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Title: Groundwater seepage mechanisms of streambank erosion and failure

item FOX, GAREY - Oklahoma State University
item Wilson, Glenn
item MIDGLEY, T - Oklahoma State University
item ALMADHHACHI, A - Oklahoma State University
item CARSON, R - Oklahoma State University

Submitted to: ASABE Annual International Meeting
Publication Type: Proceedings
Publication Acceptance Date: 8/11/2012
Publication Date: 9/18/2012
Citation: Fox, G.A., Wilson, G.V., Midgley, T., Almadhhachi, A., Carson, R. 2012. Groundwater seepage mechanisms of streambank erosion and failure. International Symposium on Erosion and Landscape Evolution, ASABE Specialty Conference, Anchorage, Alaska, Sept. 19-24, 2011. doi:10.13031/2013.39206.

Interpretive Summary: Less is known about the importance of groundwater seeping out of streambanks than other factors that cause streambanks to slough off into the stream. Studies done in fields and laboratories on erosion by seepage have shown that groundwater seepage is very important. Previous work showed that erosion by seepage results in holes in the streambank face and this undercutting of the bank results in its failure . The purpose of this research was to learn more details from laboratory and field tests to determine how erosion by seepage can combine with erosion by the flowing stream to cause streambanks to fail. This work included tests with boxes of soil to determine the amount and rate of seepage erosion as well as factors such as changes in soil water pressures and the speed of undercutting for a lot of different soil textures and other soil properties. Undercuts by seepage were found to only form in sands with a bulk density greater than 1.35 g/cm3 and in loamy sands with a bulk density greater than 1.50 g/cm3. The bulk density at which undercutting occurred increased as the silt and clay content increased. Measurements of the soil erosion properties were made using the Jet Tester, which shoots a stream of water into the soil while measuring how fast a hole erodes. In this study, a new small Jet Test device was used on a tube filled with soil in which seepage could be forced through the soil to measure the impact of seepage on the soil erosion properties. The property describing the ability of the soil to erode was greater when seepage was included than without seepage. Tests were also made in the field in which soil-water pressures in a streambank on Dry Creek, a tributary to Little Topashaw Creek, located in Chickasaw County, Mississippi, were made while the soil adjacent to the streambank had water injected into it to cause seepage. Rates of water flow out of the bank and erosion of the bank were also measured in four separate tests on three different seepage locations. Seepage caused the bank to quickly fail due to erosion resulting in undercuts. Erosion and bank failure were even more rapid when the soil that is deposited at the bottom of the bank from seepage is removed such as by the flowing water of the stream. These laboratory and field tests have shown the importance of seepage to bank failure. Groundwater factors can act over long periods of time to cause banks to fail between flow events under certain soil and water conditions.

Technical Abstract: The importance of groundwater seepage and pipeflow is unknown with respect to other fundamental processes of streambank erosion and failure, although seepage and pipeflow features are observed on streambanks throughout the world that span a range of geomorphologic conditions. Previous field and laboratory research on seepage erosion has demonstrated that groundwater seepage and pipeflow play an important role in the erosion and failure of streambanks. This previous research pointed to seepage forces and undercutting as causes, independent of fluvial forces, of bank failures in some stream systems. Specific seepage and pipeflow mechanisms that cause bank failure may never manifest themselves as transparent features on unstable banks. The objective of this research was to conduct more in-depth laboratory and field experiments to determine how groundwater flow mechanisms, potentially in combination with fluvial processes, affect the occurrence and timing of streambank erosion and failure. Current research activities include conducting three-dimensional soil column experiments to determine the occurrence and prevalence of different seepage erosion mechanisms (i.e., seepage gradient forces and undercutting) across a range of soil textures and cohesions and to identify typical undercut formations when seepage undercutting occurs. Seepage undercuts only formed in sands with a bulk density greater than 1.35 g/cm3 and in loamy sands with a bulk density greater than 1.50 g/cm3, and an increasing trend in the bulk density demarcation was observed with soils with greater silt and clay content. Usually the erosion rate of cohesive soils from fluvial forces is computed using an excess shear stress equation, dependent on two major soil parameters: the critical shear stress and the erodibility coefficient. A submerged jet test apparatus is one method for measuring these parameters. In this study, a new miniature version of the jet test device and a seepage column were utilized to measure the erodibility of cohesive soils influenced by seepage. Erodibility of cohesive soils exponentially increased at high seepage gradients. This research also monitored pore-water pressures in a streambank on Dry Creek, a tributary to Little Topashaw Creek located in Chickasaw County, Mississippi, before and during an induced seepage experiment. Seepage flow and erosion rates were also measured in four separate experiments on three seeps and demonstrated the capability of seepage to rapidly destabilize streambanks, especially when acting in concert with processes that remove deposited material from the seepage undercut. These laboratory and field experiments have further demonstrated the importance of considering seepage mechanisms relative to bank and hillslope stability. Groundwater forces can act over extended periods to destabilize banks between flow events, but specific seepage mechanisms become prevalent under certain streambank stratigraphy and hydrologic conditions.