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I I I I I I I I -~ '. I I I I I I I I I Page 5 Chunnel Committee December 2, 1994 in transition structure design. The present culvert is undersized to take the regulatory 100- year flood event. The future culvert size must be determined before additional design work can be completed on the transition structure. Modifications to the Gregory Gulch flume must also be incorporated into the design. The existing (Caldwell) pump house is being evaluated. The intent is to modify only the pump house while leaving the existing diversion structure untouched. The pump house modification will be to replace pumps and construct wet wells, controls and pumps in a below ground vault. Outlet Structure HydraulicslPreliminary Design The outlet design has not been selected at this time since the final location has not been determined. As-built plans of the waste water treatment plant expansion are necessary. However, there are several energy dissipator options available such as impact-type, U.S.B.R. dentated-type, a water fall, and others. Physical space to construct the outlet will determine the type. Sediment TransportlDebris Flow Sediment transport is an important design consideration. Stream power inside the QlUnnel will be maintained to keep all sediments through the entire length. The preliminary Chunnel design will keep sediments up to 18-inch (<Iso) moving during the loo-year event. Lower level events are being analyzed to ensure clogging does not occur at lower-than-design event discharges. The sediment transport scenario is as follows: Sediments will be contributed by North aear Creek, Gregory Gulch and local sources. The North aear Creek sediment contribution will be controlled by the relatively flatter slope upstream of Bullwhackers and the S.H. 279 culvert. In other words, upstream sediment contribution to the Black Hawk Street culvert and low flow channel will be limited to the maximum sediment size the reach between Bullwhackers and Chase Gulch can carry. Our analysis (Shields type) shows bed material up to 17-inch diameter may move along the bed. Since North aear Creek flood water will be entering from the low flow channel, sediment clogging of the low flow channel is critical. The width of the low flow channel is narrowed along the transition structure to maintain stream power. Downstream of the transition, the low flow channel is widened to provide additional hydraulic capacity around sediments which drop out. The Gregory Gulch connection will carry more sediments (in the near term or) until the Gregory Gulch channel and upper basin (mine tailings) are stabilized. Existing bed load in .