Drainage Overview - Maricopa County Department of Transportation

(PMF) events were also calculated for the future land use condition. SCS Type II rainfall
distribution was used for the 24-hour storm.
Subbasin boundaries were placed along the CAP canal alignment so that the effects of the
structure could be included in the model, but most roadways, including US 60, State Route
74, and Sun Valley Parkway, were ignored. Rainfall losses were estimated using the Green
and Ampt infiltration equation. The S-Graph method was found to be the most appropriate
method developing unit hydrographs.
Normal depth channel routing methodology was utilized in the hydrologic model to route
surface runoff through subbasins. An eight-point composite channel cross-section was
developed to represent typical wash cross-section conveyance. The longitudinal slopes and
Manning’s “n” values were estimated based on topographic mapping, aerial photographs,
and field reconnaissance estimates. Storage routing behind the CAP canal was modeled
using level pool routing. Most flow splits were not considered important for the purposes
of this model. One flow split upstream of the CAP and east of US 60 was included, which
directs some flow into the study area and some flow away from the study area.
4.2 Offsite Hydrology Results
Detailed hydrologic analysis was not performed as part of this study. The existing peak 100-year
flows for each major wash crossing within the study area depicted in Figure 6 are listed in Table
2. The wash information presented previously is also included to provide a comprehensive
summary of the offsite hydrology at each wash crossing. Table 2 indicates if the peak flow was
taken directly from an existing study or if the discharge was calculated by combining peak flows
of published values. Combined values are conservative as the peak flows generally do not occur
at the same time. The concentration point or subbasin identification and storm duration used in
each existing study are also presented. Excerpts from the original source documents of each
respective hydrologic study are included in Appendix TM3-10.
As stated previously, concentration points were located at USGS “blue lines” and 100-year
floodplains. There is no discharge reported for Wash 4 (Crossing 9). Because of the distributary
nature of the area, it is difficult to determine the area contributing to this wash.
Trilby Wash (Crossing 14) has a published effective FEMA discharge that differs from the value
reported in the Wittmann ADMSU. The peak flows presented for this crossing in Table 2 report
both the effective FEMA discharge from the Flood Insurance Study (FIS) as well as the discharge
from the Wittmann ADMSU. The effective FEMA discharges reported in the FIS for Trilby
Wash are higher than those published in the Wittmann ADMSU. It should be noted that the
drainage areas recorded in the FIS are also significantly larger than the drainage areas in the
Wittmann ADMSU.
091337137, 2011-018, TT005 Maricopa County Department of Transportation
Technical Memorandum 3 Deer Valley Parkway Feasibility Study
Drainage Overview 23 April 2012