Monitoring sediment infilling at the Ship Creek Reservoir, Fort Richardson, Alaska, using GPR

Abstract

AbstractThe Ship Creek Reservoir serves as the municipal water supply for Fort Richardson, Elmendorf Air Force Base, and Anchorage, the largest city in Alaska. The reservoir is located at the base of a ∼4.8 km-long canyon along the flanks of the Chugach Mountains, where it is fed by a 234 km2 unglaciated drainage basin. High sediment production within the watershed results in rapid infilling of the reservoir and a need for periodic dredging. During March 2001, we conducted a ground penetrating radar (GPR) survey of the reservoir to document the current state of sedimentation and to evaluate sediment trap designs. These data are presented here as a case study in which the utility of GPR for evaluating sediment infilling in a shallow reservoir is evaluatedThe survey was conducted from a 45 cm-thick ice cover in the lower reservoir that thinned to about 6 cm at a distance of 220 m above the dam. A total of 49 radar profiles were collected from the ice cover and an additional 10 profiles were collected on a gravel bar in the upper parts of the reservoir. Water depth near the dam was about 6 m and sediment in that reach forms a blanket deposit. More hummocky and asymmetric bed topography was found away from the dam, apparently controlled by palaeochannel geometry and bedrock sills. GPR profiles collected on the gravel bar document a sedimentary wedge that thickens downvalley and toward the southern edge of the reservoir, apparently the palaeothalweg of Ship Creek. Transverse profiles across the bar indicate that an average of about 2 m of sediment can be removed across this reach, with 3 m or more being available adjacent to the active channel. This survey indicates that infilling of the entire reservoir had reached a level approaching that of the pre-1995 dredging. Analysis of sediment-yield data and basin geometry showed that a weir built across the reservoir approximately 215 m from the dam would produce a trap capable of storing 60% to 70% of that currently held in the reservoir. Sediment trapped within this portion can be excavated using standard machinery, providing a more cost-effective alternative to periodic dredging.