Increased Hydraulic Roughness in Alluvial Rivers Created by Sand-Mining Sculpted Bed Features

Alluvial bedforms are a first order control on flow resistance, water levels and rate of sediment transport. Their morphologies are a direct reflection of the hydraulic and sedimentological conditions under which they are formed. Studies of natural dunes in large river systems have shown that low angled dunes are dominant, characterized by reduced flow resistance and limited recirculation of flow in their lee-slopes. However, increasingly, alluvial rivers are influenced by human activities, such as sand extraction, that directly impact the morphologies of river beds and bedforms. Here, we present a comparison of the morphologies of natural and anthropogenically influenced bedforms observed through multibeam echo sounder surveys on the Mekong River in Cambodia. We show that anthropogenic bedforms have higher amplitudes (μ = 2.8 m, σ = 1.0 m), steeper leeside angles (μ = 20.6°, σ = 5.8°) and shorter wavelengths (μ = 100.1 m, σ = 87.9 m) compared to natural dunes (amplitude: μ = 1.79 m σ = 0.86 m, leeside angle: μ = 11.8° σ = 5.7°, wavelength: μ = 117.0 m, σ = 89.1 m). Our data suggests that the form roughness of anthropogenic bedforms is higher (median k_sf = 1.23) than natural bedforms found in sections of the Mekong unaffected by mining (median k_sf = 0.49). As a result, flow patterns subsequently force suspended load over bedforms, meaning sediment is unable to infill mining pits. Anthropogenic bedforms may represent a significant and previously under-represented control of flow and sediment transport in alluvial river systems.