REV09711 - Laserfiche WebLink

Home Browse Search

Your letter does not identify a specific issue regarding the inlet but the photo #2 shows the storm runoff entering the inlet of the eight-foot diameter culvert. Review of the design information in the approved permit application reveals this large culvert was designed to accommodate the 100-year storm event. The drainage area above the culvert is about 275 acres, considerably Tess than one square mile. Rule 4.05.3 allows for permanent diversions draining areas less than one square mile to be designed to pass the 10-year storm event. In this case, it appears that the mine designed the permanent culvert in Stazkville Gulch in excess of the requirements of the Rules. This is a substantial benefit in the long-term, as the stability of the reconstructed drainage is ensured well into the future given the oversized design approved and constructed. The detailed design assumes that the head on the culvert would be slightly less than one foot during the 100-year event. As can be seen in the photo #2 taken by you, the amount of available head is much greater than one foot; comparison to the culvert itself indicates the fill over the culvert to be in excess of eight feet at the inlet end. Also noted during the field inspection was that the culvert grade at the inlet appears considerably steeper than the grade at the outlet. Average grade over the entire length of the culvert is I.8% as indicated by the design information. Your concem, as stated in the letter, A is the amount of silt that is resident Headwater (Hw) in the outlet of the eight foot culvert in the permanent diversion. The ~ -----_Watersurta°e~__` concern appears to be that if the - __ ___, culvert plugged, you would be liable projecting Inlet End-Unsubmerged for downstream damage to the interstate highway, where another j- water suAace e eight foot culvert is located. ~~ Photographs and measurements Hw taken during the bond release ~_ %'~ - -~-__- water surraca inspection (enclosed)indicate2.4 -- -'~--- ______ feet of siltation extending for some projecting or Mitered Inlet-Submerged distance east of the outlet of the INLET CONTROL eight foot culvert. Design for this Flgure 4.20 inlet con(rol is one of the two major types of culvert Ilow. Condition A culvert is as shown in condition B in with unsubmerged culvert inlet is preferred to the submerged end. Slope, roughness and length of culvert barrel are no consitleratron. Figure 4-20 (right), taken from "Handbook of Steel Drainage and Highway Construction Products"American Iron and Steel Institute, 1983. Inlet control means that the discharge capacity is controlled at the entrance (inlet) by the headwater depth, cross-sectional area, and type of inlet edge. The roughness, length, and outlet conditions aze not factors in determining capacity. In the case at the Starkville Gulch eight-foot culvert, inlet conditions at design flow assume water covering the entire inlet and extending slightly less than one foot over the top of the culvert, well below the point where the culvert would overtop the gulch and flow across the ground. As shown in Figure 4-20, condition B, flow at the outlet does not occupy the entire cross-sectional area of the outlet. -The outlet end of the culvert had 76% of its available area clear of sediment. No evidence of a high water mazk anywhere close to full capacity at the outlet was observed. The culvert has functioned properly at all times during the reclamation liability period and plugging of the inlet or outlet ends of the culvert has not been observed. Sediment load carried in the permanent drainage above and within the culvert is not a function of the mine disturbance. Only that portion of the channel below this culvert that was reconstructed by the operator is the responsibility of the operator. This