HighlightsThe JET is a key instrument for in situ and laboratory measurement of soil erodibility.Operation and reporting guidelines are needed to ensure consistency across JETs and applications.JET design improvements and hydrodynamic studies are needed to inform proper analyses and limit operator effects.Erodibility databases should be developed that report JET, soil, and fluid properties. Keywords: Cohesive soils, Critical shear stress, Erodibility, Erosion, Jet Erosion Test, Scour.
Flume tests and in situ jet erosion tests (JETs) allow for the process-based quantification of erodibility parameters of cohesive soils. In the excess shear stress model used to predict soil detachment, the critical shear stress (τ c ) corresponds to the stress at which fluvial forces can detach soil particles or aggregates and the erodibility coefficient (k d ) governs the rate at which detachment occurs when the imposed shear exceeds τ c . The primary objective of this research was to derive and compare erodibility parameters from flume tests and JETs on various root-permeated soil samples. Based on statistical analysis, erodibility coefficients from the flume tests were typically statistically similar to those derived with JETs. In order to further determine the capability of the JET to indicate differences in erodibility of root-permeated soils, this research then performed JETs on side-by-side in situ soils. The JETs on root-permeated soils estimated significantly higher τ c but insignificant differences in k d compared to JETs on adjacent bare soil. A significant correlation was observed between exposed root surface area (TRSA) and τ c , but no correlation between TRSA and k d or the equilibrium scour depth was observed. More flume and JET experiments are needed to clarify the relationship between root-permeated soils and erodibility parameters, and address a wider range of average root diameters than was considered in this research.
The objective of this study was to assess the performance of form-based and process-based models, and of local-scale and reach-scale models, used to examine bank retreat and sediment transport in stream restoration. The evaluated models were the Bank Erosion Hazard Index (BEHI), Bank Assessment for Nonpoint Source Consequences of Sediment (BANCS), Bank Stability and Toe Erosion Model (BSTEM), and HEC River Analysis System (HEC-RAS 1D). Model-to-model assessments were conducted to quantify the impact of model selection when predicting applied stress and geomorphic change in a restored stream in North Carolina, USA. Results indicated that the mobility of the bed dictated model selection at the reach-scale. The process-based HEC-RAS 1D was needed to accurately analyze the sand-bed stream, predicting amounts of geomorphic change comparable to measured data and up to three orders of magnitude higher than those from local-scale models. At the local-scale, results indicated that the bank retreat mechanism and flow variability constrained model selection. The form-based BEHI and BANCS did not directly account for geotechnical failure nor capture severe floods, underpredicting amounts of geomorphic change by an order of magnitude when compared to the process-based BSTEM, and failing to characterize erosion potential and applied stresses after short-term morphodynamic adjustments.
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