Section 1 Introduction ........................................................................................4
1.1 Propose...................................................................................................................... 4
1.2 Project Authority....................................................................................................... 4
1.3 Project Location ........................................................................................................ 5
1.3 Hydrologic and Hydraulic Methods.......................................................................... 5
1.4 Acknowledgment ...................................................................................................... 6
This study relied on assistance of RFCD GIS staff, who were integral to the
development of the models and maps............................................................................. 6
1.5 Study Results ............................................................................................................ 6
Section 2 FEMA Forms.....................................................................................10
2.1 Study Documentation Abstract for FEMA submittals............................................ 10
2.1.1 Date Study Accepted........................................................................................ 10
2.1.2 Study Contractor .............................................................................................. 10
2.1.3 Local Technical Reviewer ............................................................................... 10
2.1.4 Reach Description............................................................................................ 10
2.1.8 USGS Quad Sheets .......................................................................................... 11
2.1.9 Unique Conditions and Problems .................................................................... 11
2.1.10 Coordination of Peak Discharges................................................................... 11
2.2 FEMA Forms .......................................................................................................... 11
Section 3 Survey and Mapping Information ...................................................11
3.1 Field Survey Information........................................................................................ 11
3.2 Mapping .................................................................................................................. 12
Section 4 Hydrology .........................................................................................12
4.1 Method Description ................................................................................................ 12
4.2 Parameter Estimation .............................................................................................. 12
4.2.1 Drainage Area .................................................................................................. 12
4.2.2 Watershed Work Map ...................................................................................... 12
4.2.3 Gage Data......................................................................................................... 13
4.2.4 Spatial Parameters............................................................................................ 13
4.2.5 Precipitation ..................................................................................................... 13
4.2.6 Physical Parameters ......................................................................................... 13
4.3 Problems Encountered During the Study................................................................ 15
4.3.1 Special Problems and Solutions....................................................................... 15
4.3.2 Modeling Warning and Error Messages .......................................................... 15
4.4 Calibration............................................................................................................... 15
4.5 Final Results........................................................................................................ 15
4.5.1 Hydrologic Analysis Results........................................................................ 15
4.5.2 Verification results....................................................................................... 16
Section 5 Hydraulics.........................................................................................17
5.1 Method Description ................................................................................................ 17
5.2 Work Study Maps ................................................................................................... 17
5.3 Parameter Estimation .............................................................................................. 17
5.3.1 Roughness Coefficients ................................................................................... 18
5.3.2 Expansion and Contraction Coefficients ......................................................... 18
5.4 Cross-Section Description ...................................................................................... 18
5.5 Modeling Consideration.......................................................................................... 18
2

5.5.1 Hydraulic Jump and Drop Analysis................................................................. 18
5.5.2. Bridges and Culverts....................................................................................... 18
5.5.3 Levees and Dikes ............................................................................................. 19
5.5.4 Island and Flow Splits...................................................................................... 19
5.5.5 Ineffective Flow Areas..................................................................................... 19
5.6 Floodway Modeling ................................................................................................ 19
5.7 Problems Encountered ............................................................................................ 19
5.7.1 Special Problems and Solutions....................................................................... 19
5.7.2 Model Warnings and Errors............................................................................. 19
5.8 Calibration............................................................................................................... 20
5.9 Final Results............................................................................................................ 20
5.9.1 Hydraulic Analysis Results.............................................................................. 20
5.9.2 Verification of Results ..................................................................................... 20
Section 6 Erosion and Sediment Transport ...................................................20
Section 7 Draft FIS Report Data.......................................................................21
7.1 Summary of Discharges.......................................................................................... 21
7.2 Floodway Data ........................................................................................................ 21
7.3 Annotated Flood Insurance Rate Map .................................................................... 21
7.4 Flood Profiles.......................................................................................................... 21
List of Tables
Table 1 Methods used for a HEC-HMS analysis.............................................................. 14
Table 2 Physical Parameters for the Sub-Basins .............................................................. 14
Table 3 Summary of the Hydrologic Analysis Results for Sub-Basins............................ 16
Table 4 Summary of the Hydrologic Analysis Results at the Concentration Points ........ 16
Table 5 Comparison of a peak discharge.......................................................................... 16
Table 6 Comparison of a peak discharge.......................................................................... 17
List of Figures
Figure 1.1 Watershed Map.................................................................................................. 7
Figure 1.2 Study Limit........................................................................................................ 8
Figure 1.3 Soil Classification.............................................................................................. 9
Appendix A: References
Appendix B: FEMA MT-2Forms, General Documentation and Correspondence
Appendix C: Survey Field Notes
Appendix D: Hydrologic Analysis, Supporting Documents
Appendix E: Hydraulic Analysis, Supporting Documents
Appendix F: Erosion Analysis, Supporting Documents
Exhibit
Exhibit 1 100-yr floodplain limits for the Flecha Caida Wash
Exhibit 2 Annotated Flood Insurance Rate Map for the Flecha Caida Wash

