PERMFILE134290 - Laserfiche WebLink

Home Browse Search

• above have been evaluated and are described below. The alternatives are illustrated in Figures H-2 and H-3. Both alternatives use a 48-inch diameter corrugated steel (CSP) culvert as the conveyance section. Alternative A consists of the CSP laid at the stream grade of about 12.68. An inlet control structure constructed of riprap obtained from the portal bench excavation is recommended. The structure may be augmented by debris posts, a debris basin, or turning up the end of the culvert to settle out debris before flow enter the culvert. An outlet structure should also be constructed of riprap. The structure should serve the purpose of an energy dissipator as well as bottom and side slope lining. On-site riprap may not be large enough to withstand high velocities. If this is the case it would be necessary to grout the available boulders. A potential problem with Alternative A is the steep culvert grade. The constant movement of bed load sediment in the sand and gravel size range, and up to boulder size range transported during high flows, could abrade and damage or destroy the culvert invert. The availability of sediment of this size and the possibility of excavating a sediment trap upstream of the culvert will be evaluated during final design. • Alternative B has the same inlet control features as Alternative A but mitigates the potential abrasion problem by laying the CSP at a flatter grade on the order of 28. Velocities at this slope are not fast enough to cause the constant abrasion from smaller sediment particles or to move the damaging large sediment particles. However, this alternative requires a conveyance section from the downstream face of the embankment to the stream bed. An additional section of 48-inch CSP laid down the slope is currently proposed, however, an open channel section will also be evaluated. A vacuum relief valve would be required at the grade break to prevent possible pipe collapse during high flows. 2.8 Diversion Ditches 2.8.1 General Description There is an undisturbed watershed of about 14.6 acres above the proposed portal bench cut. It is proposed to intercept and divert overland flows from this area around the disturbed area to East Roatcap Creek. The area of the watershed that drains to the road is 13.2 acres as shown on Figure H-1. 1.4 acres drains from the road to the proposed cut. To avoid the necessity of cutting a new diversion ditch through undisturbed terrain above the portal bench cut, the existing road is tentatively proposed to be used for the ditch alignment. Final ditch alignment will be determined in conjunction with sediment pond and portal bench final design. A