2015-08-31_PERMIT FILE - C1981010 (59)

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It can be seen that for a channel slope of 15 percent, the storage capacity for each check dam is only about 31 % of the storage available for a check dam constructed in a channel with a 5% slope. A three foot high check dam in a 20% slope will only back water up about 15 feet upstream, while the same check dam in a 5% slope will back water up about 60 feet. Appendix D contains an example of a more complex SEDCAD model. In this case, a series of 10 individual check dams have been modeled in a postmining channel. This "string of pearls" model is intended to mimic Trapper's use of multiple check dams and will demonstrate that each check dam temporarily stores and detains a portion of the structure's inflow and results in reduced peak flows downstream in the channel. This modeling case is representative of early stages of reclamation. There are certain practical limitations in SEDCAD to the number of structures that can be effectively modeled, but this example is intended to show how these small upstream structures in series can have a significant cumulative effect downstream. Given the porous nature of the check dams, the dams do not actually result in water storage. However, it is assumed that temporary storage is achieved under design storm conditions and that temporary in -channel detention occurs. This assumption is further verified by the fact that the mine has seen these structures both overtop during large flow events as well as overtopping due to sediment accumulation. The runoff curve number selected for this evaluation is 78. This is arrived at by assuming that during the active mining stage approximately one third of the watershed tributary to the check dam would be at a freshly topsoiled condition (CN 86), one third at 1 year's growth (CN 80), and one third at 2 year's growth (CN 67), all the while the grass lining in the channel is still immature The object of this modeling is to find a ditch check spacing that slows the flow enough to not present erosive velocities in a bare earth channel lining, modeled as "coarse gravel, non colloidal" in SEDCAD. In the model, the outlet of each check dam flows to an individual channel downstream. With 10 check dams in the model, there are also 10 drainage channels downstream in which the flow rates are identified. All drainage channels are assumed to be unvegetated as representative of newly reclaimed lands. All of the channels were modeled as "erosive channels" with a width of 20 feet and with a 10 percent channel slope and a surface of "gravel, non -colloidal". These dimensions are representative of the typical condition at Trapper Mine. The limiting erosive velocity for this case is 4.0 feet per second, significantly less than the 6.0 feet per second for a vegetated channel. Using this design methodology should subsequently allow for the establishment of a grass -lined channel in the future. All ten check dams are identical in size and have been modeled in SEDCAD as "ponds" with a small (but not insignificant) storage capacity. The size of each of these structures is 0.022 acre feet for the 10% channel as described above. Each structure also has an identical contributory drainage area, and each structure is assumed to be empty prior to