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

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TABLE 111-3
LAND USE VS. PERCENT IMPERVIOUS
BIG DRY CREEK (ADCO) NORTH AREA TRIBUTARIES

Land Use %-Impervious
Low Density Residential 35
Medium Density Residential 50
High Density Residential 65
Community Commercial 75
Regional Commercial 75
Business Park 65
Industrial 75
Activity Center 75
Parks, Greenbelts 5

Characteristic Subbasin Slope
The weighted slope method given in the USDCM was used to develop the
subbasin slope. The subbasin length was divided into two slope reaches,
if necessary, and the corresponding lengths and elevations were read
from the USGS quadrangle maps. Prior to the slope weighting calcula-
tion, the intermediate slopes were adjusted according to Figure 4-1 in
the USDCM (Volume I "Runoff") if the subbasin slope exceeded 4 percent.

Time of Concentration
For subbasins less Ihan 90 acres, the modified CUHP procedure was used
to generate hydrographs. The time of concentration for the subbasins
was calculated using the procedure described in the USDCM.

111-6

Infiltration and Abstraction Losses
Weighted infiltration parameters (i.e.,
rate, decay rate) were calculated based
type in the subbasin.

initial and final infiltration
on the percent of each soil

The infiltration rates for the various soil types are given below:

Initial infiltration rate (in/hr)
Final infiltration rate (in/hr)
Decay rate

Type B
4.5
0.6
0.0018

Types C & D
3.0
0.5
0.0018

An impervious area storage loss of O. I inches and a pervious area stor-
age loss of 0.35 inches was used for all subbasins.

E.

IMPERVIOUS LAND DENSITY
The percent impervious values adopted for use in this study reflect the
fact that "pockets" of undeveloped and low~irnpervious parcels will most
likely exist al ultimate development even in planned high-density
areas. The percent impervious for each land use with the drainage area
is presented in Table 111-3 and is based on the existing zoning densi-
ties.

Existing Development Conditions
The limits of existing development were determined from recent aerial
photographs. Figure 111-2 shows the limits of the existing development
with the corresponding percent impervious. At present, the area is
almost completely devoted to agricultural use with a few low density
residential developments.

The percenl impervious for the various subbasins were calculated based
on the existing development density and are presented in Table 1II-2.

111-7