•

FINAL REPORT
SOlmi BRANCH, UPPER CARMACK

SUB-BASIN MANAGE)(ENT STUDY
PHASE II

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FINAL REPORT
SOUI'H BRANCH, UPPER CARMACK
SUB-BASIN MANNiE1tiENI' STUDY
PHASE II

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Prepared by:

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mDBERT L. $HAND, P.E.
Drainage & Flood-Control Engineering
1 W. William Carey Street
Tucson, Arizona 85747

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(602) 762-5668

Prepared for:
Pima County
Department of Transport at ion
and

Flood Control District
201 North Stone Avenue
Tucson, Arizona 85701-1207

July 24, 1992

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TABLE OF CXlNTENTS
I.

1.1
1.2

;.

I

II .

•

III.

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Phase I Summary
Phase II Objectives

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2

4

Floodwalls
Flood-Control Levee
Improved Channel Section
Design Parameters

MITIGATION IMPACT AND COST ESTIMATE
3. 1
3.2
3.3
3.4
3.5
3.6

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1

PRELIMINARY DESIGNS
2. 1
2.2
2.3
2.4

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INTRODUCTION

Problem
Problem
Problem
Problem
Problem
Problem

Area
Area
Area
Area
Area
Area

A
B
C
D
E
F

4
5
5

6
7
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;.

8
11
12
13
14

IV.

RECOMMENDATIONS REGARDING IMPLEMENTATION

18

VI.

REFERENCES

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LIST OF FIGURES

Figure 1

Project Location Map .

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Figure 2

Photographic Base Map of the Study Area

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Figure 3

Flood-Prone Structures Map .

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Figure 4

Typical Section of Concrete Floodwall.

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Figure 5

Typical Section of Masonry Floodwall

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Figure 6

Typical Section of Flood-Control Levee

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Figure 7

Channel Improvements, Section 113, Problem Area A

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Figure 8

Channel Improvements, Section 114, Problem Area A

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Figure 9

Cross Section Location and 100-year Flood Plain Map.

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Figure 10

Proposed Alignment of Floodwall, Problem Area A

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Figure 11

Water Surface/Ground Profile, Problem Area A .

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Figure 12

Proposed Alignment of Floodwall, Problem Area B.

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Figure 13

Water Surface/Ground Profile, Problem Area B

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Figure 14

Proposed Alignment of Floodwall, Home #11.

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Figure 15

Proposed Alignment of Levee/Floodwall, Areas C and E

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Figure 16

Water Surface/Ground Profile, Problem Area C

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Figure 17

Proposed Alignment of Floodwall, Areas D and F.

Figure 18

Water Surface/Ground Profile, Problem Area E

.

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LIST OF TABLES

Table 2.1A

Cost Estimate for Varying Wall Heights

Table 3.2A

Relative Impact of Floodwall on the South Branch Wash.

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Table 3.2B

Relative Impact of Reclamation on Lot 35C

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Table 4A

Damages versus Mitigation Cost of Proposed Improvements.

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Table 4B

Estimated Flood Insurance Premiums for Affected Homes

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LIST OF APPENDICES

Appendix A

Summary Input/Output Listing for 101I120.DAT & 101II120.DAT
including associated Scour Computation Sheet for Problem Area A
(see accompanying 5 1/4" diskettes for complete HEC-2 data files)

Appendix B

Summary Input/Output Listing for SBW-D.DAT & 300D305.DAT including
associated Scour Computation Sheet for Problem Area B (see
accompanying 5 1/4" diskettes for complete HEC-2 data files)

Appendix C

Summary Input/Output Listing for 200D212.DAT (see accompanying
5 1/4" diskettes for complete HEC-2 data files)

Appendix D

Scour Computation Sheet for Problem Area C

Appendix E

Summary Input/Output Listing for SBWDUS2.DAT including associated
Scour Computation Sheet for Problem Area D and E (see accompanying
5 1/4" diskettes for complete HEC-2 data files)

Appendix F

Summary Input/Output Listing for 401D409.DAT (see accompanying
5 1/4" diskettes for complete HEC-2 data files)

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I.

INTRODUCTION

This report presents the results of an expanded qualitative and
quantitative evaluation of the relative impact of those mitigation measures that
were identified, during the Phase I study, as being the most appropriate and
cost-effective to address the drainage problems inherent to the study area (i.e.,
the South Branch, Upper Carmack drainage basin from the Carmack Wash upstream
to the Forest Service boundary, Figures 1 and 2). In addition, preliminary cost
estimates are provided for each mitigation measure, and a recommendation is made
as to which mitigation measures should be implemented.
1.1

Phase I Summary

Phase I represented an existing-conditions analysis of the study area.
As part of that study, a hydrologic analysis and a flood plain analysis were
first conducted to identify potential f !cod-prone homes within the area.
Finished-floor elevations were then obtained for each of the identified homes
and a damage-assessment analysis was performed on selected homes to estimate
the impact from a monetary standpoint. The homes were grouped into problem areas
and concept mitigation measures were developed for each problem area.
A
preliminary evaluation, both qualitative and quantitative in nature, was then
conducted relative to each problem area to estimate the cost-effectiveness and
impact of each mitigation measure. Recommendations regarding Phase II were then
presented.
More specifically, the hydrologic analysis included watersheds or drainage
areas that were capable of generating greater than 100 cfs during the 5-year
event. As a result fifteen runoff concentration points were established and a
peak discharge was determined at each concentration point for the 2-year, 5year, 10-year, 25-year, 50-year, and 100-year events.
The flood plain analysis was performed on seven different watercourses or
watercourse segments. These watercourses and their HEC-2 (Reference 1) file
names are: (1) the South Branch Wash from its confluence with the Carmack Wash
upstream to Oracle Road (SBW.DAT); (2) the South Branch Wash from Oracle Road
upstream to the Forest Service Boundary (SBW-US!.DAT); (3) the Northern Tributary
to the South Branch Wash from Rancho Cat a! ina Avenue upstream to the Forest
Service Boundary (SBW-US2.DAT); (4) the Glenhurst Wash in the immediate vicinity
of G!enhurst Drive (401-409.DAT); (5) an historic channel braid along the South
Branch Wash located immediately upstream of Northern Avenue (300-JOS.DAT); (6)
the Southern Tributary to the Shadow Mountain Wash from Calle Buena Vista
upstream to Shadow Mountain Drive; and, (7) the Shadow Mountain Wash from its
confluence with the Carmack Wash upstream to Oracle Road. Each watercourse or
segment was analyzed to define the flood plain associated with the 5-year, 25year and 100-year flood events.
Of the twenty-six homes located in or near the 100-year flood plains (see
Figure 3), only fourteen were included in the damage-assessment analysis. Those
homes that possessed greater than one-foot of freeboard during all three of the
flood events analyzed were excluded from the analysis. Based on the location
of each home within the study area, its proximity to other flood-prone homes,
and the primary flooding source, six problem areas were identified (A through
F). The flood-prone homes were then grouped by problem area and their damage