3

Section 1 Introduction
1.1 Propose
This Technical Data notebook (TDN) has been prepared for a Letter of Map Revision
(LOMR) application for a portion of the Flecha Caida Wash (FCW) located in Pima
County, Arizona. The objective of the TDN and LOMR submission is provide regulatory
discharge rates and floodplain limits along the Flecha Caida Wash using better
topographic, hydrologic, and hydraulic data.
This TDN was prepared in accordance with the “Instructions for Organizing and
Submitting Technical Documentation for Flood Studies” prepared by the Arizona
Department of Water Resources, Flood Mitigation Section (Arizona State Standard, SSA
1-97) and FEMA Guideline. FEMA LOMR forms are included in this TDN.

1.2 Project Authority
The State of Arizona has delegated the responsibility to each county flood control district
to adopt floodplain regulations designed to promote the public health, safety and general
welfare of its citizenry as provided under the Arizona Revised Statutes, Title 48, Chapter
21, Article 1, Sections 48-3601 through 3627. More specifically, A.R.S. 3609 directs
county flood control districts to adopt floodplain regulations that:
A. Regulate all development of land, construction of residential, commercial or
industrial structures or uses of any kind which may divert, retard or obstruct flood
water and threaten public health or safety or the general welfare; and
B. Establish minimum flood protection elevations and flood damage prevention
requirements for uses, structures and facilities which are vulnerable to flood
damage; and
C. Comply with state and local land use plans and ordinances, if any.
In conformance with A.R.S. 3609, this ordinance provides for protection of the
public health safety and welfare by regulation of flood and erosion hazard areas to
control flood hazards and prevent repetitive loss from flood damage.
D. The flood hazard areas of Pima County are subject to periodic inundation which
may result in loss of life and property, create health and safety hazards, disrupt
commerce and governmental services, require extraordinary public expenditures for
flood protection and relief, and impair the tax base, all of which adversely affect the
public health, safety, and general welfare.
E. These flood losses are caused by the cumulative effect of obstructions in areas of
special flood hazards which increase flood heights, flow velocities, and cause flood
and erosion damage. Uses that are inadequately flood-proofed, elevated, or
otherwise protected from flood damage, also contribute to the flood loss. (Ord. 2005
FC-2 § 2 (part), 2005).
4

Section 16 of the Pima County Ordinance describes the provisions for floodplain
regulation in Pima County.
This study has been prepared by the Pima County Regional Flood Control District
(RFCD):
Pima County Regional Flood Control District
97 East Congress, Tucson, AZ 85701
The project was prepared by:
Akitsu Kimoto, Ph.D., C.F.M., Principal Hydrologist.
Pima County Regional Flood Control District
97 East Congress, Tucson, AZ 85701

1.3 Project Location
The study reach of the Flecha Caida Wash (FCW) is located within a Federal Emergency
Management Agency (FEMA)-designated “Zone A” flood-hazard area, as depicted on
FIRM Map Panel Numbers 04019C1644K and 04019C1645K (February 8, 1999). No
documented hydraulic analyses were found to determine the “Zone A”, and the existing
“Zone A” depiction is not consistent with current topography. The objective of the TDN
and LOMR submission is provide regulatory discharge rates and floodplain limits along
the Flecha Caida Wash using better topographic, hydrologic, and hydraulic data.
The study reach of the Flecha Caida Wash is located primarily east of Swan Rd. and
extends to Sections 23, 26, and 27 of Township 13 South, Range 14 East, Pima County,
Arizona (Fig. 1.1). The study reach was divided into three segments in the study limit for
the Flecha Caida Wash LOMR (Fig. 1.1). The western branch of the Flecha Caida Wash
enters study limit from the northeast and flows southwest until it converges with the
eastern branch. The eastern branch of the Flecha Caida Wash enters the study limit from
the northwest and flows southeast until it converges with the western branch. The eastern
and western branches converge immediately south of River Road. The Flecha Caida
Wash LOMR study limit for the western branch extends from a junction with western
branch to Paseo del Bac, in Section 23 of Township 13 South, Range 14 East. The study
limit for the eastern branch extends from the junction with the eastern branch to Avenida
del Cazador, in Section 23 of Township 13 South, Range 14 East. After the junction of
the western and eastern branches, the wash flows down until it converges with Rillito
Creek, in Section 27 of Township 13 South, Range 14 East.

