A sinuous gravel-bedded river with frequent bedrock exposures: The statistics of its planform compared with a freely meandering river and the suitability of a processed-based hydraulic model predicting its erosion.

摘要

The study area of the South River, Virginia, a sinuous, gravel-bedded river influenced by frequent bedrock exposures, consist of sections of exposed bedrock (either on the bed and/or in the banks), and islands totaling 65% of the channel length. The remaining 35% of the study reach is alluvial and is scattered between the sections of bedrock and islands. Sinuosity values of the alluvial sections can be as high as 2.5. At the map scale of 1:150,000 the South River appears to be a meandering river. However, when its planform statistics are compared to a freely meandering river (the Teklanika River, Alaska), systematic differences become apparent. The South River's planform consists of shorter bend lengths, longer radii of curvatures, a distinctive meander wavelength spectrum where the dominant wavelength is not the longest, and a lower fractal dimension of D = 1.11 (compared with the Teklanika's D = 1.4). It is within these sections that the river displays significant migration when analyzed with aerial photographs covering the period 1937 to 2005. Driven by the need to quantify the flux of mercury-contaminated sediments entering the river through bank erosion, the bend migration model of Johannesson and Parker (1989) is applied. The model, when schematized to suit the hydraulics of the alluvial sections of the South River and calibrated using the observed areal erosion along six selected bends, correctly predicts 45% of the observed locations of erosion as mapped from the aerial photographs and observed in the field. The total area of erosion computed using the predicted near bank excess velocity fell within 20% of that mapped. According to a non-parametric Kruskal-Wallis test, the predicted areas of erosion at scales of reach lengths along the river are like from the same distribution of observed erosion with P = 0.30 (alpha ≤ 0.05, two tailed test). These results show promise in that one may be able to at least apply curvature dependent hydraulic models to predict the total erosion of alluvium in rivers like the South River. However, more work is needed to improve the model's ability to predict the precise location of erosion.