FLOOD10200

• County: Boulder
• Community: Boulder County
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Simplified Mountain Canyon Flash Flood Guidance for Boulder Creek
• Date: 3/1/1990
• Prepared For: UDFCD
• Prepared By: Sabol Consulting
• Floodplain - Doc Type: Educational/Technical/Reference Information

Rainfall Losses

Hydrologic DI!slgn Manual
for Maricopa Gounly

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

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

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1. Rainfall losses are independent of th,! 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,

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2. The estimated rainfall loss rate is a func:tion 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.

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

4. The initial abstraction is equal to 0.25

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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 va:,ues rather than on scientifically substantiated findings.

6. At low rainfalls (less than 4 inches), the estimate of rainfall loss I., very sensitive
to the selection of eN.

As mentioned previously, the two recorrunended 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, a,s programmed
into HEC-l, can be used to simulate the rainfall loss model as depicted in Figure
4.2. (For a full discussion of these methods, see Sections 4.4.1 and 4.4.2.) The IL+ ULR
is a simplified model tha t has been used extensively for flood hydrology ilnd data
art,is-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 a,n option in HEC-l.

Procedures have been dE!velop~ toestirnate tile thr~~ters.Qf the G~n and
A;.npt_~iltrati.!!n.J!9.uati~.n.,./The preferred method, and the most theoretica.lly)
ccurate, is the Green ~i Ame.t ~iltration equa!~jTIie1C+ULR1i;ieC'oii:imended
as an a tema ve It AS not poss1l51e-toeSUmate the Green and Ampt equation

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