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Deriving Erodibility Parameters of a Mechanistic Detachment Model for Gravels

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Transactions of the ASABE. 59(1): 145-151. (doi: 10.13031/trans.59.11490) @2016
Authors:   David T. Criswell
Keywords:   Erodibility, Excess shear stress model, Fluvial resistance, Noncohesive soil, Wilson model.

Abstract. Recent research has proposed the use of a mechanistic detachment model, called the Wilson model, in place of the excess shear stress equation for predicting the detachment of cohesive soils during erosion. However, this mechanistic detachment model was also proposed as being valid for noncohesive soils but with limited evaluation. Such erodibility parameters are often needed in erosion models developed using a single detachment rate approach for both cohesive and noncohesive soils. Therefore, the objectives of this research were to evaluate the applicability of the Wilson model for noncohesive soils and to evaluate a procedure for deriving the erodibility parameters (b0 and b1) from flume experiments. Gravel samples were extracted from composite streambanks on the Barren Fork Creek in eastern Oklahoma. The samples were sieved into particle size classes, and then at least triplicate flume experiments were performed (gravel sizes of 0.45, 0.60, 1.30, and 1.90 cm). Flow rate, water surface elevation, and scour depth were measured to estimate the energy slope, scour rate, and effective shear stress. The Wilson model was fit to the scour depth data to derive b0 and b1 using the generalized reduced gradient method to minimize the error between the predicted and measured scour. Constraints were required within the solver routine to limit potential solutions of b1. Similar to cohesive soils, b0 and b1 had similar relationships to but different magnitudes than the erodibility coefficient (kd) and critical shear stress (τc) for these gravels. Equivalent b1c relationships were derived from the flume tests as compared to the theoretical b1c relationship in the Wilson model. The b0-b1 and kdc relationships followed power law relationships. This research supports the applicability of the nonlinear mechanistic detachment model for noncohesive gravels.

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