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& Flood-Control Engineering

Page 2

estimates, relative to edch return interval, were combined in order to evaluate
the cost effectiveness of the proposed mitigation measures relative to each area.
Since the study area is almost totally developed and the upstream portion
of the impacting watersheds are located on Forest Service property, only
structural flood-control measures seemed appropriate to resolve the drainage
problems attributed to each problem (i.e., residential flooding with minor
erosion and sedimentation damage). Three structural mitigation measures were
considered for each problem area, channelization, f lood-contro I levees, and
f loodwalls.
With the except ion of Problem Area A, channe I izat ion and/or channe I
reshaping was rejected as the least cost-effective and the least desirable
mitigation measure. With the exception of one home in Problem Area Band one
in Problem Area F, continuous floodwalls with closures or floodwalls segments,
both designed to provide protection up to and including the 100-year flood event,
seemed to be the most appropriate and cost-effective mitigation measure compared
to flood-control levees.
AI though flood-control levees are generally less
expensive than floodwal!s, the relative height of the wall; the need to protect
the levees from erosion; the amount of area required to accommodate a levee; and
the general appearance of a levee contributed to the conclusion that a floodwall
would be the best solution.
Within Problem Area B, home #11 did not exhibit a potential for flooding,
however, it did lack adequate freeboard, which suggests some minimal damage to
the lot, pad, or structure may occur. Considering the minor damage potential
associated with this home, structural improvements, other than the maintenance
program already initiated by the Town of Oro Valley, did not seem justified.
Within Problem Area F, the most appropriate means of dealing with the
hazard associated with home #23 was to purchase the home; remove it; and restore
the lot to its predeveloped condition. This was considered the most logical
approach since the actual damage potential, which might exceed the value of the
home by a substantial amount, was difficult to estimate.
Overall, the results of the mitigation analysis seemed to indicate that
cost-effective measures could be provided within most of the problem areas
especially if protection were geared toward the 100-year event. Consequently,
it was recommended that Phase II proceed with a more detailed evaluation of the
floodwall concept for all problem areas with the exception of Problem Area A and
home #23 in Problem Area F. Within Problem Area A, channelization and/or channel
reshaping should also be evaluated and compared to the approximate cost of a
f loodwall to determine which appears to be the most cost effective. Within
Problem Area F, a quantitative analysis should be performed in an attempt to
evaluate both the positive and negative impacts of removing home #23.
1.2

Phase II Objectives

Based on the results, which were both qualitative and quantitative in
nature, and the recommendations of the Phase I study, Pima County and the Town
of Oro Valley agreed that the Phase II study should be performed as originally
conceived. The main objectives of Phase II are to: (1) provide preliminary
designs for the selected mitigation measures; (2) perform an evaluation of the
impacts of each measure from hydraulic standpoint; and (3) establish preliminary
cost estimates that can be compared to the benefits derived.

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With these main objectives in mind, the Phase II study was conducted to
determine the impact and cost effectiveness of: (1) an improved channel section
versus a f!oodwall within Problem Area A; (2) a continuous floodwall along a
portion of the northern bank of the South Branch Wash within Problem Area B; (3)
a floodwall and/or levee with closures along either a portion of the south bank
of the South Branch Wash within Problem Area C or around the combined area
occupied by home #14 and #14A; (4) a continuous f!oodwall around home #17 within
Problem Area D; (5) a continuous floodwall along a portion of the northern bank
of the SBW Northern Tributary within Problem Area E; and (6) individual
floodwalls around homes #24 through #26 coupled with a quantitative evaluation
of the removal of home #23 within Problem Area f.

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II.

PRELIMINARY DESIGNS

2. 1

Floodwalls

Figure 4 provides a conceptual design of a typical, concrete-reinforced,
retaining wall that can be used as a floodwall. This section was taken from a
manual prepared by the Concrete Reinforcing Steel Institute (Reference 2, Page
14-8). Figure 5 provides a similar design for a masonry floodwall. This section
was taken from Reference 3 (Appendix G, Page 243).
The latter design (Figure 5) was based on specific design conditions
relative to a particular site. It was presented in that reference as part of
a case study. Consequently, the design is applicable only when similar site
conditions are encountered since variations in the depth of flow will affect the
height of the wall. This coupled with varying soil conditions will affect the
structural design of the wall. Therefore, the design as presented can not be
used for cost-estimation purposes.
However, since Reference 3 was intended to be used as a design manual, it
provides a table of physical dimensions and quantities for walls of varying
heights (although it does not address varying soil conditions). Since the height
of the proposed floodwall will vary from one problem area to another, Figure 4
wi 11 be used to provide the quantities necessary to estimate the approximate cost
of each floodwall. Locally, a cost of $275.00 per cubic yard is commonly used
to estimate the cost of installing a structural concrete feature similar to the
retaining represented in Figure 4. The 1inear-foot cost estimate for the various
wall heights represented in this figure are provided in the following table.

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Table 2.2.1
Cost Estimate for Varying Wall Heights

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Height
( ft)
3.0
4.0
5.0
6.0
7.0
8.0

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Concrete
(ft 3 /lf)
4.25
5.50
6.84
8.25
9.66
10.92

Concrete
(cy/lf)

Cost
($/If)

0. 1574
0.2037
0.2533
0.3056
0.3578
0.4044

43.29
56.02
69.67
84.03
98.39
111.22

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Since floodwalls must be designed with consideration for the hydrodynamic
and hydrostatic forces that impact the wall and the soil conditions that exist
at the site, they should only be designed by a qualified structural engineer with
input from a soils engineer. Consequently, these preliminary designs are for
cost estimation purposes only and are not intended for construction.

