Effects of relict levees on sediment and debris deposition in delta systems
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Relic levees and dikes not removed during restoration reduce water flow over the marsh face, reducing the input of sediment laden flood waters and reducing sediment deposition rates.
- Sheet flow over the delta plain has been proposed as a significant pathway for sediment deposition, given that most sediment is transported during flood flow and delta plain vegetation slows velocity and increases deposition.
- The formation of natural levees may mimic the function of anthropogenic levees at all but high tidal flows.
- Sediment supply issues may be most significant on sites where there is a sediment shortage due to impoundment behind dams (See Nisqually Refuge Restoration)
- Deposition regime is different between bed-load and suspended-load, with bed load being most significant during island formation, and suspended load during island accretion under vegetation (Eilers 1974).
- Distributary configuration effects on delta sediment deposition is a related topic, where the general distribution of sediment is controlled by larger scale delta routing, while smaller scale deposition may be affected by remnant dikes.
- There is some evidence in the Skagit Delta that remnant levees and dikes affect wood accumulation which in turn affects the viability of assisted vs. unassisted development of tidal fresh swamp (Hood 2007).
Delta Strategy Analysis
The uncertainties in how this topic affects delta restoration has resulted in its inclusion in the ESRP River Delta Adaptive Management Strategy. This three criteria analysis should build off the analysis above, and supports development of learning projects.
Restoration in subsided deltas depends on deposition to recovery functions. Suspended load is transported during high river flow. Out-of-channel movement of river water over the vegetated face of a restored site during high river flow events would logically increase deposition rate.
A wide range of factors may interact with a variety of structures affecting sediment input to a site. The ability to predict these dynamics are likely subsidiary to questions of system scale sediment supply and routing.
The significant decision of how much levee or dike infrastructure to leave behind, how to configure these remnants, interest in recreational access all figure heavily into design decisionmaking, and would be strongly influenced by additional evidence.