WSP03331 - Laserfiche WebLink

Home Browse Search

~~,~~.,. tiJ ..)_v....' Affected Envifonment & Consequences 3-25 quality would primarily occur downstream from the North Fork. This condition is unchanged from that described in the PElS, and a more detailed discussion is presented in that report. 3.6.2.3 Uncompahgre River Development of the proposed Project would result in widening of the active channel in response to increased magnitude and duration of in-channel flows. Channel widening would increase the amount of sediment in the river, which could subsequently reduce water quality, The Sponsors evaluated this issue using a sediment continuity model, described in Section 3.4.3. Based on an ultimate average width/depth (W /0) ratio of 35 with a minimum dimensionless shear stress of 0.03 (Shields value 'tc = 0.03), channel widening and estimated total volume of material eroded are shown in Table 3-15. Table 3.15. Total Material Eroded Over Life of Project Average Volume Eroded, Total Material Eroded, Alternative Widening, First Year, over 40 years, feet cubic yards cubic yards E-I 30 76,000 278,000 E-2 26 64,200 224,000 E-3 23 45,800 176,000 Source: Mussetter Engineering, Inc, Uncompahgre River Channel Stability Study, Final Report, February 1995; modified December, 1997, The estimates shown in Table 3-15 account for the relative width between the flood plain limits (average height 4,1 feet) and the height of the bars along the active channel (average height of approximately 2 feet). Based on the proposed bank protection plan (see Section 2.4.4) it was assumed that none of the terraced surfaces would be affected by the channel widening. The estimates also exclude those areas where the channel is currently protected on both banks and thus cannot widen. The impact of introducing this volume of material into the Uncompahgre River, and ultimately the Gunnison River, depends primarily on the rate at which it is introduced. After the Uncompahgre River has adjusted to the new flow regime, the total sediment load delivered to the Gunnison River would not exceed the current sediment load. In fact, the sediment load after channel adjustment would most likely be less because the introduced water from the proposed Project would be sediment free and the coarse grained Uncompahgre River would achieve a threshold condition (Mussetter Engineering, 1995). MEI (1995b) converted the eroded volumes to sediment loads. For the 850-cfs Alternative, the maximum (first-year) increase in sediment load would be 127 parts per million (ppm) to 190 ppm. Of this, approximately 8 ppm to 13 pprn would be silt and clays, with the balance being larger particles. For comparison, the total existing summer AS Lateral Hydropower Project July 2000 .