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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Rainfall Losses

Hydrologic Design Manual
for Maricopa County

for all soil types and land uses in Maricopa County, and this method is not
recommended for general use in Maricopa County.

The SCS CN method is the most extensively used rainfall loss ~ethod in
Maricopa County and Arizona and it has wide acceptance among many agencies,
consulting engineering firms, and individuals throughout the community. This
method is limited, however, by both theoretical and practical deficiencies, and thus
is not recommended for general use in Maricopa County. Deficiencies of the SCS
CN method include:

1. Rainfall losses are independent of the duration of rainfall. That is, for a given
depth of rainfall, the same rainfall loss results (regardless of the duration of
rainfall) and the same rainfall excess would be estimated for a given rainfall
depth occurring in, for example, either 1 hour or 24 hours.

2. The estimated rainfall loss rate is a function of rainfall intensity. Short periods
of high intensity rainfall would often result in large estimates of rainfall losses.
This is contrary to the generally accepted infiltration relation as illustrated in
Figure 4.2.

3. The infiltration rate approaches zero rather than a minimum infiltration rate
(fc).

4. The initial abstraction is equal to 0.25

where S = 1000/CN -10

This equation is not theoretically justified nor is it based on data for hydrologic
conditions that are representative of Maricopa County.

5. The selection of CN is too subjective and is often based more on traditional
acceptance of CN values rather than on scientifically substantiated findings.

6. At low rainfalls (less than 4 inches), the estimate of rainfalllo?s is very sensitive
to the selection of CN,

As mentioned previously. the two recommended methods for estimating rainfall
losses in Maricopa County are the Green and Ampt infiltration equation and the
initial loss and uniform loss rate (IL+ULR) method. Both methods, as programmed
into HEC-1, can be used to simulate the rainfall loss model as depicted in Figure
4.2. (Fora full discussion of these methods, see Sections 4.4.1 and 4.4.2.) The IL+ULR
is a simplified model that has been used extensively for flood hydrology and data
areJs-available to estimate the two parameters for this method. The Green and Ampt
infiltration equation is a physically based model that has been in existence since
1911, and has recently been incorporated as an option in HEC-1.

Procedures ha, ,ve,been develop~ to.~d;mate tl1.e_fu.~e paraJlletD'" of th~e,n and
Ampt.i!!flltration_l!fLuat~n/The preferred method, and the most theor~l!~~~V
te, is the Green and Am t infiltration equatiol!/The lL+ULK is reCommended
as an a tema ve It IS not posslD1e to estImate the Green and Ampt equation

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