Numerically predicting seepage gradient forces and erosion: Sensitivity to soil hydraulic properties

Abstract

s u m m a r yResearch has suggested that streambank seepage can be an important mechanism of bank instability; however, limited information is available on the level of soil characterization necessary to accurately predict seepage gradient forces and erosion. The objective of this research was to quantify the expected range of predicted seepage gradients for various degrees of site characterization. Uncertainty analysis on seepage gradient predictions was performed relative to variability in soil hydraulic properties. A two-dimensional unsaturated/saturated groundwater flow model was used to simulate a homogeneous soil layer for sand and loamy sand soils packed at various bulk densities, q b . A pedotransfer function (ROSETTA), designed to estimate unsaturated hydraulic properties from surrogate soil data (i.e., texture and bulk density), was used to derive the saturated hydraulic conductivity, K s , and water retention parameters for various levels of site information (i.e., only textural class; percent sand, silt, and clay (%SSC); %SSC and q b ; and %SSC, q b , and K s ). Statistical distributions were derived for each soil hydraulic parameter and Monte Carlo simulations were performed to generate distributions of maximum seepage gradient. The deviation in predicted seepage gradient was calculated using assumed baseline conditions. Ranges in predicted soil hydraulic parameters and maximum seepage gradients were considerably reduced when using %SSC as compared to soil texture. Therefore, at a minimum, soil samples should be taken for particle size analysis. For q b between 1450 and 1500 kg m À3, soil hydraulic parameters could be derived using ROSETTA and inputting %SSC, with little additional benefit provided by measuring q b and/or K s . When the q b was less than 1450-1500 kg m À3, inputting q b and/or K s consistently reduced the magnitude of deviations from the baseline and therefore should be measured from undisturbed soil samples. The opposite was observed for q b greater than 1450-1500 kg m À3 due to discrepancies between ROSETTA-derived and actual values of soil hydraulic parameters other than K s . Considerable deviations (i.e., around 20%) were observed in seepage gradients under this scenario. When ROSETTA-derived and actual values of soil hydraulic parameters more closely matched, inputting q b and/or K s benefitted seepage gradient predictions as deviations in seepage gradients were less than 5% for the sand and loamy sand soils. Therefore, it is vital to quantify all soil hydraulic parameters for high q b soils and textures with a wide range in %SSC.

Assessing the Effects of Rainfall Intensity and Hydraulic Conductivity on Riverbank Stability

“…The change in FOS was nearly the same as the change in RWL, which meant that the FOS increased with the increase in RWL, and the FOS decreased with the decrease in RWL. These results agreed with those of previous studies [39][40][41][42], which studied the effect of RWL on FOS. When the riverbank had a soil hydraulic conductivity lower than 10 −4 cm/s, the FOS always depended on the change in RWL because of confining pressure [42].…”

“…A riverbank is a special slope that relates closely to hydraulic dynamics, not only from rainfall infiltration from the surface concerning the space above the slope, but also significantly from the fluctuation of the river water level and groundwater table [27,[37][38][39][40][41][42][43]. Past research works assessed the effects of rainfall on riverbank stability using the indirect process of transient seepage due to water level changes and assumed that rainfall has no effect on the riverbank surface.…”

Oblique aggradation: a novel explanation for sinuosity of low‐energy streams in peat‐filled valley systems

“…Groundwater seepage can occur in the valley, because the sandy valley side is conductive compared with the underlying tills. Some studies have shown that seepage is an important factor in bank erosion, resulting in unstable banks which can initiate and promote lateral channel migration (Fox et al, 2007(Fox et al, , 2010Van Balen et al, 2008;Eekhout et al, 2013). Seepage does not initiate meandering in the Drentsche Aa, but it may enhance the process of oblique aggradation by lowering the resistance to erosion of the valley side.…”

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.…”