Analyses of the erosion of fine sediment deposit for a large dam-removal project: an empirical approach

ABSTRACT

Large quantities of fine sediment can be accumulated in reservoirs, and the potential impact of their downstream release is often a great concern if the dams are to be removed. Currently, there are no reliable numerical models to simulate the dynamics of the release of these fine sediments, mostly because their release following dam removal is often driven by a rapid erosional process not addressed by traditional sediment transport theory. However, precise quantification of fine sediment transport is rarely necessary to evaluate potential environmental impacts of alternative scenarios. Using the removal of Matilija Dam in southern California, USA, as an example, we quantify the likely magnitude of suspended sediment concentration and the duration of associated downstream impacts, two necessary (and most likely adequate) parameters for assessing alternatives. The analyses first estimate the general magnitude of suspended sediment concentration and duration of impacts based on field and experimental data; they then quantify the duration of impacts under both worst-case and reasonable assumptions according to the underlying physics and common sense. For rapid sediment release with fine-grained impoundment deposits, initial suspended sediment concentrations are likely to approach 10⁶ mg/L, persisting for a few hours to no more than a couple of days. Suspended sediment concentrations are expected to decline approximately exponentially after the initial peak, reaching background levels within a few hours to a few days, provided that sufficient flow is available. The general method presented in the paper should be useful for stakeholders choosing amongst dam-removal alternatives for implementation under similar conditions.

KEYWORDS:

• Dam removal
• numerical modelling and analysis
• fine sediment erosion and transport
• suspended sediment concentration and turbidity
• empirical analysis

Acknowledgements

We thank Dennis Gathard for the brainstorming sessions before and during the project that helped us to better formulate the analyses. Peter Downs and Thomas Lisle provided the review to the sediment analyses during the project that helped to shape up this manuscript. We also benefited greatly from the guidance, comments, and questions from the members of the Technical Advisory Committee composed of Peter Sheydayi, Sam Jenniches, Paul Jenkin, Steve Wickstrom, and Neil Cole, and from the members of the Design Oversight Group (DOG) during the course of the project.

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Yantao Cui http://orcid.org/0000-0001-5820-8670

Derek B. Booth http://orcid.org/0000-0002-5242-4089

Joel Monschke http://orcid.org/0000-0001-7960-6942

Additional information

Funding

Funding for this project was provided by California State Coastal Conservancy and Ventura County Watershed Protection District.