1.3 Hydrologic and Hydraulic Methods

5

Hydrologic analysis was preformed to determine proposed regulatory discharge rates at
concentration points along the Flecha Caida Wash using U.S. Army Corps of Engineers
Computer Hydrologic Modeling System, HEC-HMS. Parameterization followed
guidelines developed by Pima County Regional Flood Control District and described in
technical Policy 018 (Tech 018, Appendix A). The proposed regulatory discharges are
flow rates that have a 1-percent chance of being equaled or exceeded each year (“100year” discharge rates). Hydraulic analysis was performed to delineate floodplain limits
along the study reach of the Flecha Caida Wash using U.S. Army Corps of Engineers
Computer Backwater Model, HEC-RAS.
.
Three culverts exist within the study limit of the Flecha Caida Wash. Two culverts are
located along the western branch of the study reach. A Corrugated Metal Pipe (CMP)
culvert is located on Camino Cardenal, and on Calle de Las Chacras. A reinforced
Concrete Box (RCB) culvert is located along the downstream segment, on Swan Road,
just north of Avenida del Cazador.

1.4 Acknowledgment
This study relied on assistance of RFCD GIS staff, who were integral to the development
of the models and maps.

1.5 Study Results
The regulatory discharge rates were calculated at five concentration points along the
Flecha Caida Wash (Fig. 1.3). Peak discharges at three concentration points (CP B, C, D)
were used for the hydraulic analysis in this study. The estimated regulatory discharge
rates are 604 cubic feet per second (cfs) with a drainage area of 0.7 square mile at the
Concentration Point B (CP B), 781 cfs at CP C with a drainage area of 0.83 square mile,
and 1370 cfs with a drainage area of 1.42 square mile at CP D.

6

Figure 1.1
Watershed Map
Flecha Caida Wash

SKYLINE

(
!

Concentration Point
10 Foot Contour

KO
LB

Flecha Caida Subbasins
FCW-1
FCW-2
FCW-3

SUNRISE

FCW-4
FCW-5
Aerial Photo:2008 Pima Association of Government
Topo: 2005 Pima Association of Government
Vertical Datum: NAVD 1988

CP A

Pima County Index Map

CRAYCROFT

(
!

Index Map Scale 1:5,250,000

CP B
SWAN

(
!

The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Regional Flood Control
Department makes no claims regarding the accuracy of the
information depicted herein.

CP C CP E
(
!
(
!

This product is subject to the Department of Transportation
Technical Services Division's Use Restriction Agreement.
Pima County Regional Flood Control District

Scale 1:2000

RIVER

2,000

AL
VE

1,000

0

2,000 Feet

CP D
RN

(
!
ON

ALV E RN

ON

06/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Flecha_Caida\Flecha_Caida_wash_Watershed_Fig1_1.mxd

Figure 1.3
Soil Classification Map
Flecha Caida Wash

SKYLINE

KO

Flecha Caida Subbasins

LB

Soil Classification
Soil Group: A (100%)
FCW-1

Soil Group: B (100%)

SUNRIS E

Soil Group: B (82%) C (18%)
Aerial Photo:2008 Pima Association of Government

FCW-5
FCW-2

Index Map Scale 1:5,250,000

SWAN

FCW-3

CRAYCROFT

Pima County Index Map

The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Regional Flood Control
Department makes no claims regarding the accuracy of the
information depicted herein.
This product is subject to the Department of Transportation
Technical Services Division's Use Restriction Agreement.

FCW-4

Pima County Regional Flood Control District

Scale 1:2000'

RIV ER

ALV E RN

ON

06/2010
\\gislib\rfcd\projects\imd\xavi\mxd\AKITSU\Flecha_Caida\Flecha_Caida_wash_Watershed_Fig1_3.mxd

Section 2 FEMA Forms
2.1 Study Documentation Abstract for FEMA submittals
2.1.1 Date Study Accepted: ___________________
2.1.2 Study Contractor:
Planning and Development Division,
Pima County Regional Flood Control District
97 East Congress, Tucson, AZ 85701
(520) 243-1800
Prepared by Akitsu Kimoto, Ph.D, C.F.M., Principal Hydrologist.

2.1.3 Local Technical Reviewer:
Terry Hendricks, C.F.M, Chief Hydrologist
Planning and Development Division,
Pima County Regional Flood Control District
97 East Congress, Tucson, AZ 85701
(520) 243-1800

2.1.4 Reach Description
The study reach of the Flecha Caida Wash is located within a Federal Emergency
Management Agency (FEMA)-designated “Zone A” and “Zone AE” flood-hazard area,
as depicted on FIRM Map Panel Numbers 04019C1644K and 04019C1645K (February
8, 1999). The study reach of the Flecha Caida Wash is located primarily east of Swan Rd.
and extends to Sections 123, 26, and 27 of Township 13 South, Range 14 East, Pima
County, Arizona (Fig. 1.1). As previously mentioned, the study reach was divided into
three segments in the study limit for the Flecha Caida Wash LOMR (Fig. 1.1). The
western branch within the Flecha Caida Wash LOMR study limit extends from a junction
with western branch to Paseo del Bac in Section 23 of Township 13 South, Range 14
East, while the eastern branch extends from the junction with the eastern branch to
Avenida del Cazador, in Section 23 of Township 13 South, Range 14 East. After the
junction of the western and eastern branches, the downstream segment flows downgrade
until it converges with Rillito Creek, in Section 27 of Township 13 South, Range 14 East.
The reach becomes wider after the junction. The length of each reach segment is 5034
feet for the western branch, 3066 feet for the eastern branch, and 3066 feet for the
downstream segment.

