Hardy Road to the north, the U.S. Forest Service boundary to the east, the Carmack Wash (Northern Avenue) to the west, and Placita De La Poza, Camino Greenfield, Sunburst Circle/Place, and Mountain Sunrise Place to the south. The study area occupies portions of four Sections (23 through 26) within Township 12 South, Range 13 East, Gila and Salt River Base and Meridian, Pima County, Arizona. The limits of the study area are depicted on Figures 1 and 2 of this report. 1.2 Background The study area, as defined, includes the South Branch Wash and a relatively small parallel wash that will, for the purpose of this study, be referred to as the Shadow Mountain Wash. The South Branch wash has in the past been referred to as the Pusch Peak Wash and the Carmack Wash. For the purpose of this study, however, the Carmack Wash is the southwesterly oriented wash located directly west of the Northern Avenue- Hardy Road intersection (see Figure 2). Runoff from the south Branch Wash enters the Carmack Wash a short distance downstream of the Carmack's Hardy Road crossing. Runoff from the Shadow Mountain Wash, on the other hand, enters the Carmack a short distance upstream of the Hardy Road crossing. The study area, which is highly developed, has a documented history of drainage problems. These problems include flooding of residential structures and erosion and sedimentation damage to private property and public improvements. The Pima county Flood Control District and the Town of Oro Valley have received and continue to receive numerous complaints from area residents. Some of the first complaints were filed in the early seventies. The majority of the south Branch watershed is located in the Coronado National Forest which is managed by the U.S. Forest Service. For the most part, the boundary line between Forest Service property and privately-owned property also corresponds to the geologic boundary between the mountain and foothi 11 regions of the Santa Catalina Mountains. From a geologic standpoint, the foothill region consists of deposits of alluvium transported from the mountain region by runoff conveyed in the normally dry washes. Consequently, developments that currently exist within the foothill region are located on relatively steep erodible soils. Midway between the Forest Service boundary and U.S. 89, more gradual slopes appear which signify a transition between the foothill region and the valley floor. ~ Drainage ~ Flood-Control Engineering r r r r r r r r r r r r r l ~ Drainage l Flood-Control Engineering Page 2 Historically, the valley region of the study area was characterized by a network of shallow, braided channels that typically conveyed runoff over a much wider area than do the well-defined, steeper channels of the mountain and foothill regions. Prior to urbanization, the amount of runoff conveyed in any given braid could and would change from one storm event to another. Unlike the South Branch watershed, the Shadow Mountain watershed is located entirely within the foothill and valley regions. Considering the geologic nature of the study area, one would expect some drainage-related problems to exist as residential and commercial developments attempt to occupy these unpredictable drainage corridors that are very sensitive to any physical changes. The study area experienced its first development in 1959, when the Shadow Mountain Estates subdivision was approved by Pima County (see Figure 2). However, since this subdivision was located at the downstream limit of the study area, few problems were noted in its early years. In fact, most of the runoff entering the subdivision along its southern boundary was easily accommodated by the subdivision's dedicated drainageways. The Shadow Mountain Estates subdivision was quickly followed by the Sunnys!ope subdivision in 1965, the Rancho Catalina subdivision in 1971, and the Rancho Felix Subdivision in 1977. Both the Sunnys lope subdivision and a port ion of the Rancho Catalina subdivision are located within the foothi II region of the study area. Even though the Sunnyslope subdivision was platted prior to the Rancho Catalina subdivision, most of the lots within the Rancho Catalina Subdivision were developed before any homes appeared in the Sunnyslope subdivision. It should also be noted that the three most recent subdivisions occupy the majority of the study area and most of the development occurred before Pima County enacted its 1974 flood plain ordinance. Most of the drainage complaints on file with Pima County and the Town of Oro Valley appear to come from residents located within the Rancho Catalina subdivision. 1.3 Objectives As previously mentioned, the overall basin-management study was divided into two phases. This report which presents the results of the existingconditions analysis constitutes Phase I. Phase II, if undertaken, will provide an evaluation of alternative mitigation measures. The scope-of-work, which lists the individual components of each phase, is included as Appendix A. In general, the existing-conditions analysis had four primary objectives. The first objective was (a) to establish a range of runoff quantities at key concentration points within the study area and (b) to delineate those areas that might be subject to flooding during the 5-year, 25-year, and 100-year storm events. The second objective was to define in economic terms the potential damage that may be expected during the selected storm events. The third objective was to develop, at a concept level, s i te-speci fie and/or regional flood-mitigation measures that could be implemented to address the area's drainage problems. These measures were to be developed in a manner that would not only consider their cost effectiveness, but also their general acceptability to the community. The fourth and final objective was to determine if a moredetailed analysis of the concept mitigation measures was warranted as an aid to the final evaluation and selection process. r . col r r r r r r r i r r r r r ~ Drainage ~ Flood-Control Engineering Page 3 Phase II of the study, if deemed necessary by Pima County, was envisioned as a means to develop preliminary designs and cost estimates for each of the selected mitigation measures. The cost estimates are to consider all applicable costs (i.e., engineering, construction, right-of-way and/or land acquisition costs). Once the Phase II study is completed, a final determination will be made regarding which problem areas to improve, the order of improvement, and the source of funding for these improvements. This final determination will be made my Pima County. However, before any improvements can be made, construction plans must be prepared. The preparation of these plans may require the services of both a structural engineer and/or a soils engineer. r r r r r r r r r r r r r r r r ,.. II. GENERAL ME'I1iODOiffiY The quantitative aspects of this project can be broken down into three major areas: hydrology, flood plain delineation, and flood-damage assessment. However, the conclusions and recommendations also consider other elements which are both quantitative and qualitative in nature. 2.1 Hydrology A hydrologic analysis was performed at 15 key concentration points using the Pima County Flood Control District's method for determining peak discharges (Reference 1). A complete range of discharges was determined at each concentration point. This range included the 2-year, 5-year, 10-year, 25-year, 50-year, and 100-year return intervals. The offsite drainage basin boundaries associated with these concentration points were delineated using the Tucson North, 7.5 minute series, U.S.G.S. quadrangle (see Figure 3). The onsite drainage basin boundaries were delineated using the project's topographic map (see Figure 4). This map, which was specifically prepared for this study, has a scale of 1" = 200' and a contour interval that is equal to two feet. Only one point-precipitation depth was defined for each return interval (see Figure 5). The location of the reference point used to define these depths corresponds to the approximate centroid of the South Branch watershed relative to its confluence with the Carmack Wash. The longitude and latitude of this point is approximately 110 degrees, 57 minutes, 7 seconds and 32 degrees, 21 minutes, 40 seconds, respectively. The drainage basin and/or watercourse roughness factors were selected using information obtained from aerial photographs and field investigations. However, if an appropriate roughness factor exceeded the maximum value recommended in Reference 1, the maximum value from that reference was used. This approach was selected to ensure that the results would be conservative. The hydrologic soi 1 groups inherent to the study's watersheds were determined using the Soil Conservation Service, soil survey maps and Reference 2. When two or more soil groups existed within the boundary of a particular drainage area, a weighted value was determined. Figure 6 shows the relative extent of each soil type and its hydrologic group classification. The relative degree of imperviousness, expressed as a percent, within the associated watersheds or drainage areas was estimated using Reference 1 in conjunction with Pima County's and the Town of Oro Valley's zoning classification maps. When more that one zoning classification existed within the boundary of a particular drainage basin, a weighted value was determined. Figure 7 shows the relative extent of each zoning classification with respect to the nine major onsite drainage concentration points. Drainage areas within the Coronado National Forest were considered 100 percent natural. Therefore, the relative degree of imperviousness within the offsite drainage areas is zero. All unpaved areas within public rights-of-way were assumed to have a relative imperviousness of five (5) percent. All paved areas were assumed to be 100 percent impervious. The relative imperviousness ~ Drainasc ~ Flood-Control Bngineerin& r ,.., ,.. ~ Drainage l associated with the various zoning classifications shown on Figure 7 are as follows: ! SR or R1-144: CR1 or R1-33: R-4 through R-6: C-1: C-2: r ' r .. r r ,.. I ,.. I ,. ; r ,.. ,.. ! . I ,.. .. t r Page 5 Flood-Control Engineering 10% 20% 65% 70% 90% The type of vegetation found within the offsi te drainage areas falls within the desert-scrub classification. A vegetative cover density of 30 percent was considered appropriate for all offsite areas. Within the boundary of the study area a mixture of desert scrub and urban lawns exist. Although the relative density of desert scrub within the onsite drainage areas is closer to 20 percent, a value of 30 percent was used to account for the effect of urban landscaping. 2.2. Flood Plain Delineations In accordance with the scope-of-work associated with this project, a flood plain analysis was performed along all watercourses within the boundary of the study area that were subject to a runoff quantity in excess of 100 cfs during the 5-year event. For the most part, this analysis was conducted using the U.S. Army Corps of Engineers' HEC-2, water-surface profile model (Reference 3). The project's 200-scale topographic map was used to define the ground profile for each cross section used in the analysis. The locations of these cross sections are shown on Figure 8. Since the contour interval on the topographic map is two feet, the relative accuracy of the mapping is typically plus or minus one foot. A multiple-profile model was used to determine the hydraulic characteristics associated with the 5-year, 25-year and 100-year flow events. In accordance with Pima County flood plain delineation criteria, each model was structured assuming either critical or subcritical flow conditions exist along all the respective watercourses. If it was evident that flow conditions along a particular watercourse were well within the supercritical range, a supercritical analysis was also performed to serve as a basis for evaluating erosion hazards. All culvert crossings were evaluated using the Federal Highway Administration's culvert analysis program, HY-8 (Reference 4). As-built roadway plans provided some of the required input data (i.e., size, shape, type, invert elevations, and roadway elevations). All elevations obtained from these plans were checked against the project's topographic map to ensure conformity within a reasonable tolerance range. Since the topographic map and the as-built plans were prepared using different datums, a direct relationship between the two documents could not be used to evaluate the hydraulic characteristics of each structure and its effect on the adjacent flood plains. The tailwater condition that is expected to exist immediately downstream of the culvert outlet was determined using the results of the HEC-2 analysis as a guide. If the subcritical analysis of the downstream channel defaulted to critical depth, a typical section in the immediate vicinity of the culvert's outlet was used to obtain a tailwater rating curve that covered the full range of discharges to be analyzed. If the subcritical analysis generated a "" 6(c) r r r r r r r r r r r r r r r r r r ~ Drainage & Flood-Control Engineering Page 6 subcritical water surface elevation, the elevation was compared to the m1n1mum (default) tailwater elevation used by the HY-8 program (i.e., the elevation of the soffit plus the critical depth elevation divided by two). The greater of the two elevations was then used as the controlling tailwater elevation. Any supplemental calculations required to estimate the hydraulic characteristics of the area's f load plains were performed using Manning's equation. The applicability and use of Manning's equation is discussed in detail in numerous hydraulic engineering textbooks and other publications. Reference 5 is just one of these publications. Upon completion of the flood plain analysis, the 100-year flood plain was delineated on the projects topographic map. Although the 5-year and 25-year flood-prone limits were not delineated, the results of the respective analyses were used as part of the damage-assessment analysis to define, in economic terms, the potential damage that might be expected during these flow events. 2.3 Flood-damage Assessment A damage-assessment analysis was performed on all structures located within the 100-year flood plain and on some of the structures that are located along or just outside the flood plain boundary. The purpose of this assessment was to determine the dollar amount of damages that might be expected during the respective flow events (i.e., 5-year, 25-year and 100-year). Once the affected structures were identified, each was surveyed to obtain the lowest finished-floor elevation. This elevation was then compared to the water-surface elevation at the upstream limit of the structure to obtain the approximate depth of inundation or f loading. If the upstream I imit of the structure did not correspond to one of the cross-section locations used in the analysis, an interpolated elevation was obtained. An attempt was then made to contact each property owner and/or occupant to obtain any information they might have concerning drainage-related problems in their area. Drainage complaints on file with the Pima County Flood Control District and the Town of Oro Valley were also consulted to identify the type and frequency of problems already noted within the study area. The location of these problem areas were then compared with the results of the flood plain analysis to determine if some type of correlation existed. In order to estimate potential damages during a particular storm event, some monetary value had to be established for the structure and its contents. To establish this value, Pima County property-tax records were consulted to obtain the most recent assessment of the full-cash value of each affected parcel and its associated improvements. Although these records provide values for both the land and its improvements (which includes the structure), only the value of the improvements was used in the final determination. Since the full-cash value of a structure and other improvements on a parcel is not a true representation of its momentary value on the open market, an estimated market value had to be determined. In was assumed that the estimated market value would be approximately equal to 1.25 times the assessed value. This was considered a reasonable assumption since the state of Arizona and the local tax-assessor's office use 80% of the market value as an indication or guide to establishing the assessed value. O{[ r r r r r r r rI r r r r r r r r r r ~ Drainage l Flood-Control Engineering Page 7 In an attempt to estimate potential damages to the structure's contents, it was further assumed that the value of the contents would be approximately equal to 30 percent of the estimated market value. Since this relationship has been used in the past by the Federal Emergency Management Agency (Reference 6), it was considered appropriate for use in this study. Using the depth of flooding and the total estimated value of the structure and its contents, damages were estimated as a percentage of the value using a table compiled by the Federal Emergency Management Agency. Although Table 17 in Reference 6 could have been used for this purpose, an updated table that provides even greater detail was obtained from the Pima County Flood Control District. A copy of this table is provided as Table 2.3A. , -, --, .. -, 1 -, .