2016-08-08_PERMIT FILE - C1981019 (47)

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through less than 20 feet of a grass filter. A longer linear disturbance of 100 by 800 feet (eg., a short segment of an ancillary road, a pipeline corridor), will achieve the standard after having passed through 20 feet of a grass filter. Finally, a larger 300 by 1000 feet (6.9 acres, such as a longer segment of an ancillary road) will also achieve the standard within the first 20 feet of flow through the native grass vegetation. The details of these generic cases and the SEDCAD modeling work that leads to these conclusions are discussed in more detail below. The analysis below suggests that the other management practices such as straw bales (to trap sediment nearer the source), mulching (to lower CN and sediment generation), would only need to be applied in special cases where existing native vegetation is absent or existing slopes are much steeper than those assumed below in the analysis. Native Undisturbed Vegetation Treated as a Grass Filter A series of generic SEDCAD models was run to determine the feasibility of using grass filters to capture sediment from these small areas. Specifically, a grass filter length of 20 feet (length is in the direction of flow) and width equal to the contemplated disturbance was evaluated for performance in a 10 year, 24 hour storm. Note that the length and width dimensions used in SEDCAD's evaluation are reversed from what seems intuitive. SEDCAD refers to the "length" of the filter as the length in the direction of flow, and the width as the direction perpendicular to that. For a 20 foot wide strip 1,000 feet long parallel to the contours, with the 20 foot dimension in the direction of flow, the "length" is 20 feet, not 1,000 feet. The grass filter physical parameters were taken as the defaults in the SEDCAD drop down tables for a good stand of well established rye grass, except that the stem spacing was conservatively increased from 0.67 inches to 2.0 inches, and the roughness coefficient reduced from 0.0121 to 0.008, as described Reference 1, page 568, for a "fair stand" of grass. Both of these revisions produce a more severe loading on the grass filter by allowing the flow velocity to be higher than in a "good stand". Slope of the grass filters was set at 25% to 50%, which is representative of the land slopes in the 300 foot wide zone within the shaded area shown on Map 41B. Several generic small disturbance areas were evaluated. First a generic pad 100 ft by 100 ft (0.23 acre) was evaluated. This could accommodate for example a transformer, a dewatering well drill pad, or power pole, etc. The runoff from that pad would be routed through the grass filter immediately down -gradient of the pad along its full 100 foot width. The 10 year, 24 hour storm produces a runoff flow of 0.18 cfs, which is assumed to be distributed evenly across the pad's 100 foot wide boundary with the down -gradient grass filter. It is observed that the 20 foot long x 100 foot wide grass filter has a trap efficiency of 93.3% and is capable of reducing the peak and 24-hour average settleable solids to 0.00 ml/1. In fact the same occurs if the grass filter is only 10 feet long. Stated differently, only 10 feet of grass filter downstream of this type of disturbance is needed to achieve a settleable solids concentration well below the 0.5 ml/1 standard. A similar disturbance was evaluated for a longer linear activity such as a pipeline, 800 feet long and 100 feet wide to accommodate the actual pipeline as well as ancillary activities and materials storage. This area is 1.84 acres, assumed to be at CN 85. It produces 1.68 cfs of runoff released onto the 1,000 foot wide by 20 foot long grass filter immediately downgradient on the slope. Exh. 7-23H-2 Revision Date: 11/09/15 Revision No.: PR -04