FLOOD09746

• County: Statewide
• Basin: Statewide
• Title: Colorado Statewide Drainage Floodplain Management Criteria Manual
• Date: 8/1/2002
• Prepared By: CWCB & WRC Engineering Inc
• Floodplain - Doc Type: Educational/Technical/Reference Information

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COLORADO STATEWIDE
DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL

Step 5: Estimate the depth-area reduction factor from Figure CH5-F115.

Assuming the drainage area for Basin A is 2,140 acres, or 3.34
square miles, the area-reduction factor is approximately 0.995. (As
runoff flows through the drainage basin, the drainage area increases,
and the depth-area reduction factor will vary. To account for this, a
range of depth-area reduction factors may need to be estimated for
large basins that have several sub-basin design points. For instance,
if the drainage basin was 15 square miles, three depth-area reduction
values may be used to estimate runoff for a design point at 5 square
miles, one at 10 square miles, and one at 15 square miles. The
respective depth-area reduction values would be 0.992, 0.985, and
0.978).

The rainfall depths for durations of 5 minutes, 15 minutes, 1 hour, 2
hours, 3 hours, 6 hours, 12 hours, and 24 hours are entered on the
PH record of HEC-1 input data to define the 24-hour storm
distribution. A value of 0.001 is entered in Field 2 of the PH record.

The depth-area reduction factor( s) is entered in the J R record of the
H EC-1 input data.

1.7.2 EXAMPLE 2: TIME-INTENSiTY-FREQUENCY CURVE GENERATION

. Problem:

Develop the 1 OO-year time-intensity-frequency curve for Rocky Subdivision
located in Basin A.

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CHAPTER 5
HYDROLOGY
SECTION 1.0
RAINFALL

Solution:

Step 1: Calculate the 100-year, 1-hour rainfall depth for Rocky Subdivision as
explained in the Example 1.
D1oo.1 = 2.56 inches (Assumed for example purposes only)

Step 2: Generate the 100-year rainfall depths for storm durations of 5
minutes, 10 minutes, 15 minutes and 30 minutes.

D100,s = RATI05*D100,1 = 0.29*2.56 = 0.74 inches
D100,1D= RATI010*D100,1= 0.45*2.56= 1.15 inches
D1oo,ls= RATI015*D100,1= 0.57*2.56= 1.46 inches
D100,3D= RATI030*D1oo,1= 0.79*2.56= 2.02 inches

Step 3: Calculate 1 OO-year rainfall intensity values for storm durations of 5
minutes, 10 minutes, 15 minutes, 30 minutes, and 60 minutes.

1100,S = D100,slDuration = 0.74/(5/60) = 8.88 incheslhour
hoo,10= D1oo.101Duration = 1.15/(10/60)= 6.90 inches/hour
11oo,1s= D100,ls1Duration = 1.46/(15/60)= 5.84 inches/hour
1100,3Q'= D100,3oIDuration = 2.02/(30/60)= 4.04 inches/hour
1100,00= D100,aoIDuration = 2.56/(60/60)= 2.56 inches/hour

AUGUST 2002

CH5-110

RAINFALL