10

The study reach of the Flecha Caida Wash are primarily composed of sand channels and
the bottom of the reach is clean with no significant vegetation cover. The overbank of the
reach is covered with scattered desert brush.

2.1.8 USGS Quad Sheets
Not available for this study

2.1.9 Unique Conditions and Problems
Approximately half of the CMP culvert, located on Camino Cardenal along the western
Branch, was clogged with sand at the inlet. This might create backwater problems that
might lead to higher flood elevation upstream of the CMP.

2.1.10 Coordination of Peak Discharges
The 100-year regulatory discharge rates at the concentration points along the study reach
were computed using HEC-HMS, assuming no base flow in the watersheds or no
transmission loss within the reaches. All reaches were modeled with HEC-RAS. The
discharge rates were acceptable per Suzanne Shield, Director of the Pima County
Regional Flood Control District.

2.2 FEMA Forms
The FEMA MT-2 forms are included in Appendix B.

Section 3 Survey and Mapping Information
3.1 Field Survey Information
The survey of the culverts was conducted between November 20-24, 2008 under direct
contract with the Pima County Regional Flood Control. A signed and sealed copy of the
survey is included in Appendix C.
The site survey was performed by:
Michael C. McCredie
Arizona Registered Land Surveyor, Certificate Number. 32792

11

3.2 Mapping
The topographic data was obtained using GeoRAS and ArcGIS. A triangular Irregular
Network (TIN) derived from 2005 Light Detection and Ranging (LiDAR) data was used
to create 2-foot interval contour map. The 2005 LiDAR data was accepted by FEMA for
the LOMR revision of the Friendly Village Wash (“Friendly Village Wash LOMR
TDN”, Case No. 08-09-0473P, effective on October 23, 2008), located approximately 5
miles west of the Flecha Caida Wash.
The following data was used in this TDN;
The aerial photo: 2008 PAG aerial photo
Projection: UTM, Zone 12
Units: International feet
The contour interval of the topographic map is 2 feet.

Section 4 Hydrology
4.1 Method Description
The 100-year peak discharges for the five sub-basins of the Flecha Caida Wash (FCW-1,
FCW-2, FCW-3, FCW-4, FCW-5; Fig. 1.3) were calculated using U.S. Army Corps of
Engineers Computer Hydrologic Modeling System, (HEC-HMS) version 3.4. The HECHMS model requires the parameters regarding rainfall, topography, soil, vegetation, and
channel characteristics to determine runoff volume and peak discharge. Those parameters
were determined according to the Pima County Regional Flood Control District
Technical Policy 018 (Tech-018). Tech-018 is included in Appendix A. The HEC-HMS
model is included in Appendix D.

4.2 Parameter Estimation
4.2.1 Drainage Area
Subbasin boundaries were delineated using the hydrology function of ArcGIS with 2005
Lidar Data. A 2-ft contour map was used to make sure if the subbasin delineation was
reasonable.

4.2.2 Watershed Work Map

12

A watershed work map is included in Exhibit 1. Nine subbasins were delineated for
HEC-HMS hydrologic analysis. Five sub-basins were delineated for HEC-HMS
hydrologic analysis. Five concentration points were included in the study watershed (CP
A, B, C, D, and E). A junction was used to connect the eastern branch of the Flecha
Caida Wash and western branch. The 100-year peak discharges at the three concentration
points (CP B, C, and D) were used for HEC-RAS hydraulic analysis.

4.2.3 Gage Data
No gage data were used in this TDN.

4.2.4 Spatial Parameters
No spatial parameters were used in this TDN.

4.2.5 Precipitation
According to the Tech-018, the 3-hour storm shall be used as rainfall data in the HECHMS model in the case that a time of concentration (Tc) is equal or less than three hours.
A 3-hour storm was selected, since Tc was less than 3 hours in all the sub-basins.
A point 3-hour rainfall depth at the coordinates of the centroid of the watershed was
obtained from NOAA Atlas 14, upper 90% confidence interval precipitation frequency
estimate (NOAA 14 rainfall). Areal reduction factor was applied to watersheds larger
than 1 square mile, as described in Tech-018. The 3-hour rainfall depth for the Flecha
Caida Wash watershed is 3.3 inches. The areal reduction factor of 0.96 was applied to CP
D.