-, -, -, --. --, --, -·, e" ,. .- g '1' ,g ..a .; :; •• - ;" I~ ,."' 'W > -, ........, -, -, --, 1989 FEMA Depth Damage •" !• -~, lnundaUon Depth Commercial Residential Mobile Home Structure Content Structure Content Structure Content -1.0 0.00 0.00 0.00 0.00 0.00 0.00 ~-5 3.85 5.68 3.85 3.59 4.t4 1.63 0.0 7.78 11.31 7.78 7.t8 8.26 3.25 0.5 10.58 14.18 10.58 8.46 26.28 14.92 1.0 13.35 17.00 13.35 9.74 44.33 26.58 1.5 18.79 24.52 18.79 13.73 53.79 37.85 2.0 20.23 32.04 20.23 17.71 63.25 49.11 2.5 23.38 33.35 23.38 20.13 68.28 56.60 3.0 26.49 34.68 26.49 22.54 73.30 64.09 3.5 27.57 35.77 27.57 25.42 75.88 67.23 4.0 26.65 38.88 26.65 28.31 78.47 70.37 4.5 29.25 38.75 29.25 30.73 79.11 72.98 5.0 29.85 40.61 29.85 33.15 79.74 75.59 5.5 35.25 42.76 35.25 36.19 80.30 76.64 6.0 40.68 44.91 40.66 39.23 80.86 77.68 6.5 41.74 47.38 41.74 41.64 81.36 78.24 7.0 42.83 49.88 42.83 44.05 81.89 78.80 7.5 43.41 52.32 43.41 47.04 8.0 44.00 54.77 44.00 50.03 ' '0 ~ (1) 00 0/? ~· r r r r r r r r r r r r r r r r r r !"' ! III. DISCUSSION OF APPROACH AND RESULTS 3.1 Hydrology Table 3.1A summarizes the results of the hydrologic analysis. Included in the table are the primary output parameters from the analysis and the most significant input parameters (i.e., drainage area, basin factor and slope). Individual hydrologic data sheets for the 100-year event with peak discharges for the more frequent events are provided in Appendix B. Also included in AppendixB is a 5 1/4 inch diskette which contains the individual hydrologic data sheets for all return periods. Although these files are compressed to save space, the expansion software is included to facilitate reconstruction and printing of the individual sheets, if desired. Based on the results of the hydrologic analysis, only the South Branch Wash and its tributaries (i.e., the SBW Northern Tributary and the Glenhurst Wash) and the Shadow Mountain Wash are capable of generating greater than 100 cfs during the 5-year event. As is noted on Figure 4, the concentration points (CP) associated with the South Branch Wash are CP 1 through 9. The concentration points associated with the Shadow Mountain wash are CP 11 through 14. Although, the 5-year peak discharge at CP 11, which is located immediately upstream of Oracle Road, is well in excess of 100 cfs, this discharge is not attributable to a single flooding source. Two contributing drainage areas (i.e, the one associated with CP 10 and the one associated with CP 11A) deliver concentrated runoff to CP 11 at about the same time. However, neither of these two drainage areas are capable of generating greater than 100 cfs during the 5year event. Likewise, hydrologic calculations relative to CP 12 also indicate that this drainage area is not capable of generating in excess of 100 cfs during the 5-year event. With regard to the South Branch Wash, it was noted that the greatest peak discharge during any given return interval occurs at CP 7 as opposed to CP 9, which is the downstream limit of the watershed. This also occurs within the Shadow Mountain Wash watershed (i.e., the greatest peak discharge occurs at CP 11 as opposed to CP 13). Although both the drainage area and the degree of urbanization continues to increase in the downstream direction relative to both watersheds, the associated peak discharge decreases. This reduct ion can, to some extent, be attributed to transmission losses and channel storage. Although the Pima County method does not attempt to account for these attenuation effects directly, it does do so indirectly. In this particular instance, the computed time it takes runoff to concentrate at the respective downstream points offsets the re !at i vely small increase in both the drainage area and its degree of urbanization. In this way, the Pima County method does, indirectly, account for both transmission losses and channel storage. Although there is a peak reduction in the downstream direction, the South Branch f load plain from the Carmack Wash upstream to its tributary, the SBW Northern Tributary, and the Shadow Maintain Wash from CP 13 upstream to Oracle Road were both initially modeled using the higher peak discharges. Consequently, the results of the flood plain analysis can be considered conservative. At this point, it should also be noted that the offsite basin factors used in the hydrologic analysis of the South Branch watershed were based on the ~ Drainage & Flood-Control Engineering Dl~ r r Page 10 r r Hydrologic Summary Table r Cone. Pt. Return Period (yr) Runoff Coefficient r' Drainage Area (acres) Rainfall Intensity (in/hr) Time of cone. (min) Peak Discharge (cfs) 1 89 2 5 10 25 50 100 0.345 0.500 0.574 0.649 0.700 0.739 2.32 3.54 4.35 5.42 6.35 7.32 22 16 14 12 11 10 72 159 224 316 399 486 2 5 10 25 50 100 0.345 0.500 0.574 0.649 0.700 0.739 3.21 4. 77 5.78 7.14 8.08 9.44 11 8 7 6 6 5 60 130 181 252 308 380 2 5 10 25 50 100 0.345 0.500 0.574 0.649 0.700 0.739 2.44 3.78 4.63 5.61 6.57 7.70 20 14 12 11 10 9 52 116 163 224 283 350 2 5 10 25 50 100 0.331 0.484 0.559 0.634 0.686 o. 726 1.35 2.25 2.83 3.61 4.38 5. 13 52 36 31 27 24 22 212 516 749 1086 1424 1764 2 5 10 25 50 100 0.334 0.486 0.560 0.635 0.687 0. 727 1. 21 1.96 2.53 3.25 3.92 4.64 61 44 37 227 535 796 1156 1508 1889 ir r r r r r r r ,.. Basin Factor: 0.060 Slope: 0.2186 ft/ft 2 54 Basin Factor: 0.060 Slope: 0.2185 ft/ft 3 61 Basin Factor: 0.060 Slope: 0.1839 ft/ft 4 470 Basin Factor: 0.060 Slope: 0.1304 ft/ft 5 556 ! ' r r r r Basin Factor: O.OS6 Slope: 0.1006 ft/ft ~ Drainage l flood-Control Engineering 32 29 26 TABLE 3.1A o1< r Page 11 r r ' Hydrologic Summary Table r r r' r r r r r r r r r r Cone. Pt. Drainage Area (acres) Return Period ( yr) Runoff Coefficient 6 194 2 5 10 25 50 100 0.348 0.495 0.568 0.641 0.691 0.730 2 5 10 25 50 100 Basin Factor: O.OSS Slope: 0.1238 ft/ft 7 750 Basin Factor: 0.056 Slope: 0.1006 ft/ft 8 778 Basin Factor: Q.OS4 Slope: 0.0892 ft/ft 9 837 Basin Factor: 0.051 Slope: 0.0635 ft/ft 10 31 Basin Factor: 0.047 Slope: 0.0812 ft/ft Rainfall Intensity (in/hr) Time of Cone. (min) Peak Discharge (cfs) I. 79 2.87 3.54 4.51 5.40 6.18 34 24 21 18 16 15 122 278 393 566 730 883 0.338 0.488 0.562 0.636 0.687 0. 727 1.22 1.99 2.53 3.25 3.92 4.64 61 43 37 32 29 26 311 736 1077 1562 2037 2550 2 5 10 25 50 100 0.336 0.486 0.560 0.634 0.685 0.725 1.11 1.88 2.37 3.07 3.67 4.37 68 47 41 35 32 29 292 718 1040 1530 1976 2485 2 5 10 25 50 100 0.342 0.489 0.561 0.635 0.685 0.725 0.85 1. 54 1.98 2.58 3.14 3.69 92 62 54 46 41 38 246 635 939 1382 1815 2256 2 5 10 25 50 100 0.359 0.484 0.549 0.617 0.665 0.704 3.00 4.31 5.21 6.43 7.66 8.54 13 10 9 8 7 7 34 65 90 124 159 188 r r ~ Drainage & Flood-Control Engineering TABLE 3.1A (cont. l r r r r r r r r r r r' r r r r r r r r ~ Drainage l Flood-Control Engineering Page 13 maximum values recommended in Reference 1 (i.e., 0.060). The results of the field investigations indicate that more appropriate values would be in the range of 0.080 to 0.12. This is due to the existence of boulders, debris, natural pools, and a higher density of vegetation along the associated watercourses. All of these factors contribute to a roughness factor that is higher than the 0.060 value. The latter value was selected, however, to further ensure that the results of the flood plain analysis would be conservative. As previously mentioned, the results of the hydrologic analysis of the Shadow Mountain Wash indicates that the flood plain analysis, by definition, should be limited to the reach located between the Carmack Wash and Oracle Road (US 89). However, considering the magnitude of the 5-year discharge at CP 11 (205 cfs) and at CP 11A (98 cfs). The 100-year peak discharge associated with CP 11 was used to define and map the ponding limits at the inlet to the singlecell (8' x 5') concrete box culvert (CBC) that currently exists beneath Oracle Road at this location. As a final note to the discussion, the results of portions of this analysis were found to be consistent with the results obtained by other engineers working in the area and by members of the Pima County Flood Control District. 3.2 Flood Plain Analysis The results of the flood plain analysis are summarized in a series of tables that follow the discussion of each wash or watercourse. A table was compiled for each of the multiple-profile, HEC-2 models that were prepared as part of the overall analysis. Figure 8 shows the location of all cross sections used in the analysis and the 100-year flood-prone limits. Hard copies of both the input 1ist ing and a port ion of the output 1ist ing for each mode 1 are contained in Appendix C. Floppy disks containing the input data file and a detailed output listing for each model are also included in Appendix C. The following paragraphs provide a brief description of each wash or watercourse studied and its associated cross sections. The name given each model is also provided for reference purposes. These names are related to either the name of the wash itself or its cross-section numbers. 3.2.1 Watercourse Description and Definition For the most part, the South Branch Wash and its tributaries (the SBW Northern Tributary and the Glenhurst Wash) were modeled using four separate HEC2 runs. However, a truncated fifth run was also included in the analysis to provide both a qualitative and quantitative estimate of the extent and nature of flow within the immediate vicinity of the Carmack Wash. As previously mentioned, each run was set up to provide the 5-year, 25-year and 100-year water surface profiles. Two of these runs were indirectly linked by an HY-8 analysis of the Oracle Road box culvert, a five cell, 10' x 6' CBC. The first run models the South Branch Wash between its confluence with the Carmack Wash and Oracle Road. This run is represented by Cross Sect ions 1 through 19. The input file name for this model is SBW.DAT. The second run models the South Branch from Oracle Road upstream to the Forest Service Boundary. This run is represented by Cross Sections 20 through 38.6. The input file name for this model is SBW-US1.DAT. The Oracle Road CBC is situated between Section 19 of the SBW.DAT model and Section 20 of the SBW-USl.DAT model. A small tributary watercourse was also included in the SBW-US1 model. This tributary, r r r r r r r r r r r r r r r Page 14 ~ Drainage l Flood-Control Engineering which conveys the runoff associated with CP 3, is represented by only one cross section, Section 38.5. The third run models the SBW Northern Tributary which is represented by Cross Sections 39 through 50. The input file name for this model is SBWUS2. OAT. The fourth run models the Glenhurst Wash which provides tributary flows to both the SBW Northern Tributary and the South Branch Wash. AI though a port ion of the runoff conveyed in the Glenhurst Wash is conveyed down Glenhurst Drive toward CP 10, the historic drainage path for low-flow runoff is toward the SBW Northern Tributary. Consequently, the Glenhurst Wash is considered a tributary to the SBW Northern Tributary and, in turn, a tributary to the South Branch Wash. The Glenhurst Wash is represented by Cross Sections 401 through 409. The input file name for this model is 401-409.DAT. Pre 1iminary results of the SBW analysis indicate that a port ion of the flow conveyed in the main channel for the South Branch wi 11 be diverted down an historic channel braid that exists midway between its confluence with the Carmack Wash and Oracle Road. To define the flood plain characteristics of this braid, a series of cross sect ions were established along its upstream 1imi t. Flow along this braid is represented by Cross Sections 300 through 305. The input name associated with this model is 300-305.DAT. This run is the truncated fifth run that was used to complete the South Branch flood plain analysis. Cross Sections 200 through 212 apply to a tributary watercourse that flows into the Shadow Mountain Wash immediately upstream of its confluence with the Carmack Wash. For the sake of discussion, this watercourse will be referred to as the SMW Southern Tributary. Although the watershed that contributes runoff to this tributary did not generate greater than 100 cfs during the 5-year event, preliminary results of the SBW analysis indicated that this wash will be subject to breakover flows from the South Branch. Since the quantity of runoff entering this wash from the South Branch during its 5-year event was greater that 100 cfs, this tributary was included in the flood plain analysis and the associated flood plain was considered a part of the South Branch Wash flood plain. The Shadow Mountain Wash is represented by Cross Sections 101 through 120. These sections extend from its confluence with the Carmack Wash upstream to Oracle Road. The input name associated with this model is l01-120.DAT. As previously mentioned, a separate HY-8 analysis was performed on the Oracle Road structure that contributes flow to this reach of the wash. The results of this analysis were used to map the associated 100-year flood-prone area that exists in the immediate vicinity of the culvert inlet. The analyses just described are summarized as follows: SBW.DAT r South Branch Wash from its confluence with the Carmack to Oracle Road. SBW-US1.DAT r South Branch Wash from Oracle Road upstream to the Forest Service Boundary. SBW-US2.DAT Northern tributary to the South Branch Wash from Rancho Catalina Avenue upstream to the Forest Service Boundary. r r Dlq r r r r r r r r r r r r r r r r r r r ~ Drainage l Flood-Control Engineering Page 15 401-409.DAT Glenhurst Wash with tributary flows to the SBW Northern Tributary in the immediate vicinity of Glenhurst Drive. 300-305.DAT Historic channel braid along the South Branch Wash immediately upstream of Northern Avenue. 