Laserfiche WebLink
2.2.9 County Road 10 to Highway 285 (Reach 1) <br />Between County Road 10 and Highway 285 there are 13 ditches that have an appropriation to divert 568 <br />cfs, which is 75 percent of the two-year event. Extensive channel straightening has also taken place in <br />this reach, decreasing the channel sinuosity to 1.1. The average channel slope is 0.4 percent, indicating <br />the channel is approaching the fringe or distal margin of the alluvial fan. Deposition of fined grained <br />sediment would be expected as the river comes to the fringes of the alluvial fan and enters more of a <br />classic river Foodplain environment. Deposition in this reach has been exacerbated by the diversion of <br />flow from the Alamosa River for irrigation. <br />2.2.10 Floodplain Mapping <br />Floodplain mapping was prepared for the reach of the Alamosa River from Gunbarrel Road to County <br />Road 10 in 2002. The analysis was performed by M~XIH using the cross section data provided by Black <br />Creek Hydrology in HEC-RAS hydraulic modeling software format. The 100-year flood boundary <br />upstream of Highway 371 is shown in Figure 2-16 and downstream of Highway 371 is shown in <br />Figure 2-17. Because the lower reaches of the Alamosa River are located on an alluvial fan that is <br />"tilted" downward to the south, high flows can exceed the elevation of the drainage divide between the <br />Alamosa River watershed and the La Jara Creek watershed. In this case "breakouts" from the Alamosa <br />River can occur. Breakout locations are on the south bank at County Road 8 in Capulin, further <br />downstream near the St. Joseph Cemetery, and just upstream of County Road 10. <br />C~CB mapped the Foodplain downstream of County Line Road using approximate methods with <br />similar results to the HEC-RAS modeling (as shown in the figures). Both analyses show that structures <br />in Capulin, particularly north of Highway 15 are in the floodplain. <br />There is no available Foodplain mapping for the rest of the watershed. <br />2.2.11 Summary of Channel Issues <br />The Alamosa River has its headwaters in steep mountainous terrain of highly erodible volcanic material <br />providing a high sediment load to the Alamosa River. The river exits the confined valley near the <br />Terrace 1Vlain Canal and flows onto an alluvial fan. Under natural/undisturbed conditions sediment <br />loads would drop out as the river enters the alluvial fan area. The deposition of this coarse sediment <br />load would choke off the river channel causing it to avulse to a new location. Terrace Reservoir, <br />irrigation diversions, and channel straightening have had a significant impact on the Alamosa River <br />characteristics, helping stabilize and negatively altering the river channel. The river system appears to be <br />adjusting to changes in both water and sediment discharge. <br />The following key issues were identified as affecting the Alamosa River geomorphology: <br />• The upper watershed produces naturally high sediment loads <br />• Terrace Reservoir, irrigation diversions, and channel straightening impact the river's <br />geomorphology <br />Structures located within Alamosa River Foodplain are a flood hazard, especially in Capulin <br />Alamosa River Watershed Restoration Master Plan and Environmental Assessment Page 2-19 <br />