FLOOD02665

• County: Larimer
• County: Weld
• Community: Larimer and Weld Counties
• Stream Name: Cache La Poudre River Basin
• Basin: South Platte
• Title: Special Study Cache La Poudre River Basin Volume II - Hydrology
• Date: 10/1/1981
• Prepared By: US Army Corps of Engineers
• Floodplain - Doc Type: Floodplain Report/Masterplan

States, Atlas 2, Volume III, Colorado, published by the NOAA in 1973.

The 500-year value was estimated by extrapolsting the selected I-hour

values on semilogarithmic paper. Adjustments for depth-area were based

on the depth-area relationship presented in the NOAA publication.

Adjustments for an average length of record of 40 years were made on

logarithmic-probability paper using Beard's table of expected

probability adjustments. The I-hour rainfall distribution developed

from NOAA criteria is given in table 18.

.

Table 18
I-Hour Rainfall Distribution

End of Period
(minutf's)

Incremental Rainfall
(percent)

5
10
15
20
25
30
35
40
45
50
55
60

3
4
4
5
12
16
29
12
5
4
4
2

LOSSES

Infiltration rates for the Sheep Draw basin were obtained from

Soil Resources of Colorado, Region 2 - Larimer and Weld Counties,

published in 1976 by the CSU Expel'iment Station, Fort Collins, and the

SCS. A general soil map of Weld County used in conjunction with table

3 of the CSU publication lists ranges of infiltration permeabilities

for each soil series. Table 19 contains the soil associations that

were taken from the general soil map, the range of infiltration

permeabilities at various depths for each soil series, and the

infiltration rate that was used in the SWMM model.

.

42