Author(s): Romain Dubuis; Giovanni De Cesare

Linked Author(s): Giovanni De Cesare

Keywords: Clogging; Infiltration; Deposition; Fine sediment

Abstract: The connectivity between the hyporheic layer and the surface flow is essential for the development of benthos and the reproductive success of spawning fish. The infiltration of fine sediments leads clogging of the riverbed, reducing porosity and water exchange between surface water, the hyporheic layer and groundwater. During the two last centuries, the natural flow regimes of rivers and streams all around the world have been heavily altered by anthropogenic activities. River morphology was modified for flood protection, by dam construction and agricultural land reclamation for agriculture and settlement. Moreover, many rivers are exposed to increased inputs of fine sediments, giving rise to high degrees of clogging in terms of intensity as well as spatial expansion. Exchange between groundwater and surface flow have a large impact on the process of clogging. In the presence of groundwater infiltration (downwelling), the substrate acts as a filter that blocks suspended particles and creates a clogged layer. In the case of exfiltration (upwelling), clogging is impeded. Despite its relevance for the depth and density of the clogged layer, so far, little attention has been payed to the effect of infiltration and exfiltration on the degree of clogging. Experiments were carried out in a 6 m-long recirculating flume to reproduce the clogging process under different gradients of infiltration and exfiltration. The substrate was composed of sand and gravel ranging from 0.1 to 8 mm, forming a layer of 30 cm for a flume width of 15 cm. The groundwater table was set to different heights to obtain the gradients of interest. Fine sediment (< 63 μm) was added to water with a target concentration of 1 g/L. Results include vertical profiles of fine sediment content and water pressure in the substrate, infiltration rates, and concentration of suspended sediments. Quantitative relations between the filling of pores by fine sediment and the hydraulic conductivity could be established. Measurements showed that the deposition of fine sediment takes place in every configuration including setups with exfiltration. The depth reached by fine sediment increases with higher gradients of infiltration. Likewise, the quantity of fine sediment needed to reduce hydraulic conductivity by a given factor is increased. The porosity of the substrate near the surface of the riverbed was highly correlated to the speed of the process. This study aims to provide new insights on the effect of vertical flow exchange, along with other parameters such as (variable) flow conditions, armour layer, and sediment concentration on clogging and declogging conditions. The study provides an important basis for remediation measures that re-establish a degree of clogging closer to natural levels.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221145

Year: 2022