~ Drainage l

Flood-Control Engineering

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Page 5

Flood-Control Levee

Figure 6 provides a typical design for a flood-control levee. Again this
design was taken from a case study presented in Reference 3. However, some
exceptions must be noted. Within Pima County and the city of Tucson, the minimum
top width for a levee ranges between 10 feet and 16 feet (typically 16 feet for
Pima County and 10 feet for the city of Tucson). In addition to constructing
the levee using impermeable material, the surface on the floods ide must be
protected against erosion, and the protection must be buried deep enough below
the bed of the channel to account for scour.
For cost-estimation purposes, the sides lopes of the levee wi II be the same
as those shown in Figure 6; however, the top width will be 10 feet. The bottom
width and height will vary depending on site conditions. In addition, since this
conceptual design applies primarily to Problem Area C, where it will be used to
form a portion of the south bank of the main channel, the levee will include bank
protection in the form of gabions and a filter fabric will be used beneath the
structure to prevent the leaching of fine material from the levee. The estimated
cost of constructing the levee will be $4.00 per cubic yard. The estimated cost
for a 1. 5 foot-thick gab ion lining wi II be $50.00 per cubic yard with an
additional $1.00 per square foot for the filter fabric.
Since levees must be designed using impermeable materials and/or properly
compacted to prevent seepage, and the foundation soil must be capable of
supporting the anticipated loads, a soils engineer should be consulted during
the design of the levee and should be present to test the levee while the lifts
are being constructed. Consequently, these preliminary designs are for cost
estimation purposes only and are not intended for construction.
2.3

Improved Channel Section

Figures 7 and 8 represent two cross sect ions of the Shadow Mountain
Drainageway within Problem Area A. These cross sections, which were taken from
HEC-2 model 101-120.DAT, as presented in the Phase I report, are identified as
Sections 113 and 114 (see Figure 9).
Section 113 is located immediately
downstream of the only home located in this problem area (i.e., home #1).
Section 114 is located immediately upstream of the home. These figures also show
the geometry of the improved channel section that could be provided within the
existing drainage easement in an attempt to reduce the water surface elevation
along this portion of the drainageway.
The improved section has a 25-foot bottom width with 3:1 side slopes
(horizontal to vertical). No man-made channel lining is shown on these sections
and none currently exists. However, reshaping the existing channel will result
in the removal of existing vegetation (although removal was not taken into
consideration in the hydraulic analysis).
If the banks are not capable of
withstanding the erosive forces of flow along this reach, bank protection may
be required.
Although general guidelines are available to determine the need for bank
protection using the basic composition of the soils that make up the channel
banks and the velocity of flow, a soils analysis is normally required in addition
to a hydraulic analysis. Under most circumstances, if the compressive strength
of the soil is sufficient to withstand erosion, bank protection is not needed.

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Consequently, it is not intended that an unprotected section be provided
unless a soils engineer is consulted or a soils analysis is performed to verify
that bank protection is not required. The purpose of providing this section is
simply to demonstrate that channel reshaping can improve flow conveyance in the
area. By retaining the roughness characteristics associated with the existing
vegetation, the incorporation of bank protection will not alter the results of
the hydraulic analysis of this improved section.

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For comparative purposes, however, three cost estimates will be prepared
for Problem Area A. One will cover the estimated cost of a floodwall; the second
will assume that channel reshaping can occur without bank protection; and the
final estimate will assume bank protection will be required to protect against
erosion. Bank protection in the form of a 1.5 foot-thick gabion lining (with
filter fabric) will be used to estimate the cost of this improvement.

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Design Parameters

The design parameters· for the three types of improvement just described
wi 11 be determined using the HEC-2 program (Reference 1) and, if necessary,
uniform-flow equations (Reference 4) to obtain the appropriate hydraulic
parameters. In addition, the guidelines presented in Reference 5 will be used
to estimate anticipated scour depths (i.e., general scour, bed-form scour, bend
scour, etc.). The scour computation summary sheets will accompany the HEC-2
input/output listings in the referenced appendices.
If the associated procedures and their application criteria are limited
by the physical conditions of the site or they do not provide consistent or
reasonable results, engineering judgement will be used to select the most
appropriate design parameters. However, under no circumstances will the minimum
design values or criteria established by Pima County (References 5 and 6) be
reduced or compromised.

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I I I.

MITIGATION IMPACf AND COST ESTIMATE

3. 1

Problem Area A

The existing-conditions HEC-2 model associated with this problem area ( 101120.DAT) was manually revised to reflect the improved channel section depicted
on Figures 4 and 5 ( 101I120.DAT) and the placement of a floodwall along the
western right-of-way limit ( 101II120.DAT). Summary input and output listings
for these revised models can be found in Appendix A. In addition, the input and
output data files are provided on a 5 1/4-inch diskette which can be found inside
the back cover of this report.
Figures 7 and 8 show the modifications made to model the effect of channel
reshaping. Figure 10 shows the proposed alignment of the wall with respect to
home #1. This alignment corresponds to the approximate location of the western
limit of the Shadow Mountain Wash drainage easement. Figure 11 shows the ground
profile and the associated design water surface profile along the proposed
alignment.
The results of the hydraulic analysis of channel reshaping indicate that
weir flow is prevented during the 5-year and 25-year events and only one cfs will
escape the confines of the drainageway during the 100-year event. Considering
the relative accuracy of both the hydraulic model and the hydrologic
calculations, the one cfs of calculated outflow can be considered insignificant.
Consequently, the effect of reshaping as proposed wi II, for the most part,
eliminate breakout and thus prevent the anticipated damages associated with home
#1. Although containment of the breakout flows will result in a slight increase
in the downstream water surface elevations, the increase does not exceed onetenth of a foot at any downstream section. Therefore, channel reshaping will
not have an adverse impact on adjacent properties.
The results of the hydraulic analysis of the floodwall measure again
indicate that the downstream water surface elevations will be increased a small
amount due to flow containment along the breakout reach, but the increase does
not exceed one-tenth of a foot at any downstream section. The only location
where the increase does exceed one-tenth of a foot is at Sect ion 114. The
increase at this section is approximately two-tenths of a foot. However, flow
remains confined to the channel section and no further increase in excess of onetenth of a foot was noted in the upstream sections. Consequently, the presence
of a floodwall will not have an adverse impact on adjacent properties.

