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

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.

One possible influence on the development of in-channel deposition is the sequential, cumulative removal of water

(and therefore sediment transport energy) by ditch diversions. Immediately below Terrace Reservoir there likely is a

"hungry water" condition for sand, gravel and cobble (bedload) sediments. However, local bank erosion downstream

of point 15 may satisfy or exc~ed the sediment needs of the system. While bank erosion cumulatively increases with

downstream channellength, tIle energy to transport the eroded bank material through the system is cumulatively

decreased.

Bank erosion information obtained during the field assessment was tabulated and converted into square feet of erodable

bank as shown in Table 1. Table 1 shows the approximate length and height of each eroding bank, the square feet of

eroding bank and the amount ~f eroding bank accumulating from upstream to downstream between Terrace Reservoir

and Gunbarrel Road. River length (distance below dam) and GPS point numbers were included to assist in referencing

the locations of the photograpns on the map and graph in this report.

The cumulative bank erosion data is graphed in Figure 2. For reference purposes, Figure 2 also shows the relative

locations of the major bridges ,and a few GPS points at key locations on the assessment reach.

,
The assessment process docu~ented an approximately 35,500 square feet of eroding bank on the 9 miles of river

evaluated. It is possible that the entire assessment reach from between Terrace Reservoir and Gunbarrel Road may

contain about 44,000 square feet of eroding bank. The shape of the data trend line in Figure 2 appears to indicate that

the amount or eroding bank increases exponentially down the river system.

The first significant bank eros~on downstream of the dam was observed near GPS point 16 at about 9,400 feet bdow

the dam of the dam. From there to about 22,000 feet, the channel still contains relatively low amounts of bank erosion.

Upstream of 22,000 feet (abou~ GPS point 43) the river appeared to be steeper than downstream reaches and contains

an abundance of larger cobble bed material. Large cobble and small boulders along the toe of the riverbank appeared to

provide stability to the bank. This apparent stability is generally visible in the photographs in Appendix A.

Immediately downstream of GPS point 43 (from 22,000 feet to about 34,000 feet) eroding banks are frequent and the

cumulative amount of eroding bank is conspicuous in the graph. However, there are a few localized sections of

3