4.2.6 Physical Parameters
The physical parameters for the sub-basins and reaches of the HEC-HMS model were
summarized in Tables 1 and 2. As mentioned in 4.1, all the methods and parameters were
determined based on Tech-018. Table 1 summarizes the method used for a HEC-HMS
analysis.

13

Table 1 Methods used for a HEC-HMS analysis
Selected Method
NOAA 14, upper 90% Confidence Interval
3-hr SCS Type II Storm
SCS Curve number
SCS Segmental Method
SCS Unit Hydrograph
Modified-Puls

Rainfall Depth
Rainfall Distribution
Rainfall Loss
Time of Concentration
Transform
Routing

The SCS Curve Number (CN) method was utilized as a rainfall loss method in the HECHMS model. The CN was determined using the Curve Number tables and Hydrologic
Soils Group maps associated with the PC Hydro User Guide (Arroyo Engineering, 2007).
The CN was not adjusted for rainfall intensity or antecedent moisture conditions. The
SCS Unit Hydrograph method was used as a transform method. Impervious cover was
determined using 2008 PAG aerial photograph. The combination of the kinematic wave
time of concentration method and the U.S. Natural Resources Conservation Service
(NRCS) segmented Time of Concentration (Tc) calculation (USDA-NRCS, 1986) was
used to determine Tc, based on the recommendation on Tech-018. The Tc was calculated
by summing the travel time for sheet flow, shallow concentrated flow and channel flow.
The Tc for sheet flow was estimated using the kinematic wave equation. Manning’s
roughness coefficient for sheet flow was obtained using Table 3-1 in Technical Release
55, Urban Hydrology for Small Watersheds (USDA-NRCS, 1986). The velocity for
channel was calculated using Manning’s equation. The detail of the Tc calculation is
included in Appendix D.

Table 2 Physical Parameters for the Sub-Basins
Sub-Basin
FCW1
FCW2
FCW3
FCW4
FCW5

Area
(sq mi)
0.47
0.22
0.14
0.18
0.41

CN
84.0
84.0
83.7
83.7
83.9

Impervious Area
(%)
10.0
10.0
10.0
10.0
10.0

Vegetation Cover
(%)
30
30
30
30
30

Lag Time
(min)
25.5
16.1
8.7
14.1
22.3

Runoff from sub-basins was routed using the Modified-Puls method. A storage discharge
table for the channel routing was developed using the cross sections and slopes derived
from HEC-HMS. The number of subreaches was calculated using the following method:

14

Vw = 1.5 * Vave .........eq.1
K=

L
...................eq.2
Vw

Therefore,
K
N = ..................eq.3
Δt
where Vave is average flow velocity, L is reach length, Vw is velocity of flood wave (a
conversion factor of 1.5 is used for natural channels), K is hydrograph travel time, Δt is
the time interval for computations in the model, and N is the number of steps in the reach
routing. Eq.4 was obtained from eq.1, 2, and 3. The detail of the calculation of the
number of subreach is included in Appendix D.

4.3 Problems Encountered During the Study
4.3.1 Special Problems and Solutions
There were no problems with the hydrologic modeling.

4.3.2 Modeling Warning and Error Messages
The time interval of the rainfall data used in this study is 5 minutes, while the simulation
time interval is 1 minute. The HEC-HMS model interpolated the 5-minute time interval
of the rainfall data to 1-minute time interval.

4.4 Calibration
No calibration was conducted in this study.

4.5 Final Results
4.5.1 Hydrologic Analysis Results
The 100-year peak discharges at the concentration points along the Flecha Caida Wash
were determined using the HEC-HMS. The results are summarized in Tables 3 and 4.

15

Table 3 Summary of the Hydrologic Analysis Results for Sub-Basins
Sub-Basin
FCW1
FCW2
FCW3
FCW4
FCW5

Area
(sq mi)
0.47
0.22
0.14
0.18
0.41

Rainfall Depth
(in)
3.30
3.30
3.30
3.30
3.30

Runoff Volume
(in)
1.77
1.77
1.74
1.74
1.76

Peak Discharge
(cfs)
603.6
385.4
330.7
336.9
574.2

Table 4 Summary of the Hydrologic Analysis Results at the Concentration Points
Concentration
Point

Location

CP A
CP B
CP C
CP D
CP E

at Paseo del Bac
at Via Ra Posa
at River Rd
Confulence with Rillito River
at River Rd

Area (sq Rainfall
Runoff
Q100
Time to
mile)
Depth (in) Volume HMS (cfs)
Peak
(in)
0.47
3.30
1.77
604
1:50
0.69
3.30
1.77
781
1:53
0.83
3.30
1.74
846
1:54
1.42
3.17
1.65
1370
2:00
0.41
3.30
1.76
574
1:47

