FLOOD00909

• County: Larimer
• Community: Wellington
• Stream Name: Boxelder Creek
• Title: An Evaluation of the Designation of the Town of Wellington as Being in the Boxelder Creek 100 year Floodplain
• Date: 6/3/1974
• Prepared For: Larimer County
• Prepared By: Marcus & Burns
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Exhibit B depicts the total drainage area aL t~s merging point (cross section 27)
7f:ft,'~ r/..5.&;, 3

to he 192.9 square mil;j, o~~~~~ 50.9 square miles is contributed by Coal Creek

leaving the remaining 142.5 square miles as contribution by Boxelder Creek. There-

fore, it can be generally stated that Coal Creek contributes about 25% of said

100 year frequency peak flow or approximately 3300 cfs and Boxelder Creek contri-

butes the remaining 75% of said flow or 10,000 cfs.

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In reviewing this flood plain situation, we have theoretically assumed that the

13,300 cfs is a close indication of a 100 year frequency flood. However, later

in this report, evidence will be presented which strongly tends to refute the

validity of this 13,300 cfs figure.

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Again, assuming that 13,300 cfs is a valid figure, capacities were calculated for
specific cross-sectional locations (exhibits C and D) using cross-section data
from both the Soil Conservation Service and the Larimer County Engineer's Office.
Calculated capacities were made using the Manning formula:
Q = A 1.49 R 2/3 S 1/2
n
and were calculated for the stream channel at bankfull stage. Because a natural
channel is involved, capacities using only the general Manning formula are not the
most detai led since losses wi 11 be incurred due to the winding nature of
natural channels, and incongruencies in channel geometry; however, as an

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approximation, the Manning Equation is reasonable.

equation takes into account friction losses in the

the value 0.033 was assigned to n.

The coefficient n in the
channel. Consequently,

Our calculated capacities of the Boxelder channel at certain particular SCS
cross-section locations are as follows:

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Cross Section 27 with flume - 11,131 cfs (see exhibit E and El)
Cross Section 27 with flume removed - 20,773 cfs (see exhibit F)
Cross Section 26 - 11150 cfs (see exhibit C and C I)
Cross Section 25 - 11,100 cfs (see exhibit D and DI)

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It is the contention of the Soil Conservation Service that a flume, which cross-

es Boxelder Creek, used to transport irrigation water along the Windsor Ditch,

will create a backwater effect causing any 100 year frequency peak flow to over-

flow the Boxelder channel and merge with the Coal Creek flow to inundate

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3. ~Watershed Work Plan, Boxelder Creek Watershed~'Project MAP

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