WAVES GENERATED BY RIVER TRAFFIC AND WIND ON THE ILLINOIS AND MISSISSIPPI RIVERS.

Abstract

An excessive amount of bank erosion along a number of waterways in Illinois and surrounding states exists at the present time. Erosion of stream banks attracts public attention, reduces property values, results in the permanent loss of real estate, increases the turbidity of streams, and accelerates the silting of reservoirs and backwater lakes along stream courses. Among the main causes of bank erosion along navigable rivers are waves generated by river traffic and wind. Also associated with river traffic is the drawdown of the water level in the channel, which exposes shore area and changes the flow characteristics of tributary streams close to their outlets. To investigate and collect data on waves and drawdown associated with river traffic on the Illinois and Mississippi Rivers, six field trips were taken to four test sites. The maximum wave heights measured in the field ranged from a low of 0.1. foot to a high of 1.08 foot, while the maximum drawdown ranged from 0.05 foot to 0.69 foot. The measured maximum wave heights and drawdowns were compared.to those expected on the basis of existing predictive equations and the correlations were found to be low. Multivariate regression analyses between the measured values and the important hydraulic and geometric parameters which were felt to influence the generation of waves and drawdowns resulted in two equations which predict maximum wave heights and drawdowns fairly well. Significant wave heights for wind-generated waves were also calculated. On the Illinois River significant wave heights were in the range of 0.9 and 1.6 ft for 2-yr and 50-yr winds of 6-hr duration, respectively, while on the Mississippi River the corresponding values were 1.3 and 2.4 ft. In general, the observed and calculated waves generated by both river traffic and wind are significant enough to cause stream bank erosion along the Mississippi and Illinois Rivers. The relative significance of waves generated by river traffic in comparison with those generated by wind cannot be determined qualitatively at the present time because of the differences in frequency, duration, and magnitude. Further research is needed. The drawdown caused by loaded tows on the two rivers is also significant, exposing shore areas and changing the flow characteristics of small tributary streams during each tow passage.