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

Hydrologic Design Manual
for Maricopa County

Rainfall Losses

Vl,.~letatlon Type

hardwood tree
colton
alfalfa
meadow grass

Interception,
Inches
0.09
0.33
0,11
0.08

No interception estimates are known fornatural vegetation that occurs in Maricopa
County. For most applications in Maricopa County the magnitude of interception
losses is essentially 0.0, and for practical purposes interception is not considered for
flood hydrology in Maricopa County.

Depression storage and infiltration losses comprise the majority of the rainfall loss
as illustrated in Figure 4.1. The estimates of these two losses will be' discussed in
more detail in later sections of this manual. Three periods of rainfall losses are
illustrated in Figur,! 4.1, and these must be understood and their implications
appreciated before a pplying the procedures in this manual. First, there is a period
of initial loss when no rainfall excess (n.moff) is produced. During this initial period,
the losses are a function of the depression storage, interception, and evaporation
rates plus the initially high infiltration capacity of the soil. The accumulated rainfall
loss during this peliod with no runoff is called the initial abstraction. The end of this
initial period is notl!d by the onset of ponded water on the surface, and the time
from start of rainfall to this time is the time of ponding (Tp). It is important to note
that losses during t~,is first period are a summation of losses due to an mechanisms
including infiltration.

The second period~; marked by a declining infiltration rate and generally very little
losses due to other factors,

. -

The third, and final, period occurs for rainfalls of sufficient duration for the
infiltration rate to reach the steady..state, equilibrium rate of the soil (fe). The only 'f I~'
appreciable loss dwing the final peliod is due to infiltration. ]:rl 61ure." ,;1.) ;
..-- 4J.s~"n.!"Pf..,
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The actual loss process is quite complex and there is a good"deal of interdependence
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of the loss mechanisms on each other and on the rainfall"itself. Therefore, simplifying
assumptions are usually made in the modeling of ra~falllosses. Figure 4.2 represents
a simplified set of assumptions that can be made.;lfuat swiace retention loss is the
summation of allIO!;5eS other than those due to infiltration, and that this loss occurs
from the start of rainfall and ends when thfl accumulated rainfalll~ls Ihe magnitude
of thecapadty of the surface retention loss. It isassurned that infiltration does not occur
during this time. After the surface TE,tention is satisfied, infiltration begins. If the
infiltration capacity exceeds the rainfall intensity, then no rainfall excess is produced.
As the infiltration capacity decreases, itmay eventually equal the rainfall intensity. TIUs
would occur at the time of ponding (Tp) which signals the beginning of surface runoff.
As illustrated in boUl Figures 4.1 and 4.2, after the time of pending the infiltration rate
decreases exponentillly and may reach a sleady--slate, equilibrium rate (fe). It is these
'simplified assumptions and processes, as illustrated in Figure 4.2, that are to be
modeled by tile procedures in this rrumual.

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