Method for Estimating Sediment Mass Movement from Delta Recutting: A Case Study Using Single Beam Sonar in Deer Creek Reservoir

Abstract

The recutting of delta sediments typically occurs during reservoir drawdown in the summer months. It can affect various reservoir processes and can impact water quality because of resuspending nutrients during warm periods supports phytoplankton growth. Quantifying this sediment movement is a key element for evaluating the life and quality of a reservoir. This study targets reservoirs in the intermountain region of the U.S. These reservoirs are filled in the spring, then drawdown through the summer to provide irrigation water. Incoming sediment loads are generally restricted to spring high flows, with little new sediment entering the reservoirs during the remainder of the year. As the reservoirs undergo drawdown, the sediment deposited in the delta region during spring flows is re-cut from the exposed delta and moved into submerged delta region. The majority of flow and sediment movement both above and below the water surface occurs in channels cut into the sediments during spring deposition. During recutting, channels in the exposed sediments often move, but the submerged channels are more stationary. Traditional single-beam sonar surveys are performed on a grid and changes are used to quantify sediment movements. This approach is not applicable to delta recutting as the grid resolution is not sufficient to resolve the relevant changes that occur in the narrow excised flow channels. This study explores the ability to quantify and monitor sediment mass movement in Deer Creek Reservoir (DCR) using a single beam sonar. Our method uses surveyed cross-sections across the flow channels. It is difficult to position boat passes exactly on previous survey lines, and small location differences in an up-stream or down-stream location can be significant because of the slope of the channel. To address this, we surveyed each line in two directions, then interpolated both the position and elevation data. We performed periodic surveys over a two-month period. We were able to document and quantify both sediment deposition and erosion areas. As expected, sediment movement was from the inlet areas toward the reservoir. The data showed both deposition and erosion depending on the distance from the reservoir head, which changed over the survey period. This method can be used to quantify sediment recutting and resuspension that can affect nutrient loads during critical warm, low-reservoir conditions, but is difficult to implement accurately.