2018-08-20_PERMIT FILE - C1981012A

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The above calculation shows that the effect of the backfilled channel on the stream quality will be non-detectable. This calculation was made assuming that flow through the backfilled channel would remain confined to that channel unit it exits into the original channel below the modified reach. Actually, there will be some mixing of the waters in the backfilled channel with adjacent alluvial groundwaters. This will have the effect of reducing even further the in -stream concentrations because of additional dilution not accounted for in the computation. On the other hand, the alluvial groundwaters immediately adjacent to the backfilled channel will experience some increase in species concentrations due to this mixing. The degree to which alluvial groundwaters will be affected is difficult to estimate. A plume of affected alluvial groundwater is expected adjacent to the backfilled channel. The width of the plume will be greatest at the downstream end of the backfilled channel. Hydrodynamic dispersion is not known, but the field experience of many researchers indicates that the value of the transverse coefficient is on the order of 5 percent of the longitudinal value of most field situations. Probably more important than transverse dispersion in this case is the interaction between the stream and alluvial groundwaters. As the stream stage increases during periods of high discharge, there will be flow from the stream to the alluvium. The flow will be reversed as the stream stage falls. The mixing caused by such periodic inflows and outflows from the alluvial aquifer is expected to be the dominant factor affecting the concentrations of dissolved species in the alluvial waters. The average concentration of dissolved solids in the alluvial groundwaters was estimated assuming that complete mixing occurs. The calculation was made as follows: C = Cb Vb + Ca Va Vb+Va RN -6 2,05-88 (Repaginated 10/21/16) `