Effects of Spatial Variability on the Estimation of Erosion Rates for Cohesive Riverbanks

Human activities, such as dam construction, can cause excessive erosion of a riverbank due to alteration of the natural flow regime. This erosion can lead to degradation in water quality and aquatic habitat as well as loss of land and damage to riparian structures. Accurate estimation of erosion rates for cohesive riverbanks is a difficult task owing to the complex nature of cohesive soils and their interaction with the river flow. Past work has attempted to correlate physical and chemical soil properties with ero-sion rates, however the wide range of soil properties has made it difficult to produce a general model for cohesive soil erosion. The erodibility of riverbank soil on the lower Roanoke River, North Carolina USA, was estimated in situ employing the jet erosion test (JET) apparatus. The JET applies a water jet of uni-form velocity directly to the soil and measures the resulting scour depth evolution over time. The meas-ured data are then used to determine two empirical parameters, the erodibility coefficient and critical shear stress. Several JETs were performed on different soil layers at sites with bank materials composed primarily of silts and clays. The physical properties of each soil layer were determined through laboratory analysis. Using boundary shear stress calculations from a simple analytical model, erosion rates are calcu-lated assuming a linear excess shear stress model, and the effects of the spatial variability of soil proper-ties on calculated erosion are investigated. While previous research has shown that the JET produces con-sistent results under controlled environments, results here demonstrate the importance of considering spatial variability of soil types when estimating erosion rates.