200-212.DAT Southern tributary to the Shadow Mountain Wash from Calle Buena Vista upstream to Shadow Mountain Drive. !Ol-120.DAT Shadow Mountain Wash from its confluence with the Carmack Wash upstream to Oracle Road. 3.2.2 South Branch Wash Flood Plain The South Branch Wash flood plain, as delineated on Figure 8, represents the combined results obtained using six of the seven HEC-2 models described in the previous sect ion. The approach used to determine both the extent of flooding and the associated water surface elevations is presented in the following paragraphs. After initial runs of the SBW model indicated that a portion of the runoff conveyed in the South Branch would overtop the drainage basin divide that separates the South Branch watershed from the watershed associated with CP 12, the model was revised to include a split-flow or weir flow analysis along the divide. This analysis was used to estimate how much flow could enter the northern half of the developed portion of the Rancho Felix subdivision (Book 29, Page 24). The split-flow routine, which is an integral part of the HEC-2 program, automatically reduces the discharge in the downstream direction as flow is lost over the weir crest or in this case the basin divide. However, the program is not capable of modeling the impact downstream when flow branches away from the primary watercourse along a secondary watercourse, in this case an historic channel braid. To accomplish this, it was necessary to first estimate the quantity of flow that would be diverted along the braid. Cross Section 10 of the SBW analysis was used for this purpose. This cross section is located at the downstream limit of the weir crest, which extends upstream to Cross Section 15.5. Consequently, the discharge arriving at Section 10 had already been reduced by the program to account for losses over the weir crest. The cross section input data was then structured such that the right bank station corresponded to the boundary between the main channel and the adjacent channel braid. It was then assumed that all flow conveyed in the right overbank at this section would be diverted down the channel braid. Based on the results of the preliminary analysis, the diverted flow quantities for the 5-year, 25-year and 100-year events are approximately 462 cfs, 845 cfs, and 1194 cfs, respectively. The 300-305 model was then used to determine if all of the diverted flow would remain within the physical confines of this secondary watercourse. The results indicate that a major port ion of the flow will return to the South Branch as a result of weir flow over the southern boundary of the watercourse. The quantity of weir flow exiting this historic watercourse or channel braid was then reintroduced into the SBW model at the appropriate locations. r r r r r r r r r r r r r r r r r r r ~ Drainage l Flood-Control Engineering Page 16 Approaching the flood plain analysis in this manner provides a truer representation of the extent of the 100-year flood plain under existing conditions. If mitigation measures are implemented to prevent weir flow into the Rancho Felix subdivision or to prevent flow diversion along the channel braid, the results of the exiting-condition model can be used as the basis for defining the impact of this type of improvement. Tables 3.2.2A and 3.2.2B summarize the results of the weir flow and diverted flow analyses just described. Discharges lis ted under the "Diverted Flow" heading and under the "Weir Flow" heading represent flows lost from the system (this is further emphasized by use of the negative sign). Discharges listed under the "Incoming Flows" heading represent flows added to the system. Although the table begins at the downstream-most section and proceeds upstream, the values at each section represent the cumulative effect as flow moves in the downstream direction. Table 3.2.2A, which applies to the SBW run, indicates that weir flow over the basin divide begins between Sections 14 and 15.5 and continues to increase until it reaches its maximum at Section 11. Weir-flow values remain at this constant value for all downstream sections, thus indicating that no additional flow was lost from the SBW system as a result of weir flow. However, between Sections 9 and 10, an additional quantity of flow is lost from the system as diverted flow. Since no other flow diversions occur downstream of these sections, the value under the "Diverted Flow" heading remains constant for the downstream sections. The effect of flows returned to the SBW system as a result of weir flow from the 300-305 system begin at Section 8 and end at Section 4. These flow quantities are listed under the "Incoming Flows" heading. The results of the accounting process are presented under the "Final Discharge" heading. In contrast, the values shown under the "Incoming Flow" column of Table 3.2.2B, which applies to the 300-305 run, are the same for all sections, thus indicating that the amount of flow entering the system at the upstream limit (Section 305) remains unchanged as one moves in the downstream direction. However, weir-flow loses begin between Sections 304 and 303 and continue to increase in the downstream direction throughout the entire run. This is noted by a gradual increase in the discharge values I is ted under the "Weir Flow" heading and a gradual reduct ion in the values I is ted under the "Final Discharge" heading. For all summary tables, the computed water-surface elevations at each section are based on the discharge presented under the "Final Discharge" heading. In addition, the velocity of flow in the main channel and its depth are presented in these summary tables. Table 3.2.2C summaries the results of the analysis of the SMW Southern Tributary (Sections 200 through 212) which was performed using the breakover quantities from the SBW analysis. The quantity of weir flow entering the 200212 system from the South Branch is represented under the "Incoming Flow" heading. A split-flow routine was since preliminary runs indicate would overtop its western bank crossing. However, breakout at year events. The total amount also included in the analysis of this tributary that a portion of the flow conveyed in this wash in the immediate vicinity of the Hardy Road dip this location is limited to the 25-year and 100of flow exiting the main channel was determined r Page [7 r ' r r r r r r r r Hydraulic Summary Table for the South Branch Wash from Confluence with the Carmack Wash upstream to Oracle Road Cross Section No. !. 00 Return Period (yr) ( cfs) Diverted Weir Flow (cfs) Flow (cfs) Incoming Flow (cfs) Final Discharge (cfs) Water Channel Surface Velocity (ft-m.sl) (fps) Maximua Depth ( ft) 5 25 100 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 233.00 544.00 826.00 315.03 708. 14 1076.47 2532.75 2533.37 2533.77 4.74 6.03 6.75 2. IS 2. 77 3. I 7 s 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 233.00 544.00 826.00 315.03 708. 14 1076.47 2538.19 2538.57 2538.84 3.53 4. 75 5.55 1.69 2.07 2.34 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 233.00 544.00 826.00 315.03 708. 14 1076.47 2544.51 2545.10 2545.34 4.i6 5.30 6.31 2.51 3. 10 3.34 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 233.00 544.00 826.00 315.03 708. 14 1076.47 2547.95 2548.54 2548.90 4.43 5.22 5.69 3.45 4.04 4.40 2.00 25 100 3.00 25 100 3.50 25 100 4.00 25 100 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 233.00 544.00 826.00 315.03 708.14 1076.47 2550.99 255!. 58 2551.94 4.47 5.32 5.84 3.69 4.28 4.64 5 25 100 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 214.00 516.00 790.00 296.03 680.14 1040.47 2554.09 2554.63 2555.03 4.60 5.99 6.61 !.09 1.63 2.03 s 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 169.00 425.00 659.00 25!. 03 589. 14 909.47 2557.85 2558.53 2558.98 4.90 6.15 6.63 !.85 2.53 2.98 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 62.00 209.00 353.00 144.03 373. 14 603.47 256 !.20 2561.70 2561.99 3.23 4.13 4.85 1.20 !. 70 !.99 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 14.00 71.00 137.00 96.03 235.14 387.47 2565.15 2565.50 2565.78 3.45 4.18 4.56 0.85 1.20 1.48 740.00 1560.00 2550.00 -462.00 -845.00 -1194.00 -195.97 -550.86 -1105.53 o.oo 0.00 82.03 164.14 250.47 2570.25 2570.44 2570.57 2.86 3.57 4.06 1.25 1.44 !. 57 740.00 1560.00 2550.00 o.oo o.oo 0.00 -195.97 -550.86 -1105.53 o.oo 544.03 1009. 14 1444.47 2574.28 2574.63 2574.89 3.88 4. 52 5.01 !. 58 1.93 2. 19 s s s r s.oo r 6.00 25 100 7.00 25 100 r 8.00 25 100 r 9.00 25 100 r Peak Discharge its s s I r s s 10.00 25 100 0.00 0.00 0.00 ! r r r I ~ Drainage A Flood-Control Engineering TABLE 3.2.2A D';)1- r r Page 18 r r r r r r r r r r r r r Hydraulic Summary Table for the South Branch Wash from its Confluence with the Carmack Wash upstream to Oracle Road (continued) Diverted Cross Return Peak. Section Period (yrl Dischar!e No. (cfs) Flow (cfs) Weir Flow ( cfs) 11.00 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo -195.97 -550.S6 -ll05.53 12.00 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo -190.20 -527.07 -1062.51 13.00 5 25 100 740 00 1560.00 2550.00 o.oo o.oo o.oo 14.00 5 25 100 740.00 1560.00 2550.00 o.oo o.oo 5 25 100 Incoaing Flow (cfs) o.oo final Discharge (cfs) Water Channel Surface Velocity ( fps) (ft-msl) Maximum Depth ( ft) 544.03 1009 14 1444.47 2577.71 257S.09 257S.36 4.20 5.01 5. 72 I. 71 2.09 2.36 o.oo o.oo 0.00 549.SO 1032.93 14S7.49 25SJ.l2 25S!.47 25S!. 7S 6.02 7.44 7.95 3.12 3.47 3.7S -4.77 -101.43 -419.05 o.oo o.oo o.oo 735.23 145S.57 2130.95 25S5.34 25S5.9S 25S6.36 7,04 s.J6 9.02 2.34 2.9S 3.36 o.oo o.oo 0.00 0.00 -77.76 -372. OS 740.00 14S2 24 2177.92 2590.23 2590.73 2591.17 5.03 6.79 7.69 2.03 2.53 2.97 740.00 1560.00 2550.00 o.oo o.oo o.oo 0.00 -10.19 -S6.95 740.00 1549.SI 2463.05 2594.04 2594,94 2595 54 6. IS 6.97 7.62 2.S4 3.74 4.34 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo 740 00 1560.00 2550.00 2596.15 2596.91 2597.45 7.45 S.75 10 ,IS 2.95 3 71 4.25 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo o.oo 0.00 0.00 o.oo 0.00 o.oo 740.00 1560.00 2550.00 2599.5S 2600.2S 2600 70 5.77 6.49 7.52 3.3S 4.0S 4. 50 17.00 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo 0.00 0.00 o.oo 0.00 o.oo 740.00 1560.00 2550.00 2603.51 2604.41 2605,02 7.38 S.42 9.36 3.21 4.10 4.72 IS.OO 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo 0.00 o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2608.04 2608.67 2609.37 6.64 S.55 9.2S 3.14 3.77 4.47 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo 0.00 0.00 0.00 19.00 740.00 1560.00 2550.00 2611.43 2612.61 2613.56 7 72 9.25 10.6S 2.63 3.SI 15.00 15.50 16.00 0 0.00 o.oo o.oo 0.00 0.00 0.00 0.00 o.oo o.oo 0.00 o.oo o.oo 0 0 0 0 0 0 0 4. 76 r' r' r r ~ Drainage ~ flood-Control Engineering TABLE 3. 2. 2A (cont.) {)d.~ r r r r r r r r r r r r r r r r r r r Page 19 Hydraulic Summary Table for Cross Sections 300 through 305 (Historic Channel Braid) Cross Section Return Period Discharge No. (yr) (cfs) s 300.00 25 100 s 300.50 25 100 s 300.70 25 100 301.00 25 100 s s 302.00 25 100 s 303.00 25 100 s 304.00 25 100 305.00 25 100 s Peak. N/A N/A N/A Flow Weir Flow (cfs) (cfs) 0.00 o.oo 0.00 Final Water Channel Surface Velocity ( ft-IIS I) ( fps) Inco•ing Flow (cfs) Discharge -237.71 -545.51 -825.06 462.00 845.00 1194.00 224.29 299.49 368.94 2556.29 2556.35 2556.40 2.06 2.31 2.51 0.29 0.35 0.40 2.63 2.92 1.33 I. 39 1.43 Diverted (cfs) Maximua Depth ( ft) N/A N/A N/A o.oo 0.00 0.00 -219.09 -517.65 -789.03 462.00 845.00 1194.00 242.91 327.35 404.97 2558.43 2558.49 2558.53 3 17 N/A N/A N/A o.oo o.oo o.oo -200.63 -489.23 -752.16 462.00 845.00 1194.00 261.37 355.77 441.84 2559.50 2559.62 2559.71 3.22 3.72 4.11 N/A N/A N/A 0.00 -171.81 -425.80 -658. 13 462.00 845.00 1194.00 290. 19 419.20 535.87 2561.30 2561.51 2561.65 4.26 4.83 5.25 N/A N/A N/A 0.00 0.00 -61.82 -209.88 -352.33 462.00 845.00 1194.00 400. 18 635. 12 841.67 2564.19 2564.45 2564.63 3.43 4.01 4.42 I. 29 462.00 845.00 1194.00 448.27 773.35 1057.19 2567.46 2567.70 2567.86 3.68 4.41 4.98 1.46 462.00 845.00 1194.00 462.00 845.00 1194.00 2570.50 2570.77 2570.97 3.24 4.03 4.67 1.80 2.07 2.27 462.00 845.00 1194.00 462.00 845.00 1194.00 2574.31 2574.63 2574.90 4.65 5.92 6.64 I. 31 o.oo 0.00 o.oo N/A N/A N/A 0.00 0.00 -13.73 -71.65 -136.81 N/A N/A N/A 0.00 o.oo o.oo o.oo N/A N/A N/A o.oo 0.00 o.oo o.oo ~ Drainage ~ Flood-Control Engineering 0.00 0.00 0.00 o.oo o.oo 0 I. so 1.62 1.71 I. 30 I. S I 1.65 !.55 I. 73 1. 70 1.86 1.63 1.90 TABLE 3.2.2B r r Page 20 r ' r r r r Hydraulic summary Table for the SMW Southern Tributary, Sect ions 200 through 212 Incoaing Plow (cfs) Final Discharge (cfs) 197.00 550.00 1105.00 197.00 503.31 873.35 2540.42 2540.91 2541.21 4. 53 5.69 6.76 2.42 2. 91 3.21 -46.69 -231.65 197.00 550.00 1105.00 197.00 503.31 873.35 2542.32 2543.01 2543.36 5.35 5.94 6.92 2.32 3 .01 3.36 o.oo o.oo o.oo 0.00 -46.69 -231.65 197.00 550.00 1105.00 197.00 503.31 873.35 2544.53 2545.13 2545.48 5.33 6.43 7.35 2.53 3. 13 3.48 N/A N/A N/A o.oo o.oo o.oo o.oo -46.69 -231.65 197.00 550.00 1105.00 197.00 503.31 873.35 2546.27 2546.87 2547.31 3. 20 4.65 5.77 I. 77 2.37 2.81 5 25 100 N/A N/A N/A o.oo 0.00 o.oo -46.69 -231.65 197.00 550.00 1105.00 197.00 503.31 873.35 2548.74 2549.60 2550.52 5.58 7,07 5. 71 1.44 2.30 3.22 200.00 5 25 100 N/A N/A N/A o.oo 0.00 o.oo -46.69 -230.74 197.00 550.00 1105.00 197.00 503.31 874.26 2551.47 2552.50 2552.77 4.88 5. 22 6.87 2.47 3.50 3.77 201.00 5 25 100 N/A N/A N/A o.oo o.oo 197.00 550.00 1105.00 197.00 545.67 1068.20 2552.56 2553.45 2554.04 3.16 4.66 6.07 2.56 3.45 4.04 5 25 100 N/A N/A N/A o.oo o.oo o.oo 0.00 202.00 o.oo o.oo 197.00 550.00 1105.00 197.00 550.00 1105.00 2554.29 2555.11 2555.85 4.30 6.18 8. 18 1.29 2.11 2.85 5 25 100 N/A N/A N/A o.oo o.oo o.oo 0.00 203.00 o.oo o.oo 197.00 550.00 1105.00 197.00 550.00 1105.00 2558.75 2559.72 2560.68 5.76 7.53 8.42 I. 75 2. 72 3.68 5 25 100 N/A N/A N/A o.oo o.oo o.oo 197.00 204.00 197.00 550.00 1105 .oo 2562.49 2563.36 2564.26 5.14 7.19 8.40 1.99 2.86 3.76 Diverted Flow ( cfs) Weir Cross Section Return Period Peak Discharge No. (yr) ( cfs) 101.00 5 25 100 N/A N/A N/A o.oo o.oo o.oo -46.69 -231.65 101. so 5 25 100 N/A N/A N/A o.oo o.oo o.oo 102.00 5 25 100 N/A N/A N/A r 103.00 5 25 100 r r r r r r 103.50 r r r Cross I Flow (cfs) o.oo o.oo o.oo o.oo 0.00 o.oo -4.33 -36.80 0.00 o.oo o.oo sso.oo 1105.00 Water Channel Surface Velocity (ft-losl) ( fps) Waxiaua Depth ( ft) r r r ~ Drainage ~ Flood-Control Engineering TABLE 3.2.2C f:JO..< r Page 21 r r r r r r r r r r r r r r r Hydraulic Summary Table for the SMW Southern Tributary, Sections 200 through 212 (continued) Cross Section Return Peak Period No. (yr) Discharge (cfs) 205.00 5 25 100 N/A N/A N/A 206.00 5 25 100 N/A N/A N/A 207.00 5 25 100 N/A N/A N/A 208.00 5 25 100 209.00 209.50 Diverted Plow (cfs) Weir Plow (cfs) Incoaing Flow (cfs) Final Discharge ( cfs) Water Surface (ft-ul) Cross Channel Velocity (fps) Maxiaum Depth ( ft) o.oo o.oo 0.00 0.00 0.00 0.00 197.00 550.00 1105.00 197.00 550.00 1105.00 2565.97 2566.74 2567.63 4.52 6.69 7.90 I. 97 2. 74 3.63 0.00 0.00 o.oo 0.00 o.oo 191.00 526.00 1062.00 191.00 526.00 1062.00 2570.66 2571.25 2571.82 3.64 4.82 6,14 0.66 I. 25 I. 82 0.00 o.oo o.oo 0.00 o.oo o.oo 5.00 !02.00 419.00 5.00 102.00 419.00 2574.14 2574.74 2575.55 0.90 2.66 4. 13 0.14 0.74 I. 55 N/A N/A N/A o.oo o.oo o.oo o.oo 0.00 0.00 1.00 77.00 372.00 I. 00 77,00 372.00 2580.03 2580.43 2581.10 0.76 3.43 5.38 0.03 0.43 I. 10 5 25 100 N/A N/A N/A o.oo o.oo o.oo 0.00 o.oo o.oo I. 00 77.00 372.00 1.00 77.00 372.00 2583.57 2584.28 2585.25 0.88 4.25 5.93 0.07 0.78 I. 75 5 25 100 N/A N/A N/A 0.00 0.00 o.oo o.oo 0.00 I. 00 77 .oo 372.00 1.00 77.00 372.00 2586.15 2586.86 2587.62 I. 56 3.75 5. 18 0. 15 0.86 1.62 5 25 100 N/A N/A N/A o.oo 210.00 0.00 0.00 o.oo o.oo o.oo 1.00 77 .oo 372.00 I. 00 77.00 372.00 2588.75 2589.37 2590. 11 I. 20 3.60 4.39 0.15 0.77 I. 5 I 211.00 5 25 100 N/A N/A N/A o.oo o.oo o.oo o.oo o.oo o.oo !99.00 430.00 643.00 199.00 430.00 643.00 2577.32 2577,91 2578.30 4.43 5.73 6.65 I. 32 I. 91 5 25 100 N/A N/A N/A o.oo o.oo 0.00 !99.00 430.00 643,00 199,00 430.00 643.00 2578.84 2579.35 2579.74 4.01 4.46 4.58 0.84 1.35 I. 74 212.00 o.oo o.oo 0.00 o.oo o.oo 2.30 r r r ~ Drainage & Flood-Control Engineering TABLE 3.2.2C (cont.) -.,.w \; !' r r r r r r r r r r r r r r r r r r r ~ Drainage l Flood-Control Engineering Page 22 to be approximately 47 cfs and 232 cfs, respectively. The values attributable to each section are presented under the "Weir Flow" heading in Table 3.2.2C. Sections 101 through 103.5 of the Shadow Mountain Wash analysis were used at the downstream end of the 200-212 analysis to complete the flood plain study of the impact of breakover flows from the South Branch down to the Carmack Wash confluence. When the combined effect of all three flood plain analyses are considered, all of the area within the immediate vicinity of the Hardy Road - Northern Avenue (Calle Buena Vista) intersect ion wi 11 be located within the 100-year f loadprone area for the South Branch Wash. However, depths of flow in excess of one foot are 1imi ted to the area surrounding the primary watercourses (i.e., the main channel for the South Branch, the SMW Southern Tributary, the downstream limit of the Shadow Mountain Wash, and the upstream I imi t of the historic channel braid). All other areas are subject to shallow sheet flooding where the average depth of flow is less than one foot. For flood plain management purposes, it will be necessary to consult either the summary output listing contained in Appendix C or the detailed output listings contained on the floppy disk to determine the regulatory depth of flooding. To accommodate flow conveyed in the South Branch Wash in the immediate vicinity of Oracle Road, the Arizona Department of Transportation (AllOT) provided a five cell, 10' x 6' CBC (with inlet and outlet wingwa!ls) beneath Oracle Road. Upon completion of the initial SBW run, it was noted that the analysis of Section 19 defaulted to critical depth. Consequently, the HY-8 analysis of this structure was setup such that a tai !water rating curve would be generated as part of the analysis. This rating curve was based on critical depth. A typical rectangular channel section, which was considered representative of the geometry of the outlet channel, was used for this purpose. Based on the results of the HY-8 analysis, which are provided in Appendix D, it was noted that the South Branch CBC is operating under inlet control. The inlet control headwater elevation was then used as the starting water surface elevation for the SBW-US1 analysis. No special hydraulic modeling techniques were performed along this reach of the South Branch Wash. However, the results indicate that under existing conditions a portion of the runoff conveyed along this reach witt break out of the main channel between Sections 30 and 31. Using the flow distribution associated with Section 31, breakout quantities during the 5-year, 25-year, and 100-year events were estimated to be approximately 109 cfs, 313 cfs, and 562 cfs, respectively. Breakout occurs in an easterly direction along an historic branch of the main channel. Since the area impacted by this breakout is outside the limits of the study area and its topographic map, there was no means available to define the impact of this breakout or its significance from a flood plain management standpoint. Consequently, the HEC-2 analysis of this reach of the South Branch (SBW-US1) assumes that no breakout occurs (i.e., the entire peak discharge generated upstream of the breakout point is conveyed in its entirety through the downstream reach. The results of the SBW-US1 model are summarized in Table 3.2.2D. Since it was assumed that no breakout occurs, the "Diverted Flow" and "Weir Flow" columns ref teet a zero change in the peak discharge quantity used in this particular HEC-2 analysis. r r r r r Page 23 Hydraulic Summary Table for the South Branch Wash from oracle Road upstream to the Forest Service Boundary Cross Sect ion Period No. (yr) 20.00 25 100 Return 5 r r 21.00 25 100 22.00 r 25 100 23.00 r 25 100 23.50 25 100 r 5 5 5 5 5 23.60 r 23.70 r 23.80 r r r r r 25 100 5 25 100 5 25 100 5 24.00 25 100 5 25.00 25 100 5 26.00 25 100 27.00 25 100 28.00 25 100 5 5 Peak. Discharge ( cfs) Diverted Flow (cfs) 740.00 1560.00 2550.00 o.oo 740.00 1560.00 2550.00 0.00 740.00 1560.00 2550.00 o.oo o.oo 740.00 1560.00 2550.00 o.oo o.oo o.oo 740.00 1560.00 2550.00 o.oo o.oo 0.00 Weir Incoaing Flow (cfs) Discharge 0.00 o.oo o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2618.74 2620.60 2622.70 7.34 6.99 5. 