Releasing sediments while reconnecting rivers: how do channels respond and how long does it take?
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Author/Creator ORCID
Date
2023-04-03
Type of Work
Department
Program
Citation of Original Publication
Collins, Mathias, et al. "Releasing sediments while reconnecting rivers: how do channels respond and how long does it take?" EGU General Assembly 2023, EGU23-7225 (Vienna, Austria & Online, 23–28 April 2023). https://doi.org/10.5194/egusphere-egu23-7225.
Rights
This work was written as part of one of the author's official duties as an Employee of the United States Government and is therefore a work of the United States Government. In accordance with 17 U.S.C. 105, no copyright protection is available for such works under U.S. Law.
Public Domain Mark 1.0
Public Domain Mark 1.0
Subjects
Abstract
Sediment management is an important aspect of river reconnection projects, often driving costs
and influencing community acceptance. At sites with uncontaminated sediments, downstream
release is an attractive option because it is often the cheapest and most practical approach and
the sediment can be ecologically beneficial to downstream areas deprived of it for years by the
dam. To employ this option, project proponents must estimate the sediment quantity to be
released and, if substantial, estimate how long it will take to erode, where it will go, and how long
it will stay there. We investigated these issues for sediments released by the 2018 removal of
Bloede Dam on the Patapsco River in Maryland, USA. The dam was about 10 m high and its
impoundment filled with sand and mud. Taking the surface elevations of these sediments
surveyed immediately before removal and subtracting estimates of the pre-dam valley elevations
derived from 21 cores and post-removal surveys of exhumed pre-dam surfaces, we estimate there
was approximately 186,600 m3
of stored sediment composed of 70% sand and 30% mud. These
proportions match estimates made during pre-removal engineering studies, but our total stored
sediment estimate is about 20% less. The difference between estimates reflects a real change in
stored sediment quantity between 2018 and 2012 when the engineering studies were completed,
additional data available to us after removal, and different estimation methods. After removal,
using elevation surveys generated by traditional methods as well as UAS-based aerial imagery and
structure-from-motion (SfM) at high temporal resolution, we documented rapid erosion of the
stored sediments in the first six months (~60%) followed by greatly reduced erosion rates for the
next couple of years. A stable channel was developed in the impoundment during the rapid
erosion phase. These results are similar to a two-phased erosion response reported for sediment
releases at dam removals around the world across a range of dam and watershed scales,
indicating what practitioners and communities should expect when reconnecting rivers in similar
settings. Downstream, repeat surveys combined with discharge and sediment gaging show rapid
transport of eroded sediments through a 5 km reach, especially during the first year when
discharges were above normal, and little overbank storage.