4.5.2 Verification results
There were comparable 100-year peak discharges available near the concentration points
A, C and E in a previous study (prepared by Simons, Li and Associates, Inc. 1986). The
peak discharges in the previous study were determined based on Hydrology Manual for
Engineering Design and Flood Plain Management within Pima County, Arizona (Pima
County Department of Transportation and Flood Control District, 1979). The 100-year
discharge near the CP A, B, and E were shown in Table 5. The comparison shows that the
100-year peak discharges estimated in this study is lower than the values from the
previous study.
The calculated 100-year peak discharge was also compared with the peak discharge
obtained from USGS Regression Equation 13 (Thomas et al., 1997) (Table 6). The
comparison showed that the peak discharge from the HMS-derived peak discharge was
lower than the one derived from the regression equation.

Table 5 Comparison of a peak discharge
Concentration Point

CP #20
CP #21
CP #24

Concentration Point Location

Area

Q100

Near CP A in this TDN
Near CP C in this TDN
Near CP D in this TDN

(sq mile)
0.5
0.81
1.33

Peak Discharge
(cfs)
766
1046
1604

16

Table 6 Comparison of a peak discharge
Concentration
Point

Location

CP A
CP B
CP C
CP D
CP E

at Paseo del Bac
at Via Ra Posa
at River Rd
Confulence with Rillito River
at River Rd

Area (sq
Q100
Q100
mile)
HMS (cfs) RRE (cfs)
0.47
0.69
0.83
1.42
0.41

604
781
846
1370
574.2

742
977
1110
1584
671

Section 5 Hydraulics
5.1 Method Description
The hydraulic modeling for the Flecha Caida Wash was performed using Hec-RAS,
Version 4.0 (HEC-RAS), HEC-GeoRAS, Version 4.1.1 (HEC-GeoRAS), and ArcGIS,
Version 9.3.
Hydraulic analysis was performed in the area currently mapped as FEMA Zone A. Steady
flow analysis was performed to determine a 100-year floodplain limit for the Flecha
Caida Wash by using HEC-RAS. Normal-depth with a slope of 0.027 was assumed for a
downstream boundary condition. Corrected model is proposed in this study. The model
name is FC, and the plan name is Plan 01.
The locations of the stream centerline, cross-sections, and bank of the Flecha Caida Wash
were determined using the 2-ft contour map and 2008 PAG aerial photos. The geometric
data, including stream centerline, flow paths and cross-sections, were digitized in HECGeoRAS. The digitized data was exported to create geospatially referenced geometric
data (cross section, reach profile) in HEC-RAS. As previously mentioned, the DTM
derived from 2005 LiDAR data was used. Other parameters for the steady-state analysis
in HEC-RAS, such as Manning’s n-values, expansion and contraction coefficients,
boundary condition, and ineffective flow areas were manually input into HEC-RAS. The
hydraulic data obtained from HEC-RAS were imported into HEC-GeoRAS to delineate a
floodplain boundary for the Flecha Caida Wash.

5.2 Work Study Maps
The work study map is included in Exhibit 2.

5.3 Parameter Estimation

17

5.3.1 Roughness Coefficients
Manning’s n values were determined using 2008 PAG aerial photo. Manning’s n value of
0.06 was assigned for the overbank with scattered desert brush along the all reaches
including western, eastern, and downstream branches. The value of 0.04-0.045 was
assigned for the channel.

5.3.2 Expansion and Contraction Coefficients
The channel of the Flecha Caida Wash is assumed to have generally gradual transitions
with minimum curvature, except for the upstream and downstream of the culverts. The
expansion coefficient of 0.30 and contraction coefficient of 0.10 were used for the entire
study reaches of the FC except the upstream and downstream of the culverts. The
expansion coefficient of 0.50 and contraction coefficient of 0.30 were used for the
upstream and downstream of the culverts.

5.4 Cross-Section Description
A 2-foot interval contour map derived from 2005 LiDAR data was used to select the
location of cross sections. Cross-section locations were determined primarily based on
the channel topography. The cross-section lines were drawn to be perpendicular to flow
paths in Geo-RAS and ArcGIS.

5.5 Modeling Consideration
5.5.1 Hydraulic Jump and Drop Analysis
The HEC-RAS profile showed a drop of the 100-year flood elevation over the two
upstream culverts (Culverts #1 and 2) along the Flecha Caida Wash. No hydraulic, drop
analyses or adjustment of the floodplain for the Flecha Caida Wash was conducted in this
study.

5.5.2. Bridges and Culverts
There are three culverts in the study limit. Two corrugated metal pipe (CMP) culverts are
located along the western branch of the study reach. A 4-cell, arch (3.5-ft span and 2.4-fe
rise) CMP is located on Camino Cardenal, and a 4-cell, circular culvert with a diameter
of 3 feet CMP is located on Calle de Las Chacras. A 2-cell, 9-ft wide by 6-ft high
reinforced concrete box (RCB) culvert is located along the downstream segment, on
Swan Road, just north of Avenida del Cazador. All the culvers were surveyed by Michael
C. McCredie, Arizona registered land surveyor.

