Groundwater seepage mechanisms of streambank erosion and failure

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. For soils with greater clay content, seepage erosion undercuts may not occur under typical bulk densities. 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 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. 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 with higher seepage gradients. 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.