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• (horizontal) to 1 (vertical), the slope of the nonh embankment was 6:1, and the slope of the bottom of the <br />emergency spillway across the embankment was -0.02:1 (-2% slope)(Figure Q-2, 1027/94, attached). <br />1.3 Criteria <br />The pond was originally constructed as a sedimentation pond. Upon Phase II bond release, the pond was <br />retained as a permanent -ivestock and wildlife pond. This analysis is based on the following criteria. <br />1.3.1 Rule 4.05.6(3)(d), Rule 4.05.6(5)(b) <br />The combination of a primary and an emergency spillway must safely pass a 25-year, 24hour event . <br />Emergenty spillway grades and allowable velocities shall be approved by the Division. <br />1.3.2 Rule 4.05.6(4) <br />The elevation of the crest of the emergency spillway shall be a minimum of 1.0 foot above the crest of the <br />principal spillway. There shall be no out-flow through the emergency spillway during the passage of runoff <br />resulting from the 10-year, 24-hour precipitation event. <br />1.3.3 Rule 4.05.6(11)(e) <br />The minimum elevation of the top of the settled embankment shall be 1.0 toot above the water surface in the <br />pond with the emergency spillway flowing at design depth (in effect, the freeboard of the emergenry spillway). <br />2. SEDCAD+ INPUT PARAMETERS <br />2.1 Storm Type and Rainfall Depth <br />The Storm Type was selected as SCS Type II. The SEDCAD+ model (hydrograph convolution interval) time <br />increment was selected as 0.1 hour. The rainfall depth fora 10-year, 24-hour storm event was determined to <br />be 1.60 inches. The rainfall depth for a 25-year, 24-hour storm event was determined to be 2.00 inches <br />[National Oceanic and Atmospheric Administration (NOAA) Atlas 2, Precipitation -Frequency Atlas of the <br />Western United States, Volume III -Colorado, & Previous ReportsJ. <br />2.2 Soil Type <br />The soil of the emergency spillway site was considered as colloidal alluvial silt. The soils of the watershed <br />areas were considered being within the "D" hydrologic soil group (a worst case scenario). <br />2.3 Vegetation <br />The emergency spillway was considered as unvegetated (bare soil). The vegetation on undisturbed portions <br />of the watersheds was taken as equivalent to the permit vegetation reference area. The Phase II Vegetation <br />Survey was used to describe vegetation on reclaimed portions of the site. <br />• <br />(Canadian Strip Mine, Pond 1 Riptap Review, 27 Dec 94, Page 2 of 5) <br />