18

5.5.3 Levees and Dikes
There are no levees or dikes located within the study limit.

5.5.4 Island and Flow Splits
There were no islands or flow splits modeled.

5.5.5 Ineffective Flow Areas
Ineffective flow option was modeled in the following situations;
• Floodplain areas are not hydraulically connected (Cross Section # 860.9661)
• A contraction and expansion of flow through the culvert openings occurs at the
upstream and downstream of the Culvert #3 (Cross Section#.2344.541, 2199.356,
and 1992.327). 4:1 expansion and 1:1 contraction ratios were used.

5.6 Floodway Modeling
No floodway modeling was performed in this study.

5.7 Problems Encountered
5.7.1 Special Problems and Solutions
There are no special problems in the study limit.

5.7.2 Model Warnings and Errors
The FEMA guidelines state that it is required to run hydraulic models under subcritical
flow conditions. Since the study area of the Flecha Caida Wash has steep slopes, the flow
regime of the Flecha Caida Wash is expected to be critical or supercritical. The HECRAS modeling produced warnings stating “During the standard step iterations, when the
assumed water surface was set equal to critical depth, the calculated water surface came
back below critical depth. This indicates that there is not a valid subcritical answer. The
program defaulted to critical depth at many cross-sections along the Flecha Caida Wash.
Most of the errors force a critical solution which is reasonable for these steep
watercourses.

19

The following warning messages occurred:
Divided flow
Energy loss greater than 1.0
Energy equation could not be balanced and defaulted to critical.
Cross-section extended vertically.
Multiple critical depths calculated.
Conveyance ratio is less than 0.7 or greater than 1.4.
The warnings stating that “The energy equation could not be balanced within the
specified number of iterations. The program used critical depth for the water surface and
continued on with the calculations”, “The energy loss was greater than 1.0 ft (0.3 m)
between the current and previous cross section. This may indicate the need for additional
cross sections”, and “The conveyance ratio (upstream conveyance divided by
downstream conveyance) is less than 0.7 or greater than 1.4. This may indicate the need
for additional cross sections” are produced at many cross sections through the Flecha
Caida Wash. These warming messages were produced mainly due to the steepness of the
slope of the Flecha Caida Wash and the subcritical flow requirement of FEMA.

5.8 Calibration
The model was not calibrated in this study.

5.9 Final Results
5.9.1 Hydraulic Analysis Results
The HEC-RAS modeling results are included in Appendix E.

5.9.2 Verification of Results
The floodplain limit produced in this Flecha Caida Wash LOMR study was compared to
the existing FEMA floodplain limit. The floodplain limit of this study was extended to
the Calle dos Cabezas. The proposed floodplain limit tends to follow the existing
floodplain limit, although the proposed limit tends to be narrower than the exiting limit.
The results suggest that the proposed floodplain limit is reasonable based on the
topography of the Flecha Caida Wash.

Section 6 Erosion and Sediment Transport

20

No erosion or sediment transport analysis was conducted in this study.

Section 7 Draft FIS Report Data
7.1 Summary of Discharges
Peak discharges at three concentration points (CP B, C, E) were used for the hydraulic
analysis in this study. The estimated regulatory discharge rates are 604 cubic feet per
second (cfs) with a drainage area of 0.7 square mile at the Concentration Point B (CP B),
781 cfs at CP C with a drainage area of 0.83 square mile, and 1370 cfs with a drainage
area of 1.42 square mile at CP E.

7.2 Floodway Data
Not applicable.

7.3 Annotated Flood Insurance Rate Map
An annotated Flood Insurance Rate Map (FIRM) is included in Exhibit 2.

7.4 Flood Profiles
Flood profiles are available in HECRAS model included in Appendix E.

21

2600

2610

26

Exhibit 1
100-year Floodplain
with cross sections
Flecha Caida Wash

70

40

26

2620

26

261

0

2550

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0
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26

79793377

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0

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28

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9. 7 5

567

98

70

60

25

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254 3.0 61

1

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24
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24

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254 0

Pima County Index Map

30

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Aerial: 2008 Pima Arizona Government
Topo: 2005 Pima Arizona Government
Vertical Datum: NAVD 1988

54

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Index Map Scale 1:1,500,000

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This product is subject to the Department of Transportation
Technical Services Division's Disclaimer and Use Restrictions.
Pima County Regional Flood Control District

Scale 1:200'

25

00

2450

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246 0

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242 0

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The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Department of
Transportation Technical Services Division makes no
claims regarding the accuracy of the information depicted
herein.

