FLOOD04416

• Designation Number: 304
• County: Adams
• Community: Unincorporated Adams County
• Stream Name: Big Dry Creek, (ADCO), North area tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Big Dry Creek, ADCO, North Area Tributaries
• Date: 7/1/1989
• Designation Date: 1/1/1990
• Floodplain - Doc Type: Floodplain Report/Masterplan

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

The study area was superimposed onto the USDCM rainfall isohyet maps
(Figures 2-1 to 2-10 USDCM) and the point rainfall for the center of
the study area was recorded. The point rainfall depths were recorded
for the IO~, 50-, and 100-year recurrence intervals for a storm dura-
tion of I hour and 6 hours. The point rainfall values for the 2-hour
storm duration were obtained by interpolation using the procedure pre-
sented in the USDCM. The unadjusted point rainfall values are shown in
Table III -1. The point rainfall values were then distributed using a
5-minute time interval for each storm frequency based on the distribu~
tion given in Table 3~1 of the USDCM.

in no significant differences. Consequently, the unadjusted storm was
used for Preble Creek for consistency with the other drainage basins in
this study.

D. SUBBASIN CHARACTERISTICS
The procedure for developing the subbasin characteristics for input to
the CUHP model is described below. A summary of the subbasin para-
meters is presented in Table 111-2.

TABLE lll-l
DESIGN RAINFALL DEPTHS
NO AREA ADJUSTMENT

Delineation of Subbasins
The subbasins were delineated using the USGS quadrangle maps. The sub-
basin limits were defined based on eXlstmg features (Le. streets,
irrigation ditches, land use changes, and jurisdiction). The subbasin
boundaries were field verified when questions regarding the flow pat-
terns developed. A copy of the subbasin map (presented in Figure II-I)
was presented to the project sponsors for review prior to incorporating
the information into this study.

Storm 2-Year 5-Year 10~Year 50-Year 100- Year
Duration (inches) (inches) (inches) (inches) (inches ) Area
I hour 0.95 1.35 1.61 2.32 2.67 The subbasin areas were calculated utilizing a planimeter. The subbasin
2-hour 1.10 1.56 1.86 2.68 3.09 areas varied from 20 to 160 acres.
6-hour 1.47 2.00 2.38 3.25 3.69

Subbasin Length and Centroidal Length
The length to centroid and the characteristic subbasin lengths were
manually measured from the USGS maps.

For drainage areas greater than 5 square miles, the rainfall distribu-
tion was adjusted. In this study, only Preble Creek has a watershed
area of greater than 5 square miles. A test evaluation was made where-
m the design rainfall was adjusted for this watershed in accordance
with the USDCM. The rainfall adjustmenl was relatively minor and a
comparison of the peak flows in the downstream portion of the creek
using both the unadjusted and adjusted rainfall distributions resulted

Subbasin Percent Impervious
The percentage of each land use in each subbasin was estimated and rec-
orded. The percent impervious was then calculated using representative
impervious values for the various land uses as presented in Table
III-3. This subject IS discussed in more detail in Section III.E
Impervious Land Density.

111-3

111-4