2011-08-29_REVISION - M1986002 (13)

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• • • Craig Ranch Pit — General Stormwater Comments Page 2 June 23, 2011 a. The Applicant shall provide sufficient information (e.g., hydrologic analyses and hydraulic computations) for the Division to evaluate all sediment pond sizing in accordance with Rule 6.3.3(g). If the northern ponds in Mining Area 1 are to be both sediment and wash ponds during mining operations, required wash pond volumes need to be included in the sediment pond sizing calculations. Also note Comment No. 5 below. b. All sediment ponds are required to include a spillway sized to pass flows exceeding the design storm in a controlled manner to an existing drainage or the adjacent river. For ponds only operated during mining operations, the design storm shall be the 25 -year, 24 -hour storm. For ponds remaining after reclamation, the spillway shall be designed to pass the unattenuated peak flow resulting from the 100 -year, 24 -hour storm. The Applicant shall provide calculations for appropriately sized spillways. See Comment No. 11 g. c. Spillway channels are also required to convey flows to existing drainages or the adjacent river. The Applicant shall provide calculations for appropriately sized channels using the same peak flows used to size the associated spillway. See Comment No. 11. 3. Page E-1, last paragraph and Map F. The reclamation plan proposes to use overburden "to create islands of thick growth medium." The Division is concerned about the long term sustainability of these "islands ". Specifically, how erosion from water and wind will be controlled around the perimeter of these islands. The Division's understands these islands will be constructed using overburden material on the exposed underlying sandstone. This is an intriguing concept. However the Division requires more information to evaluate it. Please provide the following: a. Conceptual cross sections (both longitudinally and laterally) to show thickness, slopes, and erosion control concepts on the perimeter, b. Hydrologic analyses (10- and 100 -year, 24 -hour storm events - also note Comment No. 5 below) estimating peak flows and volumes reporting to the upstream face of the islands in each drainage shown on Map F that is crossed with a proposed island, c. Estimated available storage volume (also note Comment No. 5 below) on the upstream face of the islands in each drainage and infiltration rates through the islands, and d. If the Applicant is aware of similar concepts being implemented elsewhere, provide any available photographs, and/or documentation describing the success, problems, or failures of the related project(s). e. Alternative Concepts for consideration: i. On the steeper slopes (>5 %), if the sandstone is rippable, the Applicant should consider a similar `island" concept, but use trenches approximately 1.5 feet deep on the contours and a few hundred feet long crossing the drainages. These would act as filter strips, intercept runoff and support plant growth, while being less susceptible to erosion. m lndn11c1bn 1988 -002 c alg ranchlstormwaterconunents _ mem2lJuntl.doox