Polyacrylamide effects on critical shear stress and rill erodibility

Joel Ryan Peterson, Purdue University

Abstract

The effect of polyacrylamide (PAM) application rate on critical shear stress and erosion was determined for 7 different soils. At least four PAM application rates were tested on each of the soils, ranging from 0.1 to 80 kg ha−1. Flow rates ranging from 4 to 56 L min −1 were introduced to a 4.5 m long, 15 cm wide rill set to a 10% slope. Flow shear stress was estimated using the equation τ = R hγS0 and assuming constant 15 cm rill width. A nonlinear power-law model with a critical shear term best fit the data collected in this series of experiments. This model resulted in the least sums of squared errors (SSE) of any of the three models and r2 values were greater as well. Parameters for a power-law model without a critical shear term were found using three different regression methods, a nonlinear regression method, a weighted linear regression procedure on log-transformed data, and an unweighted linear regression on log-transformed data. The nonlinear method resulted in the second least SSE. Both regression methods on log-transformed data resulted in the greatest SSE. The linear excess shear model resulted in a slightly greater SSE and slightly poorer fit than either of the power models but was satisfactory. A two term multiplicative parameter was used to predict the detachment model parameters based on soil properties and PAM application rate in the form τc = τcu*τ cp, where the two terms on the right hand side of the equation are the baseline critical shear on untreated soil and a modification to account for the application of PAM, respectively. While both power models provided better fits to the data, the parameters for the linear excess shear model were better described by average soil properties and PAM application rate and resulted in less SSE than either power models. The erodibility factor, Kru, was estimated using sand and organic matter content. The effect of PAM on erodibility was modeled using the PAM application rate, the concentration of the PAM in solution, and representative aggregate diameter, d84. Critical shear stress on untreated soil was a function of bulk density and d16. The additional effect of PAM on critical shear stress was modeled using the clay content, PAM application rate, d16, and calcium content. Additional research is needed to determine whether these relationships are valid in natural erosion field conditions.

Degree

Ph.D.

Advisors

Flanagan, Purdue University.

Subject Area

Agricultural engineering|Hydrology|Soil sciences

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