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

I
I
I
I
I
I
I
m
I
I
I
I
m
I
I
I
I
I
I

Lower Big Dry cpreek Hydrologic Study

I
Wulliman and Urbonas (2005) suggest that ~etter design guidance for detention facilities could
help to achieve peak flows closer to pre-dev~lopment conditions for the full spectrum of nmoff
events with greater confidence. Even though Wulliman and Urbonas (2005) propose a procedure
to help better achieve this goal, they note that implementing better designs will not totally
eliminate stream erosion and degradation and that in-stream measures will still be needed to keep
streams from excessive degradation and loss 0f aquatic habitat or ecological function.
I
The details of the Wulliman and Urbonas (2005) full-spectrum procedure can be accessed from
the UDFCD website (www.udfcd.org) wheJe it is posted in review-draft form, but the basic
design concepts include:

.

The difference between urban runoff volume and predevelopment volume, called
excess urban nmoff volume, appJars to be relatively consistent for a wide range of
design stonn sizes at any given !level of imperviousness. When this excess urban
nmoff volume is captured and hel~ back for an extended period, the remaining runoff
I
from a site approximates the nmoffvolume for predevelopment conditions.
I
I
I
The first stage of a two-stage fun-spectrum detention basin would be designed to
capture the excess urban runoff +Iume. The release rate for this first stage is very
slow to reduce the energy of flow in receiving streams. A 70-hour drain time was
I
selected for this volume because it is an extrapolation of the 40-hour drain time used
I
for the WQCV for a perforated plate outlet.

The upper stage of a full-SPecJ detention basin would be sized to control the pre-
I
development 100-year peak flow rate from the tributary sub-watershed. When using
I
the design guidance for the Denter area, the total full-spectrum detention volume
. I
approxImates the volume required currently to control the 100-year plus the WQCV

with the recommended 20 perclnt of WQCV sediment storage. Full-spectrum

detention sizing appears to reqUirJless total volume than the current sizing methods
. i:'.' . h' . I d'
reqUIre ~or SItes WIt ImpervIOusness excee mg 50 percent.

.

.

971-179.092
June 2005

Wright War Engineers. Inc.

Page 61