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<br />b. Colowyo used a maximum headwater of 1.2 times the culvert height. [f the <br />culvert is partially full of sediment, the culvert height would decrease by the <br />amount of sediment accumulation in the culvert. ~, <br />Please also note that Rule 4.03.1(4)(e)(i) states that culverts shall be designed <br />to safely pass the 10-year, 24-hour precipitation event without a head of water <br />at the entrance uriless suitable protection against erosion or fill saturation is <br />in place a the entrance and exit of the culvert. If Colowyo chooses to use a <br />maximum headwatervariable in the SEDCAD+ model, designs for protection <br />against erosion or fill saturation is required. <br />c. Colowyo used a Manning's N value of .024 which is for corrugated metal pipe. <br />If the culvert is full of sediment, the Manning's N value would be different. <br />Please revise the SEDCAD+ model to account for a Manning's N value that <br />represent sediment accumulation in the culvert. <br />d. The Division compared the pipe slope submitted in the SEDCAD+ designs <br />to the Tipton & Kalmbach inputs currently in the permit. The slopes of the <br />pipes vary significantly between the two designs. Please clarify this <br />discrepancy with the Division. <br />2. Colowyo revised the Carve Number (CN) to 63. The CN used by Tipton & <br />Kalmbach was 67. The Division believes that a CN of 67 more appropriate based <br />on some minimal disturbances on the watersheds such as run-away truck ramps and <br />old exploration roads. Please further justify the CN of 63 or revise the SEDCAD+ <br />model using a CN of 67. <br />3. Colowyo states that for watershed I, a curve number of 63 was used even though <br />there is a gravel pit in the watershed that is self contained. Colowyo has not <br />submitted any designs of the gravel pit sump to make the demonstration that the 10- <br />year, 24-hour event would be contained in the gravel sump. The Division is not <br />requesting at this time that the gravel sump be designed (due to the ongoing issue <br />of whether the gravel pit should fall under a Minerals permit rather than a Coal <br />Permit), but that the culvert be designed to handle the flow of all undisturbed and <br />disturbed areas. Please revise the SEDCAD+ model accordingly. <br />4. The following questions pertain to time of concentration variables used in the <br />SEDCAD+ model; <br />a. Colowyo generally used two landflow conditions to estimate the time of <br />concentration. Overland flow is noted as a 3 type landflow condition and 7 <br />is noted as Paved area (sheet flow) and small upland gullies. According to, <br />Design H d~rolo_gy and Sedimentology for Small Catchments, the flow length <br />for overland flow is less than 300 feet. In a number of the watersheds, <br />Colowyo used an overland flow length of over 4,000 feet. Please revise the <br />SEDCAD+ model showing overland flow of less than 300 feet. <br />