Drainage Overview - Maricopa County Department of Transportation

channel slopes at the base of the White Tank Mountains can result in high flow velocities
with the energy to move significant volumes of sediment and debris during floods.
The SVADMP analyzed several alternatives for mitigating the flood hazards of the alluvial
fans including structural and non-structural strategies. The result of the alternatives
evaluation process was to recommend large in-line detention basins at the apex of each
alluvial fan together with downstream corridors protected by levees. The detention basins
were designed to outlet 10 percent of the 100-year inflow volume from each alluvial fan.
The corridors were designed to act as regional flood control trunk systems and were sized to
include local drainage as well as sediment from adjacent watershed areas. Numerous drop
structures would be constructed as part of the wash corridors. A conceptual cross-section of
the leveed corridor and the location of the recommended flood control improvements are
included in Appendix TM3-07. The Deer Valley Parkway alignment will almost certainly
cross the proposed leveed drainage corridor associated with alluvial fan #2. Similar to
typical wash crossings, the final design of this crossing will need to accommodate the
hydraulic design and sediment load anticipated within the corridor, including the effect of
contraction and expansion of flows at the structure.
3.4.4 Scour and Sedimentation
Transport and distribution of sediment within the study area are significantly impacted by
the CAP canal according to the Wittmann Area Drainage Master Study Update (ADMSU)
Geomorphic and Sedimentation Analysis Report (FCDMC, 2005). The CAP canal is
located upstream of the eastern portion of the study area and traverses the western portion
of the study area. Concrete drainage over-chutes convey storm runoff and sediment across
the canal upstream of and within the study area. Typically, sediment is deposited upstream
of the over-chutes and scour occurs downstream of the over-chutes. According to the
Wittmann ADMSU Sedimentation Report, all of the areas upstream of the CAP canal
within the 100-year floodplain should be considered to be depositional hazard areas.
The report also identified several sedimentation problems that are common with roadways,
including the following:
� Undersized bridges or culverts can cause significant deposition to occur
upstream of the roadway and erosion to occur downstream of the roadway;
� Gathering numerous washes into a single wash to reduce bridge construction
costs can disrupt existing drainage patterns and have significant impacts
downstream; and
� Trapping of debris can have a major impact on the ability of culverts and bridges
to adequately convey runoff and sediment.
Alluvial fans are built up from sediment that drops out of flood waters when flows start to
spread out at the base of mountains. Therefore, excessive sedimentation can be expected in
active alluvial fan areas.
3.4.5 Lateral Erosion
Bank erosion from flood events is another critical concern for potential Deer Valley
Parkway infrastructure. Most of the soils in the study area are composed of sands and noncohesive
materials that water can easily erode. Avulsions can readily occur where some
091337137, 2011-018, TT005 Maricopa County Department of Transportation
Technical Memorandum 3 Deer Valley Parkway Feasibility Study
Drainage Overview 15 April 2012