2011-06-29_PERMIT FILE - C1981019A (22)

Home Browse Search

• A similar situation exists in The East Taylor Watershed except that there is no undisturbed land, and the reclaimed lands consist of only two conditions, CN =74 for the fill face (3 year revegetation) and CN = 77 for the remainder of the watershed (1 -3 year reclamation growth). Attached in this Part E are four separate SEDCAD model runs to evaluate the peak flow and total runoff volume for each of the four ditch segments under the 100 year, 24 -hour storm event. Map 41A shows the location of these ditches, and Map 33B presents the postmining channel profiles. Where riprap is needed, all four ditches have been designed with a minimum D50 rip -rap dimension of 18 inches as shown on Table 1. The SEDCAD model indicates that riprap smaller than the 18 -inch diameter specified would be adequate for most segments, but for construction expediency, the larger size has been selected for all segments requiring riprap. Finally, in cases where channel slopes are less than about 5 percent, the SEDCAD model shows that a permanent vegetative lining will be adequate. Secondary Channels As shown on Map 41A, a system of eight secondary channels will also be provided. These secondary channels are also summarized on Table 1. Profiles for these channels are presented on Map 33B. Although Rule 4.09.2 only requires a 10 year 24 hour event, these have been designed for a 100 year design event to produce a more conservative design. This results in all of these channels requiring a riprap lining for erosion protection. It should be noted that under the 10 year event, many of these ditches would not require riprap assuming the entire area has been revegetated for at least 1 -2 years. Because other SEDCAD runs provided in this submittal represent a particular period in time when only portions of the area have been reclaimed, the selection here of the 100 year design basis and the need for erosion protection is consistent with those analyses. The Trib. G ditch, which has the largest drainage area and largest resultant peak flow has been used as the representative example for design of all of the secondary ditches. Although dimensions for all secondary ditches are presented in attached Table 1, the flow results are only provided for Trib. G Ditch. Flows in the other ditches would be proportionately less. The following SEDCAD4 run for Trib. G Ditch is provided for this long term case where the 100 year, 24 hour design storm is used and the entire drainage area is assumed to be both contributory and recently reclaimed (Curve Number 77). The resultant peak flow generated was 40.1 cfs, and requires a riprap size with D50 of 12 inches in its lowermost (steepest) reach. For design simplicity and as an added conservative measure, a D50 of 12 inches has been selected for all of the secondary ditches. As such, all of the secondary channels will be constructed as trapezoidal ditches with a 12.0 foot bottom width, 2H:1 V side slopes, and a minimum D50 riprap diameter of at least 12 inches. Exh. 7 -20E -3 Revision Date: 6/6/11 Revision No.: TR -81