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 <j;reek Hydrologic Study

"manage the quantity, velocity and dlection of resulting storm water runoff in a manner
which otherwise adequately protects Jealth and property from possible injury." Debo and
Reese state that detention storage fac~lities are almost always the mechanism to achieve
these goals. They further note that cbmmunities are increasingly asking questions about
whether the use of detention facilitieJ on all developments is the. answer to flooding and
drainage problems. They recommen~ three factors should be present in any stormwater
detention ordinance, including:

1. The municipality should have a mechanism to encourage and implement regional
storage facilities.

2. Detention ponds should be designed to control downstream impacts, not just peak
flows at the outlet.

3. A comprehensive inspection and maintenance program should be implemented.

With regard to item #2, Debo and Jeese (2003) state that municipalities are starting to
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realize that limiting the hydraulic analysis of stormwater management facilities to the exit
of the proposed development or ddtention site is not sufficient to prevent increased
downstream flooding and drainage broblems. They identify the need for a policy to
address this problem that can be incoborated into local ordinances and regulations. The
difficulty with addressing downstrearh problems is determining how far downstrearn the
limits of hydraulic analysis should go]

One relatively new approach suggestl by Debo and Reese based on modeling conducted
in North Carolina and South Caro1inJ is use of a specific criterion regarding the total area
to be considered for downstream effJcts termed the "10 percent rule," which is stated as
follows:

In determining downstream effects from storm water management
structures and development,1 hydrologic-hydraulic engineering studies
shall extend downstream to a point where the proposed development
represents less than ten (10) percent of the total watershed draining to
that point.

971-179.092
June 2005

Wright Water Engineers, Inc.

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