Erosion Processes of a Novel Configuration of Sediment Replenishment During an Artificial Flood

Floodplains downstream of dams with constant residual flow discharge often lack sediment supply and periodic inundation due to the absence of flood events. At the Sarine floodplain in Fribourg, western Switzerland, an artificial flood was released from Rossens Dam in 2016. This flood was combined with a novel configuration of sediment replenishment. The replenishment design was tested previously in laboratory experiments but not in the field. To investigate the erosion processes of the replenishment design, 489 stones with an average b-axis of 5.7 or 11.3 cm were equipped with RFID PIT tags and mixed in the replenished sediment. Downstream of the replenishment area, a pass-over loop fix-antenna was installed in the river to record passing PIT tags. With a mobile antenna, PIT tags were relocated after the flood. Peak discharge of the flood corresponded to a flood with a return period of two years; bankful discharge was observed during the flood peak. With the information of the fix-antenna, an average transport velocity for both PIT-tagged sediment sizes was calculated. Applying this measured transport velocity on all transported and post-flood detected stones allowed calculation of the erosion time for each stone and sediment deposit. The results show that the rising limb of the hydrograph is significantly more effective in mobilizing replenishment material than the decreasing limb. These findings are in line with observations made in the laboratory experiments.