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...
Artificial neural networks (ANNs) were developed to accurately predict highly time-variable specific conductance values in an unconfined coastal aquifer. Conductance values in the fresh water lens aquifer change in response to vertical displacements of the brackish zone and fresh water-salt water interface, which are caused by variable pumping and climate conditions. Unlike physical-based models, which require hydrologic parameter inputs, such as horizontal and vertical hydraulic conductivities, porosity, and fluid densities, ANNs can "learn" system behavior from easily measurable variables. In this study, the ANN input predictor variables were initial conductance, total precipitation, mean daily temperature, and total pumping extraction. The ANNs were used to predict salinity (specific conductance) at a single monitoring well located near a high-capacity municipal-supply well over time periods ranging from 30 d to several years. Model accuracy was compared against both measured/interpolated values and predictions were made with linear regression, and in general, excellent prediction accuracy was achieved. For example, although the average percent change of conductance over 90-d periods was 39%, the absolute mean prediction error achieved with the ANN was only 1.1%. The ANNs were also used to conduct a sensitivity analysis that quantified the importance of each of the four predictor variables on final conductance values, providing valuable insights into the dynamics of the system. The results demonstrate that the ANN technology can serve as a powerful and accurate prediction and management tool, minimizing degradation of ground water quality to the extent possible by identifying appropriate pumping policies under variable and/or changing climate conditions.
This paper presents the analytic element modeling approach implemented in the software AnAqSim for simulating steady groundwater flow with a sharp fresh-salt interface in multilayer (three-dimensional) aquifer systems. Compared with numerical methods for variable-density interface modeling, this approach allows quick model construction and can yield useful guidance about the three-dimensional configuration of an interface even at a large scale. The approach employs subdomains and multiple layers as outlined by Fitts (2010) with the addition of discharge potentials for shallow interface flow (Strack 1989). The following simplifying assumptions are made: steady flow, a sharp interface between fresh- and salt water, static salt water, and no resistance to vertical flow and hydrostatic heads within each fresh water layer. A key component of this approach is a transition to a thin fixed minimum fresh water thickness mode when the fresh water thickness approaches zero. This allows the solution to converge and determine the steady interface position without a long transient simulation. The approach is checked against the widely used numerical codes SEAWAT and SWI/MODFLOW and a hypothetical application of the method to a coastal wellfield is presented.
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