Analysis and evaluation of round hay bale breakwaters at Lake Sharpe, South Dakota

Abstract

Lake Sharpe in central South Dakota is one of three reservoirs constructed along the Missouri River by the U.S. Army Corps of Engineers in the 50's and 60's. The Lake is underlain by Cretaceous Pierre shale and various Quaternary glacial deposits including gravel, sand, clay, and loess. Shoreline erosion along much of the 200 mi shoreline has not significantly decreased through the years, especially in the areas underlain by finer grain-size materials. This erosion is threatening and destroying Government and private lands, structures, and archaeological sites. The addition of sediments from this erosion is reducing the design life of the reservoir as well as decreasing the water quality and lake bioproductivity. In a few critical areas the standard solution of an armored revetment of stone riprap has successfully halted this erosion, but it is too costly a method for general use in an area without a plentiful source of suitable stone. Because of the high cost of traditional methods, more economical alternatives are being sought. The Corps has experimented with using local materials such as driftwood logs and hay bales to prevent this erosion and allow the establishment of protective shoreline vegetation. The round hay bales were placed offshore as a breakwater to block the waves and cause sedimentation. This creates a suitable substrate for the establishment of shoreline vegetation which will act to protect the shore after the hay bales begin to deteriorate. The mechanisms of erosion and the performance of these structures was studied in the field. The addition of the hay bales decreased the dissolved oxygen locally up to 50% but otherwise did not appear to be harming the lake water. The hay bale breakwaters had a number of problems such as early disintegration and placement too close to the bank. It is recommended that the bales be placed further offshore and that an aggressive revegetation program be implemented following bale placement, possibly in conjunction with a controlled drawdown of the reservoir and/or sediment placement.