FLOOD04286

• Designation Number: 23
• County: Adams
• Community: Westminster
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Major Drainageway Planning - Master Plan - Big Dry Creek - Westminster - Volume I
• Date: 3/1/1973
• Designation Date: 7/1/1973
• Floodplain - Doc Type: Floodplain Report/Masterplan

IV-2

Corps of Engineers' studies for flood plain purposes define the
Intermediate regional flood, which generally Is a 100-year flood. In
addition, the 25-year flood is often inadequate for a prosperous and
growing community to use for planning on major drainage systems. The
main use of the 5 and 25-year floods In this study Is to point out
critical design locations In the study area, to provide sufficient In-
formation for optimization of costs comparIson,;, and to provide for de-
sign criteria at specific locatIons, or for specific flood plain re-
ductions.

Rainfall

Three series of point rainfall curves were dev.eloped for the basin
one at the upper end, one in the center, and one at the lower end. The
point rainfall curves for the center of basin were used In all hydrolo-
gical computations since it is representative of the entire basin.
Figure No.1 is the Depth-Duratlon-Frequency curve for the study area
for the 5, 25, and 100-year storms. Table No. IV-I represents the dis-
tributed, re-arranged rainfall Increments that were used for the unl t
hydrograph. An average area reduction factor .:0 85 per cent was used In
analyzing basins and combinations of basins between fifteen and forty
five square miles. An average reduction factor.. to 75 per cent was used
for basins from 45 to 85 square miles. Reduction factors of point raln-
fa 11 s were based on Weather Bureau T'~chn I ca 1 P'lper I~o. 4 and used for a II
hydrograph work.

TABLE IV-l

100-YEAR DESIGN ~AINFAU,
BIG DRY CREEK_BASIN
ADAMS AND JEFFERSO~.5~I:1 E5,

Point Rainfall

Time from Beginning
of,Storm

Total
Rainfall

o
15
30
45
60
75
90
105
120
135

o
1.39
1.82
2.07
2.25
2.38
2.50
2.61
2.70
2.77
(Cant I nued Next:

I ncrenental
~J..!:U~~_
o
1.39
.11.3
.:'5
.18
. 13
. 12
.11
,09
,07
Pagel

Point Design
RaInfall

o
.18
,43
1.39
,25
.13
.12
.11
.09
.08