The Reattainment of Dynamic Channel Equilibrium Following Dam Removal

Authors: Jordan Fields*, Dartmouth College Department of Earth Sciences, Francis Magilligan, Dartmouth College Department of Geography, Carl Renshaw, Dartmouth College Department of Earth Sciences, Evan Dethier, Dartmouth College Department of Earth Sciences, Rebecca Rossi, Dartmouth College Department of Earth Sciences
Topics: Geomorphology, Coupled Human and Natural Systems, Water Resources and Hydrology
Keywords: Fluvial geomorphology, dams, equilibrium, hydrology
Session Type: Virtual Paper
Day: 4/8/2021
Start / End Time: 3:05 PM / 4:20 PM
Room: Virtual 3
Presentation File: No File Uploaded

Two primary frameworks exist for identifying the onset of river equilibrium post-disturbance: (1) tracking changes in channel form towards invariant dimensions and (2) the attenuation of sediment flux (dq) towards a regime where flux is longitudinally invariant (dq/dt = 0). Yet, it is unclear whether these frameworks measure the same phenomena. In this study, we take advantage of the removal a small (4 m tall) upland dam in New England as an opportunistic experimental design to contrast the channel’s recovery from form and process-based perspectives. Results indicate the recovery of channel sediment transport processes is rapid and outpaces the establishment of stable channel dimensions. Critical Shields values in the former reservoir – calculated from shear stresses from 2D flow modeling and from the transport of tracer rocks equipped with Radio Frequency Identification (RFID) tags and accelerometers – increased from 0.012 +/- 0.001 immediately post removal to 0.048 +/- 0.003 over the course of few weeks to months, nearly matching the pre-removal value of 0.055 +/- 0.003. Bankfull Shields values were high immediately post-removal (> 0.1 – 0.2), and returned to more typical values (0.03 – 0.09) only after a bankfull flow event six months post-removal, marking the onset of process-based equilibrium. Channel dimensions stabilized synchronously with bankfull Shields values but characteristic reach types have yet to develop, indicating that the evolution of channel form requires timescales longer than the recovery of an equilibrium sediment transport regime.

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