Waves
Generated from River Traffic and Wind
Bhowmik and Demissie (1982) report excessive bank erosion along both the Illinois and Mississippi Rivers. Along some reaches of the Illinois River, as much as 75% of the stream banks are being eroded away. They attribute the cause of this erosion to the waves generated by river traffic and wind. A secondary factor is the drawdown of the water in the river channel that exposes the shore area and changes the flow characteristics of tributary streams.
The wave formation process of boats is complex. In brief, as a barge or boat moves at or near the surface of the water, it causes a disturbance that ultimately results in a change in pressure and water level. Waves are generated where there are changes in the boat’s hull geometry that causes disturbances in the flow field. As the boat moves forward, the energy is transferred to the water and moves away laterally. These waves move to shore, causing bank or shoreline erosion. Factors that influence the degree of erosion include the type and speed of boat (e.g., barge, tugboat, and powerboat), proximity of the boat to the shoreline, and the characteristics of the bank, including soil type and steepness.
Another factor affecting soil erosion is drawdown. As a boat moves forward, it pushes the water in front of it sideways and down under the boat. An open space is left behind the boat causing water to flow in from all directions to fill the void. Propellers also draw large amounts of water from beneath the boat. All of these changes result in a change in water elevation. As water levels drop, the boat also drops. This lowering of the boat is known as “squat”, and the drop of water level is known as the drawdown. This drawdown exposes the shore area to increased erosion (Bhowmik 1982).
Bhowmik and Demissie (1982) report that the maximum wave height due to river traffic ranged from 0.1 foot to 1.08 foot, while the maximum drawdown ranged from 0.05 to 0.69 foot. Wave height for wind-generated waves on the Illinois River range from 0.9 and 1.6 feet for 2-year and 50-year winds of 6-hour duration.
These researchers find that the observed and calculated waves generated by both river traffic and wind are significant enough to cause shoreline erosion on the Illinois River, although it is not possible to separate wind-generated from boat-generated erosion. Drawdown caused by loaded barges is also found to contribute to shoreline erosion.
A second study looks specifically at the impacts of waves generated from recreational traffic. This study looks at the waves generated from 246 runs made with 12 different boats at two sites. The results indicate that recreational boats can generate from 4 to 40 waves per event, with a median of between 10-20 waves. These waves can last from 6 to 40 seconds or more. Average wave heights for these controlled runs vary from 0.01 to 0.25 meters, with a median of 0.06 to 0.12 meter. The maximum wave height is as much as 0.6 meter. This study also demonstrates that sustained recreational boat traffic can create essentially continual waves. While shoreline erosion is not quantitatively calculated, it is clear that recreational traffic can generate waves sufficient in height and duration to cause erosion to shorelines with erodible soils.
Asplund (2000) provides a summary of other studies conducted to examine the relationship between waves from boats and shoreline erosion. One researcher, Nanson (1994) monitored bank erosion and wave characteristics from three ferryboats. Nanson finds that most of the measurements are positively correlated to rates of bank recession. Erosive wave heights are found to range from 30 and 35 cm (12-14 in.). Above these levels all bank sediments erode.
Apslund also reports on a study conducted by Johnson (1994) where iron stakes are placed along transects to monitor shoreline erosion on the Mississippi River. Over a 3.5 year time period, shoreline recession of up to 14 feet is observed in portions of the river channel with intense boat traffic. This compares to only 3 feet of bank recession where there is light boat traffic.
Johnson also reports the results of a study conducted on the Lower St. Croix National Scenic Riverway. He finds that of 14 sites examined, 9 have net erosion, 2 have net deposition, and 3 have no changes. When river traffic and foot-traffic trampling are examined, those sites with no boat waves or foot traffic on the bank have net deposition or no change to the shoreline. Erosion occurs at all sites with boat traffic and shoreline foot-trampling. Johnson finds that wave heights below 0.4 feet do not erode sediments, but there are greater amounts of erosion where more boat waves 0.4 feet and higher occur in a 30-minute period. Of the types of boats examined, runabouts and cruisers are found to have the highest wave heights and therefore the higher amounts of soil erosion.
Some conclusions can be drawn about the impacts of boat traffic on shoreline erosion from the limited data available. Waves produced by boats are the primary way in which boats influence shoreline erosion. Wave heights depend on the speed, size, and draft of a boat, but can reach heights of 40-50 cm (15-20 in.); however, waves dissipate rapidly as they move away from the boat, while wind-generated waves increase with larger distances. Based on these factors, river systems, channels connecting lakes, and small lakes are most likely to be affected by boat-induced waves because the boats may operate closer to shore and wind-induced waves are reduced.
