Comparison of test methods for erodibility of bank materials on the Lower American and Sacramento Rivers, adjacent to the City of Sacramento, California

Authors: Ursic, Michael - Mick; Langendoen, Eddy

Submitted to: Technical Report
Publication Type: Government Publication
Publication Acceptance Date: 4/1/2021
Publication Date: 4/1/2021
Citation: Ursic, M.E., Langendoen, E.J. 2021. Comparison of test methods for erodibility of bank materials on the Lower American and Sacramento Rivers, adjacent to the City of Sacramento, California. Research Report No,. 81. U.S. Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory, Oxford, MS. 150 pp..

Interpretive Summary: The assessment of cohesive bank-soil erosion utilizes two soil erosion-resistance parameters: critical shear stress and erodibility coefficient. A variety of field and laboratory resistance-to-erosion measurement techniques (REMTs) have been used to characterize soil erosion resistance, most commonly: Jet Erosion Test (JET), Erosion Function Apparatus (EFA) and the Borehole Erosion Test (BET). Unfortunately, erosion-resistance values derived with the various measurement methods generally differ widely for the same soil. ARS scientists in Oxford, MS, compared the soil erosion-resistance parameters derived by the JET, EFA, and BET REMTs and grouped by Unified Soil Classification System (USCS) soil types for bank soils on the American and Sacramento Rivers adjacent to the City of Sacramento. It was found that the discrepancies between REMTs are primarily caused by: derived erosion-resistance parameters representing different erosion regimes, and not accounting for surface roughness. After adjusting measured erosion-resistance parameters for these differences, both the distributions of critical shear stress and erodibility coefficient determined by the EFA and JET methods compared well for silts and silty sands. The study's findings are used by the US Army Corps of Engineers to combine data from several REMTs into distributions of erosion-resistance parameters by soil type. These distributions are used to perform probabilistic simulations with the ARS Bank Stability and Toe Erosion Model (BSTEM) to prioritize bank stabilization measures along reaches of the American and Sacramento Rivers to protect the City of Sacramento as authorized by the Water Resources Development Act of 2016.

Technical Abstract: The Water Resources Development Act (WRDA) of 2016 provides for engineering and design activities associated with implementing projects to reduce the flood risk on the Lower American River and the Sacramento River near Sacramento, California. The integrity of levees that protect Sacramento could be adversely affected by bank erosion. The rate of bank erosion depends, among others, on the resistance-to-erosion properties of the bank soils. The U.S. Army Corps of Engineers (USACE) Sacramento District requested both the U.S. Department of Agriculture (USDA), Agricultural Research Service, National Sedimentation Laboratory, U.S. Geological Survey (USGS) California Water Science Center, and Texas A&M University (TAMU) to use a variety of field and laboratory resistance-to-erosion measurement techniques (REMTs) to characterize the erodibility of soils in the study area. Methods used include the Jet Erosion Test (JET, NSL), Erosion Function Apparatus (EFA, TAMU) and the Borehole Erosion Test (BET, USGS & TAMU). The tests were performed during the period 2018-2020. The collected data were supplemented with JETs and EFAs conducted by USACE Engineer and Research Development Center, Vicksburg, MS, from samples collected in 2011. The purpose of this report is to compare the soil erosion-resistance parameters derived by the JET, EFA, and BET REMTs and grouped by Unified Soil Classification System (USCS) soil types. The main findings of comparing soil erosion-resistance characterized by BET, EFA, and JET methods for various soils along the Lower American River and Sacramento River are: 1. The BET method provides a similar range in critical shear stress as the EFA and JET methods, but the corresponding erodibility coefficients are typically two orders of magnitude smaller than those provided by the EFA and JET methods. This is likely caused by errors/uncertainty in measuring the erosion rate of the borehole wall and estimating the applied shear stresses acting on the borehole wall. 2. The erosion-resistance parameters determined by the EFA and JET methods have similar magnitude and cluster by soil type when plotted on an erodibility coefficient versus critical shear stress chart. Both measurement techniques show the more cohesive the soil, the more erosion-resistant it is; that is greater critical shear stress and lower erodibility coefficient. 3. The distribution of erodibility coefficients determined by the EFA and JET methods compare well for silts and silty sands. The distributions of critical shear stress determined by the EFA and JET methods compare reasonably well for silts and silty sands. The critical shear stresses determined by the EFA method are about half those determined by the JET method. Comparison of the measured erosion-resistance parameters against those calibrated using the BSTEM bank erosion model against observed bank erosion along the Lower American River shows: 1. Erosion-resistance parameters for silts only need little calibration when used in bank erosion modeling. 2. Erosion-resistance parameters for less cohesive and cohesionless soils, such as silty sands and sands, need careful calibration when used in bank erosion modeling, because the simulated rate of erosion is sensitive to the model’s bank shear stress approximation and transport-capacity limitation effects on entrainment.