PERMFILE68505 - Laserfiche WebLink

Home Browse Search

wildlifelfish habitat area. Upstream of the inlet, there are similar habitat features at Walter Walker Wildlife Area. At my property alone, I've observed deer, pheasants, bald eagles, vultures, cranes„ ravens, river otters, muskrat, beaver, weasel, coyote, a wide variety of waterfowl and small birds, and an occasional bear sign. Several species of typical river fish utilize this reach ofthe river, including the Colorado Pike Minnow and other endangered species. Post-mining Conditions: There will be a 100-105 acres in the mine-out pit; depths will be 15-20'. The proposed flow- through inlet is 100' wide, 2' above normal water, and will discharge into a pit that locally is possibly 10-IS' lower in elevation than the present riverbed. The design (on the U.S. C. of E. Notice) is not very flood-resistant. The consensus of several geologists, engineers, and myself, is that it mi t survive a 10-year flood, at most. Its function is to allow the upper part of the flood- stage water column to enter the pit, depositing sediment so as to eventually build up a mazsh, which will discharge via an outlet structure. As noted below, several factors suggest this probably will not be a simple process. A. Several small floods ~ build up considerable sand and silt in the pit. (The suspended sediment in the upper water column in flood water is mostly sand and silt.) I have no estimate of the "trap efficiency" of the pit. B. Under the above scenario, because of the high velocity/high volume inflow, there will be a permanent "plunge" pool l00' wide, 10-15' deep, and, extending downstream through the pit at shallower depths perhaps several hundred feet. C. Moderate to large flooding will quickly scour out a channel across the pit in the newly deposited soft sand and silt. Depending on the nature of the rise portion of the flood hydrograph, this large volume of sand/silt could leave the outflow and enter the river as a short-term "slug" of highly concentrated suspended sediment. Typically, the declining portion of a fload hydrograph contains lower concentrations of suspended sediment, probably not enoueh volume to replace the material scoured out. This could leave a long pond across much of the pit, to be partly refilled with sediment during subsequent smaller floods. D. A moderate-to-large flood would discharge several 1,000 cubic feet per second over the inlet into the pit, at high velocities (perhaps 5-8' per second over the crest, up to 10-15' per second down the 1:3 slope. Such flows would likely destroy the structure, allowing rapid down-cutting to the level of the riverbed, or lower. Such conditions would quickly divert most of the river flow through the pit, and result in very low flow, or most likely no flow in the "island"area, except for a single fast-flowing channel along the south side (Appendix II). The new gradient along this channel would be about twice as steep as at present and would tend to deepen the channel. E. Following this uncontrolled flow-through condition, head cutting in the main channel would progress upstream from the inlet area until a new profile at hydrologic equilibrium is established. This head cutting will deepen the main channel. Depending on the amount