The influence of groundwater on surface flow erosion processes during a rainstorm

Abstract

Surface erosion rates on a disturbed natural soil in a 10 m indoor flume increased by an order of magnitude when a water table developed at a 10 cm depth during simulated rainstorms. Erosion rate increases did not correlate well with surface hydraulic flow conditions, and all significant erosion increases began before the full soil depth was saturated, before the water table reached the soil surface, and before seepage was possible. Groundwater influenced erosion processes primarily by increasing unsaturated pore-water pressures and decreasing soil shear strength in surface rainflow, rather than through the direct entrainment of soil particles by seepage flow.There was no unique morphologic expression of the influence of groundwater during a rainstorm. Subsurface processes influencing surface erosion were detected only by appropriate subsurface instrumentation, which included micropiezometers, tensiometers and time domain reflectometry. Erosion rate increases occurred all along the slope, and were not concentrated at the base of slope due to a seepage zone. Soil depth was crucial to determining surface erosion increase. It is likely that confusing trends in surface flow erosion rates in past studies have occurred due to unrecorded groundwater development or an emphasis on seepage effects. Groundwater must be monitored along hillslopes under all moisture and soil conditions in order to avoid misleading and inconsistent conclusions derived solely from surface flow or seepage data.

Modeling seepage erosion and bank retreat in a composite river bank

“…This increased pore water pressure was recognized as the most important factor for mass failure. Flume studies pointed out that erosion rates of the bank surface increase in magnitude when the unsaturated pore water pressure is near to saturation and soil strength is decreased (Owoputi and Stolte, 2001;Rockwell, 2002;Fox et al, 2006).…”

Headcut erosive regimes influenced by groundwater on disturbed agricultural soils

“…The wetting front reached the flume floor just before initial headcut formation in several experiments (Table 3). Rockwell (2002) shows that wetting fronts reaching an impermeable lower layer at a 10 cm depth in a soil column can increase surface pore-water pressures. Slightly increased pore-water pressures could have reduced surface soil shear strengths enough to contribute to early hydraulic headcut formation on the 5 Pontypool/Peel slopes in these experiments.…”

“…Bryan et al (1999) and Rockwell and Bryan (in press-b) found very little delay in measuring water tables with these micro-standpipes, and Rockwell (2002) used TDR data to ascertain that this Pontypool/Peel soil was not saturated throughout when initial water table rise occurred. Other studies have found sapping can cause severe headcut erosion, and some have found headcuts only formed with seepage, with no headcuts under ''free drainage'' (Huang and Laflen, 1996).…”

“…Rockwell (2002) shows how groundwater development increases soil pore-water pressures and decreases surface soil shear strength values, resulting in increased surface flow erosion on a Pontypool/ Peel soil at a 5 gradient. Although shear strength values were not measured locally at the headcut in these experiments, the processes indicated by Rockwell (2002) can be applied to accepted theories of headcut advance caused by increased surface flow energy at the overfall (DePloey, 1989;Hanson et al, 1997) and plunge-pool scour (Bennett, 1999). Decreased surface soil shear strengths likely have effects similar to increased surface flow shear stress.…”

“…Unfortunately, these experiments were conducted on a very shallow soil (3.5 cm), without subsurface data collection. Considerable improvements in automated subsurface instrumentation (Bryan et al, 1999) have since shown the influence of pore-water pressures, water tables, and other subsurface processes on surface flow erosion (Rockwell, 2002) and rill initiation . Bryan and Poesen's (1989) and Bryan's (1990) shallow soils were likely affected by groundwater development during the two to four hour rainstorms (Rockwell and Bryan, in press-a).…”