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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BOX aoER cru:EK a.OMA

most watersheds, but not for the long, narrow Box Elder Creek watershed. Discharge
profiles for Box Elder Creek would be expected to be more "flat", as the UDFCD and COE
studies indicate in Figure 2.

The results of USGS Water Supply Paper 1680 are inconsistent within a relatively small
geographic region near Box Elder Creek. If the method is applied based on the adjacent
hydrologic region to the west (Region 9, where Plum Creek is located), the 100-year
discharge for the 200 square mile Box Elder Creek watershed would be 6000 cfs, compared
to 26,500 cis. In other words, the WS 1680 procedure produces significantly different results
for creeks like Plum Creek and Box Elder Creek or Cherry Creek, even though the creeks are
adjacent and parallel and all drain from the Palmer Divide.

3. USGS Water Resources Investigations Report 87-4094 does not represent best available
hydrology for Box Elder Creek.

The FHvIA letter states that the regional regression equation in USGS Water Resources
Investigations Report 87-4094 (1987) was used to assess the reasonableness of the submitted
discharges, particularly on Coyote Run. The letter states that the rainfall-runoff model
results for Coyote Run appear reasonable when compared to the regression equation
results.

The regional regression equation published in USGS Water Resources Investigations Report
87-4094 was based on data from watersheds ranging in size from 1.2 to 23.0 square miles.
The report states that the equation is applicable to watersheds up to 20 square miles in size.
As such, the equation does not apply to the Box Elder Creek watershed. A reference is
provided in the 1987 USGS report to a regional regression approach for watersheds greater
than 100 square miles. TItis reference is to l:SGS Technical Manual No.1 (wI-I).

4. USGS Technicallyfanual No.1 does not represent best available hydrolog'-j for Box Elder Creek.

USGS Technical Manual No.1, published in 1976, provides a regional regression equation
for a large geographic region in eastern Colorado. The region extends east to west from the
Kansas border to elevation 7500 to 9000 feet in the mountain foothills and from north to
south across the entire State. Two gage stations in the Cherry Creek watershed are the only
data points near Box Elder Creek that were factored into the equation. As discussed
previously, the Cherry Creek watershed has a smaller length to width ratio than the Box
Elder Creek watershed.

As with other regression equations, predicted discharges are shown to increase
proportionally as watershed area increases, a relationship that holds true for most
watersheds, but not for the long, narrow Box Elder Creek watershed. Discharge profiles for
Box Elder Creek would be expected to be more "flat", as the UDFCD and COE studies
indicate in Figure 2.

In summanj, none of the regional regression e!{WItions referenced in the FEMA letter represent the
hydrologic effects of the long, narrow shape of the Box Elder Creek watershed. None of the regression
e!{WItions represent best available hydrology for Box Elder Creek, since they are based on limited data
from watersheds that Iuroe different hydrologic characteristics than the Box Elder Creek watershed.

OfNICl.OlARHVC.OCC

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