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Approximately 171 cubic yards of material must be removed to accommodate
the reshaped channel section. At an approximate cost of $4.00 per cubic yard,
the reshaping measure would cost approximately $685.00. If a soi Is analysis
determines that bank protection is required, the estimated cost for 690 linear
feet (total length required for both banks) would be approximately $22,232.00.
This figure includes $17,871.00 for the gabions and $4,361.00 for the filter
fabric. The bank protection design includes a height, relative to the flow line
of two feet and a minimum toe-down depth of three feet. The latter value was
selected when the results of the scour analysis indicated that the depth of scour
would be less than three feet. When the cost of channel reshaping is added to
the cost of the bank protection, the total cost of channelization is estimated
to be approximately $22,917.00

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Using Figure 11 as a guide, the average height of the wall required to just
contain the 100-year water surface would be approximately 1.25 feet. When an
additional one foot of freeboard is added to the top of the wall and a three
foot toe-down is applied relative to the flowline elevation, the total height
of wall required would be approximately 5.25 feet. Again. using Figure 11 as
a guide, approximately 200 linear feet of wall would be required. Consequently,
the cost of the wall at a unit price of $73.26 per linear foot would be
approximately $14,652.00.
The costs associated with the three a! ternat i ves
summarized as follows:
Channel Reshaping:
Channelization:
Floodwall:

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just discussed are

$
685.00
$ 22,917.00
$ 14,652.00

The results of the damage-assessment analysis from the Phase
indicated that the maximum damage potential during the 100-year event
approximately $10,800.00.
Consequently, only channel reshaping
effective. The other alternatives will cost far more than the benefits
3.2

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flood-Control Engineering

I study
would be
is cost
derived.

Problem Area B

Two of the three, existing-conditions, HEC-2 models associated with this
problem area (SBW.DAT, 300-305.DAT, and 200-212.DAT) were revised as needed to
reflect the existence of a floodwall along a portion of the ridge that separates
the South Branch Wash from the SMW Southern Tributary. The alignment of this
wall is shown on Figure 12. Summary input and output I ist ings for the two
revised models (SBW-D.DAT and 300D305.DAT) are provided in Appendix B. Complete
listings are provided on the 5 1/4-inch diskettes.
The SBW-D.DAT model was created by removing the split-flow routine from
the SBW.DAT model. Since the revised model reflects containment of all runoff
conveyed into this problem area (i.e., no weir flow occurs over the ridge that
separates the two watercourses), it was apparent that the amount of flow diverted
along the historic channel braid would be changed. For this reason, the 300305.DAT model had to be revised accordingly.
However, for the purpose of
modeling the impact of the floodwall, there was no reason to revise the 200212 model. If weir flow from the South Branch is prevented, the quantity of
runoff conveyed in the SMW South Tributary will be the amount associated with
Concentration Point 12, which is less than 100 cfs during the 5-year event.
Consequently, a revised flood plain analysis of this tributary was not required
for this scenario.
The results of the containment analysis relative to the South Branch Wash
( SBW-D. OAT) indicate that the proposed wall wi II significantly increase the water
surface elevations along the majority of the downstream reach and the reach that
parallels the wall. The increase wi II exceed one-tenth of a foot at all sections
during all return intervals with the exception of Section 9 where, during the
5-year event, the increase was limited to 0.09 feet. The average increase during
the 100-year event is in excess of 0.5 feet for all downstream sections. An
increase in the water surface elevation results in a corresponding increase in
the width of the associated flood plain. The maximum increase occurs during the
100-year event at Section 4, where the change was determined to be approximately
107 feet. In addition, during the 100-year event, the velocity of flow in the

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main channel wi II be increased in excess of one foot per second at several
sections. Table 3.2A summarizes the changes at all sections during the three
return intervals analyzed.
As previously mentioned, the wall will also increase the quantity of flow
diverted along the historic channel braid. During the 5-year, 25-year, and 100year events the increased quantity was determined to be approximately 162 cfs,
438 cfs, and 806 cfs, respectively. However, most of this flow will return to
the South Branch Wash before it exits the downstream limit of the study reach
(i.e., Section 300).
The increased quantity of flow impacting the area
immediately downstream of Section 300, during the three respective events, will
be approximately 35 cfs, 86 cfs, and 158 cfs.
All of the changes just described, including those associated with the
South Branch, are generally unacceptable from a flood plain management
standpoint.
The presence of the wall canst i tutes an encroachment into the
existing 100-year flood plain with undesirable consequences.
Assuming that the results of flood plain analysis had not created an
adverse impact on the area, approximately 1215 I inear feet of wall would be
required along the alignment shown on Figure 12. The relationship between the
100-year water surface and existing grade along this alignment is shown on Figure
13. The 1215 linear feet assumes that a continuous wall is provided. If several
walls were constructed to fill the gaps between the existing patio walls, the
total linear footage would be reduced to 585 feet.
The average height of wall required to contain the 100-year water surface
was estimated to be 1.5 feet. When an additional one foot is added for freeboard
and 4.0 feet is added as a toe-down to guard against the combined effect of scour
and migration of the channel thalweg, the total height of wall required would
be approximately 6.5 feet. The cost per linear foot for this wall would be
approximately $91.21.
Consequent !y, the total cost would be approximate !y
$110,820.00 for a continuous wall and approximately $53,358.00 for a fragmented
wall. Since the total estimated damages during the 100-year event was determined
to be approximately $64,700, the cost of providing the wall is not justified by
the benefits derived unless a fragmented wall is provided. However, the cost
of a fragmented wall does not consider the additional cost of reinforcing the
existing patio walls if they are determined to be unsuitable to withstand the
hydrodynamic and hydrostatic forces of flow along this reach.
Considering the negative impacts of the South Branch floodwall and its
cost effectiveness, the problem associated with home #11 was considered
separately. For home #11, an individual floodwall could be constructed around
its upstream perimeter (see Figure 14). To determine if the construction of this
wall would have a negative impact on the area, model 200-212.DAT was modified
to reflect the existence of this wall and the in-effective flow areas created
in both the upstream and downstream directions. The results of this analysis
(200D212.DAT) indicate that the wall will not have an adverse impact (i.e., no
significant increase in the water surface elevations or flow velocities). A
summarized version of the input and output listing is contained in Appendix c.
A complete version is provided on the accompanying 5 1/4-inch diskettes.
To accommodate this measure, approximately 220 linear feet of wall will
be required.
The home is located on the northern overbank adjacent to the
confluence of the Shadow Mountain Wash and the SMW Southern Tributary where the

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Page 10

Relative Impact of Floodwall on the South Branch Floodplain

w.s.

