2016-10-12_PERMIT FILE - M2016010

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Rationale for Recommendation to Approve 18 September 29, 2016 Precipitation data used for the design of stormwater structures were determined from the National Oceanographic and Atmospheric Administration(NOAA) annual maximum point frequency estimates for the site area. The NOAA values were generated for a point near the center of the proposed operation. The intensity-duration-frequency values for the 100 year storms were calculated using equations that consider the travel time/time of concentration duration. This equation was used in the peak flow determination spreadsheet to determine the precipitation intensities for each watershed based on the time of concentration values. A Natural Resources Conservation Service(NRCS)Type Il rainfall distribution was applied to the 24-hour storm. The Type Il distribution applies to mid-continent locations where there are intense localized storms. The 100-year, 24-hour storm event was modeled for the design of culverts and ditches, and for the design of all sediment ponds in accordance with state and county regulations. 8. Concern regarding Exhibit G — the application states "sediment levels should be examined annually following highest runoff season and sediment removal should be undertaken in fall to prepare sediment pond for next rainy season"- will the operator commit to this plan instead of"should be (N.Reed) In the discussion of the sediment detention basins provided in Exhibit G, the application states that a key element of the stormwater management plan is maintaining a high available sediment capacity in the sediment detention basins. Norwest Corporation designed the basins to have capacities no less than the runoff volume from a 100-year, 24-hour design storm, plus 20% for sediment detention. The basins will require periodic maintenance to ensure adequate sediment storage volume is available as described below. In discussion of Best Management Practices (BMPs) for sediment control, the application states that maintenance of sediment deposited in the existing ponds is another BMP. Sediment levels should be examined annually following the highest runoff season (i.e., summer), and sediment removal should be undertaken in the fall to prepare the sediment pond for the next rainy season. This is a critical element of the successful operation of the water management plan. For effective use of BMPs,regular inspection and maintenance of the structures must occur. The above statements made in the application, upon approval of the permit, become enforceable permit conditions. During routine inspections of permitted sites, the Division inspects stormwater management structures to determine whether they are functioning as designed. 9. Concern regarding Exhibit G—Sedcad 4—Hitch Rack Ranch Quarry access road- 100 year, 24-hour event—why is this 100 year storm design included? Will culverts be designed for the 10 year storm event(as previously mentioned) or the 100 year event? (N.Reed) Portions of Exhibit G, including the section on culvert design and Table G-6 - Culvert Summary, were revised in the adequacy review response submitted on July 21, 2016. This section now clarifies that culverts have been designed to convey runoff from a 100-year, 24-hour storm event with various headwater to diameter (HW/D) ratios. 10. Concern regarding Exhibit G—Sedcad 4—TSI-CC-1 and LTC-CC-1 thru LTC-CC-5 are shown as 100 foot culverts-is that the planned length for these culverts? (N.Reed) CO 1313 Snennan Stiee', Prom 215, Denver. CO 80203 P 303.866.3567 F 303.832.8106 h'* :: mimmn;.state.coAll> 90�, ;. 1 John W. Hrcke".tr:,00r. Governor ( ?�;�t:�rt W. Rand, !!, Exv3c_,=,:,- Dircctoi I 4'ir;rt� 8�arron, DriectGr 0*�( , ,• *) \*1876/*