David H. Schoellhamer
Paul A. Work
2018
<div><div><span>This report describes work performed to quantify the </span><span>erodibility of surface soils in the Yolo Bypass (Bypass) near </span><span>Sacramento, California, for use in the California Department </span><span>of Water Resources (DWR) Yolo Bypass D-MCM mercury </span><span>model. The Bypass, when not serving as a floodway, is heavily </span><span>utilized for agriculture. During flood events, surface water </span><span>flows over the soil, resulting in the application of a shear stress </span><span>to the soil. The shear stress is a function of flow speed and </span><span>is often assumed to vary as the square of flow speed. Once </span><span>the shear stress reaches a critical value, erosion commences, </span><span>and the erosion rate typically increases with applied shear </span><span>stress. The goal of the work described here was to quantify </span><span>this process and how it varies throughout the major land uses </span><span>found in the Yolo Bypass.</span></div><div><span><br></span></div><div><span>Each of the major land uses found in the Bypass was </span><span>targeted for sediment coring and two side-by-side cores, </span><span>10 centimeters in diameter, were extracted at each site for </span><span>testing in a Gust erosion chamber. This device consists of a </span><span>cylinder with a piston and cap installed to contain a sediment </span><span>sample and overlying water. In most instances, coring was </span><span>done with the cylinder, the piston and cap were installed, and </span><span>testing commenced immediately. The cap at the top of the </span><span>cylinder contains vanes to induce rotation of the flow and is </span><span>driven by an electric motor, simulating the bed shear stress </span><span>experienced by the soil in a flood event. Ambient water is </span><span>introduced to the cylinder, passes through the device, and </span><span>carries eroded sediment out of the chamber. The exiting water </span><span>is tested for turbidity, and water samples obtained to relate </span><span>turbidity to suspended sediment concentration are used to </span><span>compute erosion rates for each of the applied shear stresses.</span></div><div><span><br></span></div><div><span>The result for each sediment core is (1) definition of the </span><span>critical shear stress required to initiate sediment erosion and </span><span>(2) estimation of coefficients required to relate erosion rate </span><span>to applied shear stress once this critical shear-stress threshold </span><span>has been exceeded. These quantities were computed for each </span><span>of the sites sampled. In total, 10 locations were sampled, </span><span>representing 10 land uses ranging from wild and white rice </span><span>fields to the flooded Liberty Island and the Toe Drain that </span><span>receives runoff from much of the cultivated land (table 1).</span></div><div><span><br></span></div><div><span>The Gust chamber test causes the erosion of a very small </span><span>layer of sediment, typically less than a millimeter thick. The </span><span>strength of the soil within this layer increases with depth, </span><span>typically, and this soil strength versus depth is measured in the </span><span>testing process.</span></div><div><span><br></span></div><div><span>Results for each land use type tested are presented as the </span><span>initial critical shear stress at which erosion began and the rate </span><span>at which erosion increases as shear stress increases (table 2). </span><span>Of the land use types sampled, irrigated pasture displayed </span><span>the lowest critical shear stress, meaning that it required the </span><span>smallest flow speed to initiate erosion. But in this case, the </span><span>rate of increase of the subsequent erosion, given higher flow </span><span>speeds, was small. The wild rice field samples exhibited a </span><span>higher critical shear stress but also exhibited a much higher </span><span>erosion rate once the critical shear stress was exceeded. The </span><span>erosion rate for wild rice was about three times greater than </span><span>that for white rice. Bear in mind that these results are based on </span><span>only two cores tested per site, and variability between fields </span><span>with the same crop could be significant. Approved digital data </span><span>can be viewed and downloaded from ScienceBase, at </span><span><a href="https://doi.org/10.5066/F7BV7DQC" target="_blank" data-mce-href="https://doi.org/10.5066/F7BV7DQC">https://doi.org/10.5066/F7BV7DQC</a>. These results are being </span><span>used to calculate erosion rates in the DWR Yolo Bypass </span><span>D-MCM mercury model.</span></div><div><span><br></span></div><div><span>The Toe Drain was very difficult to sample, owing to </span><span>hard, consolidated sediments on the channel bed. On the </span><span>first visit, two cores were obtained successfully, and testing </span><span>revealed very different results. A second visit was made, but </span><span>it was not possible to obtain cores suitable for testing with the </span><span>coring equipment used. The available results suggest that Toe </span><span>Drain soil is highly erodible (low critical shear stress and high </span><span>erosion rate once initiated) despite being difficult to sample. </span><span>As a collector of runoff, it also has the potential to accumulate </span><span>soils eroded from adjacent areas, subsequent to storm events, </span><span>as flows subside. This deposited material will typically be </span><span>more erodible than the material that it lands on. The deposition </span><span>and resuspension of material was not simulated in the testing </span><span>described here because the applied shear stress increases </span><span>monotonically during testing.</span></div></div><div><span><br></span></div><div><div><span>The spatial distribution of mean grain size, loss on </span><span>ignition, and percent fines of Yolo Bypass soils are also </span><span>presented. Sediment sampling for this effort was performed </span><span>by DWR; the U.S. Geological Survey (USGS) performed </span><span>the sample analysis. These data should thus be considered </span><span>provisional, but the remainder of the data presented here, and </span><span>this report, have been through the formal U.S. Geological </span><span>Survey review process.</span></div><div><span><br></span></div><div><span>A separate effort has been made by others to develop </span><span>numerical model results defining the spatially varying, time-dependent </span><span>hydrodynamics in the Yolo Bypass. These model </span><span>results are being used to quantify shear stress on the soil </span><span>surface, which together with the Gust chamber results shown </span><span>here, are used for the DWR Yolo Bypass D-MCM mercury </span><span>transport model to compute erosion rates for each time step.</span></div><div><span><br data-mce-bogus="1"></span></div></div>
application/pdf
10.3133/ofr20181062
en
U.S. Geological Survey
Measurements of erosion potential using Gust chamber in Yolo Bypass near Sacramento, California
reports