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