RDR 3 Report - Flood Control District of Maricopa County

V. RECOMMENDED PLAN
A. Introduction
The Recommended Plan is shown on Figure V-1. The plan elements
are shown in plan and profile on the Preliminary Design Plans at the
end of this report. This section is intended to be used with the
Preliminary Design Plans to further describe the planned
improvements, project costs, and special issues to be considered during
final planning and design. The project elements are described as well
as environmental considerations, right-of-way requirements, and utility
conflicts. In addition, the area benefitting from the project is described,
and the agencies with an interest in the project are identified as possible
participants in project implementation.
B. Floodplain Impacts
A number of floodplains were delineated as a part of this study. These
delineations identify areas of potential flooding due to stormwater
runoff from within the study area. The areas identified include the
UPRR from 69 th Avenue to 35 th Avenue, the BFC from the confluence
with the Agua Fria River to approximately 105 th Avenue, and the
Sunland Avenue Tributary to the BFC from the confluence with the
BFC to 91 st Avenue. After the floodplain delineations were completed
and the Preliminary Recommended Plan was developed, the residual
floodplains were delineated with the intent to show the floodplain
reduction benefit derived from the ADMP projects. The existing and
residual floodplains are shown on Figure V-2.
1. Southwest Area Re-Study
After evaluation of the residual floodplains, based on the Preliminary
Recommended Plan, the southwest portion of the study area was
determined to receive a less than desired benefit due to the quantity of
stormwater runoff generated by the local sub-basins. The southwest
area was then re-analyzed to determine if there was a better solution
which would have a greater impact on reducing the residual floodplains.
The Southwest Area Alternatives Analysis Report can be found in
Appendix C. The results of the preferred alternative have been
incorporated into this final Recommended Plan.
2. Northeast Area Re-Study
The northeast area was also found to have minimal reduction in the
floodplain from the Preliminary Recommended Plan improvements.
Additional analysis was performed to determine the feasibility and cost
of a flood control solution in the northeast portion of the study area to
remove existing homes and businesses from the floodplain. A solution
for this area had not been identified in the previous alternatives analysis
due to the high cost of constructing open channels in the high density,
highly developed and populated area. Typically, industrial businesses
are too close to the railroad and to each other to allow an open channel
to be constructed in a continuous path.
Upon further investigation, it was determined that a large concrete box
culvert storm drain system could be constructed in local east-west
roadways, approximately 1/4 mile north of the railroad with laterals in
the north-south streets to capture runoff from the sub-basins. Exhibits
of the analyzed system are shown in Appendix D. In order to analyze
the cost versus benefit of the box culvert storm drain system, three
alternatives were formulated to convey runoff from the 10-year, 50-year
and 100-year storms. The three alternatives all used the same
alignment, but were sized according to the storm they were intended to
convey. The residual floodplain along the railroad due to the 100-year
storm was then analyzed for each of the alternatives.
The costs of the three different alternatives ranged from $68.6 million
dollars to $99.4 million dollars. The residual floodplain benefit of the
alternatives is described based on the area of impact:
69 th Avenue to 57 th Avenue
- No residual floodplain with 10-yr, 50-yr, or 100-yr system
57 th Avenue to 51 st Avenue
- Very minor benefit with any system in place
51 st Avenue to 43 rd Avenue
- Minor benefit with any system in place
43 rd Avenue to 31 st Avenue
- No residual floodplain with 100-yr system
- Very minor benefit with 10-yr or 50-yr system
The greatest benefit occurred between 69 th and 57 th Avenues because the
greatest number of residences were removed from the residual
floodplain. Most of the other areas received very little benefit with any
system in place. It was found that the spurs and obstructions along the
railroad cause such a large ponding effect, that conveyance of the runoff
does not occur with even the smallest amount of flow. Therefore, even
the local runoff within the sub-basin was causing nearly as much
flooding due to ponding regardless of whether the box culvert storm
drain system was in place 1/4 mile north. The only way to completely
alleviate flooding along the railroad is to provide a system of
conveyance immediately adjacent to the railroad, allowing runoff to
flow under the spurs and other obstructions. This is infeasible due to the
number of existing buildings so close to the tracks and the number of
high-cost utilities that would have to be relocated within the railroad
right-of-way.
In lieu of providing a complete box culvert storm drain system, a
portion of the analyzed alternatives was incorporated into the
recommended plan to provide a significant benefit to a majority of the
residences which are located between 69 th and 57 th Avenues. A section
of vertical concrete open channel at grade was identified along the
railroad from the east edge of the previously identified retention basin
(DRC Basin #4) at 69 th Avenue, east to approximately the 63 rd Avenue
alignment. This section of channel connects to the bypass channel of
DIBBLE & ASSOCIATES 16 DURANGO AREA DRAINAGE MASTER PLAN
August 2002
RECOMMENDED DESIGN REPORT