Effects of May through July 2015 storm events on suspended sediment loads, sediment trapping efficiency, and storage capacity of John Redmond Reservoir, east-central Kansas

Abstract

The Neosho River and its primary tributary, the Cottonwood River, are the main sources of inflow to John Redmond Reservoir in east-central Kansas. Storage loss in the reservoir resulting from sedimentation has been estimated to be 765 acre-feet per year for 1964–2014. The 1964–2014 sedimentation rate was almost 90 percent larger than the projected design sedimentation rate of 404 acre-feet per year, and resulted in a loss of about 40 percent of the original (1964) conservation (multi-purpose) pool storage capacity. To help maintain storage in the reservoir, the Kansas Water Office has implemented more than two dozen stream bank erosion control projects to reduce the annual sediment load entering the reservoir and initiated a dredging project to restore nearly 2,000 acre-feet of storage near the dam to provide additional water supply to downstream water users. Storm events during May through July 2015 caused large inflows of water and sediment into the reservoir. Initially, flood waters were held back in the reservoir in order to decrease downstream flooding in Oklahoma. Later, retained reservoir flood waters were released at high rates (up to 25,400 acre-feet per day, the maximum allowed for the reservoir) for extended periods. The U.S. Geological Survey, in cooperation with the Kansas Water Office, computed the suspended-sediment inflows and retention in John Redmond Reservoir during May through July 2015. Computations relied upon previously published turbidity-suspended sediment relations at water-quality monitoring sites located upstream and downstream from the reservoir. During the 3-month period, approximately 872,000 tons of sediment entered the reservoir, and 57,000 tons were released through the reservoir outlet. The average monthly trapping efficiency during this period was 93 percent, and monthly averages ranged from 83 to 97 percent. During the study period, an estimated 980 acre-feet of storage was lost, over 2.4 times the design annual sedimentation rate of the reservoir. Storm inflows during the 3-month analysis period reduced reservoir storage in the conservation pool approximately 1.6 percent. This indicates that large inflows, coupled with minimal releases, can have substantial effects on reservoir storage and lifespan.