70 1. 74 3.60 5.70 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2622.54 2624.34 2624.71 7.68 5.97 7. 17 4.84 5.21 0.00 o.oo o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2625.68 2627.47 2628.95 8.78 9.43 8. 18 3.68 5.47 6.95 0.00 o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2628.33 2631.43 2632.43 9.91 8. 14 7.73 4.33 7.43 8.43 o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2631.93 2634.40 2634.77 9.69 6.66 7. 77 4.93 7.40 7.77 0.00 0.00 o.oo o.oo o.oo 740.00 !560.00 2550.00 2635.60 2636.47 2636.96 6. 71 7,52 8. 75 5.60 6.47 6.96 0.00 Flow (cfs) 0.00 0.00 Final (cfs) Water Surface ( ft-OISI) Channel Velocity ( fps) Yaxiaum. Depth ( ft) 3.04 740.00 1560.00 2550.00 o.oo o.oo 0.00 o.oo 740.00 1560.00 2550.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2637.13 2638.18 2638.78 6.82 8.07 3. 13 4. 18 4. 78 740.00 1560.00 2550.00 o.oo o.oo 0.00 o.oo o.oo o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2639.64 2640.55 2640.95 6.99 6. 71 7.83 2.14 3.05 3.45 740.00 1560.00 2550.00 o.oo o.oo o.oo o.oo 0.00 0.00 o.oo o.oo o.oo 740.00 1560.00 2550.00 2643.23 2643.71 2644.25 3.73 5.46 6. 16 2.43 2.91 3.45 740.00 1560.00 2550.00 o.oo o.oo 0.00 0.00 o.oo o.oo o.oo 740.00 1560.00 2550.00 2650.19 2651.23 2651.68 6.86 6.73 7.91 2.19 3.23 3.68 o.oo o.oo 0.00 o.oo 0.00 o.oo o.oo o.oo o.oo 740.00 1560.00 2550.00 2657.75 2658.44 2658.80 5.36 5.37 5.96 1. 75 2.44 2.80 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2666.14 2666.65 2667.12 5.02 6.34 7,30 1. 64 2. IS 2.62 540.00 1160.00 1890.00 2672.03 2672.53 2673.00 4.96 6.23 7.22 1.43 1. 93 2.40 740.00 1560.00 2550.00 540.00 1160.00 1890.00 540.00 1160.00 1890.00 0.00 0.00 o.oo 0.00 7.45 f r r ~ Drainage & Flood-Control Engineering TABLE 3.2.20 '··'~. ,., ~' \_; ' r r r r r r r r r r r r r r r r r r r Page 24 Hydraulic Summary Table for the South Branch Wash from Oracle Road upstream to the Forest Service Boundary (continued) Cross Weir Flow Incoaing Final Water Channel Surface Velocity (ft-msl) (fps) Yaxiaum Section Return Period Peak Discharge No. (yr) (cfs) 29.00 5 25 100 540.00 1160.00 1890.00 0.00 0.00 0.00 o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2678.99 2679.78 2680.23 6.01 6.31 7.28 1.49 2.28 2.73 5 540.00 1160.00 1890.00 0.00 0.00 0.00 o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2683.96 2684.51 2685.04 5.17 6.62 7. 71 0.96 1. 51 2.04 540.00 1160.00 1890.00 0.00 0.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2687.19 2687.58 2687.95 4.43 5.69 6.59 1. 19 1. 58 1. 95 540.00 1160.00 1890.00 o.oo o.oo 0.00 o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2694.52 2694,93 2695.27 5. 22 6.61 7. 85 2.52 2.93 3.27 540.00 1160.00 1890.00 0.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2701.43 2701.87 2702.33 4. 58 5.66 6. 15 2.23 2.67 3.13 540.00 1160.00 1890.00 o.oo o.oo 0.00 0.00 o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2709.09 2709.48 2709,87 4.92 6.49 7.49 1.69 2.08 2.47 540.00 1160.00 . 1890.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2716.71 2717.26 2717.77 5. 19 6.54 7.56 1.41 1. 96 2.47 540.00 1160.00 1890.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890,00 2723.87 2724.44 2724.98 5.26 6.65 7.70 1.27 1.84 2.38 540.00 1160.00 1890.00 o.oo o.oo 0.00 o.oo o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890,00 2731.91 2732.33 2732.74 4.60 5.84 6.82 1. 21 1. 63 2.04 540.00 1160.00 1890.00 o.oo o.oo 0.00 0.00 o.oo o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2740.34 2740.90 2741.32 5.21 6.02 7.00 1. 94 2.50 2.92 540.00 1160.00 1890.00 o.oo o.oo o.oo o.oo 0.00 540.00 1160.00 1890.00 2749,56 2750.15 2750.68 5.30 0.00 o.oo o.oo o.oo 6.68 7.79 1.16 1. 75 2.28 540.00 1160.00 1890.00 o.oo o.oo 0.00 o.oo 0.00 o.oo o.oo o.oo o.oo 540.00 1160.00 1890.00 2759.69 2760.72 2761.66 6.95 8.62 9.79 1.69 2. 72 3.66 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 116.00 224.00 350.00 2748.19 2748.66 2749. 11 5. IS 6.27 7.03 1. 59 2.06 2.51 29.50 25 100 5 30.00 31.00 25 100 5 25 100 32.00 5 25 100 33.00 5 25 100 34.00 5 25 100 35.00 5 25 100 36.00 5 25 100 37.00 5 25 100 38.00 5 25 100 38.60 5 25 100 s 38.50 25 100 116.00 224.00 350.00 Diverted Flow (cfs) ( cfs) 0.00 ~ Drainage ~ Flood-Control Engineering Flow (cfs) Discharge (cfs) TABLE 3.2.2D Depth ( ft) (cont.) [ '· ' r r r r r r r, r r r r r r r r r r r r rnHJre Drainage l Flood-Control Engineering Using the results of the SBW-US1 analysis with through 26, the SBW-US2 analysis was performed on the A weir-flow routine was used at the downstream limit initial runs indicated that the entire peak discharge within confines of the channel section. Page 25 respect to Sections 24 SBW Northern Tributary. of the tributary after could not be contained Weir flow at this location occurs in a southwesterly direction over the drainage basin divide that separates the tributary watershed from the primary watershed associated with the South Branch Wash. Weir flow is limited to that portion of the divide that is located between Sections 39 and 40. The results of the analysis indicate that a small insignificant quantity of flow will overtop the divide at this location. The total weir-flow quantity associated with the 5-year event was determined to be less than one cfs. The quantity associated with the 25-year and 100-year events was determined to be approximately 8 cfs and 55 cfs, respectively. The results of the SBW-US2 analysis are summarized in Table 3.2.2E. The weir-flow quantities just described are noted under the "Weir Flow" heading. Again, since no additional flow is diverted from the system, there are no values presented under the "Diverted Flow" heading. Table 3.2.2F summarizes the results of the Glenhurst wash analysis (Cross Sections 401 through 409). As briefly mentioned in a previous section, not all runoff entering the study area along the Glenhurst Wash will be conveyed toward the SBW Northern Tributary. Under existing conditions, a portion is diverted down Glenhurst Drive. Historically, it appears that a small quantity of flow was always directed towards CP 10. Today, however, the roadway and other local obstructions govern how much flow is directed towards CP 10 and how much is directed towards the SBW Northern Tributary. Historic photographs indicate that the primary low-flow channel for the Glenhurst Wash has always been directed towards the SBW Northern Tributary. In addition, almost all hydrologic studies conducted on this wash have considered its drainage area to be part of the SBW Northern Tributary watershed. If fact, it appears that the drainageway constructed between CP 10 and CP 11 was never designed to accommodate runoff generated within the Glenhurst Wash drainage area. Consequently, for the purpose of this study, the Glenhurst Wash drainage area is considered part of the SBW Northern Tributary watershed. The quantity of flow diverted down Glenhurst Drive was estimated by placing one of the HEC-2 cross sections immediately upstream of the retaining wall that was constructed on Lot 35C of the Sunnyslope subdivision. When Lot 35C was originally developed, the house was placed in the middle of the wash. Consequently, it has been subject to inundation from both runoff and sediment conveyed along the wash. In a effort to prevent future damage to the house and its contents, the owner constructed the retaining/diversion wall immediately upstream. In an effort to approximate the quantity of flow diverted down Glenhurst Drive, Cross Section 405.5 was structured such that the right bank station corresponded to the upstream limit of the wall. It was then assumed that all flow conveyed in the right overbank at this section would be diverted down Glenhurst Drive. Therefore, all runoff conveyed in the main channel at this section will be directed towards the northeastern portion of the Rancho Catalina subdivision which lies between Lot 35C and the SBW Northern Tributary. The peak discharges used to define the water surface elevations for Sections 401 through r r r r r r r r r r r r r r r r r r r Page 26 Hydraulic Summary Table for the SBW Northern Tributary Peak Section Return Period No. (yr) 5 25 100 740.00 1560.00 2550.00 o.oo o.oo o.oo 0.00 24.00 0.00 o.oo o.oo o.oo 5 25 100 740.00 1560.00 2550.00 0.00 0.00 0.00 o.oo o.oo 26.00 5 25 100 740.00 1560.00 2550.00 o.oo 0.00 o.oo 39.00 5 25 100 278.00 566.00 883.00 40.00 5 25 100 41.00 Cross Discharge (cfs) Diverted Flow (cfs) Weir Flow ( cfs) Incoming Flow (cfs) Final Discharge (cfs) Water Channel Surface Velocity ( ft-msl) ( fps) Maxi•um Depth ( ft) 740.00 1560.00 2550.00 2643.25 2643.75 2644.25 3.68 5. 31 6. 15 2.45 2.95 3.45 0.00 740.00 0.00 1560.00 2550.00 2650.19 265!. 23 265!. 68 6.86 6. 75 7.90 2.19 3.23 3.68 o.oo 0.00 o.oo o.oo 0.00 o.oo 740.00 1560.00 2550.00 2657.75 2658.44 2658.86 5. 35 5.37 5.63 !. 75 2.44 2.86 o.oo o.oo o.oo -0.31 -8.33 -55.41 o.oo o.oo 277.69 2668. 14 2668.47 2668.73 4.30 5. 35 6.04 1. 14 557,67 827.59 278.00 566.00 883.00 o.oo 0.00 o.oo o.oo 0.00 0.00 o.oo o.oo o.oo 278.00 566.00 883,00 2674.79 2675.13 2675.38 4. 12 4.83 5,52 0.79 1.13 1.38 5 25 100 278.00 566.00 883.00 o.oo o.oo o.oo 0.00 o.oo 0.00 278,00 566.00 883.00 2681.47 2681.88 2682.25 4.53 5.29 5.47 1.47 0.00 0.00 1.88 2.25 5 25 100 278.00 566.00 883.00 0.00 o.oo o.oo 0.00 o.oo o.oo 0.00 278.00 566.00 883.00 2687.14 2687.81 2688.28 5.67 6,32 6.85 2. 14 2.81 3.28 43.00 5 25 100 278.00 566.00 883.00 o.oo o.oo o.oo o.oo 0.00 o.oo o.oo 0.00 o.oo 278.00 566.00 883.00 2692.37 2692.88 2693.25 5.43 6.40 7.02 2.87 3.38 3.75 5 25 100 278.00 566.00 883.00 0.00 44.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 278.00 566.00 883.00 2696.10 2696.51 2696.80 5.00 6.03 7.09 2. 10 2.51 2.80 45.00 5 25 100 278.00 566.00 883.00 o.oo 0.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo 278,00 566,00 883,00 270!. 31 2701.80 2702.24 4.96 5.71 5.84 !. 31 1.80 5 25 100 278.00 566.00 883.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 278.00 566.00 883.00 2708.24 2708.67 2709.04 4. 77 46.00 5.89 6.77 !. 54 !. 97 2.34 46,50 5 25 100 278.00 566.00 883.00 o.oo o.oo o.oo o.oo o.oo o.oo 0.00 0.00 o.oo 278.00 566.00 883.00 2713.66 27!4.22 2714.69 5.38 6.34 7. 11 1.66 2.22 2.69 47.00 5 25 100 204.00 409.00 632.00 o.oo 0.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo 204,00 409.00 632.00 2715.42 27!5.86 2716.22 3.54 4.05 4.42 1.02 1.46 !. 82 5 25 100 130.00 252.00 380.00 o.oo o.oo 130.00 252.00 380.00 272!.65 2721.95 2722.17 4.04 4. 73 0.65 0.95 o.oo o.oo o.oo o.oo o.oo 48.00 5.32 1. 17 5 25 100 130.00 252.00 380.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo 49.00 0.00 0.00 130,00 252.00 380.00 2728.82 2728.96 2729.08 2. 72 3.57 4. 22 0.32 0.46 o. 58 50.00 5 25 100 130.00 252.00 380.00 o.oo o.oo o.oo o.oo o.oo o.oo 0.00 o.oo 130.00 252.00 380.00 2735.79 2736.08 2736.22 3.70 3.88 4.50 0.79 1.08 !. 22 25.00 42.00 o.oo o.oo 0.00 ~ Drainage ~ Flood-Control Engineering o.oo o.oo 0.00 o.oo o.oo 0.00 0.00 1.47 !. 73 2.24 TABLE 3.2.2E f 1\'l,\ r r Page 27 r r r r r r r Hydraulic Summary Table for the Glenhurst Wash Cross Section No, s r r r r r (cfs) Final Discharge (cfs) Water Channel Surface Velocity (ft-msl) ( fps) Maxiaum Depth ( ft) 73.00 162.00 259.00 2773.90 2774.14 2774.30 3. 31 3. 76 4.38 0.40 0.64 0.80 -86.00 -154.00 -227.00 o.oo o.oo o.oo o.oo 0.00 0.00 73 .oo 162.00 259.00 2782.81 2783.02 2783.22 3.16 4. 13 4.83 0.31 o. 52 0.72 159.00 3!6.00 486.00 -86.00 -154.00 -227.00 0.00 0,00 0.00 o.oo 73.00 162.00 259.00 2788.91 2789.39 2789.76 4. 56 5.43 5.97 0.91 I. 39 5 25 !00 !59. 00 316.00 486.00 -86.00 -154.00 -227.00 o.oo o.oo o.oo o.oo 0.00 2796.25 2796.76 2797.18 4. 76 5.93 6.67 I. 25 I. 76 0.00 73.00 162.00 259.00 2.18 5 25 100 159.00 3!6.00 486.00 -86.00 -154.00 -227.00 o.oo 0.00 o.oo o.oo o.oo 73.00 162.00 259.00 2802.32 2802.64 2802.91 3,98 5.02 5.69 0.82 1.14 1.41 s 159.00 316.00 486.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 2805.80 2806.05 2806.25 3. IS 4.03 0.00 159.00 316.00 486.00 0.80 1.05 I. 25 159.00 316.00 486.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo !59. 00 316,00 486.00 2809. 16 2809.42 2809,67 2.31 3.59 4.38 I. 16 25 100 5 25 100 159.00 3!6.00 486.00 o.oo o.oo o.oo 0.00 o.oo o.oo o.oo o.oo o.oo 159.00 316.00 486.00 2816.47 2816.73 2816.93 3.82 4.72 S.S5 0.47 0.73 0.93 s 159.00 3!6.00 486.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 159.00 316.00 486,00 2826.32 2826.57 2826.81 4.15 5.23 5.80 I. 32 159.00 316.00 486.00 o.oo o.oo o.oo 0.00 0.00 0.00 0.00 159.00 316.00 486.00 2835.01 2835.50 2835.85 4.93 5.60 6.18 159.00 3!6.00 486.00 25 100 25 !00 s 407.00 Flow o.oo o.oo o.oo s 406.00 Incoming 0,00 s 405.50 (cfs) Weir Flow (cfs) o.oo o.oo 25 !00 405.00 (cfs) Diverted Flow -86.00 -154.00 -227.00 402.00 403.00 Peak Discharge 159.00 3!6.00 486.00 25 !00 r I Period (yr) 401.00 404.00 r Return 408.00 25 100 409.00 25 100 s 0.00 o.oo 0.00 0.00 0.00 o.oo 4. 78 1. 76 1.42 1.67 !.57 !.81 1.01 I. so !.85 r r r ~ Drainage A Flood-Control Engineering TABLE 3.2.2F \j'l:J(]- r r r r r r r r r r r r r r r r r r r ~ Drainage l flood-Control Engineering Page 28 405 reflect the loss of flow down Glenhurst Drive. Based on these assumptions, the 5-year, 25-year, and 100-year flow quantities entering Glenhurst Drive were estimated to be 85 cfs, 154 cfs, and 227 cfs, respectively. The flow quantities entering the Rancho Catalina subdivision were estimated to be 74 cfs, 162 cfs, and 259 cfs, respectively. Historically, the Glenhurst Wash immediately downstream of Lot 35C possessed a physical characteristic that had a tendency to create a wide and shallow flow pattern. However, this flow pattern was drastically altered when the home and retaining wall were constructed on Lot 35C. These improvements (to the lot, not the wash) concentrate flow into a narrow cross section and direct this flow toward adjacent downstream properties. Although the physical definition of this constricted sect ion quickly diminishes in the downstream direction, the effective flow boundaries within the downstream area are a function of the hydraulic characteristics of the constricted section. High discharge flows along this reach of the Glenhurst Wash are well within the supercritical range. For supercritical flow, the effective flow boundary approximates a 10:1 expansion ratio (i.e., one foot laterally for every 10 feet along the direction of flow). In this particular instance, expansion begins on the downstream side of the patio wall associated with Lot 35C, which corresponds to Section 403. However, since the effective flow area is almost immediately altered by local obstructions (i.e., homes, fences, etc.) and other land-form modifications introduced by local residents, it is difficult to accurately predict during a major flow event either the direction of the primary flow path or the distribution of flow within the associated flood plain. This situation is further complicated by 1imitations inherent to the HEC-2 model and the relative accuracy of the project's topographic map. The HEC-2 model is a gradually-varied flow model that should not be applied when slopes exceed 10 percent. Within this area, flow is subject to rapid changes in both its magnitude and direction and the slope of the land surface fluctuates between 8.5 and 10 percent. Consequently, for this area, the potential for flooding during the respective flow events must be addressed more from a qualitative standpoint than from a quantitative