Controls on and spatial variability of sustained coarse sediment mobility following dam removal

Keywords: dam removal, bedload, virtual velocity, Shields parameter
Session Type: Virtual Paper
Day: Thursday
Session Start / End Time: 4/8/2021 11:10 AM (Pacific Time (US & Canada)) - 4/8/2021 12:25 PM (Pacific Time (US & Canada))
Room: Virtual 3

Abstract

Dam removal has become an important component of river restoration with > 1,900 dams having been removed nationwide. Despite this recent progression of removals, the lack of monitoring limits our understanding of the magnitude, rate, and sequence of geomorphic recovery, especially the fate of released sediment. Taking advantage of the November, 2012 removal of a ~190 yr-old, 6-m high, run-of-river dam on Amethyst Brook (26 km2) in central Massachusetts, we have been monitoring the progressive mobility of coarse sediment over a four year period. To quantify sediment mobility, we deployed > 300 RFID pit tagged clasts and 25 clasts embedded with accelerometers. Accelerometers log changes on each of the three grain axes and when its internal clock is linked to stage recorder data, its movement can be subsequently synched to the storm hydrograph. This field design thus allows quantifying clast travel distance and virtual velocity and to test hypotheses regarding flow conditions generating partial and/or full mobility and the role of bed architecture. Our results suggest an important seasonal effect where particles are mobilized more easily in spring and transport at a faster rate. We adduce this seasonal effect to progressive armoring and conditioning of the bed throughout the summer with lower bed mobilizing flows. Moreover, reach morphology (pool, plane bed, step-pool) has a significant effect on critical Shields. We find for critical Shields that particles are more easily entrained in pools; this effect may be due to the to greater macro-roughness in step-pool morphologies.