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

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Lower Big Dry Creek Hydrologic Study

Table 3.
Portions of Jllig Dry Creek Susceptible to Erosion (from WWE 2000)

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. Vaious l..eIIeI d Erosion

L..o.v Erosion Nbderate Erosi
Pctential (<5 ft Potential (5-10 ft

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As a follow-up to the WWE (2000) report, the Watershed Association collaborated with two
Colorado State University (CSU) engineering students who completed more detailed evaluations
of several parcels in the agricultural portion of the watershed (Gossenauer and Wachob 2(01).
These students interviewed several landowners along the creek, documented conditions on their
properties and providt::d recommendations to mitigate the problems. The common concerns of
most of the landowners were loss of property to erosion, an increase in the rate of erosion,
increases in the flow rate and occurrence of high flow events, loss of infrastructure from
heightened flows, and the high cost of even simple bank stabilization techniques such as riprap.

Gossenauer and Wachob (2001) identified accelerated erosion present at several locations
studied, primarily along bends with naturally high banks, sites with significant cattle impacts, or
points where infrastructure such as bridges and headgates were present. In some cases, the
erosion appeared to be due to increased peak flows and significant fluctuations in flow
conditions. In other cases, impacts appeared to be due to a combination of increased flows
exacerbated by cattle impacts. Many landowners had one or two especially troublesome points of
erosion along their reaeh of stream.

Gossenauer and Wachob (2001) characterized the influence of the significant variations
in the flow regime of Big Dry Creek on the stream channel as follows:

Variable flows cause increased bank erosion by preventing the stream from
reaching a state of equilibrium. At low flows, the banks dry out. As a higher flow
progresses down the channel, water moves into the pore spaces in the soil, the
tension in the pore spaces is reduced, and the soil becomes less cohesive (Bledsoe
2001). The now weakened bank is subjected to greater hydraulic forces and a

971-179.092
June 2005

Wright Water Engineers, Inc.

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