FLOOD10420

• County: Adams
• County: Broomfield
• County: Weld
• Community: Broomfield, Westminster and Thornton
• Stream Name: Lower Big Dry Creek
• Title: Lower Big Dry Creek Hydrology Study
• Date: 6/1/2005
• Prepared For: Big Dry Creek Watershed Association
• Prepared By: Wright Water Engineers, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Lower Big Dry Creek Hydrologic Study

Table 5.
Allowable Release Rates (ds/acre) of Detention Ponds
Values Used for Cities of Broomfield, Westminster, Thornton and Adams County

Allowable Release Rate (cfs/acre)
NRCS Soil Groupl
Design Storm Control Frequency A B C&D
2-year 0.02 0.04 0.06
5-year (BroomfieldIW estminster/ Adams Cnty) 0.07 0.13 0.17
5-year (Thornton) 0.10 0.20 0.20
10-year (not required for detention) 0.13 0.23 0.30
100- year 0.5 0.85 1.00

Table Notes:

INatural Resource Conservation Service (NRCS) soil groups are classified according to their infiltration and
transmission rates. The hydrologic soil groups are:
. Type A. Soils having high infiltration rates even when thoroughly wetted. These consist chiefly of deep, well to
excessively drained sands or gravel. These soils have a high rate of water transmission.
. Type B. Soils having moderate infiltration rates when thoroughly wetted. These consist chiefly of moderately
fine to moderately coarse textures. These soils have a moderate rate of water transmission.
. Type C & D. Soils having slow infiltration rates when thoroughly wetted. These consist chiefly of soils with a
layer that impedes downward movement of water, or soils with moderately fme to fine texture. These soils have
a slow rate of water transmission.

In addition to these requirements, several of the communities allow for more stringent
requirements, if conditions warrant. A few examples include:

. City of Westminster: Special Planning Area 1anguage--"Presently, there may be areas
where drainage problems currently exist. Any development or redevelopment in these
areas may compound the existing drainage problems. The City may define Special
Planning Areas within the City due to specific drainage concerns such as undersized
drainage facilities, areas of uncontrolled flows, or areas with inadequate upstream
detention." "The policy of the city shall be to require additional drainage analysis and to
identify additional facilities required for development and redevelopment in areas where
drainage problems may currently exist."

971-179.092
June 2005

Wright Water Engineers, Inc.

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