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Mitigation and Modification Alternatives <br />Temporary Gravel Berm Structures <br />The common element of these diversions is the use of a temporary instream gravel berm <br />to direct water to the actual point of diversion from the river. Whether the water is removed <br />by pump or gravity is largely determined by economics, local water use, and the morphologic <br />character of the river. As the river generally flattens and takes on a more meandering form <br />below Craig, removal of water by pumping is more common. Upstream of Craig these <br />withdrawals are usually made by gravity. <br />Because they are temporary in nature, to be fully functional they must be constructed <br />each year. This characteristic enables their position and configuration to be modified each time <br />they are constructed in order to adjust to river movement, annually varying flow patterns, to <br />refine previous "designs" that were not effective, etc. This also results in a more frequent and <br />larger disturbance area, creation of a braided approach channel, and extra costs to annually <br />reconstruct the base elements of each structure. While it is acknowledged that each structure is <br />somewhat site specific, there are some general diversion structure construction practices which <br />could be implemented to reduce their adverse impact and improve their performance. Some of <br />these possible practices include: <br />• Construction of a permanent structure footprint to facilitate low flow diversions while <br />passing a navigable instream flow. This could consist of a full-width cutoff wall up <br />to the minimum flow level, with a constructed low-flow channel. The remainder of <br />the gravel embankment could then be constructed annually, as needed, to create local <br />conditions suitable for the desired range of flows. A more expensive, permanent <br />option could involve an inflatable dam mounted on a permanent footing. <br />• Construction of a defined low-flow passage channel upstream and downstream of <br />each diversion. This can be accomplished as a part of building the diversion by <br />simply giving a little extra care in the earthmoving process. <br />• Construction of a permanent low-flow passage channel of large rock, sloped 8:1 or <br />flatter, aligned in a concave downstream direction. This should be positioned to <br />survive the mean annual high flow and establish a permanent low flow approach <br />channel (refer to preceding paragraph). <br />• Installation of an impermeable geotextile blanket on the upstream face of the <br />diversion embankment to reduce water loss through the permeable gravel and direct <br />low flow to a defined, navigable channel. <br />d <br />ll <br />d f <br />i <br />or contro <br />e <br />gne <br />• Construction of a selective fill, zoned embankment des <br />progressive lateral breaching, in order to limit the amount of annual embankment <br />loss. <br />• Conversion of gravity diversion arrangements to use an in-river infiltration gallery <br />and bank-mounted pump. <br /> <br />4-8