Erosion, transport and deposition of a sediment replenishment under flood conditions

Abstract

River reaches downstream of dams with constant residual discharge often lack sediment supply and periodic high flows due to dam sediment retention and flow regulation, respectively. To test a novel multi-deposit methodology for defining environmental flows for activating the dynamics of the river morphology downstream of dams, a flood was released from Rossens Dam in Switzerland. This event was combined for the first time with a multi-deposit configuration of sediment replenishment consisting of four artificial deposits allocated as alternate bars along the riverbanks as a restoration measure. To validate the sediment transport behaviour observed in laboratory tests, stones were equipped with radiofrequency identification (RFID) passive integrated transponder (PIT) tags, a fixed antenna was installed at the river bed and a mobile antenna was used to enable the investigation of the erosion, transport and deposition of replenished sediments. The duration of the erosion period was determined for the tracked stones, and average transport velocities were found to be on the order of 10-3 m/s. To estimate the erosion efficiency of the flood, defined as the eroded tagged stones compared with the released water volume, the hydrograph was divided into different periods: rising limb, constant peak discharge, decreasing limb. During the rising limb of the flood, which lasted for 20% of the total flood duration, more than 40% of the PIT tags were transported. The defined erosion efficiency is a measure to support the hydrographic design of artificial flood releases from dams. The deposition of tagged stones resulted in a repeating cluster formation, as expected from previous laboratory experiments, creating an increase in hydraulic habitat diversity. Comparison of the results obtained in the field and from laboratory experiments confirmed the robustness of the multi-deposit sediment replenishment method. Combined with the knowledge gained on the erosion efficiency, these results could motivate further applications and research into multi-deposit sediment replenishment techniques as a habitat-oriented river restoration measure.

The e-RFIDuino: An Arduino-based RFID environmental station to monitor mobile tags

“…For our currently performed trials, we report a 100% detection rate of tracked objects, even though they were transported in the bottom of the river flow channel and immersed at depths estimated to be up to 1 m during some of the monitored floods. This detection rate is higher than the detection rate obtained so far with RFID station using PIT tags, ranging from 16% to 39% [12,14]. This detection rate is defined as the number of tracked objects that were located upstream of the e-RFIDuino station once it was installed and that were detected before e-RFIDuino station removal divided by the number of tracked objects that were located upstream of the e-RFIDuino station once it was installed and that were recovered downstream of the e-RFIDuino station by pedestrian operators or UAV survey.…”

“…The Slender III antenna, which has a semi-directional sensing field, was fixed on the right bank with an angle of approximatively 45°between the antenna nadir and the vertical axis, and with an angle of 45°b etween the antenna plan and the river channel axis. In that location, about 3 m above the channel bottom, the e-RFIDuino station cannot be damaged by river flow like it has been previously experimented [13,14].…”

Sediment Augmentation for River Rehabilitation and Management—A Review

“…The ecomorphological state of the Sarine River in Switzerland, which can be adequately represented by, e.g., the hydro-morphological index of diversity (HMID) [41], was reported to benefit from an SAM that was the first of its kind to be implemented near a reach downstream of the river's residual flow section [42,43]. At the Drôme River in France, investigations on basin-scale environmental changes suggested that sediment augmentation caused by floodplain deforestation can potentially still negatively impact the ecological value of the river network, even while morphological diversity increases [44].…”

“…For example, in residual flow reaches that are downstream of large dams, new riffle structures can be created to free the benthic zone from the nuisance caused by attached algae that travel from the reservoir [20]. Furthermore, the hydro-morphological diversity can be increased [42,43]. Another theoretical benefit of sediment augmentation for the riverbed structure is the deconsolidation and mobilization of the top layer [60] and thus the reduction of clogging of the interstitial pore space.…”

“…The sediment volume of a single SAM in the case studies varies from small (<10 3 m 3 ) (e.g., [21,25]) to medium (10 3 -10 4 m 3 ) (e.g., [34,43]) and large (10 4 -5 × 10 4 m 3 ) (e.g., [32,35]).…”