EFFECT OF JETTIES ON EROSION RATES ALONG THE WABASH RIVER, NEW HARMONY, INDIANA

A permeable jetty system was installed in 1986 to control erosion along the eastern cutbank of the Lower Wabash River near New Harmony, Indiana. Composed of specially engineered cargo netting and steel poles, the jetties were designed to decrease flow velocity along the meander, causing sediment deposition in areas experiencing extensive erosion. Pre-jetty annual erosion rates, determined by comparing bank locations on 1959 and 1980 topographic maps, were calculated as 10.8 m/yr (226.5m/21 yrs) proximal to the jetties, and 4.8 m/yr (100.3 m/21 yrs) at downstream sites. Average erosion rates were determined for the interval 1980 to 2001, using GPS to document nine new bank locations on a 1980 topographic map. Annual erosion rates in the zone of protection, were determined to be 10.2 m/yr (214 m/ 21 yrs). Average erosion rates downstream of the jetties during the same interval were calculated as 13.1 m/yr (275 m/21 yrs). Downstream sites were monitored for erosion during the spring of 2001 using metal stakes at regular intervals. During this short time, the banks downstream from the jetties lost an average of 18.6 cm. Erosion along the outside of the meander bend occurs as normal flow undercuts the banks. However, during flooding groundwater sapping and piping contribute to bank erosion. The data suggest that the jetties control erosion locally by affecting the flow velocity. However, the jetties do little to inhibit sapping or piping, allowing some erosion to occur. At locations downstream, the structures may have accelerated erosion by causing the current to become sediment starved. The current attempts to transport sediment, which it erodes from the banks downstream. This activity, coupled with normal groundwater related erosion, is likely responsible for the large annual erosion rates observed at locations the jetties were intended to protect.