Seepage-Induced Streambank Erosion and Instability: In Situ Constant-Head Experiments

Abstract

The effects of seepage on streambank erosion and failure are less understood compared to fluvial processes, especially the linkage between surface water and groundwater mechanisms. Field data are needed to validate laboratory seepage erosion and instability conclusions and to understand how engineering tools and methods may be applied to field conditions. An innovative trench injection system was utilized to provide a constant head on a near-streambank groundwater system when filled with stream water. This research was performed on a streambank of Dry Creek, a deeply incised stream with near-vertical banks located in Mississippi. Experiments included installing a trench (2.8 m from the bank and 2 m below ground surface) and a network of tensiometers and observation wells to measure soil-water pressures and water table elevations. Bank stratigraphy consisted of a sloping, conductive loamy sand layer between cohesive streambank layers. Groundwater conditions were monitored during a series of induced-seepage experiments. The bank face was outfitted with a seepage collection device to measure seep flow rates and sediment concentrations. Seepage flow rates (as high as 0.4 L=min) and corresponding erosion rates (as high as 0.86 kg=min) were proportional to estimated hydraulic gradients in the near-streambank region and followed an excess flow rate equation. However, flow paths and hydraulic gradients were largely nonuniform due to local variability in streambank stratigraphy, suggesting difficulty when attempting to apply engineering analyses of bank erosion and stability for seepage processes without accounting for this heterogeneity. Seepage flow and erosion became restricted when small-scale bank failures due to undercutting blocked flow pathways and limited particle mobilization, termed temporary self-healing. Seepage erosion was shown to be an important mechanism of streambank failure, especially when acting in concert with fluvial erosion processes that prevent permanent self-healing of seeps.

Laboratory soil piping and internal erosion experiments: evaluation of a soil piping model for low‐compacted soils

“…The mechanistic model of Bonelli et al . () was developed with a focus on well‐compacted levees and dams, with limited research performed to evaluate the model in less compacted soils more typical of streambanks (Midgley et al ., , ).…”

Bank undercutting and tension failure by groundwater seepage: predicting failure mechanisms

“…Seepage velocity is given by Darcy's Law: where v s is the seepage velocity, K sat is the saturated hydraulic conductivity of the soil, i is the hydraulic gradient, and n is the porosity of the soil. Seepage forces are proportional to the hydraulic gradient ( i ): where τ s is the seepage stress, ρ is the density of the fluid, g is gravity, and d is the grain or aggregate diameter (Fox and Wilson, ; Fox et al ., ; Midgley et al ., ). Iverson and Major () noted that a seepage force vector is responsible for destabilizing hillslopes subjected to subsurface flow and claimed seepage forces played a bigger role on slope destabilization than excess soil–water pressures.…”

“…Along with pore‐water pressure effects, seepage gradient forces are exerted as seepage flows through a soil profile. Seepage can occur near the bank toe in streambanks that are uniform and at various elevations on the streambank in composite or layered streambanks (Fox et al ., ; Wilson et al ., ; Midgley et al ., ). Seepage velocity is given by Darcy's Law: where v s is the seepage velocity, K sat is the saturated hydraulic conductivity of the soil, i is the hydraulic gradient, and n is the porosity of the soil.…”

A Mechanistic Detachment Rate Model to Predict Soil Erodibility due to Fluvial and Seepage Forces

“…Recent studies have demonstrated the importance of groundwater seepage exfiltration and gradients on erosion and bank or hillslope failure (Fox et al, 2006;Fox et al, 2007;Wilson et al, 2007;Fox and Wilson, 2010). The intricate linkage between seepage and fluvial forces has recently been emphasized in field seepage experiments (Midgley et al, 2013). Seepage commonly occurs at the toes on streambanks and hillslopes where bank stored water returns to the streams following storm events.…”

“…Seepage commonly occurs at the toes on streambanks and hillslopes where bank stored water returns to the streams following storm events. Such locations on a bank are a critical location for creating geotechnical instability due to fluvial undercutting (Midgley et al, 2013).…”