A Mechanistic Understanding of Channel Evolution Following Dam Removal

Abstract:

Besides its role as a restoration tool, dam removal can provide insights into channel recovery. The re-attainment of dynamic channel equilibrium post-disturbance has been largely considered through morphologic adjustments. However, recent investigations of the geomorphic responses to extreme floods suggests that sedimentological effects may precede channel morphological changes. We take advantage of the removal of a small (4 m tall) upland dam removal in New England to investigate the sedimentological and morphological arc of the channel’s re-attainment of dynamic equilibrium conditions. Results indicate that sedimentologic recovery is rapid and outpaces the attainment of an equilibrium channel form. Critical Shields (θ_Cr) values in the former reservoir – calculated from the transport of tracer rocks equipped with accelerometers and corresponding shear stress rasters produced in HEC-RAS 2D – increased from 0.019 ± 0.003 immediately post removal to 0.062 ± 0.007 during one storm, falling within the expected range of 0.03 to 0.09 and nearly matching the pre-removal value of 0.055 ± 0.003. Bankfull shields (θ_Bf) shields values lagged behind, increasing to high values immediately post-removal (>0.1 – 0.2), but returned to the expected range at all reservoir and downstream cross-sections by six-months post-removal, facilitated by a bankfull flow. Shields values’ return to the expected range was driven by coarsening grain size and the redistribution of shear stress, which resulted from increased slope in the reservoir coupled with decreasing slope and increasing width in downstream reaches. These mechanisms explain the “two-phased” recovery at dam removal sites: a period of intense sediment flux driven by imbalanced bankfull and critical Shields values, rapidly mediated by channel adjustments that drive both to the expected range, where critical Shields values are then only achieved at the bankfull flow. Finally, despite rapid bed stabilization, characteristic morphologies of upland streams beyond incipient step-pool sequences have yet to develop in most reaches. The observed disparity in flux and form indicates that while morphologic and sedimentologic phenomena covary, the re-attainment of channel equilibrium occurs at various spatial and temporal scales, depending on the metric used.