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CRDA #2 Upper Diversion Ditch <br />Permit Exhibit 53 approved design specifies trapezoidal ditch <br />with 4' base width, 18' top width, and 3.5' depth (cross <br />sectional area of 38.5 sq. ft.); slope of .5%. Field <br />measurements indicated that the ditch gradient may be somewhat <br />higher (1-2%) than specified in some segments, however the <br />amount of variance is within the error range of the <br />measurement technique. Some ditch segments exhibit a "V" <br />configuration rather than trapezoidal, and cross-sectional <br />dimensions were less than specified in several sections. <br />SEDCAD evaluation indicated that ditch capacity was adequate <br />to pass design flow in all but one measured section, but <br />projected velocities of design flow were indicated to be <br />erosive. To date, no channel erosion has been observed within <br />the ditch, and it is probable that the rocky nature of the <br />channel provides sufficient roughness to minimize erosion, <br />given the relatively low ditch gradient. If channel erosion <br />is observed in the future, the operator will be required to <br />implement appropriate protective measures. <br />Measured sections are addressed below. Ditch segments <br />represented by DM~7, DM/8, and DH/9 will need to be modified <br />to comply with approved design, or permit requirements will <br />need to be properly amended to reflect field conditions and <br />sizing requirements for the design storm event. Necessary <br />field work should be completed by December 1, 1997, and any <br />permit modifications should be initiated promptly to <br />accomodate that time frame. Locations where the ditch <br />crosses steep ephemeral washes are critical segments due to <br />the potential for sediment deposition, a factor which is not <br />accounted for in the SEDCAD modelling, and it is recommended <br />that these locations be ^overbuilt^ where feasible. <br />DM#7. Approximately 30' from western terminus of ditch. 4' <br />bw, 14' tw, 2.7' d. Trapezoidal, 24.3 sq. ft cross section, <br />2% grade to west, rocky/soil bottom. Cross sectional area <br />substantially lower than approved design, however SEDCAD <br />evaluation indicates sufficient capacity for design flow. <br />DM#8. Approximately 150 yards east of the western ditch <br />terminus, at a point where ditch crosses a steep dry wash. <br />"V" shape ditch, 14' tw, 2.4' d, outside slope 25%, inside <br />slope 65% (cross sectional area of 16.8 sq. ft.). 2% grade to,. <br />west, rocky soil bottom. Cross sectional area substantial=ly <br />lower than approved design. SEDCAD evaluation indicates <br />insufficient capacity for design flow with required freeboard. <br />Note that subsequent to the special focus inspection, storm <br />runoff from the arroyo above the ditch at this location filled <br />the ditch with sediment and the ditch breached. This location <br />was included in NOV C-97-016, issued October 3, 1997, for <br />"Failure to ensure proper surface drainage control on coal <br />5 <br />