standpoint. Even though this portion of the Rancho Catalina subdivision is subject to flooding during the 100-year event, the flood-prone limits are not definable. This is why the flood-prone 1imi ts associated with the Glenhurst Wash do not join the SEW Northern Tributary flood-prone limits shown on Figure 8. For the most part, the average depth of flooding in this area should not exceed 0.5 feet. In addition, no attempt was made to delineate the flood-prone limits associated with Glenhurst Drive drainage path. This area was not included in the mapping due in part to the limitation of the HEC-2 program and because the quantity of flow diverted down Glenhurst Drive was less than 100 cfs during the 5-year event. However, it is important to note that appropriate measures must be taken to regulate future development in both areas. Regarding existing developments, observations made during field trips to the Glenhurst Drive area indicate that local residents and the Town of Oro Valley have both taken measures to control nuisance runoff. Consequently, it appears that most of the existing homes in the area should remain free from flooding up to and including the 100year event. r r r r r r r r r r r r r r r r r r r ffimJre Drainage l Flood-Control Engineering Page 29 AI though it is difficult to predict the primary flow path through the northeastern port ion of the Rancho Cat a! ina subdivision, it appears that discharges from the Glenhurst Wash will have their greatest impact on the first three structures located immediately downstream of Lot 35C of the Sunnyslope subdivision. Other homes in the immediate vicinity of these three structures may experience some nuisance runoff problems, however, it is unlikely that they will be subjected to any major flood damage. In order to assess any flood damage that these three structures might experience during the 5-year, 25-year, and 100-year events, uniform-flow calculations were performed using an assumed, representative, channel section. The boundaries of this section were established using what was considered to be the average width of the effective flow area. The results of this analysis were then used to determine the anticipated depth of flooding for these structures. The input and output parameters associated with the uniform-flow analysis are provided in Table 3.2.2G. 3.2.3 Shadow Mountain Wash Flood Plain Table 3.2.3A summarizes the results of the Shadow Mountain Wash analysis (i.e., Sections 101 through 120). The majority of this wash exists as a drainageway that was constructed during the development of the Shadow Mountain subdivision. Low-profile levees were provided along each bank of this drainageway, which extends for Calle Buena Vista upstream to Hardy Road. From Hardy Road upstream to Oracle Road most of the wash remains in its natural state. A split-flow routine was again included in the analysis of this wash after initial runs indicated that a portion of the flow will overtop the western bank in the immediate vicinity of Hardy Road. However, the quantity of flow exiting the channel during the 25-year and 100-year events is less than seven percent of the computed discharge for the 100-year event. These weir flow quantities were determined to be 6 cfs and 29 cfs, respectively. Most of the breakout occurs between Sections 112 and 113. The results indicate that no break out occurs during the 5-year event. The 100-year flood plain as delineated on Figure 8 indicates that the entire 100-year peak discharge is, for the most part, contained within the confines of the drainageway. However, when the associated water surface elevations are compared to the top of levee elevations, which basically define the banks of the drainageway, very little freeboard is available. Consequently, if the drainageway is not maintained on a regular basis and the wash is subjected to a flow event that approximates the 100-year event, the drainageway may be overtopped. Should this occur, property on both sides of the drainageway would be impacted. However, the depth of flow should be limited to a few inches unless local obstructions cause breakout flows to concentrate at a higher depth. To accommodate flow conveyed in the Shadow Mountain Wash at Oracle Road, ADOT provided a single cell, 8' x 5' CBC (with inlet and outlet wingwalls). Upon completion of the initial Shadow Mountain run, which used the peak discharges associated with CP 11, it was again noted that the analysis of Sect ion 120 defaulted to critical depth. Consequently, the HY-8 analysis of this structure was again setup such that a tailwater rating curve would be generated during the analysis. This rating curve was based on critical depth. A typical rectangular channel section was again used to represent the geometry of the outlet channel. !".?.'\ \j / r r r r r r r r r r r r r r r r r r r Page 30 Hydraulic Properties within the Glenhurst Wash Transition Zone under Normal-Flow Condtions 5-yr Input discharge n-value bottom width slope side slope = = = = = Output depth velocity Froude # critical depth sequent depth hydraulic depth velocity head specific head top width x-sectional area wetted perimeter hydraulic radius ~ Drainage ~ flood-Control Engineering 73 0.045 30 0.085 30 5-yr = = = = = = = = = = = = 0.4 4.4 1.39 0.48 0.58 0.31 0.3 0.7 53.78 16.61 53.8 0.31 25-yr 162 0.045 30 0.085 30 25-yr 0.6 5.57 1.48 0.75 0.92 0.44 0.48 1.09 66.28 29.11 66.3 0.44 100-yr 259 0.045 30 0.085 30 cfs ft. 100-yr 0.77 6.35 1. 53 0.96 1. 19 0.54 0.63 1.39 76.12 40.79 76.15 0.54 ft. fps ft. ft. ft. ft. ft. ft. sq. ft. ft. TABLE 3.2.2G r r r r r r r Page 31 Hydraulic Summary Table for the Shadow Mountain Wash from its Confluence with the Carmack Wash upstream to Oracle Road Return Cross Section Period No. (yr) 5 101.00 25 100 5 Peak Discharge (cfs) Diverted Flow 206.00 385.00 604.00 0.00 0.00 0.00 0.00 -5.86 -28.81 o.oo o.oo o.oo 206.00 379.14 575.19 2540.40 2540.78 2540.94 4.87 5.16 6.23 2.40 2. 78 2.94 206.00 385.00 604.00 0.00 o.oo o.oo o.oo -5.86 -28.81 o.oo o.oo o.oo 206.00 379.14 575.19 2542.36 2542.84 2543. 10 5.32 s.s3 6.07 2. 36 2.84 3. 10 206.00 385.00 604.00 o.oo o.oo 0.00 0.00 -5.86 -28.81 o.oo o.oo o.oo 206.00 379.14 575.19 2544.54 2544.89 2545.21 5.48 6.41 6.66 2.54 2.89 3.21 206.00 385.00 604.00 o.oo o.oo o.oo o.oo -5.86 -28.81 o.oo o.oo o.oo 206.00 379. 14 575.19 2546.30 2546.69 2546,97 3.24 4.06 4.89 !. 80 2.19 2. 47 !. 99 ( cfs) Weir Flow (cfs) Incoming Flow (cfs) Final Discharge (cfs) Water Channel Surface Velocity (ft-asl) ( fps) t.lax imum Depth ( ft) 101. so 25 100 102.00 25 100 103.00 25 100 r 103.50 25 100 206.00 385.00 604.00 0.00 o.oo o.oo o.oo -5.86 -28.81 o.oo o.oo o.oo 206.00 379.14 575.19 2548.77 2549.29 2549.73 5.64 6.61 7.45 2.43 r 104.00 5 25 100 205.00 360.00 475.00 o.oo 0.00 o.oo 0.00 -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446. 19 2551.81 2552.40 2552.65 5.69 5.78 5.82 !. 5 I 2. 10 2.35 r 105.00 5 25 100 205.00 360.00 475.00 0.00 o.oo 0.00 o.oo -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446. 19 2555.38 2555.93 2556.30 6. IS 6.99 6.87 !. 78 2.33 2.70 106.00 5 25 100 205.00 360.00 475.00 0.00 o.oo 0.00 o.oo -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446.19 2560.55 2561.09 2561.30 5.92 6.36 6. 77 2.55 3.09 3.30 5 25 100 205.00 360.00 475.00 o.oo 0.00 o.oo o.oo -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446. 19 2564.32 2564.87 2565.13 6.04 6.64 6.93 2.32 2.87 3.13 108.00 5 25 100 205.00 360.00 475.00 o.oo o.oo o.oo o.oo -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446. 19 2568.63 2569. 14 2569.35 5.69 6.05 6.33 2.43 2.94 3. 16 109.00 5 25 100 205.00 360.00 475.00 o.oo 0.00 o.oo 0.00 -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446.19 2571.19 2571.78 2572.09 6.30 7. 10 7.33 2.19 2. 78 3.09 5 205.00 360.00 475.00 0;00 0.00 o.oo o.oo -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446. 19 2573.75 2574.36 2574.66 6.25 6.80 6.97 !. 75 2.36 2.66 ! r r 5 5 • r r r r 5 107.00 110.00 25 100 1.47 r • r r ~ Drainage l Flood-Control Engineering TABLE 3.2.3A ,, o?F r Page 32 r r r r r r r r r r r r r r r r r r Hydraulic Summary Table for the Shadow Mountain Wash from its Confluence with the Carmack Wash upstream to Oracle Road (continued) Cross Sect ion No. Return Peak Period (yr) Discharge 113.00 114.00 115.00 115.50 116.00 Depth ( ft) 0.00 o.oo o.oo o.oo -5.86 -28.81 o.oo o.oo o.oo 205.00 354. 14 446. 19 2579.38 2579.91 2580.43 6. 16 7. 10 5.92 l. 78 2.31 2.83 o.oo o.oo o.oo 205.00 354.58 450.31 2582 .!4 2582.65 2582.85 5.63 6. 12 6.66 l. 74 2.25 2.45• o.oo o.oo o.oo 205.00 360.00 474.97 2584.02 2584.37 2584.64 4.90 6.32 6. 74 2.02 2.37 2.64 0.00 o.oo o.oo o.oo 205.00 360.00 475.00 2586.94 2587.31 2587.50 4.68 5.33 5.91 1.94 2.31 2.50 5 25 100 205.00 360.00 475.00 5 25 100 205.00 360.00 475.00 0.00 5 25 100 5 0.00 o.oo o.oo 0.00 o.oo o.oo -5.42 -24.69 o.oo o.oo 0.00 0.00 -0.03 o.oo o.oo o.oo o.oo 205.00 360.00 475.00 o.oo o.oo o.oo o.oo o.oo 0.00 o.oo o.oo o.oo 205.00 360.00 475.00 2588.51 2589.!8 2589.55 6.55 7.39 7.91 2.51 3 .!8 3.55 205.00 360.00 475.00 o.oo o.oo 0.00 o.oo o.oo o.oo o.oo o.oo 205.00 360.00 475.00 2592.03 2592.26 2592.41 4.05 4.79 5. 20 l. 03 1.26 1.41 205.00 360.00 475.00 2597.91 2598.42 2598.51 5.65 4.74 5.54 !. 91 2.42 2.51 0.00 205.00 360.00 475.00 0.00 o.oo o.oo o.oo o.oo 0.00 o.oo o.oo o.oo 205.00 360.00 475.00 o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo o.oo 205.00 360.00 475.00 2603.57 2603.81 2603.89 4.21 4.80 5.61 2.07 2.31 2.39 o.oo o.oo o.oo o.oo 205.00 360.00 475.00 2610.21 2610.80 2611.01 5.55 0.00 0.00 5. 16 5.27 2.21 2.80 3.01 o.oo o.oo o.oo o.oo o.oo 205.00 360.00 475.00 2614.62 2615.01 2615.23 5.54 6.62 7.31 2.62 3.01 3.23 118.00 25 100 119.00 25 100 205.00 360.00 475.00 o.oo o.oo o.oo 5 25 100 205.00 360.00 475.00 o.oo o.oo o.oo 120.00 (fps) l. 69 2.23 2.48 205.00 360.00 475.00 s Maximum 5.92 6.42 6.68 5 25 100 5 (ft-msl) Channel Velocity 2576.09 2576.63 2576.88 0.00 25 100 Water Surface 205.00 354. 14 446. 19 205.00 360.00 475.00 5 117.00 (cfs) Final Discharge (cfs) o.oo o.oo o.oo 25 100 25 100 (cfs) Incoming Flow 0.00 -5.86 -28.81 o.oo 25 100 5 112.00 (cfs) 205.00 360.00 475.00 5 Ill. 00 ( cfs) Weir Flow Diverted Flow ~ Drainage ~ Flood-Control Engineering 0.00 TABLE 3 , 2. 3A (cont. ) ,..,,··; \)1-,./ ! r r r r r r r r r r r r r r r r r r r Jm~ Drainage l Flood-Control Engtneering Page 33 The results of the HY-8 analysis, which are contained in Appendix E, indicate that this structure is operating under inlet control. However, not all of the 25-year and 100-year peak discharges associated with CP 11 wi II be conveyed through the structure. A portion overtops the access drive co the south and enters the South Branch for conveyance through its CBC. This is why the 100-year flood-prone area in the immediate vicinity of the CBC's inlet, which is based on the computed headwater elevation, joins the South Branch floodplain. The results indicate that during the 25-year event approximately 360 cfs, compared to 368 cfs, will be conveyed through the structure during the 25-year event. During the 100-year event only 413 cfs will be conveyed through the structure, compared to the computed peak discharge which is 542 cfs. Consequently, the initial discharges used in the Shadow Mountain analysis (i.e., Sections 101 through 120) were reduced to account for this flow diversion. However, the reduced value for the 100-year event was held at 475 cfs which corresponds to the 100-year peak discharge at CP 13. In this way, the results of the floodplain analysis could be considered conservative from a flood-damage assessment standpoint 3.3 Flood-damage Assessment Using Figure 8 and the results of the 100-year flood plain analyses as a guide, twenty-six homes that appeared to possess the greatest potential for flooding were identified. Each home was given an identification number which will be used for reference purposes throughout the remainder of this report. The location of each of these homes is shown on Figure 9 as is the 100-year floodprone limits. Using the surveyed finished-floor elevation associated with each of the twenty-six homes, the potential depth of flooding was determined for each return interval. Table 3.3A summarizes the results of this determination. The table also provides the surveyed finished-floor elevation, the water-surface elevation for the cross section that is located immediately upstream and downstream of the affected structure, and the data used to compute an interpolated water surface elevation when one of the cross sections shown on Figure 8 did not correspond to the upstream limit of the structure in question. Although each of the identified homes appeared to be subject to flooding during the 100-year event, the results summarized in Table 3.3A indicate that only fourteen homes will actually be affected. Of these fourteen homes, only eleven will be impacted during both the 5-year and the 25-year events. The affected homes are denoted by a positive sign under the "Depth of Flooding" heading in the table. The negative values under this heading reflect how much freeboard exists between the related water surface elevations and the finishedfloor elevations. Those homes that possessed greater than one-foot of freeboard during either the 5-year, 25-year, or 100-year event were excluded from the damage-assessment analysis. The results of the damage-assessment analysis are provided in Table 3.3B. As stated earlier, estimated damages are based solely on the estimated market value of the structure and its associated improvements. Based on the field notes provided by the project's land surveyor, a secondary structure (i.e., a guest house) exists on Lot 118 of the Rancho Catalina subdivision. This is the lot associated home #14. The results shown r r r r r r r r r r r r r r r r r r ,. ~ Page 34 Water Surface Elevation versus Finished-Flood Elevation with the Relative Depth of Flooding for each Return Interval Home No. finished Floor Elevation {ft-msl) 2583.05 2 2581.27 Flood U/S Return Water Surface Elevation (ft-msl) D/S Water Surface Elevation (ft-msl) 5 25 100 2584.02 2584.37 2584.64 2582.14 2582.65 2582.85 5 25 100 2578.85 2579.35 2577.91 2579.74 2578.30 Period (yr) Distance Related Depth Adjacent Sections Upstream D/S Sect ion Water Surface Flooding ( ft) ( ft) Elevation ( ft-msl) ( ft) Distance Between 130.00 90.00 of 2583.44 2583.84 2584.09 0. 79 0.39 1.04 60.00 30.00 2578.09 2578.63 2579.02 -3.18 -2.64 -2.25 2577.32 3 2585.88 5 25 100 2590.23 2590.73 2591.17 2585.34 2585.98 2586.36 240.00 50.00 2586.36 2586.97 2587.36 0.48 1.09 1.48 4 2588.93 5 25 100 2588.75 2589.37 2590.11 2586.15 2586.86 2587.62 so.oo 25.00 2587,45 2588.12 2588.87 -1.48 -0.81 -0.06 5 2583. 14 5 25 100 2584.13 2584.60 2585.50 2580.07 2580.43 2581.10 100.00 20.00 2580.88 2581.26 2581.98 -I. 88 6 2573.13 5 25 100 2570.66 2571.25 2571.82 7 2571.75 5 25 100 2574.31 2574.63 2574.90 2570.50 2570.77 2570.97 165.00 8 2575.95 5 25 100 2577.71 2578.09 2578.36 2574.28 2574.63 2574.89 9 2579.01 5 25 100 2581.12 2581.47 2581.78 10 2580.63 5 25 100 II 2550.62 12 -2.26 -1.16 2570.66 2571.25 2571.82 -I. 88 -I. 31 40.00 2571.42 2571.71 2571.92 -0.33 -0.04 0.17 160.00 30.00 2574.92 2575.28 2575.54 -1.03 -0.67 -0.41 2577.71 2578.09 2578.36 130.00 25.00 2578.37 2578.74 2579.02 -0.64 -0.27 0.01 2581.12 2581.47 2581.78 2577.71 2578.09 2578.36 130.00 115.00 2580.73 2581.08 2581.39 0.10 0.45 0.76 5 25 100 2551.47 2552.50 2552.77 2548.74 2549.60 2550.52 130.00 35.00 2549.48 2550.38 2551.13 -1.14 -0.24 0.51 2554.54 5 25 100 2551.47 2552.50 2552.77 2551.47 2552.50 2552.77 -3.07 -2.04 -I. 77 2698.94 5 25 100 2701.43 2701.87 2702.33 2696.57 13 2696.99 2697.37 -2.37 -1.95 -1.57 14 2685.30 5 25 100 2683.96 2684.51 2685.04 2683.96 2684.51 2685.04 -1.34 -0.79 -0.26 ~ Drainage ~ flood-Control Engineering N/A N/A N/A N/A N/A N/A 2694.52 2694.93 2695.27 N/A N/A N/A N/A N/A 185.00 N/A N/A N/A 55.00 N/A -2.47 TABLE 3.3A •f I l;)'j' r r r r r r r r r r r r r r r r r Page 35 Water Surface Elevation versus Finished-F load Elevation with the Relative Depth of Flooding for each Return Interval (continued) Home No. Finished Floor Elevation (ft-ar;l) Flood Return Period (yr) Water Surface Elevation D/S Water Surface Distance Between Adjacent Distance Upstream D/S Elevation Sections Section (ft-msl) (ft-msl) U/S Related Water Surface Depth of Floodin8 Elevation (ft-asl) ( ft) (ft) (ft) N/A N/A 2687.19 2687.58 2687.95 2.09 2.48 2.85 14A 2685.10 5 25 100 2687.19 2687.58 2687.95 15 2680.79 5 25 100 2678.99 2679.78 2680.23 2672.03 2672.53 2673.00 200.00 165.00 2677.77 2678.51 2678.96 -3.02 -2.28 -1.83 16 2675.31 5 25 100 2678.99 2679.78 2680.23 2672.03 2672.53 2673.00 200.00 10.00 2672.38 2672.89 2673.36 -2.93 -2.42 -1.95 17 2710.93 5 25 100 2713.66 2714.22 2714.69 2708.24 2708.67 2709.04 140.00 75.00 2711.14 2711.64 2712.07 o. 71 N/A N/A N/A 0.21 1.14 18 2695.65 5 25 100 2696.10 2696.51 2696.80 2692.37 2692.88 2693.25 130.00 90.00 2694.95 2695.39 2695.71 -0.70 -0.26 0.06 19 2690.26 5 25 100 2692.37 2692.88 2693.25 2687.14 2687.81 2688.28 140.00 101.00 2690.91 2691.47 2691.87 0.65 1. 