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

Att8Cnment ~

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Memorandum

RECEIVED
DEe 24 1998

U _.~,--_._,.-.
.D. & F.C.D.

To:
From:
Date:
Re:

Bill DeGroot
Brian Hyde biC t(
12/22/98

Appropriateness of Regressions for Box Elder Creek Flood Hydrology

The notion of using regression equations either to develop new peak flow values or
to check flow values developed by other methodologies has raised a great deal of
controversy with regard to Box Elder Creek. Rather than belabor the specific details
of Ihat controversy, I would like to examine the appropriateness of the concept of
regressions for that particular watershed. Examining the appropriateness of
regressions requires assumptions about the hydrologic homogeneity of the Eastern
Plains in Colorado, or, at least, the homogeneity of portions of the Eastern Plains. I
think there is far less homogeneity in the Eastern Plains than some people believe.

The Eastern Plains in General
Before I get into details about Box Elder Creek, I would like to look at the Eastern
Plains as a whole. You are aware that there are three major watersheds in the
Eastern Plains. The northern part of the Eastern Plains is located in the South Platte
River basin. The southern part of the Eastern Plains is located in the Arkansas River
basin. In between those two basins is the Republican River basin. All of the
detailed hydrologic analyses that have been performed in those three basins and
reviewed by the CWCB demonstrate substantial hydrologic differences between the
three basins. J think thai aggregating locations from more than one of these three
major basins for any hydrologic analysis is a fundamental mistake. The three basins
and hydrologic locations must be analyzed separately just like the streams in the Rio
Grande, the Colorado River and the South Platte River basin are analyzed
separately. Box Elder Creek flows should, therefore, not be compared to flows in
the Arkansas River basin or the Republican River basin.

1

Specific Characteristics of Box Elder Creek
Box Elder Creek flows should be compared to flows for other streams within the
South Platte River basin only after a great deal of care has been taken to understand
the similarities and differences between Box Elder Creek and the other streams.
Finally, the specific location on Box Elder Creek makes a difference, because the