Erodibility of bank materials on the Lower American and Sacramento Rivers, adjacent to the City of Sacramento, California

Authors: Langendoen, Eddy; Ursic, Michael - Mick

Submitted to: Laboratory Publication
Publication Type: Government Publication
Publication Acceptance Date: 5/1/2020
Publication Date: 5/1/2020
Citation: Langendoen, E.J., Ursic, M.E. 2020. Erodibility of bank materials on the Lower American and Sacramento Rivers, adjacent to the City of Sacramento, California. Laboratory Publication. Research Report No. 80, U.S. Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory, Oxford, MS, 138 pp.

Interpretive Summary: The assessment of cohesive bank-soil erosion utilizes two soil erosion-resistance parameters: critical shear stress and erodibility coefficient. The magnitude of the parameters is highly site-specific and is sensitive to small variations in soil physical properties, which introduces uncertainty in computed bank erosion rates. ARS scientists in Oxford, MS, collaborated with the US Army Corps of Engineers (USACE), Sacramento District to quantify the range in erosion-resistance parameters of bank soils on the American and Sacramento Rivers adjacent to the City of Sacramento. Soils were tested in situ using a Jet Erosion Test to characterize soil erodibility. The measurements were used to derive distributions of critical shear stress and erodibility coefficient for the following soil textures: silts, sandy silts, and silty sands. The distributions will be used by USACE 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 and Sacramento Rivers 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 and grain-size distribution of the bank soils. At the request of the U.S. Army Corps of Engineers, Sacramento District, the US Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory, Oxford, Mississippi characterized soil texture and conducted jet-erosion testing (JET) for resistance-to-erosion properties (critical shear stress and erodibility coefficient) of the bank material along the Lower American and Sacramento Rivers. Ten study sites on the Lower American River and eight study sites on the Sacramento River were selected by the U.S. Army Corps of Engineers, Sacramento District. At these sites bank soil stratigraphy was characterized, and soil samples were collected for grain-size and bulk density analysis in the laboratory facilities of the National Sedimentation Laboratory. JETs were conducted using a mini-jet testing device on soils that were cohesive. Based on the Unified Soil Classification System the bank soils at the study sites were classified as: poorly graded sand (SP), poorly graded sand with silt (SP-SM), silty sand (SM), and silt (ML). The median grain size ranged from 0.032 mm (0.00126 in) to 0.285 mm (0.0112 in) with an average value of 0.121 mm (0.00476 in). Dry bulk density varied between 0.95 g cm^-3 (59.3 lb ft^-3) and 1.39 g cm^-3 (86.8 lb ft^-3). At higher applied shear stresses, that is for the mass erosion regime, measured critical shear stresses of the bank soils varied between 0.408 and 24.7 Pa (0.00852 and 0.516 lbf ft^-2). The corresponding erodibility coefficients varied between 0.835 and 27.2 cm hr^-1 Pa^-1 (1.31 and 42.7 ft^3 lbf^-1 hr^-1). These bank soils are therefore classified as erodible to very erodible.