248 0

24

2

1 66

2480

24

50

243 3.

0

2480

166

25

10

Pima County Regional Flood Control
97 East Congress Street - 3rd Floor
Tucson. Arizona 85701-1207
(520)243-1800 - FAX (520)243-1821
http://www.rfcd.pima.gov

01/2010

gislib\rfcd\projects\imd\xavi\mdx\akitsu\Flecha Caida wash_100yr_exh1.mxd

Exhibit 2
Annotated Flood
Insurance Rate Map
04019C1644 K
Flecha Caida Wash
AR

CO

SWA N

M

IN
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23

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22

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çç çç ç

HILL FARM

TUCSON

HILL RANCH

ZONE X - SHADED

WELL

ZONE X

LOMR Case 00-09-616P Effective Date7/26/2000

ASA CARR

ZONE X - SHADED

This product is subject to the Department of Transportation
Technical Services Division's Disclaimer and Use Restrictions.

EE

K

ING

CR

QUARTERMASTER

ZONE X

Streets
The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Department of
Transportation Technical Services Division makes no
claims regarding the accuracy of the information depicted
herein.

OW

SP R

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34

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35

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CAMP LO

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ç ç ç ç ç ç ç çç

LOMR Case 04-09-0465X Effective Date4/22/2004

2420

çççç

LOMR Case 04-09-0621P Effective Date11/4/2004

ZONE AE

çç ç ç ç ç ç ç ç ç ç çç

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27

RIVER HOUSE

2417

ç ç ç ç 240
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26

CAMINO RIO DE LUZ

EN A

AN

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A

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ç ç ç ç ç ç ç ç çç

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Proposed
100 year
Floodplain

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ZONE X

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INO S

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NI

CA

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E
AV

L
DE

FIRM X-Sections

çç çç

0

250

500

1,000
Feet

Base Flood Elevations
Proposed Floodplain
LOMRs
FIRM - Flood Insurance Rate Map
Sections
Jurisdictions
Floodway

Pima County Index Map

Existing Floodplain Zone
ZONE A
ZONE AE
Pima County Regional Flood Control
97 East Congress Street - 3rd Floor
Tucson. Arizona 85701-1207
(520)243-1800 - FAX (520)243-1821
http://www.rfcd.pima.gov

ZONE AO 1
ZONE AO 2
ZONE X
ZONE X - SHADED

gislib\rfcd\projects\imd\xavi\mdx\akitsu\Flecha_Caida\Flecha_Caida_watershed_Anno_FIRM8x11_1644K.mxd

Exhibit 2
Annotated Flood
Insurance Rate Map
04019C1645 K
Flecha Caida Wash

O

CALL

E FA

JA

ZONE X

C

H

AP

ES
LL
E
CA

14

CRAYCROFT

Q

SPL E
C AM

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CALLE BOSQUE

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BRE

INO E

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CAL

15

ITA
D

UI
NA

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RA

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AC

PA
S

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EN
RD

DE

LB

AC

R

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ZONE X
22

DO
ZA

A

HEATHER

G
IN
O
M
CA

23
Proposed
100 year
Floodplain

RA

RVO

CIE

VI
A

LE
CAL

DEL

NO
MI

AR

ZONE X

0
The information depicted on this display is the result
of digital analyses performed on a variety of databases
provided and maintained by several governmental agencies.
The accuracy of the information presented is limited to
the collective accuracy of these databases on the date
of the analysis. The Pima County Department of
Transportation Technical Services Division makes no
claims regarding the accuracy of the information depicted
herein.
This product is subject to the Department of Transportation
Technical Services Division's Disclaimer and Use Restrictions.

Pima County Regional Flood Control
97 East Congress Street - 3rd Floor
Tucson. Arizona 85701-1207
(520)243-1800 - FAX (520)243-1821
http://www.rfcd.pima.gov

CO

CA

IBRI

AMA

CO L

DA R

INO

IN
LA L

CAM

PO

SA

VIA NUEZ

SWAN

EL

L
DE
NI
DA

PIMA COUNTY

ZONE A

WOOD

CA

IN
O

AV
E

AVENIDA DE PAZ

PLACIT
A SHE

LLY

CA

M

PLACITA PROVIDA

AC

ISTRAN
CALLE CAP

CA

Proposed
100 year
Floodplain

VI
A

RRIL
CALLE BA

Streets
FIRM X-Sections

250

500

1,000
Feet

ç ç ç ç ç Base Flood Elevations
Proposed Floodplain
LOMRs
FIRM - Flood Insurance Rate Map
Sections
Jurisdictions
Existing Floodplain Zone
A
AE
AO
X
X (SHADED)

Pima County Index Map

gislib\rfcd\projects\imd\xavi\mdx\akitsu\Flecha_Caida\Flecha_Caida_watershed_Anno_FIRM8x11_1645K.mxd