Shape and dynamics of seepage erosion in a horizontal granular bed

Abstract

We investigate erosion patterns observed in a horizontal granular bed resulting from seepage of water motivated by observation of beach rills and channel growth in larger scale landforms. Our experimental apparatus consists of a wide rectangular box filled with glass beads with a narrow opening in one of the side walls from which eroded grains can exit. Quantitative data on the shape of the pattern and erosion dynamics are obtained with a laser-aided topography technique. We show that the spatial distribution of the source of groundwater can significantly impact the shape of observed patterns. An elongated channel is observed to grow upstream when groundwater is injected at a boundary adjacent to a reservoir held at constant height. An amphitheater (semi-circular) shape is observed when uniform rainfall infiltrates the granular bed to maintain a water table. Bifurcations are observed as the channels grow in response to the ground water. We further find that the channels grow by discrete avalanches as the height of the granular bed is increased above the capillary rise, causing the deeper channels to have rougher fronts. The spatio-temporal distribution of avalanches increase with bed height when partial saturation of the bed leads to cohesion between grains. However, the overall shape of the channels is observed to remain unaffected indicating that seepage erosion is robust to perturbation of the erosion front.

Evolution of Porosity and Channelization of an Erosive Medium Driven by Fluid Flow

“…In contrast with studies in which air is injected into a fluid-particle mixture and where surface tension is important [25], the interface appears more diffused and no significant piling of grains at the interface can be observed in our experiments. Thus, the observed growth is more similar to curvature driven growth of seepage channels observed in granular beds [26] and river delta formation [27] than the invasion of a front as in Saffman-Taylor instability [22,23].…”

“…Small precursors are observed (see magnified plot in the inset) before a large avalanche occurs when J f /J c ∼ 0.34. Even after this event, the erosion is not continuous in time but rather progresses in avalanches as also observed near channel heads in seepage driven erosion [26]. To elucidate the packing dynamics, we plot in Fig.…”

Valley formation by groundwater seepage, pressurized groundwater outbursts and crater-lake overflow in flume experiments with implications for Mars

“…Our experiments complement previous work as we focus on morphological processes in the entire valley which is absent in the experiments of Fox et al (2007) and Berhanu et al (2012). In order to simulate natural turbulent flow we used poorly sorted, angular sediment with grains up to 1.6 mm, in contrast to uniform spheres as used by Schorghofer et al (2004) and Berhanu et al (2012).…”

“…Fox et al (2007) studied seepage erosion with a specific aim at undermining of stream banks, which is on a much more detailed scale than useful for our study where we try to explain landscapes. Recent experiments using glass beads (Berhanu et al, 2012) focused on different patterns when the groundwater source is distant or fed by local precipitation. In their experiments, the outflow had fixed dimensions and they mainly studied valley head development.…”

Experiment of Channelization Due to Seepage Erosion

“…Channel bifurcation by uniform rain on the sediment surface has been studied by [5]. They show that the groundwater flow field is governed by the Laplace equation if water fed from the upstream end.…”