21 1.61 20 2683.93 5 25 100 2687.14 2687.81 2688.28 2681.47 2681.88 2682.25 180.00 95.00 2684.46 2685.01 2685.43 0.53 1. 08 1. 50 21 2680.08 5 25 100 2681.47 2681.88 2682.25 2674.79 2675.13 2675.38 160.00 85.00 2678.34 2678.72 2679.03 -1.74 -1.36 -1.05 22 2676.68 5 25 100 2674.79 2675.13 2675.38 2668.14 2668.47 2668.73 190.00 170.00 2674.09 2674.43 2674.68 -2.59 -2.25 -2.00 23 2791.62 5 25 100 2796.25 2796,76 2797.18 N/A N/A 2796.25 2796.76 2797.18 4.63 5.14 5.56 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A 24 25 2765.19 2756.63 5 N/A 25 100 N/A 5 25 100 5 26 2744,37 25 100 N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A o. 50 0.75 1.00 N/A N/A N/A o. 50 0.75 1.00 N/A N/A N/A o. 50 0.75 1.00 N/A r l r ~ Drainage ~ Flood-Control Engineering TABLE 3. 3A (cont.) r r Page 36 r r r r r r r r r r r r r r r r r • Damages for the Twenty-Six Selected Flood-Prone Structures Based on a Percentage of the Estimated Market Value Home No. Legal Description Assessed Value W/0 Land ($) Estimated Market Estimated Content Value Value ($) ($) Depth of water ( ft) 45535.00 56918.75 17075.63 5 25 100 0.39 0.79 1.04 Lot 25 Shadow Ntn. Estates Flood Return Period (yr) Percent Damage Structure (~) 9.95 12. 18 13.63 (~) Total Estiaated Daaages ( s) 13.53 15.81 17.60 Lot 364 Rancho Pel ix 56828.00 71035.00 21310.50 5 25 100 -3.18 -2.64 -2.25 3 Lot 363 Rancho Pel ix 56629.00 70786.25 21235.88 5 25 100 0.48 1.09 1.48 10.45 13.97 16.65 14.05 18.35 24.22 51940.00 64925.00 19477.50 -1.48 -0.81 -0.06 0.00 1.46 7.31 o.oo Rancho 5 25 100 o.oo 4 2. 15 10.63 1366.67 6816.48 61998.00 77497.50 23249.25 5 25 100 -2.26 -I. 88 -1.16 N/A N/A N/A N/A N/A N/A N/A N/A N/A 49883.00 62353.75 18706.13 5 25 100 -2.47 -I. 88 -1.31 N/A N/A N/A N/A N/A N/A N/A N/A N/A 49204.00 61505.00 1845 I. 50 5 25 100 -0.33 -0.04 0. 17 5.19 7.47 8. 73 7.58 10.86 12.28 4590.73 6598.26 7635.23 55935.00 69918.75 20975.63 5 25 100 -I. 03 -0.67 -0.41 0.00 2.54 4. 56 0.00 3.74 6.68 0.00 2560.42 4589.47 56457.00 70571.25 21171.38 5 25 100 -0.64 -0.27 0.01 2.77 5.66 7.84 4.08 8.26 II. 37 2818.62 5743.09 7939.97 66161.25 19848.38 5 25 100 o. 10 52929.00 8.34 10.28 12.01 11.88 13.88 15.64 7875.84 9556.33 11050.25 5 52339.00 19627.13 25 100 -1.14 -0.24 0.51 o.oo 65423.75 0.00 8.60 14.22 0.00 5541.39 9738.98 41749.00 52186.25 15655.88 25 100 46266.00 57832.50 17349.75 69650.96 87063.70 26119.11 Pel ix 5 Lot 361 Rancho Felix 6 Rancho Lot 368 felix Lot 372 7 Rancho Felix 8 Lot 371 Rancho Felix Lot 370 9 Rancho Pel ix: Lot 370 10 Rancho Felix Lot 37 11 Shadow Ntn. Estates 12 Shadow Ytn. Lot 38 Lot 131 13 14 Rancho Catalina Lot 118 Rancho ~ Drainage ~ Flood-Control Engineering 10380.80 13785.62 16929.24 N/A N/A N/A N/A N/A N/A N/A N/A N/A 25 100 -2.37 -I. 95 -I. 57 N/A N/A N/A N/A N/A N/A N/A N/A N/A 5 25 100 -I. 34 -0.79 -0.26 5 Catalina 5.89 10.62 N/A N/A N/A -3.07 -2.04 -I. 77 5 Estates 0.45 0.76 N/A N/A N/A 7973.75 9632.36 10763.34 2 Lot 362 N/A N/A N/A Percent Daaage Contents o.oo I. 61 5.89 0.00 2.38 8.60 o.oo 2023.36 7374.30 TABLE J.JB ~""'\ r r r r r r r r r r r r r r r r r r r ' Page 37 Damages for the Twenty-Six Selected Flood-Prone Structures Based on a Percentage of the Estimated Market Value (continued Home No. Legal Description Assessed Value wfo Land ($) Estimated Market Value Estimated Content Flood Return Depth Value Period water ( $) ($) (yr) Rancho Catalina Rancho Lot 116 Rancho Lot 185 Rancho 58858 75 17657.63 5 25 100 -3.02 -2.28 -I. 83 N/A N/A N/A N/A N/A N/A N/A N/A N/A 45972 00 57465.00 17239.50 5 25 100 -2.93 -2.42 -I. 95 N/A N/A N/A N/A N/A N/A N/A N/A N/A 64445.00 80556.25 24166.88 5 25 100 0.21 0.71 I. 14 8.95 II. 73 14.31 12 51 IS. 35 19.11 10233.06 13158.86 1614S.89 s 43776.00 S4720.00 16416.00 2S 100 -0.70 -0.26 0.06 2.29 5.82 8.34 3.Sl 8.49 II. 88 1829.29 4S78.42 6Sl3.87 4086S.OO Sl081.2S 15324.38 25 100 0.6S I. 21 1.61 II. 56 14.73 17.48 IS. 18 20.01 26.02 8231.23 IOS90.68 12916.40 70909.00 88636.2S 26590.88 2S 100 O.S3 1.08 I. SO 10.84 13.76 16.72 14.44 17.90 24.37 13447.89 l69S6. II 21300.18 48663.00 60828.75 18248.63 2S 100 6S1Sl.OO 81438.7S 24431.63 2S 100 72S83.00 90728.75 27218.63 2S 100 61221.00 76S26.2S 229S7.88 2S 100 5 2S 100 0 Lot 142 Rancho Catalina s Lot 143 19 Rancho Catalina 20 Rancho s Lot 144 Catalina s Lot 145 21 Rancho Catalina s Lot 146 22 23 Rancho Catalina Lot 3SC Sunnyslope s 'Lot 176 24 Rancho Catalina 2S Lot 17S Rancho 71006.00 88757.SO 26627.25 49337.00 61671.2S 18501.38 Catalina Lot 174 26 Rancho ($) 47087.00 0 Catalina ~ Drainage ~ Flood-Control Engineering 20.79 23.23 25.55 Daaages 2.09 2.48 2.85 Catalina 18 (~) 5 25 100 Catalina 17 (~) Total Estimated 8248.41 Catalina 16 Structure Percent Damage Contents 27494.70 Lot 117 15 (ft) Percent Damage 21995.76 Lot 118 14A of 32 28 33.30 34.27 0 0 8378.73 9133.74 9851.63 -I. 74 -1.36 -LOS N/A N/A N/A N/A N/A N/A N/A N/A N/A -2.S9 -2.2S -2.00 N/A N/A N/A N/A N/A N/A N/A N/A N/A 4.63 s .14 S.S6 29.41 31.69 3S.90 39.23 41.34 43.02 37361.19 40004 12 44281.07 o.so 0.7S 1.00 10.56 11.96 13.3S 14 16 1S.S8 17.00 11332.01 12729.38 14119 09 0.50 0,7S 1.00 IO.S6 11.96 13.3S 14 16 IS.S8 17.00 13143.21 14763.92 1637S.76 s o.so 2S 100 0.7S I. 00 10.S6 11.96 13.35 14.16 IS. 58 17.00 9132.28 10258.40 11378.35 s 0 0 TABLE 3.3B 0 0 (cont.) t\\1- r r r r r r r r r r r r r r r r r ,.. ! r ~ Drainage l Flood-Control Engineering Page 38 in Table 3.3A indicate that the home itself is not flood-prone (although damages were assessed due to a lack of adequate freeboard), but the guest house (#14A) is or may be flood-prone. In order to establish a separate market value for each structure, the total market value associated with the improvements on this lot was proportioned between the two structures on a square footage basis. 3.4 Observations by Local Residents During the course of this phase of the study, an attempt was made to contact the individual owners and/or occupants of the twenty-six selected homes in an effort to obtain, first hand, any accounts of drainage-related damages or problems that currently exist. Although all were contacted by letter, two were interviewed in person. These latter two individuals are the owners of homes #25 and #26. Of those contacted by letter, one provided a written response (#11), one responded by phone (#17), and one contact letter was returned since the home (#12) is not occupied at the present time. The owners of home #25 and #26 have on numerous occasions filed complaints with Pima County and the Town of Oro Valley regarding the problems they continue to experience. For the most part, these problems, although a nuisance, have been relatively minor in nature (i.e., concentrated runoff near their home coupled with erosion and sedimentation damage). However, on a few occasions, runoff has actually entered portions of their homes. They have seen the severity of their problems increase over time. They attribute most of this increase to recent developments on upstream lots and "drainage improvements" performed by adjacent property owners. The most significant increase in both the frequency and the severity of these problems occurred immediately following construction of the home (#23) that currently occupies Lot 35C of the sunnyslope subdivision. This is the structure that was bui It in the middle of the Glenhurst Wash. Their observations seem to support the results of flood plain analysis of this problem area. Further downstream, the owner of home #17 has also experienced drainage and erosion problems that can to some extent be attributed to runoff from the Glenhurst Wash watershed. His home is located immediately downstream of a drainageway that was originally constructed to accommodate local runoff from a portion of the Rancho Catalina subdivision, primari !y runoff collected and conveyed in Windy Peak Place. However, the drainageway also serves as a collector channel for a portion of the runoff that enters the subdivision from the Glenhurst Wash. The intended outlet for this drainageway is the SBW Northern Tributary. Since the drainageway lacks the capacity to convey local runoff from the street section with minor contributions from the Glenhurst Wash, it will not convey runoff of the magnitude that can be expected should the Glenhurst Wash experience a major flow event. Consequently, any major problems that could be attributed to this flooding source have yet to occur. The fact that the area surrounding the lot has yet to experience a more significant runoff event could be explained in two ways. Either the Glenhurst Wash has not experienced a major flow event or most of the runoff entering the subdivision has been detained and/or diverted by upstream properties. A second source of concern for this homeowner is runoff conveyed in the SBW Northern Tributary. Although no problems have been noted, to date, the owner r r r r r r r r r r r r r r r r r r r ~ Drainage & Flood-Control Engineering Page 39 is still concerned that high flows in this wash will eventually cause significant damage to his property and/or flood his home. Likewise, the owners of home #11 are concerned that flows conveyed in the Shadow Mountain Wash and/or the SMW Southern Tributary will eventually overtop the bank and flood their property. They have lived in the neighborhood since 1962 and have watched the area change from a rural setting to an urban one. On one occasion, they have seen the Shadow Mountain Wash filled to capacity. Shortly thereafter, they asked the Town of Oro Valley to elevate the bank on their side of the wash (which Oro Valley did) in hopes of preventing any future breakout. However, to date, most of their actual drainage problems can be attributed to local runoff from their street and the upstream areas to the east. One point worth noting is their observations regarding a reduction, over time, in the capacity of that portion of the southern tributary that is located between Hardy Road and its confluence with the Shadow Mountain Wash. Considering the existing breakover potential from the South Branch, a reduction in the capacity of this reach will increase the quantity of flow spilling over the Hardy Road dip section. r r r r r r r r r r r r r r r r r r r IV. CONCEPT MITIGATION MEASURES 4.1 Economic Feasibility Analysis In order to justify the expense of providing flood control improvements within an area, the estimated cost of the improvements must be compared to the benefits derived. In this particular instance, the benefits derived are the savings realized by individual property owners and/or damage-re 1ief organizations or agencies when flood damages are prevented during a particular storm event. If the damage/cost ratio for the selected or design event is greater than one, the costs of improvements are usually justified, at least from an economic standpoint. The damage-assessment analysis associated with this project considered three storm events, the 5-year, 25-year and 100-year events. Typically, engineering studies of this type develop a mitigation measure for each storm event. Their associated damage/cost ratios are evaluated independently to determine the feasibility of each level or type of improvement. However, Reference 6 suggests the use of an alternative method for determining the benefit/cost of retrofitting improvements. Using the damages associated with each return interval, Reference 6 guides the user through a probability analysis that is used to determine an expected annual-damage value or average-annual damage value over the range of return periods considered. The purpose of this analysis is to determine the feasibility of each level of improvements based on a payback period. The cost of each level of improvements is divided by the payback period and then compared to the expected annual damage value to determine if the benefit/cost ratio is greater than one. The payback period over which these improvements are applied is either based on the life expectancy of the structure itself or the current owner's investment period (i.e., the length of time he or she plans to keep the structure). However, some very relevant factors or benefits that are not considered in this type of analysis include: (1) a reduction in flood insurance premiums if 100-year protection is provided, (2) the higher value associated with flood free property versus flood-prone property, (3) the availability of bank loans for either the owner or a potential buyer, and ( 4) the peace of mind a residential property owner would have in knowing that his or her property is not continually threatened by flooding within generally acceptable confidence 1imi ts. From a computational standpoint, one must be able to determine within reasonable limits, the point or return interval at which zero damage occurs. Since the threshold return interval used in the floodplain analysis was the 5year event and several structures were impacted during this return interval, it was not possible to define the zero-damage interval except by extrapolation. Considering the importance of the zero-damage interval in determining averageannual damages, it was felt that extrapolation of the computed results may produce unreliable estimates. Finally, when one considers the infrequent and unpredictable nature of flooding, the term "average-annual damages" would seem misleading to most individuals. In addition, the payback period in and of itself is not a standard method of economic analysis for projects of this type. Consequently, for the purpose of this study, only the benefits derived during each flow event wi 11 be used to determine the most appropriate means of addressing the problems inherent to each problem area. ~ Drainage l Flood-Control Engineering r r r r r r r r r r r r r r r r r r r ~ Drainage l 4.2 Flood-Control Engineering Page 41 Identified Problem Areas The results of the flood plain analyses indicate that there are six distinct problem areas (identified as Problem Areas A through F on Figure 9). Since each of these problem areas can not be addressed by a single flood improvement project, the cost of improvements within each area must be weighed against the total benefit derived from within that area. The homes associated with each problem area and a brief description of the flooding source they share are provided in the following paragraph. Problem Area A, includes only one home, #1. This home is located within the direct path of flow that overtops a portion of the west bank of the Shadow Mountain Wash between Sections 114 and 115. Problem Area B includes homes #2 through #12. These homes are within the path of breakover flows from the South Branch that are conveyed within the SMW Southern Tributary and the Shadow Mountain Wash itself. Problem Area C includes homes #13 through #16. This problem area is located along the South Branch immediately upstream of its confluence with the SBW Northern Tributary. This is the area where flow breaks out of the South Branch between Sections 30 and 31. Problem Area D includes only one home, #17. The home in this problem area is located within the 100-year flood plain for the SBW Northern Tributary and is located in the direct path of reconcentrated runoff from the Glenhurst Wash. Problem Area E includes homes #18 through #22. The homes within this problem area are located within the 100year flood plain for the SBW Northern Tributary. Problem Area F includes homes #23 through #26. Homes within this problem area are located on the fan region of the Glenhurst Wash and in the wash itself. Based on the results of the damage-assessment analysis, not all homes within each of the identified problem areas are affected and, in some cases, the potential damage associated with one home is far greater than the potential associated with another home. Consequently, the cost of improvements versus the benefit derived relative to each problem area must be evaluated from both a single-structure standpoint and from a group standpoint. The single-structure benefit is provided in Table 3.3B. The group benefit is provided in Table 4.2A. 4.3 Site-Specific Mitigation Measures 4.3.1 Problem Area A Before proceeding with the discussion of this problem area, it should be noted that the estimated damages associated with the only home in this area (i.e., #1) may be high considering its location relative to the west bank and the quantity of breakout expected. The flood plain analysis indicates that no breakout occurs during the 5-year event; only 6 cfs breaks out during the 25year event; and only 25 cfs breaks out during the 100-year event. The latter value represents