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-~- C. Diversions ~3nd Conveyances of Overland Flow. The regulations address two types of diversions and conveyances; Those having a drainage area of more than one square mile, Rule (4.05.9) and those having a drainage area of less than one square mile. 1'he second demarcation in the Regulations is if the diversion is permanent or temporary. 1. A]agpie Creek Diversion The Magpie Creek Diversion has a drainage of more than one square mile. Dorchester Coal Company requested that the diversion be permanent. If the request to leave the diversion permanently is to be approved, the following concerns must be resolved: a. Probable Hydrologic Consequences - The applicant must provide a quantitative discussion assessing the probable hydrologic consequences of leaving the Pfagpie Creek diversion as a permanent diversion. This assessment is necessary for the Division to find that the operation is designed to prevent material damage outside the permit area. Since the most likely effect would be increased erosion in the unnamed tributary to Flagpie Creek which receives the flow, the extent, magnitude, and consequences of this erosion should be assessed. This erosion could effect thestability of G.E.C. Minerals' segment of the diversion. (The effect could be reduced or eliminated by enlarging the natural channel to pass the increased flow.) b. Capability of the diversion to transport the supplied sediment load. - The Magpie Creek diversion is constructed at gradients lower than those in the natural channel, and as a result, would likely be unable to transport the natural sediment loads of the basin. The diversion may, therefore, be subject to siltation which would reduce channel capacity and, without maintenance, may make the diversion non- functional. ~(In addition, Rule (9.05.9(6)) would require that the diversion be capable of transporting natural sediment loads.) If the diversion is to be left following reclamation, the applicant should technically demonstrate that the diversion can pass the runoff and sediment supplied by the basin above without erosion or deposition. c. Side slope stability - The diversion is cut into the base of a steep hi11s1ope and the stability of the hillslope may be affected. Landslides would reduce channel capacity. The Division will require that the applicant demonstrate that the side slopes of the diversion will have a static safety factor of 1.3, utilizing current prudent state-of-the-art geotechnical analyses.