Velocity
Change
( fps)

Width
Change
( ft)

Discharge
Change
(cfs)

0.31
0.49
0.72

0.63
0.90
1. 22

23.87
37.12
54.66

165.97
466.86
948.53

5
25
100

0. 18
0.33
0.53

0.60
0.97
1. 43

8. 70
16.65
26.55

165.97
466.86
948.53

3

5
25
100

0.25
0.33
0.62

0.61
1.02
1. 15

17.06
5.00
9.40

165.97
466.86
948.53

3.5

5
25
100

0.30
0.40
0.53

0.37
0.78
1.23

29.15
40.44
54.82

165.97
466.86
948.53

4

5
25
100

0.28
0.47
0.58

0.43
0.51
0.96

26.04
90.63
107.14

165.97
466.86
948.53

5

5
25
100

0.20
0.46
0.68

0.68
0.69
1.09

16.09
34.19
50.83

138.97
416.86
873.53

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6

5
25
100

0.27
0.54
0.73

0.94
0.61
0.97

16.01
53.23
71.72

140.97
405.86
833.53

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7

5
25
100

0.28
0.35
0.50

0.24
0.92
1.61

17.56
24.98
57.00

91.97
292.86
628.53

8

5
25
100

0.16
0.36
0.49

0.45
0.49
1. 21

11.44
26.04
44.49

53.97
195.86
458.53

9

5
25
100

0.09
0.16
0.31

0.30
0.58
0.90

18.28
52.24
17.23

33.97
112.86
299.53

10

5
25
100

0.16
0.35
0.59

0.32
0.49
1. 23

6.22
10.40
39.52

195.97
550.86
1105.53

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Cross
Section
No.

Return
Period
(yr)

1

5
25
100

2

I"'

Change
( ft)

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TABLE 3.2A

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Page 11

average depth of flow is approximately 0.5 feet. If one foot is added to the
average depth of flow to provide freeboard, the height of the wall above existing
grade would be 1.5 feet. When the minimum toe-down depth is applied, the design
height of the wall will be 4.5 feet.
The unit cost for a 4.5-foot wall 1 s
approximately $62.85 per linear foot. Consequently the total estimated cost of
this wall is approximately $13,827.00.
Since the anticipated damages for this home during the 100-year event was
estimated to be approximately $9740.00, the cost of protecting the home with a
floodwall is greater than the benefits derived.
It should be noted that the
design toe-down depth for the wall does not account for the possibility of
channel migration or bank erosion. Since the wall will be located less than
fifty feet from the bank, it will be located in the erosion-hazard area. If
the toe-down depth were increased to account for the possibility of channel
migration, the total cost of the wall would be even greater.
3.3

Problem Area C

Within Problem Area C, two mitigation measures were evaluated to determine
which would be the most cost effective. One measure involves either the combined
use of a levee and a floodwall to protect the two affected homes (#14 and #14A).
The second measure involves the construction of a floodwall around the upstream
perimeter of the combined area occupied by the two homes. The proposed a! ignment
of the levee and the floodwalls are shown on Figure 15.
The advantage of the first measure is that it not only protects homes #14
and #14A but it also affords some protection to the homes located downstream
along a secondary channel that conveys breakout flows from the South Branch.
However, since the majority of this secondary channel is outside the limits of
the study area, there was not enough topographic information available to include
this channel in the Phase I floodplain analysis. Consequently, the damageassessment analysis did not include any of the homes that may be impacted by
flows conveyed in this secondary channel.
The second mitigation measure simply addresses the problems associated with
the two identified homes. However, since adjacent property owners may object
to the existence of a levee or wall along the mouth of the secondary channel,
and it appears that the entire structure can not be built on the lot associated
with homes #14 and 14A, this secondary measure was also evaluated to determine
its cost effectiveness.
When the existing-conditions, flood plain analysis (SBW-USl.DAT) was
conducted for this reach of the South Branch, the possible loss of flow down the
secondary channel was disregarded. This was done to ensure conservative results
relative to the downstream water surface elevations. In addition, the proposed
alignment of the wall and the levee corresponds to the effective flow boundary
re !at i ve to Sect ions 29.5 and 30, respective Jy.
Consequently, a revised
floodplain analysis is not needed to model the impact of constructing the two
proposed mitigation measures.
Figure 16 shows the profile of the 100-year water surface from the SBWUS1.DAT analysis and the ground profile along the proposed alignment of the wall
and the levee. Using this profile as a guide, the average height of the wall
required to contain the water surface is approximately 1. 5 feet.
When an
additional one foot is added for freeboard, the average height above the ground

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Page 12

profile will be 2.5 feet. Again, the minimum toe-down depth of three feet must
be applied to the wall since the results of the scour analysis indicates that
the computed depth will be less than three feet (see Appendix D). Consequently,
the total height of wall required will be 5.5 feet.
Approximately 245 linear feet of wall wi 11 be required for the first
mitigation measure and approximately 375 feet will be required for the second
mitigation measure. Using a unit cost of approximately $76.85 per linear foot,
the estimated cost of the floodwall for the first mitigation measure will be
approximately $18,828.00. For the second mitigation measure the estimated cost
will be $28,819.00. However, the total estimated cost of the first mitigation
measure must also include the estimated cost of the levee.
Again using Figure 16 as a guide, the average height of the proposed levee
will be approximately 3.5 feet which includes an allowance for freeboard.
Consequently, the cross-sectional area will be 79.38 square feet.
Since
approximately 205 linear feet will be required, the total estimated volume of
material will be approximately 600 cubic yards.
Using a unit cost of
approximately $4.00 per cubic yard, the estimated cost to construct the levee,
excluding bank protection will be $2,400.00.
To protect the levee against erosion, approximately 124.0 cubic yards of
rock will be required to construct the gabions. At an estimated, in-place, cost
of $50.00 per cubic yard, the cost of the bank protection will be approximately
$6,200.00. Consequently, the total cost of the bank protected levee will be
approximately $8,600.00 or $42.00 per linear foot.

