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
I
m
I
I
I
I
I
I
I
I
I

Lower Big Dry q;reek Hydrologic Study

I
enabled the site to match pre-development ~onditions more closely. Combining the extended
detention BMP with the 100/l-year flood coJtrols proved to be even more effective in both Fort
Collins and Atlanta at lowering the flow frlquency curve for flows smaller than the 1.5..year
event (flow exceedance of 0.7 times per yeJ). Finally, Nehrke and Roesner (2004) found that
for Fort Collins if the detention facility was dbsigned to provide the water quality capture volume
with 24-hour drawdown time and peak flow controls were provided for the lower "breakpoint"
storm (lA-year storm) and the 100-year storm, the post-development flow frequency curve
followed the predevelopment curve very well! a shown in Figure 10.
I
Nehrke and Roesner (2004) stated that in both Fort Collins and Atlanta that the use of traditional
design storm development does not produce la post-development flow frequency curve that is an
I
adequate match to the pre-development curre for storms smaller than the 2-year storm. The
authors concluded that by utilizing the best available tools and local long-term data, flow control
that is superior to the current state of practice is obtainable. Multi-outlet controls, sized using the
best available technology, that regulate the discharge over the entire spectrum of the flow
frequency curve are required to make this imJrovement.
I
An additional issue that Nehrke and Roesnerlraised was that although the post-development flow
frequency curves can be better controlled, this still solves only part of the problem. The question
of post-development flow duration relative to flow duration in the predevelopment state still
remains. I

Applications of the concept proposed by Nlhrke and Roesner (2004) have been independently
I
implemented in several different locations. For example, at a development site in Denver where
an acceptable outfall did not exist for a nbw development, the City and County of Denver
required that the developer provide contrl1 of a broader range of storm events (personal
I
Communication with Alan Sorrel, City and Oounty of Denver, May 2005). WWE implemented a
similar concept at a development in Miss1uri that discharged to sensitive receiving waters

(WWE 2004).

971-179.092
June 2005

I
I
I
I
Wright Water Engineers, Inc.
I
I
,

Page 58