FLOOD08228

• County: Boulder
• Community: Boulder
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Urban Drainage and Flood Control District
• Date: 3/1/1990
• Prepared For: Boulder Creek
• Prepared By: Goerge V. Sabol
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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Hydrologic Design Manual
for Maricopa County

Rainfall Losses

Hydrologic soil group D should be retained only for:

>> clay soils,

>> soils with a permanent high water table, and

>> rock outcrop.

Hydrologic soil group D should probably not be retained in all situations where the
classification is based on shallow soils over nearly impervious layers; site specific
studies and sensitivity analyses should be performed to estimate the loss rates to be
used for such soils.

ifmil!i~~il~
Rli Recommended Methods for Estimating
f'j Rainfall Losses
...!11t:._.,;:;.;.~ .

Many methods have been developed for estimating rainfall losses; five are listed as
options in the HEC-1 Flood Hydrology Package. They are:

1. Holtan Infiltration Equation

2. Exponential Loss Rate

3. SCS Curve Numbers (CN) Loss Rate

4. Green and Ampt Infiltration Equation
5. Initial Loss Plus Uniform Loss Rate OL+ULR)

Of these five, however, only two-Green and Ampt and IL+ULR-are recom-
mended for estimating rainfall losses in Maricopa County for the reasons discussed
below.

The Holtan Infiltration Equation is an exponential decay type of equation for
which the rainfall loss rate asymptotically diminishes to the minimwn infiltration
rate (fc). The Holtan equation is not extensively used and there is no known
application of this method in Arizona. Data and procedures to estimate the
parameters for use in Maricopa County are not available. Therefore, the Holtan
equation is not recommended for general use in Maricopa County.

The Exponential Loss Rate Method is a four parameter method that is not exten-
sively used, but it is a method preferred by of the U.s. Army Corps of Engineers.
Data and procedures are not available to estimate the parameters for this method
for all physiographic regions in Maricopa County, but Exponential loss rate
parameters have be~n developed from the reconstitution of flood events for a flood
hydrology study.>>:rli" portion of Maricopa County (U.S. Army Corps of Engineers,
1982). However, adequate data j,5'nota vailable to estimate the necessary parameters
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