2015-09-09_PERMIT FILE - C1981019A (11)

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done using the Simons/OSM method in SEDCAD. For channels to be protected by a vegetative liner, the permissible velocities are also determined using SEDCAD's routines. Some of the segments of the permanent primary channels are flat enough that a vegetative liner will provide sufficient erosion protection, even for the 100 year flow. This situation exists in the middle reaches of the West Taylor West and West Taylor East Tributaries. The general uniform section selected for the rip -rap ditches is as follows: • a trapezoidal channel with a 12 foot bottom width at the channel slopes shown below, • a total depth of 4.0 feet to ensure 1.0 feet of freeboard, • 3H:1 V side slopes, and • a minimum D50 riprap diameter of 18 inches on all rip -rapped portions The general uniform section selected for the portions that are vegetated channels are as follows: • a trapezoidal channel with a 20 foot bottom width at the channel slopes shown below, • a total depth of 4.0 feet to ensure 1.0 feet of freeboard, • 3H:1 V side slopes, and In addition, as a means of retaining some of the runoff in the upper reaches of the watershed to reduce flows and achieve the sediment performance standards in the 10 year 24 hour storm, two small "stock tanks" are included in -stream in the upper portion of the watershed. For modeling purposes at this time, they are set as nominal 2.0 ac ft incised ponds, and are assumed to be prismatic with a nominal depth of 10 feet. The actual stock tanks to be constructed once the reclaimed area has achieved final post -mining topography will be installed as 2.0 ac ft incised ponds, but they may be shallower and shaped to more naturally blend with the final topography. Attached in this Part E is a single SEDCAD model to evaluate the peak flow and total runoff volume for each of the ditch segments under the 100 year, 24-hour storm event. Map 41 A shows the location of these ditches, and Map 33B presents the postmining channel profiles. Where riprap is needed, both 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. 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 Exh. 7-20E-2 Revision Date: 07/20/15 Revision No.: TR -105