Desktop Method for Estimating Vessel-Induced Sediment Suspension

Riverbank erosion involves engineering and environmental concerns. Among several dominant mechanisms of riverbank erosion, navigation effects caused by the passage of vessels are quite important. Field data indicate that large vessels generate large drawdown and small wave heights, whereas small vessels such as pleasure craft generate small drawdown and large wave heights. Passage of both types of vessels may result in bank erosion and a substantial increase in suspended-sediment concentration. Since comprehensive numerical modeling is time-consuming, a desktop computational approach has been offered that helps in providing preliminary answers to several questions related to vessel-induced sediment resuspension. Due to the uncertainties inherent in sediment calculations, order-of-magnitude values are often adequate for decision making. A preliminary assessment of possible impact on vegetation or benthic organisms can then be made based on this information. PC-based FORTRAN programs were developed for (1) computation of time series of vessel-induced waves; (2) erosion and deposition of cohesive sediment under waves and nearshore currents; and (3) computation of noncohesive suspendedsediment concentration caused by river current alone. An application to the Upper Mississippi River Navigation Study is presented. The riverbank sediments were labeled as soft, medium, and hard using their erodibility characteristics. Three water depths (0.5, 1.0, and 1.5 m) were considered. Vessel-induced wave heights ranged from 10 to 60 cm. The maximum concentrations were obtained in 0.5 m water depth with a wave height of 30 cm and in 1.0 m depth with a wave height of 60 cm. The estimated maximum concentrations were 1,463, 56, and 4 mg/L for soft, medium, and hard beds, respectively. Such results will be useful for preliminary assessment of relative impact of increased barge traffic in the river and identification of potential areas along the riverbanks that are likely to be sensitive from the point of view of environmental considerations.