FLOOD10415

• County: Conejos
• Stream Name: Alamosa River
• Title: Assessment Report on the Alamosa River, Between Terrace Reservior and Gunbarrel Road
• Date: 10/1/2002
• Prepared For: Alamosa River Watershed Restoration Project
• Prepared By: Black Creek Hydrology, LLC
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

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The sections of river that tend to be more stable include the relatively steeper and less sinuous sections such as between

points 32 to 43 and 86 to 96. Those reaches also contain larger bed materials than observed in the less stable sections

of channel. The stream reach between points 91 and 96 contains sections of channel that appear to be abnormally

straight and, not knowing the history of that area, there is speculation that some of the normal meander pattern may

have been mechanically removed.

The reaches that contain local sediment deposition likely act as areas of temporary sediment storage. However, high
flow and meander migration processes will eventually allow this stored material to be re-mobilized into the stream
system. The sediment will the~ eventually be available to be re-deposited at other locations and create other problems

including flow deflection resulting in additional bank erosion.

Mitigation Options

The sources of excess sedimeIit in the assessment reach will need to be addressed during the design of the proposed

river restoration downstream qf Gunbarrel Road. The river restoration work will need to incorporate efforts to remove

the excess sediment materials from the river system or design modifications to create a channel able to transport the

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material all the way through 1:l1.e restored system. Sediment removal would likely require the installation of facilities to

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capture perhaps several hundred yards of material each year, a means to remove the material from the collection areas

and a location to dispose of or:a means to use the captured material. Use of the material would require some

determination of the expected size distribution of the captured sand, gravel and cobble.

Conveyance of sediment in excess of the river system's needs will require that all structures and channel shaping be

done to maintain a high sediment transport competence in the river. This could be done by keeping channel and

structure width/depth ratios low and maintaining high energy gradients. These actions will require that all structures

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and bed armoring be done to withstand the higher energy required to transport materials. This effort will need to be

implemented the entire length of the restoration reach.

Alternatively, efforts could be:implemented to stabilize the eroding banks downstream of Terrace Reservoir and reduce

the sediment sources. A total of about 8,500 linear feet of eroding bank was identified at 60 locations. The average

eroding bank is about 145 feet'long and about 4 feet tall. It is estimated that an additional 2,000 to 2,500 linear feet of

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