1991-05-16_REVISION - C1981035 (2)

Home Browse Search

Watershed Parameters for Eastside Drainage Drainage Area = 43.5 acres Length to Divide = 3000 ft Curve Number = 76 Average Slope = 12 .8% Lag Time = 15 minutes 100yr-24hr Rainfall Depth = 3 . 25 in 25yr-24hr Rainfall Depth = 2 .90 in 10yr-24hr Rainfall Depth = 2 . 30 in flow with more than 1 ft. of freeboard. The invert slope was obtained from a topographic map of the site provided by National King Coal. See appendix 1 for computer runs. The outside bank on all curved sections of the channel should be protected from erosion with rock rip-rap (6" - 24" well graded) . Rip-rap should extend below the invert of the channel a distance of approximately 2 ft. and be backfilled and compacted. 3 . East Sediment Pond Hydrology The MLRB regulations impose several constraints on the hydrologic design of sedimentation ponds. First of all the pond must be capable of storing the 10yr-24hr event volume. In addition to this volume it must also be constructed and maintained to account for watershed sediment yield. In time, sediment inflow to the pond will decrease the effective storage capacity. The next hydrologic requirement stipulates that the outlet works must be capable of safely passing the 25yr-24hr event immediately following the 10yr-24hr event. What this effectively means is that with the pond full from the 10yr-24hr event, the outlet works must pass the 25yr-24hr event with 1 ft of freeboard from the maximum water surface elevation to the top of the pond embankment. This is accomplished through the combined effect of low level outlet and a primary spillway. These outlet works must be separate structures. The final hydrologic regulation specifies that the entire 10yr- 24hr volume, stored in the pond, must drain through the low level outlet in 48 hrs. The 10yr-24hr and 25yr-24hr design events were computed and routed through the East Pond using the HEC-1 model (HEC, 1981) . 2