Preliminary hydraulic analysis and implications for restoration of Noyes Slough, Fairbanks, Alaska

The present-day channels of the Chena River and Noyes Slough in downtown Fairbanks, Alaska, were formed as sloughs of the Tanana River, and part of the flow of the Tanana River occupied these waterways. Flow in these channels was reduced after the completion of Moose Creek Dike in 1945, and flow in the Chena River was affected by regulation from the Chena River Lakes Flood Control Project, which was completed in 1980. In 1981, flow in the Chena River was regulated for the first time by Moose Creek Dam, located about 20 miles upstream from Fairbanks. Constructed as part of the Chena River Lakes Flood Control Project, the dam was designed to reduce maximum flows to 12,000 cubic feet per second in downtown Fairbanks. Cross-section measurements made near the entrance to Noyes Slough show that the channel bed of the Chena River has been downcutting, thereby reducing the magnitude and duration of flow in the slough. Consequently the slough slowly is drying up. The slough provides habitat for wildlife such as ducks, beaver, and muskrat and is a fishery for anadromous and other resident species. Beavers have built 10 dams in the slough. Declining flow in the slough may endanger the remaining habitat. Residents of the community wish to restore flow in Noyes Slough to create a clean, flowing waterway during normal summer flows. The desire is to enhance the slough as a fishery and habitat for other wildlife and for recreational boating. During this study, existing and new data were compiled to determine past and present hydraulic interaction between the Chena River and Noyes Slough. The U.S. Army Corps of Engineers Hydrologic Engineering Center River Analysis System (HECRAS) computer program was used to construct a model to use in evaluating alternatives for increasing flow in the slough. Under present conditions, the Chena must flow at about 2,400 cubic feet per second or more for flow to enter Noyes Slough. In an average year, water flows in Noyes Slough for 106 days during the open-water season, and maximum flow is about 1,050 cubic feet per second. The model was used to test a single method of increasing flow in Noyes Slough. A modified channel 40 feet wide and about 2 feet deeper within the existing slough channel was simulated by changing the cross-section geometry in the HECRAS model. The resulting model showed that flow in such a modified slough channel would begin at a flow of about 830 cubic feet per second in the Chena River and would increase to a maximum flow of about 1,440 cubic feet per second. In an average year, flow would continue for 158 days during the open-water season. Theoretically, enlarging the slough channel by lowering its bed could increase flow, but other solutions are possible. Possible obstacles to excavating the channel, such as bridges and utility crossings, and the destruction of desirable features such as beaver dams were not considered in the study. Further engineering and economic analyses would be needed to assess the cost of excavation and future maintenance of the modified channel. A computer-modeling program such as HECRAS may provide a means for testing other solutions.