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bedding material underlayments will be used for riprap linings as needed to ensure structure stability (Figure B-3). Check dams az critical locations along a diversion or channel may be used to promote channel stability (Figure B~). Check dams are oriented perpendicular to the flow of water and may be constmcted using hay bales, loose rock fills or a combination of compacted earthen dams or embankments with a loose rock outlet structure that retards surface flow without pernranemly impounding water (Figure B-5 and Figure Bpi). Their primary use is to reduce flow velocities and to a lesser extent the trapping of sediment. In lower gradient drainages multiple check dams may be used as an intermittent channel liner, replacing riprap or other hard continuous lining materials. A check dam may be a temporary or permanent fixture. Check dams are less effective in continuous flow channels or in drainages that are more than ] 0 acres in size. Areas prone to flash flooding may further reduce their effectiveness. They can be extremely effective in repairing erosion in large ephemeral gullies. n Straw bale filters and check dams may be used for temporary erosion and sediment control during establishment of vegetation in a concentrated flow structure. Straw bale check dams are usually effective for one growing season or less. Maximum channel slopes for effective use of straw bale filters is less than 20`%. Straw bales will be placed end to end and keyed into the channel to avoid undercutting. Excessively rocky soils will be avoided when placing straw bate check dams. Properly installed bales will reduce runoff velocities and trap sediment during the vegetation establishment period. Straw bale check dams will be used where runoff volumes are low. They will be placed on the contour. When used in series the maximum spacing between bales will vary based on percentage slope between dams (Table B-1). Straw bale dams will be inspected after significant nmoff events and maintained as necessary. Table B-1 Straw Bale Check Dam Maximwn Spacing o ( Marumum traw Bale Interv ( ) >2<5 1 >5<10 5 >1 < > < Rock check dams will be used when channel erosion control is required for more than one growing season or the flow velocities and volumes are larger. Rock check dams may be placed and constructed using typical designs provided that rock is sized for the anticipated flow volumes and pemrissible velocities. A combination of earthen embanlmrent and loose rock check dam (the rock portion functions as aflow-through, non-impounding outlet) may be used to improve construction economics. Use of earthen materials with loose rock will be determined az American Soda's discretion. Gabions may be used to secure rock in check dams in high velocity flow anus (Figure B-7), particularly in narrow restricted channels. The spacing of loose rock check dams may be calculated using the formula: Where: D =Distance Between loose Rock Check Dams H =Height of Dam in Feet B-3