Laserfiche WebLink
<br />d. Grade Control: One new grade control structure is proposed along the alignment <br />of the pilot channel. The purpose of the grade control structure is to prevent excessive bed scour <br />and channel degradation along the flow path of the pilot channel, as well as to help control the <br />overall width and location of the chute channel. The location for the grade control structure <br />would be at chute station 54+00. This structure would consist of dumped riprap material and <br />bedding, and would be constructed with a bottom width of 200 feet and side slopes of 3H to 1 V. <br />A portion of the existing closure dike located at chute station 54+00 would be lowered to <br />accommodate the new grade control structure. The approximate riprap and bedding material <br />quantities for the grade control structure are 2,430 tons and 590 tons, respectively. <br /> <br />e. Operation: The operation of this alternative would initially include complete and <br />frequent burning to remove flow obstructions. As shallow water areas develop, some woody <br />debris accumulation will be beneficial to provide substrate for macroinvertebrate colonization and <br />as cover for fish species, in which case frequent burning would not be required. Periodic <br />inspections of the inlet and outlet structures and the grade control structures will ensure that timely <br />maintenance operations keep the structures functioning properly. <br /> <br />9. ENVIRONMENTAL EFFECTS <br /> <br />Table 8 provides a generalized overview of the enviromnental effects anticipated for each <br />of the alternatives, including the No Federal Action alternative. The following is a detailed <br />discussion of the enviromnental effects of a lO-foot wide constructed chute channel. <br /> <br />a. Physiography and Topography: There would be no significant adverse effects <br />on the topography and physiography of the Langdon Bend site due to the proposed plan. A chute <br />will be excavated immediately east and along the approximate alignment of the historic chute <br />channel on the northern end of the property. The spoil material from this section of the chute will <br />be placed as a wide, flat berm located approximately 250 feet to the east of the pilot channel, with <br />a width of 90 feet and a height of 6 feet. The excavated spoil material from the middle to the <br />southern exit of the natural chute will be side cast on either side of the pilot channel. As a result <br />of annual flooding and erosion, the spoil material will be incorporated into the natural topography. <br />The proposed plan will also allow for more of the project area to revert to its natural <br />physiographic character by eliminating agriculture on most of the site. In conclusion, the project <br />site would remain part of the Missouri River floodplain and alluvial soils would continue to be <br />deposited and eroded. <br /> <br />b. Water Quality. Water quality within the flow-through chute is expected to be <br />adequate for the establislunent of a backwater fishery resource. These types of environments are <br />expected to have highly variable water quality parameters such as temperature and dissolved <br />oxygen concentration. Areas of reduced flow which are exposed to summer light conditions are <br />expected to have high diurnal temperature fluctuations. Water quality is expected to improve due <br />to the reduction of land dedicated to agricultural use and the associated elimination of herbicide, <br />pesticide, and fertilizer applications. <br /> <br />36 <br />