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Roadside Memorandum September 21, ?000 Page 3 minimize the risk of a breach. The middle, multi-culvert concrete crossing area will require additional work, as discussed below. Drainage passing along the road, originating above the multi-culvert structure, should be diverted to the Coal Creek channel at locations above the structure. This should be done so that runoff is dispersed along the road, and so as to minimize erosion. This runoff should not be allowed to concentrate at the concrete crossing location. ~BLM requested that the railings along the crossing be removed so that the rails will not cause debris to snag at the crossing. BLM also suggested that the straw bales along the bridge should likewise be removed, so we should consider whether they are still needed to provide sediment control. My observations during the precipitation event indicate that the bales are only diverting runoff to the north side of the crossing, and are not providing any sediment control. Wing walls should be installed at the upstream edges of the crossings, such that the walls aze keyed to the crossing, set into the channel at an elevation below the crossing base, and with a top elevation at or about on the top of the crossing. ~BLM suggested that during large peak flow events, runoff will pass over the crossing, and the overflow runoff energy will concentrate at the northeastern bank of Coal Creek. This may be the case. We also agreed that bank erosion downstream from the crossing along the narrow southeastern bank was significant risk, especially since the culverts aze situated to direct flow to that location. My observations during the rain event verify that flow through the culverts is concentrated both in depth and direction toward this southeastern bank. This area must be protected from erosion so that the downstream southern RDA drainage control system and sedimentation pond aze notjeopazdized. It was agreed that certain measures can be implemented to achieve this goal, while retaining the multi-culvert crossing. Removal of the downstream rock check structures must occur to allow the channel gradient to adjust vertically downwazd. The material residing in the channel can be excavated and used for site reclamation, thus managing the inevitable incision. The rock can be used to provide erosion protection at the crossing. When the channel gradient drops, this will create the potential for undercutting at the downstream edge of the crossing. Undercut protection will be required, both along the northeast and southeast lateral edges of the crossing, and along the crossing base. Flow constriction at the crossing must also be relieved by removing material now located on the northeast side of the crossing and channel so that flood flows can spread to the northeast. In addition, the channel should be widened to the southeast, using the space available up to the approximate location of the Tamarisk tree. This widened area must be protected using the fill material and the rock available on site. The southem RDA ditch and pond inlet area will also require minor adjustment to accommodate this work. We will need to be sure that necessary southem RDA ditch and pond capacities aze not jeopardized as this plan is developed.