only five percent of the computed 100-year peak discharge which is well within the tolerance I imits generally associated with hydrologic calculations (i.e., accurate to within 10 15 percent). Additional consideration must be given to the relative accuracy of the project's topographic map and the flood plain model itself. As previously mentioned, an effort was made to ensure that both the hydrologic calculations and the flood plain model would provide conservative results. In addition, the average depth of flow overtopping the west bank during the 100-year event is only approximately 0.5 feet. Yet, the depth of flooding r r r r Page 42 r r r r r r r r r r r r r r r Damages Grouped by Problem Area Affected Residences Only Problem Area Affected Residences Return Period (yr) Combined Damages ($) A 1 5 25 100 8000.00 9700.00 10800.00 B 3, 4, 7-11 5 25 100 25700.00 45200.00 64700.00 c 14, 14A 5 25 100 8400.00 11200.00 17300.00 D 17 5 25 100 10300.00 13200.00 16200.00 E 18-20 5 25 100 23600.00 32200.00 40800.00 23-26 5 25 100 71000.00 77800.00 86200.00 F ~ Drainage l Flood-Control Engineering TABLE 4.2A 4">, \) r r r r r r r r r r r r r r r r r r r ][~ Drainage ~ flood-Control Engineering Page 43 is estimated to be approximately one foot. This occurs because home #1 is located on the leeward side of poorly defined levee that constitutes the west bank. Consequently, the finished-floor elevation is one foot lower than the contained water surface elevation. However, the home may not be inundated by sheet flow escaping the confines of the channel since its depth would probably be less than 0.5 feet. Any relief between the finished floor of the home and the surrounding ground may be enough to prevent inundation. These facts must be considered in selecting the most appropriate and cost-effective mitigation measure. Four different mitigation measures could be implemented to address the problem associated with this home. These measures are: (1) widen the existing channel section to convey more runoff; (2) line a portion of the exiting channel section to improve flow conveyance; (3) elevate the west bank with fill material to contain the entire 100-year peak discharge; or (4) provide a floodwall along the top of the existing bank to contain flows. Both the second and fourth alternatives will probably require more money to implement than the benefit derived. This is especially true when the conservative nature of the flood-damage assessment is considered. Consequently, a more-detailed analysis would probably demonstrate that either the first or third alternative would be the most cost-effective approach. In fact, both a! ternatives could be implemented by placing the material removed from the channel along the west bank. To confirm the effectiveness of the first and/or third alternative, the amount of area required to facilitate the improvements must be matched with the availability of right-of-way or the size of the established drainage easement. However, these improvements could easily be performed by roadway maintenance crews from either Pima County or the Town of Oro Valley. Consequently, the services of a private contractor would not be needed. Therefore, it does not appear that structural flood control improvements are warranted for this area. 4.3.2 Problem Area B This problem area appears to represent one of the most significant problem areas within the study area since ali of the affected homes within this area, with the exception of #11 and #12, are located along the drainage divide where flow escapes the South Branch Wash. However, ail of these drainage-divide homes appear to have a masonry block wall around their rear yards. These walls actually separate the homes from the South Branch and thus offer some protection from overtopping flows. In addition, of these nine drainage-divide homes, only five (#3, #7, and #9 through #11) are directly affected (i.e., inundated) by flood waters. The remaining four homes were included in the damage-assessment analysis because they lack adequate freeboard (i.e., the water-surface elevation is less than one foot below the finished-floor elevation), not because the results show that they are actually inundated. Of the five affected homes, two (#3 and #10) are expected to be inundated during all three return intervals and three (#7, #9 and #11) are only impacted during the 100-year event. The two most appropriate mitigation measures for this problem area appear to be the construction of either a floodwall or a levee. If the existing patio wails are capable of withstanding the force of floodwaters, it may be possible to limit the extent of either of these two measures to the open space between adjacent walls. Even if this is the case, some floodproofing or retrofitting r r r r r r r r r r r r r r r r r r r ~~ Drainage l Flood-Control Engineering Page 44 of the existing walls may be required to guard against seepage and/or erosion along the base of the wall which could cause failure. Channelization in any form would not be considered a good alternative from the community's standpoint since most of the South Branch within this area is still in its natural state. In addition, a significant about of right-of-way would have to be obtained to accommodate a channelization alternative. Of the two more desirable alternatives (i.e., a floodwall or a levee), the floodwall would probably be the more acceptable from the communities standpoint and more cost-effective. If a discontinuous levee were provided, much of the work would have to be done by small equipment since the space between the existing walls is limited. Although a continuous levee could be constructed using heavy equipment, the impact on the area may not be acceptable to local residents and a great deal more material would be required. In addition, the face of the levee would have to be protected to prevent erosion. Consequently, it appears that the floodwall alternative would be more appropriate. Based on a comparison the existing 100-year water surface profile along this reach of the South Branch and the elevation of the drainage divide, the average height of a floodwall would be approximately two feet. Approximately 450 linear feet would be required to close the gap that currently exists along the divide. If this number is compared to the total estimated damages for the area during the 100-year event, the wall would provide a benefit equal to approximately $144.00 per linear foot ($64,700 divided by 450 linear feet). During the 5-year and 25-year events, the benefit derived would be equal to approximately $57.00 and $100.00 per linear foot, respectively. The approximate cost of a structurally sound floodwall could range between $60.00 and $100.00 per linear foot. Therefore, if the existing patio walls are structurally sound, the benefits derived during the 25-year and 100-year events are both within the anticipated cost range for the wall. However, if the patio walls are not adequate, approximately 1200 linear feet of wall would be required. The benefit derived during the 100-year event alone would only be $54.00 per linear foot ($64,700 divided by 1200 linear feet). Under this scenario, the benefit does not appear to justify the cost. Even if the above described improvements occur, special consideration may be required with regard to home #11 since it may also be impacted by flows conveyed in the Shadow Mountain Wash. AI though the results of the damageassessment analysis relative to the Shadow Mountain Wash indicate that this structure would not be directly impacted (i.e., inundated) during the three flow events analyzed, it was noted that the home lacks adequate freeboard during both the 25-year and 100-year events. Consequently, a small amount of damage would have been assessed this structure separately had it not been included in the South Branch analysis. During the two less-frequent events, the assessed damage relative to the Shadow Mountain Wash would have been 1imi ted to approximately $1527.00 and $4436.00, respectively. Considering the minor amount of damages incurred, it is unlikely that any improvements would be justified for this area alone. Consequently, either Pima County or the Town of Oro Valley must continue to maintain the drainageway and ensure that the adjacent leveed banks are sufficient to contain the 100-year peak discharge associated with this watercourse r r r r r r r r r r r r r r r r r r r ~ Drainage l Flood-Control Engineering Page 45 4.3.3 Problem Area C The only lot impacted within this problem area is Lot 118 of the Rancho Catalina subdivision. Of the two homes on this lot (#14 and #14A), only #14A is directly impacted during all three flow events. However, since #14 lacks adequate freeboard relative to the 25-year and the 100-year events, it was also included in the damage-assessment analysis. In addition to the damage estimates associated with these two homes, damage may occur to other homes located along the breakout channel. However, since the breakout channel is outside the limits of the study area and its topographic coverage, the extent of this damage cannot be assessed. Consequently, the cost of flood control improvements for this area can only be compared to the potential damages associated with homes #14 and #14A. Other than channelization, the damages associated with these two homes could be reduced or eliminated by floodproofing each individual home or an area that includes both the main house and the guest house. To a certain extent, the actual damages associated with each home needs to be clarified to more accurately determine the benefit derived. Since the combined value associated with Lot 118 was proportioned between the two homes and the guest house is impacted to a greater degree than is the main house, most of the damages are attributed to the guest house. If the value of the guest house and its contents were overestimated, then the anticipated damage amount was also overestimated. In either case, the most feasible or cost-effective flood mitigation measure would be site-specific floodproofing measures. This could be One approach would involve the construct ion a accomplished in two ways. floodwall or levee along the ill-defined south bank of the South Branch from Section 29.5 upstream to Section 31. Another approach would involve floodproofing each home or the area defined by both homes and the adjacent yard. This measure would also include a floodwall if no structurally-sound masonry wall currently exists. In addition, closures may be required to provide access through the wall when the homes are not threatened. The first approach would not only address flooding problems inherent to the two identified homes (#14 and #14A), but also those problems that may exist downstream along the breakout channel. This approach would require a minimum of 320 linear feet of wall or levee. Since the average height of either the wall or the levee would be limited to approximately two feet, a wall may prove more cost effective than a levee. Based on the estimated damages associated with the two identified homes, the benefit derived would only be equal to approximately $54.00 per linear foot for the 100-year event ($17,300 divided by 320 linear feet). In this particular instance, the benefit does not appear to justify the cost. However, until a preliminary design is prepared for the wall, it is impossible to determine at this time the actual feasibility of this a! ternat ive. In addition, the approximate the cost of floodproofing each home or the area shared by both homes can not be determined at this time. However, Figure 10, which was taken from Reference 6, provides an excellent example of how individual floodproofing measures can be made a part of the existing home and its associated improvements. If a masonry wall similar to the one shown in this figure does not already exists around the perimeter of the structure, one could r lffiJB Dratnage l Flood-Control Engineering Page 46 be provided. This approach may shorten the length of wall required to adequately floodproof the home. r r r r r r r r r r r r r r r r r However, it is not necessary that a floodwall with closures be attached to the homes themselves. A totally detached wall or levee could be provided. The simplest way to layout a detached flood-control improvement of this type is to use the characteristic horseshoe shape with the open end pointing in the downstream direction. Floodwaters are then be diverted around the perimeter of the homes and their improvements. In this particular instance, the length of wall required to partially encircle the combined area will exceed the length required to close the gap between Section 29.5 and Section 31. Consequently, the feasibility of floodproofing this structure is questionable. Although a levee could be used in lieu of a floodwall, the levee would occupy more area than a wall and it would need to be protected from erosion. Consequently, a floodwall would probably cost less than a levee. However, unless both alternatives are analyzed, it is difficult to determine which would be more cost effective. 4.3.4 Problem Area D As previously mentioned, this problem area includes only one home, #17. This home occupies Lot 185 of the Rancho Catalina subdivision. Two flooding sources impact this home. One source, the Glenhurst Wash, impacts the structure on a much more frequent basis than does the more obvious flooding source (i.e., the SBW Northern Tributary). However, it appears that flows conveyed in the SBW Northern Tributary wi II cause most of the damage associated with this home. Based on the home's location relative to the surrounding area, both onsite and offsite improvements may be warranted Offsite improvements to the Windy Peak Place drainageway would eliminate some of the nuisance problems associated with Lot 185. This drainageway, which begins on the south side of Windy Peak Place between Lots 182 and 183, was originally designed to accommodate low-flow runoff collected by the street section. However, in order to properly design the necessary improvements, a more-detailed study of the drainageway would be· required. AI though street maintenance crews could faci I i tate some of the improvements, a small-scale engineering study that offers design recommendations would be more appropriate. However, improving the drainageway will not completely eliminate the problems inherent to Lot 185. Consequently, onsite improvements are needed to adequately address the impact of runoff from both flooding sources. To accomplish this, site-specific floodproofing measures similar to those already discussed are needed. This includes the construction of a continuous floodwall or levee around the northern, eastern, and southern boundary of the improvements associated with this lot. The northern and eastern portion of this floodwall or levee would protect the home from runoff released by the Glenhurst Wash. The eastern and southern portion would protect the home from flow conveyed in the SBW Northern Tributary. The average height of the wall or levee would range between one and three feet. The upper limit could be reduced with some cooperation from an adjacent property owner. Some time ago, the owner of Lot 189 which is located on the opposite side of the SBW Northern Tributary constructed a portion of the rear yard fence in the main channel. The degree of encroachment associated with this r r ' r r r r r r r r r r r r r r r r r ~ Drainage l Flood-Control Engineering Page 47 fence significantly increases the flood hazard associated with Lot 185. Removal of the encroached section would lower the water surface elevation in its immediate vicinity. Consequently, the height of the floodwall or levee could be reduced. Even if a floodwall or levee is not provided, a reduction in the water surface elevation would in turn reduce the damage potential. Again, a floodwall would probably be the best floodproofing measure available. Approximately 300 linear feet would be required to protect the home on Lot 185. The benefit realized during the 5-year, 25-year and 100-year events would be approximately $34.00, $44.00, and $54.00 per linear foot, respectively. However, its important to note that the estimated damages associated with this problem area apply only to the SBW Northern Tributary. The unpredictable nature of flow released by the Glenhurst Wash prevent a reasonably accurate determination of the damages associated with this flooding source. Considering only the potential damages associated with the SBW Northern Tributary, it appears that the potential cost of improvements is not justified by the benefits derived. 