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Using the cost estimates just derived, the total estimated cost of the
first mitigation measure will be approximately $27,428.00 which is very close
to the estimated cost of the second measure ($28,819.00). However, the cost of
both mitigation measures far exceed the estimated damages ($17,300) associated
with this problem area during the 100-year event. Consequently, it does not
appear that either of the proposed alternatives are cost effective.
3.4

Problem Area D

Within this problem area, the only reasonable solution is to construct a
floodwall around the upstream perimeter of the only affected home (#17). Figure
17 shows the proposed location of this wall with respect to the home. Since the
presence of the wall will constitute an encroachment into the floodplain for the
SBW Northern Tributary, the SBW-US2.DAT model from the Phase I study was revised.
In order to model the presence of the wall, one cross sect ion was added
to the SBW-US2.DAT model at the most constricted section created by the wall.
The location of this section is also shown on Figure 17. The results of the
impact analysis (SBWDUSII.DAT) indicate that the wall will not have a significant
effect on the SBW Northern Tributary floodplain. In fact, there was no change
worth noting in the upstream water surface elevations during any of the return
intervals analyzed. A summarized version of the input and output listing is
contained in Appendix E. A complete copy of these listings is provided on the
5 1/4-inch diskettes.
In order to protect home #17 from flows conveyed in the SBW Northern
Tributary and overflow runoff from the Windy Peak Place drainageway,

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approximately 320 linear feet of wall will be required. Since a portion of this
wall (approximately 160 linear feet) will be located along the northern bank of
the SBW Northern Tributary and a portion will be located within the sheet flow
area created by overflows from the drainageway, the design requirements will vary
along the wall.

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Flood-Control Engineering

Page 13

The average height of the tributary segment will be approximately 2.5 feet
which includes one foot of freeboard.
Based on the results of the scour
analysis, a four-foot toe-down depth is applicable to this segment of the wall.
Consequently, the total height of the tributary segment will be approximately
6.5 feet.
The unit cost for this height of floodwall was estimated to be
approximately $91.21 per linear foot. Therefore, the total cost of 160 linear
feet will be approximately $14,594.00.
The average height of the segment located in the sheet flow area will be
1.5 feet. This assumes the average depth of flow will not exceed 0.5 feet and
that one foot of freeboard is applicable. Again, a minimum toe-down depth of
three feet should be applied to protect against local scour. Consequently, for
cost estimation purposes, the total height of the drainageway segment will be
approximately 4.5 feet.
The unit cost for this segment of floodwall was
estimated to be approximately $62.84 per linear foot. Therefore, the total cost
of 160 linear feet will be approximately $10,055.00.
The total estimated damages associated with this home during the 100-year
event was determined to be approximately $16,200.00. Since the total estimated
cost of 320 linear feet of floodwall is approximately $24,649.00, the proposed
improvements will cost approximately $8,500 more than the benefits derived.
3.5

Problem Area E

Figure 15 shows the proposed alignment of the floodwall for this problem
area. Although the floodprone limit shown on Figure 9 extends to the north of
most of the homes in this problem area, the proposed location of the wall will
only constitute a very minor encroachment since most of this floodprone area is
located in an ineffective flow-conveyance area. Consequently, this area was not
included in the existing-conditions floodplain analysis (SBW-US2.DAT) as an
effective conveyance area.
The revised floodplain analysis that models the
existence of the wall is the same one used for Problem Area D (i.e., SBWDUS2.DAT
as contained in Appendix E).
The results of the impact analysis indicate that the presence of the wall
will not have a significant effect on adjacent and/or upstream water surface
elevations. The maximum increase noted was 0.09 feet at Section 41.
Figure 18 shows the profile of the design water surface with respect to
existing grade along the proposed alignment of the wall. Using this profile as
a guide, the average height of the wall, including one-foot of freeboard, will
be approximately 2.0 feet. Based on the results of the scour analysis, the
required toe-down depth wi II be approximately 5. 0 feet. Consequently, the design
height of the wall will be approximately 7.0 feet.
Based on
of wall will be
At a unit cost
the estimated

the alignment shown of Figure 15, approximately 425 linear feet
required to protect the affected homes within this problem area.
of approximately $98.39 per linear foot for a seven-foot wall,
..;ost of the wall is approximately $41,816.00.
The total

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Page 14

anticipated damages in this area during the 100-year event were estimated to be
approximately $40,800.00. Therefore, given the proximity of the two estimates,
it appears that the proposed floodwall may be a cost effective solution for this
problem area.
3.6

Problem Area F

Within this problem area. it was not possible during the Phase I study to
quantify, using the HEC-2 model, the anticipated depth of flooding relative to
homes #24 through #26. Consequently, the depth was estimated using uniformflow parameters and the effective-conveyance boundary of flow released from the
confined channe 1 sect ion created between home #23 and the adjacent hills ide.
This depth was determined to be approximately one foot during the 100-year event.
The HEC-2 model, 401-409.DAT, was used in the Phase I study to estimate
the depth of inundation associated with home #23. Based on the results of that
analysis, the results of the corresponding damage assessment analysis and the
results of a qualitative assessment of the negative impacts associated with this
home, it was recommended that home #23 be purchased for removal. If the home
and its improvements were removed, it was further recommended that the lot be
physically returned to its natural or pre-developed condition. It was felt that
reclamation of this lot would reduce the severity of flooding and mitigate some
of the drainage problems plaguing the downstream properties under existing
conditions.
In an effort to quantify the impact that removal would have on the flow
characteristics of the Glenhurst Wash in this area, the 401-409.DAT model was
revised to approximate pre-developed conditions (i.e., the conditions that
existed before home #23 and its retaining wall were constructed). The input
listing and a portion of the output listing for the revised model (401D409.DAT)
is provided in Appendix F. Table 3.6A summaries the changes noted between the
existing-conditions model and the approximated, pre-developed conditions model.
The results of the pre-developed conditions analysis indicate that during
both the 100-year event and the more frequent flow events, the anticipated depth
of flooding on the downstream properties would not be significantly altered.
Although the width of flow is increased in excess of 50 feet at Section 401, a
maximum difference of only 0.12 feet was noted with respect to the calculated
water surface elevation. However, there is a significant change in the velocity
of flow in the main channel during the 100-year event (i.e., a reduction of 1.91
feet per second). A significant reduction is also recognized during both the
5-year and 25-year events.
The most significant reduction in the velocity of flow occurs at Section
402. At this section, flow velocities were reduced between three and five feet
per second over the range of events analyzed. This section represents the last
confined section created by the improvements on Lot 35C. Homes #24 and #25 are
located in the direct path of flow exiting this section.
Under existing
conditions, there is not enough distance between this section and the downstream
homes to allow flow to return to a state that more closely approximates predeveloped conditions. Consequently, these downstream homes are significantly
impacted by the effects of high-velocity flows that exit the confines of this
section.

