WSP03331 - Laserfiche WebLink

Home Browse Search

nr.1,~:5"7 lJ..._ wi Affected Environment & Consequences 3-15 design flow, the amount of water diverted from the Gunnison River would be less. Consequently, vegetative encroachment into the channel would be reduced. However, this effect is also minimized by the periodic high flows, which are unaffected by any of the development alternatives. 3.4.3 Impacts of Alternatives - Uncompahgre River 3.4.3.1 Alternative A (No Action) Under the No Action alternative, the Uncompahgre River upstream of the study area would become a more narrow and stable river, influenced by Ridgway Reservoir. Within the study area (downstream from the South Canal), infrequent major flow events could cause significant river channel changes. Bank stabilization by individuals and local governments in response to these events would continue. 3.4.3.2 Development Alternatives The AB Lateral penstock would be used to convey some of the irrigation flows that now flow into the Uncompahgre River. Specifically, irrigation demands from the Selig, Ironstone, and East that are met with diversions from the Gunnison River will be diverted from the South Canal, into the AB Lateral and through the penstock to the Uncompahgre River. In addition, these flows could be supplemented by new diversion, when flows are available and in accordance with priorities discussed in Chapter 2. This change would increase the volume of flow in the Uncompahgre River. Several individuals expressed concern during the PElS that this change would lead to substantial erosion and degradation of the Uncompahgre River. The Sponsors have conducted studies regarding the river's hydraulic and morphologic conditions to quantify potential impacts. Method of Analysis The Sponsors collected samples of the river sediments found in the bed and banks and combined this information with hydrologic and hydraulic models to determine the impacts of project flows, Two approaches were used in this analysis: critical particle size and the ratio of the channel width to depth when flows are at bankfull. A stable riverbed is one in which bed material exceeds a certain particle size threshold. This size is a function of the channel slope, flow, width, and material characteristics. If flows exceed a critical stable value based upon this threshold, the stream can generally react in any of 3 ways: . Degradation, where the bed lowers until slope is reduced to the point where critical shear is no longer exceeded, or the transport capacity equals the sediment supply. . Bed coarsening, where larger and larger cobbles are left as the smaller-sized materials are moved downstream, This will slow and eventually halt degradation as mean particle sizes increase to and exceed the critical sizes. AS Lateral Hydropower Project '< r 'i 'i . July 2000 . J , . t ~ ~ ..fuf,.L. ,M,- .1(1: <~ << ~ '''.-,"-2.<1__,<,$-,