4.3.5 Problem Area E The homes in this problem area also located in the 100-year flood plain for the SBW Northern Tributary. Of the five homes identified in this problem area only three are directly impacted during the 100-year event. Of these three, only two are directly impacted during both the 5-year and 25-year events. Since channelization is not likely to be an acceptable alternative from a community standpoint, a floodwall or levee is again the best means available to address the problems associated with this area. In this particular instance, a linear wall or levee (as opposed to a horseshoe alignment) could be provided along the alignment of the ill-defined northern bank of the wash. The upstream limit of the wall or levee should correspond to either the eastern boundary of the lot associated with home # 18 (i.e. , Lot 142 of Rancho Catalina) or the downstream limit of the fill slope associated with Lot 140. The downstream limit of the wall or levee should be located a short distance downstream of home #20. Approximately 460 linear feet of wall or levee would be required. The average height of this wall or levee would be approximately two feet. Again, a wall may prove to be more cost effective that a levee and it will occupy less space. In addition, some form of bank protection would be required for the levee. Based on the estimated cost of damages, the benefit realized by this wall during the 5-year, 25-year, 100-year events would be approximately $5 !.00, $70.00, and $89.00 per linear foot, respectively. Therefore, it appears that floodproofing measures are justified within this problem area. 4.3.6 Problem Area F Only four homes were included in the damage-assessment analysis of this problem area. Three of these homes (#24 through #26) are located on Lots 174 through 176 of the Rancho Catalina subdivision and one (#23) is located on Lot 35C of the Sunnyslope subdivision. The results of the flood plain analysis clearly show that these four homes have been and wi II continue to be adversely impacted by flows conveyed in the Glenhurst Wash. However, the potential also exists, but to a lesser degree, for r r r r r r r r r r r r r r r r r r r ~ Drainage l Flood-Control Engineering Page 48 neighboring homes within the Rancho Catalina subdivision to be impacted in much the same way as are homes #24 through #26. Once flow leaves the confines of the low-flow channel created between the hillside and home #23 and interacts with the first three Rancho Catalina homes (#24 through #26), the downstream flow path is diverse and to some extent unpredictable. Although neighboring homes within the Rancho Catalina subdivision may benefit to some degree from mitigation measures similar to those required to protect homes #24 through #26, the need for these improvements could not be determined within reasonable limits. To address the hazards associated with each of the three identified Rancho Catalina homes (#24 through #26), a continuous floodwall in the shape of horseshoe could be provided. However, care must be taken in the construction of these walls to maintain as much open space as possible between adjacent homes. This is why a flood control levee would not be appropriate in this situation. If a continuous wall or levee were provided along the upstream boundary of all three lots, in lieu of three separate walls, runoff would be diverted to the west. This would adversely impact the homes located on Lots 171 through 172 of the Rancho Catalina subdivision. Consequently, the continuous wall or levee approach is not an acceptable mitigation measure to address the problems associated with these three homes. The individual-wall approach will require approximately 200 linear feet of wall for each home. Since the average damage estimate for these three homes is approximately $14,000.00 during the 100-year event, the benefit realized would be approximately $70.00 per linear foot. Therefore, this alternative may be feasible only for this level of protection. The only other alternative available is a collector channel. However, this channel would have to extend from Glenhurst Drive south to the SBW Northern Tributary. Consequently, a minimum of 600 I inear feet would be required. Considering the sediment transport capacity of the Glenhurst Wash, the upstream port ion of this channel would have to be oversized to account for sediment deposition. Due to the natural slope of the ground surface between the upstream limit and the downstream limit, excessive flow velocities would be created. To prevent erosion, the channel would have to be totally lined and several drop structures would probably be required. In addition, an energy dissipator would be required at the channel's outlet. Since most of the property along the proposed alignment is privately owned, right-of-way would have to be acquired and some utilities would have to be relocated. A drainage structure would also be required beneath a portion of Windy Peak Place. Considering the magnitude of the 100-year peak discharge (486 cfs), a concrete box culvert is the only drainage structure that would be adequate. Therefore, considering the cost and appearance of these improvements and the disrupt ion that construct ion would cause within neigh~rhood, this alternative would probably not be feasible nor would it be desirable. If floodwalls are designed to address the drainage problems inherent to this area, care must be taken to account for the possibility of sedimentation on their upstream side. In addition, the foundation or footing must be designed to account for erosion. Erosion wi 11 be a major factor in the design since concentrated runoff will be forced between the confines of adjacent walls. In conjunction with the walls, all property owners in the area should avoid or at least be conscious of the fact that any "drainage improvements" they perform r ~ Drainage l Flood-Control Engineering Page 49 without the assistance of a professional engineer (either private or public) may have a significant impact on their neighbors. r r r r r r r r r r r r' r r r r Improvements discussed thus far are not applicable to home #23. The owner of this home has already constructed a wall along a portion of the upstream limit of his lot, Lot 35C of the Sunnyslope subdivision. Although the primary purpose of this wall was to prevent inundation of the home, it merely acts to divert low-flow runoff around the side of the house. The home itself is still subject to inundation during all three return intervals. Based on the results of flood plain analysis, the maximum depth of flooding ranges between approximately 4.5 feet and 5.5 feet. The results of the damageassessment analysis indicate that damages during the 100-year event are approximately equal to 50 percent of the home's market value. Consequently, the wall and the adjacent low-flow channel do little to protect the home except during annual runoff events. In addition, there is some concern regarding the structural stability of the wall. As a result, the possibility of bodily harm and/or additional damages should the wall fall have not been considered in the damage-assessment analysis. If a value could be attributed to these items, the assessed damages associated with this home would far exceed its value. By today's standards, it is unlikely that a permit would be issued to construct this home in what is most certainly the floodway for the Glenhurst Wash. Consequently, if the structure is damaged to the extent anticipated, reconstruction would probably not be permitted either. The only way to protect this home is to channelize the wash. Channelization must begin on the upstream side of the existing wall and extend downstream along the alignment of the existing low-flow channel. However, an acceptable channel would be difficult to design using the current standards and criteria established by Pima county, especially if the channel were used solely to address the problem associated with home #23. In addition to the potential damages that would be sustained by downstream property owners immediately following channelization, the structural requirements, coupled with other design elements, would most likely require far more funds than the benefits derived. Even if the channelized section were to extend all the way downstream to the SBW Northern Tributary and all assessable damages were combined (i.e., those associated with homes #23 through #26), the cost required would most likely exceed the benefits derived by a substantial amount. In addition, it is highly unlikely that the community would accept this type of improvement. Considering the problems already created by the existence of this home and its wall (i.e. the concentration of runoff during both major and minor flow events), the only reasonable alternative is to purchase the home on Lot 35C and remove it. In addition, Lot 36 of the Sunnyslope subdivision, which is located immediately downstream of Lot 35C, should be purchased to prevent future development. AI though stringent standards would be imposed on any proposed development of Lot 36, one can only ensure that development does not occur by purchasing the lot. If steps are taken to ensure that Lot 36 remains in its natural state and all traces of development are removed from Lot 35C, the damage potential associated with the three identified downstream properties during major flow events should be diminished considerably. r r r r r r r r r r r r r r r r r r r ~ Draina~e l flood-Control Engineering Page 50 Under this scenario, during major flow events, runoff in the Glenhurst wash would be distributed over a wider area, which approximates the condition that existed before Lot 35C was developed. This would, in turn, reduce the depth of flow, the average velocity of flow, and the amount of sediment currently impacting the downstream properties. Reducing the depth of flow will lower the damage potential from a monetary standpoint (based on the procedure applied in this study). AI though low-flow runoff could cant inue to impact downstream properties in a manner similar to that which currently exists, the flow pattern would be less predictable, thus creating a state that more closely approximates the pre-developed condition. Consequently, the risks associated with living in the Glenhurst Wash flood plain would be more equable in the sense that they would be more a function of where one chose to reside within the natural floodprone area rather than a function of where one resides relative to an upstream owner, who is forced, for his own protection, to implement improvements that significantly alter natural flow patterns. There could be some drawbacks associated with the removal of improvements on Lot 35C and its reclamation. The most significant will be felt by the owner of Lot 35B and, quite possibly, the owner of Lot 37, both of which are located in the Sunnyslope subdivision. Under existing conditions, the wall on Lot 35C acts, to some degree, to stabilize the vertical profile of the channel in this area. Removal of the wall, even with reclamation of the Jot, could result in the formation of a headcut during low-flow events. As the headcut moves upstream, the access drive to the home on Lot 35B will be damaged unless a cut-off wall is provided along its downstream edge. Another drawback, which may or may not be beneficial depending on ones perspective, is that removal of the wall may reduce the quantity of runoff diverted down Glenhurst Drive. This may be of benefit to most of the individuals who live along Glenhurst Drive, but it could have an adverse impact on Lot 37. The existence of the wall on Lot 35C appears to significantly limit the amount of runoff impacting Lot 37 during major flow events. Its removal could increase the extent of flooding during these less-frequent events. However, the extent of flooding would be no different than that which existed prior to the development of Lot 35C. Although the above discussion is qualitative in nature, a more-detailed evaluation will be performed as part of Phase II. This evaluation will include a flood plain analysis of the Glenhurst Wash that will attempt to model the impact of removing the home on Lot 35C. However, based solely on the results of the qualitative analysis, the benefits derived by removing all improvements on Lot 35C seem to outweigh the potential drawbacks. r r r r r r r r r r ,.. ! r r r v. REaJt,t.IENJJATIONS RIDARDIOO PHASE I I The Phase II scope-of-work for this project calls for: (1) preliminary designs for the selected mitigation measures; (2) an evaluation of the impacts of each measure from hydraulic standpoint; and ( 3) the establishment of preliminary cost estimates that can be compared to the benefits derived. Based on the results of the Phase I investigation, only one mitigation measure seems appropriate to address most of the problems associated with five of the six identified problem areas. This single mitigation measure involves the construction of a series of independent floodwalls within each problem area, with the possible exception of Problem Area A. Within Problem Area A, channel reshaping and/or levee-height extensions may be more cost-effective than a f loodwall. It should be noted that within Problem Area F, floodwalls are also the preferred alternative to address the problems associated with homes #24 through #26. Since there does not appear to be a reasonable and cost-effective solution to the flooding potential associated with home #23, a suggestion was made that this home be purchased and removed. AI though removal of this home wi II not completely eliminate the drainage problems associated with homes #24 through #26, it may reduce the design requirements for the f loodwall. Consequently, an evaluation that attempts to quantify both the positive and negative impacts associated with removal of this home should be performed as part of the Phase II study. If the results of this evaluation are inconclusive (i.e., removal can not be justified from a quantitative standpoint), the only alternative is for the owner to purchase enough flood insurance (if he has not already done so) to offset the anticipated damages. In addition, the preliminary f!oodwall designs for homes #24 through #26 should consider the full impact of the hazards that currently exist. Although the required height of each floodwall is similar in all areas, the foundation depth will vary depending on the depth of scour and other local constraints. Once these design parameters are established, preliminary designs can be prepared and the approximate cost of each wall can then be established and compared to the benefits derived. Based on the results of this study, it appears that the benefit/cost ratio will be greater that one in most of the problem areas especially if 100-year protection is provided. If the Phase II analysis verifies this and no adverse impacts are identified, a funding source can be sought and appropriate improvements made. Consequently, it is recommended that the study proceed with Phase II as outlined. The floodwall concept is the only mitigation measure that should be evaluated with the except ion of channel reshaping within Problem Area A. However, the removal of home #23 should be investigated from a quantitative standpoint in an attempt to fine tune the floodwall designs for homes #24 through #26. r r If Pima County does not feel the expected damages warrant proceeding with a detailed analysis of the floodwall concept, including its anticipated costs, area residents should be notified of the results of this phase of the study so ~ Drainage l Flood-Control Engineering r r r r r r r r r r r r r r r r r r r ffim~ Drainage l Flood-Control Engineering that they can decide, on an individual basis, independent evaluation. Page 52 rather to proceed with an In any event, area residents should be notified of the flood hazards that exist and informed that they have the option of obtaining flood insurance to help offset any damages that may result should their homes be subjected to flooding. r r r r r r r r VI. REFERENCES 1. Pima County, Department of Transportation and Flood Control District, "Hydrology Manual for Engineering Design and Floodplain Management within Pima County, Arizona," September, 1979. 2. Pima County, Department of Transportation and Flood Control District, letter entitled "Revised Soi 1 Hydrologic Groups," addressed to "All Engineers," September 22, 1986. 3. U.S. Army Corps of Engineers, "HEC-2: Water Surface Profiles, Manual," September, 1982, updates, September, 1988. 4. U.S. Department of Transportation, Federal Highway Administration, Bridge Division, "FHWA Culvert Analysis (H¥8), Version 3.2," November, 1990. 5. Chow, V.T., Open Channel HYdraulics, 1959. 6. Federal Emergency Management Agency, "Design Manual for Retrofitting Floodprone Residential Structures," September, 1984. 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