This paper, published in the International Journal of River Basin Management, 2016, provides a scientific framework for natural erosion of fine sediments in dam removal projects like Matilija Dam.
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 10^6
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.
