EP012-0008
Channel response to a dam-removal sediment wave captured at high-temporal resolution using routine streamflow data

Tuesday, 8 December 2020
Poster
Matthew Joseph Cashman1, Allen C Gellis2, Eric Boyd3, Mathias J Collins4, Scott Wallace Anderson5, Brett McFarland6 and Ashley Ryan6, (1)USGS Baltimore, Baltimore, MD, United States, (2)USGS, Baltimore, MD, United States, (3)U.S. Geological Survey, Frostburg, United States, (4)NOAA, Gloucester, MA, United States, (5)USGS Washington Water Science Center, Tacoma, WA, United States, (6)USGS Baltimore, Baltimore, United States
Abstract:
The response of river channels to changes in sediment supply and flow has been a central issue in fluvial geomorphology, yet field-based monitoring to address this can be time and labor intensive, often limiting the temporal resolution of data capturing channel adjustments.

In this study, we used existing routine U.S. Geological Survey data from 3 surface water gages along the same river to track channel changes due to a sediment wave released by a dam removal and subsequent large storm events. Paired analyses of river stage and bed elevation indicated large, rapid responses in channel form immediately downstream of the dam removal. During this time, there were corresponding increases in Froude number and channel velocities due to bed fining and/or slope adjustment. Downstream gages captured travel times of the leading edge and peak of the sediment wave, which underwent attenuation, and suggest both translation and dispersion. At the most downstream gage, recovery in bed elevation, stage, Froude number and velocities were slower and remained above pre-removal conditions but suggested a trajectory of recovery.

In contrast, persistent changes occurred to out-of-channel flood responses. Stage associated with flooding discharges increased after dam removal, suggesting sediment deposition at the channel margins and nearby floodplain. This resulted in flood stages being attained by smaller discharges, with National Weather Service-indicated flood stages obtained by 3 – 48% smaller discharges compared to earlier in the study period.

Suspended sediment transport increased following dam removal, particularly at low to mid flows, with rapid recovery. However, the patterns and timing of channel change was not driven by large flow or sediment transporting events, with change mostly occurring during lower flows.

This study used preexisting data to capture a two-signal channel response from a sediment wave released by dam removal: (1) short to medium-term translation and dispersion within the channel, resulting in bed aggradation and changing Froude number and velocities; (2) persistent longer-term effects of sediment deposition on overbank surfaces. This study suggests continued use of routine USGS gage information to quantify patterns of geomorphic change over varying spatial and temporal scales.

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