Check dam impact on sediment load: a catchment scale experiment from the Gurbe River, Swiss Alps
Authors: do Prado Ariel , Mair David , Garefalakis Philippos , Schmidt Chantal , Whittaker Alexander , Castelltort Sébastien , Schlunegger Fritz 
Published: February 2023

The construction of check dams is a common human practice around the world where the aim is to reduce the damage by flooding events through mountain streams. However, quantifying the effectiveness of such engineering structures has remained very challenging and requires a specific case study, since it is very sensitive to the geometrical, geological, and climatical conditions. One key parameter to measure the check dams&#8217; effectiveness is to quantify the change in the amount of the bedload carried by the stream after the check dam construction. A permanent lowering of this parameter not only points to success in reducing the probability of sediment transport occurrence but also implies that the catchment sediment input into the system is likely to decrease. Here, classical methods to estimate the sediment transport in a stream depend on measurements of the riverbed slope, grain size percentiles, water discharge, and channel widths.&#160;In this work, we selected the G&#252;rbe River situated in the Swiss Alps as a case study, which has been engineered with more than 110 check dams along a c. 5 km reach where sediment has been continuously supplied from adjacent hillslopes, primarily by landsliding. We measured the riverbed grain size, energy gradients, and river widths within selected segments along this reach. Additionally, a gauging station downstream of the check dams yielded information to calibrate the hydroclimatic situation, thus yielding ideal conditions for our catchment scale experiment. Using the acquired data and the historical runoff dataset covering the time interval between 2009 and 2021, we estimated a total volume of bedload transport ranging from 1.0x104 to 7.0x104 m3 over this period. These estimates are consistent with previous studies in this catchment (Delunel et al., 2020; Ramirez et al., 2022), thereby validating our approach. We then envisaged possible channel geometries for the time before the check dams were constructed. We considered (1) higher energy gradients for the scale of the check dams, representing a proxy for the river slope under natural conditions, (2) channel widths that are smaller than those measured today, thereby anticipating that the channel was more confined in the past due to lateral sediment supply by landslides, and (3) larger grain size percentiles, which we consider to be similar to the values measured from preserved landslides in the region. Using such potential natural scenarios as constraints, we found a significant increase in the predicted bedload in comparison with the current situation, and we calculated a total bedload transport ranging from 7.0x105 to 6.0x306 m3 for a period of 13 years. These results suggest that the check dams in the G&#252;rbe River are highly efficient not only in decreasing the probability of torrents with high sediment flux but also in stabilizing the channel bed by avoiding incision. The most likely consequence is a stabilization of the terrain around it by reducing the landslide occurrence.&#160;