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Dra<nage • Flood-Control Engineering

Page 16

The most drastic change in the water surface elevation occurs at Section
404 which, like Sections 402 and 403, models the full impact encroachment (by
home #23) imposes on the Glenhurst Wash flood plain. During the 100-year event,
in the vicinity of Sect ion 404, reclamation wi II reduce the water surface
elevation by approximately 2.55 feet and the flow velocity by approximately 2.90
feet per second. The depth was only reduced by !.55 feet because the revised
model assumes some of the low-flow area will be filled during reclamation.
Based on the results of the HEC-2 analysis, reclamation does not appear
to have any significant effect on upstream properties. However, as noted in the
Phase I report, removal of the retaining wall on Lot 35C may result in the
formation of a headcut if an incised low-flow channel forms after reclamation.
The propagation of this headcut in the upstream direction could damage the access
drive for Lot 35B
In general, the quantitative assessment of the impact of removal is limited
to the immediate area surrounding Lot 35C. Although the last downstream section
used in the revised model reflects a small change in the water surface elevation
tn relation to the upstream sections, the change is still greater than that
normally allowed when encroachment studies are performed to gain approval for
the construction of improvements in a floodprone area. However, the change is
not significant enough to warrant a revision to the damage-assessment analysis
relative to homes #24 through #26. In addition, it is impossible to determine
what impact removal of home #23 wi II have on the quantity of flow that is
currently diverted down Glenhurst Drive. Therefore, it is difficult to conclude
from strictly a quantitative standpoint that removal of the home will
significantly reduce the flooding problems attributed to these downstream homes.
AI though there is no hard evidence to justify reducing the estimated
damages for homes #24 through #26 (even if home #23 were removed), the results
of the quantitative analysis, with respect to the velocity of flow, does warrant
an addendum to the qualitative discussion presented in the Phase I report
regarding the impact that concentrated flow has on these downstream homes. The
adverse effect improvements on Lot 35C have had on the velocity of flow in this
area is very apparent. Within the confines of the constricted channel (i.e.,
Sections 402 through 404, under existing conditions), high flow velocities,
similar to those defined by the existing-conditions analysis, erode material
from the confined section. This material is then transported out of the section
and deposited immediately downstream. Since flow leaving Section 402 can not
expand fast enough to reduce its kinetic energy and downstream improvements force
it to reconcentrate, the erosion potential relative to homes #24 and #25 is
greater than the potential that existed prior to the development of Lot 35C.
Again, this situation could be mitigated by removing the home and its
improvements.
AI though adjacent downstream property owners have created problems for
themselves and their neighbors by performing improvements that divert and
concentrate runoff, the associated impacts are not nearly as significant as those
created by the improvements on Lot 35C. This is especially true for homes #24
and #25. To some extent, some of the problems related to home #25 and most of
those related to home #26 may be the result of localized improvements, as opposed
to the improvements associated with Lot 35C. However, it seems obvious that the
severity of some of the problems associated with home #24 and those associated
with the upstream portion of home #25 could be reduced by removal of home #23
and its associated improvements. Again, removal would prevent the concentration

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Page 17

of flow which will significantly reduce the velocity of flow and in turn the
erosion and sedimentation potential.
Since the damage potential associated with home #24 through #26 remains
unchanged from that reported in the Phase I study, preliminary floodwall designs
and cost estimates were prepared for each home. Figure 17 shows the location
of each floodwall with respect to the home it will serve. Using an approximate
depth of flow of one foot, the height of the proposed wall above existing grade
would be two feet when freeboard is considered in the design. Assuming the
minimum toe-down depth is applied, the overall height of the wall would be five
feet. The linear footage required for home #24, #25 and #26 was estimated to
be approximately 220 feet, 300 feet, and 300 feet, respectively. At an estimated
unit cost of $69.67 per linear foot, the estimated cost of the three walls would
be $15,327.00, $20,901.00, and $20,901.00, respectively.
The individual damage estimates prepared as part of the Phase I assessment
for these three homes were $14,119.09, $16,375.76, and $11,378.35 respectively.
Consequently, the benefits derived by the proposed improvements do not appear
to justify the anticipated costs. However, if the owners of these homes are
willing to place the upstream limit of the wall closer to the house itself, the
overall length as depicted on Figure 17 could be reduced. To be cost-effective,
the length of each wall must be limited to 200 feet, 235 feet, and 160 feet,
respectively. However, a site-specific investigation would be required to ensure
that these reduced lengths provide adequate protection.
At the present time, there does not appear to be any justifiable
improvements that can be used to protect home #23.
The only reasonable
alternative for the owner of this home is to purchase flood insurance to offset
the estimated losses that are, for the most part, definable.

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RECOMMENDATIONS RffiARDING IMPIDIENTATION

Table 4.1 summaries the damages and estimated costs of the proposed
mitigation measures associated with each problem area .

...

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.
..

Based on the results of the Phase II study, the floodwall is still the
preferred (but not necessarily recommended) mitigation measure for all problem
areas with the exception of home #23 within Problem Area F. Although channel
reshaping within Problem Area A is cost effective, it can not be considered a
preferred measure unless a soils analysis confirms that the existing banks are
erosion resistent. Within Problem Area B, the preferred measure would be a
continuous floodwall since it is unlikely that the existing patio walls are
properly designed to withstand flood waters; however, a floodwall is not
recommended to protect the South Branch homes (#3, #4, and #7 through #10) since
it will have an adverse impact on adjacent properties. Within Problem Area C,
the levee/floodwall alternative is less expensive than the floodwall alternative;
however, the levee will occupy more area, require a more intensive construction
effort, and may be less desirable from an aesthetic standpoint. Consequently,
it is not the preferred mitigation measure for this problem area .
~ Drainage ~ Flood-Control Engineering

