FLOOD06897

• County: Larimer
• Community: Wellington
• Stream Name: Boxelder Creek
• Basin: South Platte
• Title: Boxelder Creek Watershed Hydrologic Analyses
• Date: 12/1/1998
• Prepared For: FEMA
• Prepared By: UDFCD
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Colorado Water Conservation Board
Department of Natural Resources
721 Centenni<11 Building
1313 Shermiln Street
Denver, Colorilda 80203
Phone: ()O)) 866-3441
FAX: [phone]

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March 7, 1997 r. ' ii
. loi;.\R 11 In UI

Roy Romer
Covt!fr101

Jo1mes S. LochheMJ
Execuuve Director. ONR

O"ri~ C. lire. PE.
Director, ewes

U. D. & F. C. D.

Mr. Scott Tucker
Urban Drainage and Flood Control District
2480 W. 26th A venue, Suite 156B
Denver, Colorado 80211

Attachment 5

Dear Mr. Tucker:

The Colorado Water Conservation Board has reviewed the hydrology study that was completed for the
District's recent Flood Hazard Area Delineation for Upper Box Elder Creek in Adams and Arapahoe
Counties. On May 14, 1996 our Board designated and approved the lOO-year floodplain information
contained in the subject report, which gives local officials and floodplain administrators legal authority
to use that information for floodplain regulation, planning, and other purposes. The CWCB staff is
quite comfortable with the District's floodplain study process, and was involved with the progress
meetings and review of the Box Elder Creek study prior to its finalization and publication. Brian Hyde
of our staff was in attendance at numerous progress meetings at which he provided input during the
course of the study. We feel that the District and its study contractor did an excellent job of addressing
any concerns or issues that were raised.

In addition, our staff feels that the District's Colorado Urban Hydrograph Procedure (CUHP) is a valid
hydrologic methodology for use in the Denver metropolitan area. The CUHP rainfall-runoff model is a
well established and accepted procedure, and has been applied to numerous basins in the Denver area.
To our knowledge, it is the only model in Colorado that was developed based on observations of local
rainfall and runoff data. The model takes into account numerous basin-specific parameters as well as
localized rainfall data which is more accurate than data found in the NOAA Atlas. Over the years, the
CWCB has designated and approved many floodplain studies that have made use of CUHP.

For the Box Elder Creek study, the CUHP model was compared to the 1973 estimated peak flood flow
on that stream. The 1973 rain storm over the basin resulted in the flood of record on upper Box Elder
Creek, which was estimated to have a peak flow of about 6,000 cfs at Bootleg Reservoir. In the 1973
flood, the most intense part of the storm was centered directly over part of the Box Elder Creek basin
(see attached isohyetal map). Compared to the regional regression equations, the CUHP study model
appears to make a more reasonable prediction of flood flows based on data from the 1973 flood event.

Box Elder Creek has not experienced the extremely high peak flows and unit discharges that have been
documented on other streams along the Front Range and in the eastern plains of Colorado over recent
history. This is primarily due to its long and narrow basin shape, its south to north orientation, its soil