Erosion of cohesionless sediment by groundwater seepage

Abstract

Surface grains of noncohesive sediment eroded by emerging groundwater are acted upon by three forces, the tractive force of the cumulative surface flow contributed by upslope seepage, the local seepage force, and gravity. The balance of the force moments determines the mode and rate of transport. Seepage. forces are strong in a narrow "sapping zone" at the upstream end of the emerging flow, where erosion occurs by mass movement and the surface gradient is determined by the balance of the seepage and gravity moments. Most of the erosion occurs in this zone, and the resultant backcutting triggers intermittent failure of overlying slopes in a "undermining zone" maintained at the angle of repose of the dry or damp sediment. In the "fluvial zone" downstream from the sapping zone the seepage force is small compared to the tractive force, and transport occurs by normal fluvial traction. The overall rate of sapping erosion in noncohesive sediments is determined by the capacity of fluvial transport to remove sediment eroded in the sapping zone. Prediction of sapping rates is complicated by the interaction between the geometry of the fluvial and sapping zones and the quantity and spatial distribution of seepage. A simulation model incorporating a groundwater flow model and sediment transport relationships closely replicates the observed evolution of sapping erosion in a two-dimensional tank filled with noncohesive sand subjected to lateral groundwater flow. INTRODUCTION This paper examines the erosion of noncohesive sediments by groundwater seepage and outflow using experiments and theoretical modeling. In recent years geomorphologists have realized that erosion by emerging groundwater is important in many landscapes. The earliest quantitative geomorphic studies, such as those of Horton [ 1933, 1945•, followed the conventional views that overland flow erodes slopes and channels. By the mid-1960s the recognition grew that overland flow is not universally important. Kirkby and Chorley [1967• suggested that throughflow dominates hillslope hydrology where vegetation cover is present and soils are moderately thick and permeable. However, they felt that overland flow is required to erode or extend channels. DeVries [1976] recognized that seepage forces may enhance the erodibility of soils. Recently, Dunne [1980, 1988] and Higgins [!982, 1984] have suggested that sapping erosion occurs in many natural landscapes. Groundwater contributes to slope erosion and channel development by several processes. The process most similar to overland flow is erosion of pipelike channels in cohesive soils or sediments having strong topographic relief and abundant near-surface fractures or cracks. Such hydraulic erosion is generally termed "piping" or "tunnel scour" [Dunne, 1988] and is common in shale badlands and alluvium in arid and semiarid regions. The pipes commonly collapse to extend the surface gully network. Piping is enhanced by expansive clay minerals that slake during rainstorms and develop dessication cracks between storms. The cohesi...

Origin and development of theater-headed valleys in the Atacama Desert, northern Chile: Morphological analogs to martian valley networks

“…The latter studies suggest that a groundwater sapping origin of bedrock valleys may not have been uniquely demonstrated anywhere on Earth, and that a positive relationship between spring discharge and weathering rate similarly lacks empirical support. However, groundwater sapping in unconsolidated materials has been demonstrated in the laboratory (Howard and McLane, 1988;Marra et al, 2014) and in at least one field setting in Florida, where it is less controversial than sapping in bedrock (Schumm et al, 1995;Lobkovsky et al, 2007;Perron and Hamon, 2012). These explanations are not mutually exclusive, and more than one may apply in a given location.…”

“…Numerical models (e.g., Pelletier and Baker, 2011) indicate a positive relationship between discharge and erosion rate in the development of theater-headed valleys, but that relationship has only been demonstrated empirically in loose sand, not in bedrock (e.g., Howard and McLane, 1988;Perron and Hamon, 2012;Marra et al, 2014). These models do not include strong-over-weak stratigraphy, vertically jointed rock, or the physics of waterfalls.…”

Amphitheater-headed canyons formed by megaflooding at Malad Gorge, Idaho

“…For example, groundwater sapping is a key process in forming amphitheater-headed canyons in unconsolidated sand (e.g., refs. 8,13,14), but its importance is controversial in rock (5,12,15). Amphitheater-headed canyons are also common to plateaus with strong-over-weak sedimentary rocks (3,16); however, here the tendency for undercutting is so strong that canyon-head morphology may bear little information about erosional processes, whether driven by groundwater or overland flow (e.g., refs.…”

Geomorphology and groundwater origin of amphitheater-shaped gullies at Fort Gordon, Georgia, 2010-2012

“…Erosion by groundwater occurs where groundwater seepage causes local pore-water pressure to increase and lead to sediment detachment and collapse (called sapping) of the slope sediments once pressure exceeds the shear resistance of the sediments (Howard and MacClane, 1988;Whiting and Stamm, 1995). Sapping is not unique to groundwater, as sapping can be caused by the headward advance of waterfall plunge pools-sapping is simply the result of the various processes that lead to sediment collapse along slopes (Howard and MacClane, 1988).…”

“…Erosion by groundwater occurs where groundwater seepage causes local pore-water pressure to increase and lead to sediment detachment and collapse (called sapping) of the slope sediments once pressure exceeds the shear resistance of the sediments (Howard and MacClane, 1988;Whiting and Stamm, 1995). Sapping is not unique to groundwater, as sapping can be caused by the headward advance of waterfall plunge pools-sapping is simply the result of the various processes that lead to sediment collapse along slopes (Howard and MacClane, 1988). Howard (1988a) suggested that five factors are necessary to support the headward erosion of sediment by groundwater seepage: (1) a permeable aquifer, (2) a substantial recharge area, (3) the intersection of the water table with an undisturbed land surface, (4) a local inhomogeneity beneath the permeable aquifer, and (5) a means of transporting the eroded sediment away from the site of seepage.…”