Shape and dynamics of seepage erosion in a horizontal granular bed

TL;DR

This study explores seepage erosion patterns in a horizontal granular bed through experiments, revealing how groundwater distribution influences channel shape, growth mechanisms, and the impact of saturation on erosion dynamics.

Contribution

It introduces experimental insights into how groundwater source distribution affects seepage erosion patterns and channel growth in granular beds.

Findings

Elongated channels grow upstream with boundary groundwater injection.

Amphitheater-shaped channels form under uniform rainfall conditions.

Channel growth involves discrete avalanches influenced by bed height and saturation.

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.