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Page 19

With the possible exception of Problem Area E, the preferred mitigation
measure is not cost effective in the sense that the estimated cost of improvement
It may be possible to reduce the estimated
exceeds the benefit derived.
improvement costs presented in this study by establishing alternative design
methods (i.e., masonry walls versus concrete walls, levees as opposed to
floodwalls, reducing the length of wall required by utilizing existing walls,
reducing the size of the protected area, etc.). However, the ability to analyze
the costs associated with these alternatives is beyond the scope of this project
since site-specific information must be obtained and the services of a structural
engineer and perhaps a soils engineer may be required. In addition to selecting
alternative design methods, the criteria used in this study to establish the
design parameters for the floodwall (i.e, toe-down and freeboard requirements)
could be adjusted downwards; however, site-specific information must be compiled
to justify these reductions.
Nevertheless, it should be noted that the estimated costs presented in this
study do not consider any additional engineering that may be required to
accommodate the proposed improvements; nor do they consider the need for
floodwall closures or contingency costs. If engineering costs and contingency
costs, which are usually estimated to be approximately 10 or 15 percent of the
total costs that can be assessed, are included in the cost of the alternative
design methods, the conclusions of this study (i.e., improvements for the most
part are not cost effective) would probably be the same.
In addition, the conceptual masonry wall design shown in Figure 5 includes
a concrete footing overall and a reinforced-concrete stem wall at all closure
openings. However, due to the potential for local scour, the entire be lowgrade portion of the floodwall should be constructed using reinforced concrete.
Consequently, only the above-grade portion of the proposed concrete floodwall
could be replaced with grout-filled masonry block.
Since the below-grade
component of the required wall height exceeds the above-grade component,
replacing the above-grade portion with grout-filled masonry block may not change
the estimated cost enough to warrant an expanded study of this alternative.
However, if an area-wide study were performed that included both a soils analysis
and a structural-design analysis for a masonry wall with varying heights, it
would be a simple matter to compare the associated unit costs of the masonry wall
to those developed as part of this study to determine which wall type is more
cost effective.
It may be possible to obtain a cost effective design for three of the four
homes located within Problem Area F if the individual length of floodwall
required to protect these three homes (#24 through #26) can be reduced. For the
purpose of this study, the length of wall required was estimated in a
conservative manner using the location of existing improvements as a guide. If
a site-specific design were prepared for each home that effectively relocates
the upstream limit of the proposed floodwall closer to the house, it may be
possible to reduce the overall length of each wall enough to obtain a costeffective design. Although removal of home #23 would, in a qualitative sense,
mitigate some of the hazards associated with these three downstream properties,
removal will not eliminate the hazards, nor will it reduce the design height of
the associated floodwalls. Consequently, it was concluded that a floodwall was
still needed to protect these three homes. However, no acceptable or feasible
mitigation measure seems to exist to address the problems associated with home
#23 .

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Page 20

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Assuming the alternative design methods just discussed would not be cost
effective either, flood insurance is the best means available to offset the
damage potential associated with all affected homes. Although the minimum amount
of insurance purchased should match the anticipated damages, it may be more
prudent for the homeowner to purchase insurance using the market value of the
home as a guide. Since the affected homes do not reside in a federally-mapped
or regulated f loodprone area, the minimum rates per $100 of coverage would
probably apply. Assuming either the pre-FIRM or post-FIRM construction rates
associated with Zone A99, B, C, or X are applicable, the annual premium rates
per $100 of coverage would be approximately $0.32 for the home and $0.52 for the
contents.
Table 4B summaries the approximate cost, for each affected home, of the
annual premiums for flood insurance. Two premium rates are presented. One rate
assumes only enough insurance is purchased to cover the anticipated damages.
The second rate assumes the estimated market value of the home is used to
establish the annual premium. Although both rates include the $45.00 per year
Expense Constant, the $25.00 per year Federal Policy Fee, and a five percent CRS
(Community Rating System) discount, they are only estimates. Actual quotes must
be obtained from a licensed insurance agent or broker.
In conclusion, there are four basic recommendations regarding further
studies and/or the implementation of the proposed mitigation measures or their
alternatives. These recommendations are summarized as follows:
( 1)

The purchase of flood insurance is recommended for all affected homes
within all problem areas, with the possible exception of the single home
in Problem Area A (see (l) below).

(2)

Within Problem Area A, the results of a soils analysis may support the
cost effectiveness of channel reshaping.
Consequently, it is
recommended that a geotechnical study of the Shadow Mountain Drainageway
be performed within this problem area before the feasibility of this
alternative is ruled out.

( 3)

Since the results of this study seem to support the possible cost
effectiveness of a floodwall within Problem Area E, it is recommended
that this measure be investigated in greater detai 1 including the
preparation of preliminary construction drawings. To facilitate this
design, a more-detailed survey of the project site may be required.
In addition, since a masonry wall may be less expensive and more
attractive than a concrete wall, it may be beneficial to explore the
possibility of using a masonry floodwall as opposed to a concrete one.
However, it should be noted that implementation of the f loodwall concept
does not preclude the need for flood insurance as an additional
safeguard, although it could significantly reduce the annual premium.

( 4)

It is further recommended that site-specific, concept designs be
prepared and evaluated with respect to homes #24 through #26 within
Problem Area F to determine if the required length of the floodwall
can be reduced enough to make this measure cost effective. Again, the
implementation of the floodwall concept does not replace the
recommendation that flood insurance be obtained by the respective
property owners.

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REFERENCES

1.

U.S. Army Corps of Engineers. "HEC-2: Water Surface Profiles,
Manual," September, 1982, updates, September, 1988.

2

Concrete Reinforcing Steel Institute, CRSI Handbook, 1978.

3.

Federal Emergency Management Agency, "Design Manual for Retrofitting Floodprone Residential Structures," September, 1984.

4.

Chow, V.T., Open Channel Hydraulics, 1959.

50

City of Tucson, Department of Transportation, Engineering Division,
"Standards Manual for Drainage Design and Floodplain Management in Tucson,
Arizona," December 1989.

6.

Pima County Department of Transportation and Flood Control District,
"Drainage and Channel Design Standards for Local Drainage for Flood Plain
Management Within Pima County, Arizona," Adopted on May 15, 1984, Effective
on June 1, 1984.

7

Pima County, Arizona, "Floodplain and Erosion Hazard Management Ordinance
No. 1988-FC2 for Pima County, Arizona," Passed and Adopted by the Board
of Supervisors sitting as the Board of Directors of the Pima County Flood
Control District, December 6, 1988.

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FLOOD-PRONE HOME
PROPOSED FLOODWALL ALIGNMENT

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PROBLEM AREAS D AND F
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