Once a continuous wetland conveying water from Lake Okeechobee to Florida Bay, the Everglades landscape has been partitioned by 1000 miles of canals and levees, draining or impounding the system. Sheetflow and sediment transport once maintained the corrugated ridge-and-slough patterning and topography, by redistributing sediments from sloughs to ridges. While levee removal is essential to restoring sheetflow, the canals pose the potential to inhibit sediment transport and eutrophy marshes downstream. The Decomp Physical Model (DPM) is a landscape-scale experiment (15-km2) quantifying benefits of sheetflow and canal-backfilling on Everglades ridge-and-slough wetlands. DPM uses 10 gated culverts to provide sheetflow into the “pocket” between the L-67A and L-67C levees. Other features include three 1,000-foot canal backfill treatments and 3,000-feet of removal of the L-67C levee. Field monitoring during no flow (baseline) and 3 high flow (impact) conditions have been completed from 2012 to 2016. Sediment movement was quantified using several methods, including horizontal and vertical sediment traps, sediment molecular biomarker analyses, synthetic sediment tracers and traditional water column particulate sampling and velocity monitoring.
Results/Conclusions
In two flow events, sediment transport in sloughs increased 5- to 20-fold under high flow, while ridge transport increased subtly. The first few hours of flow generated disproportionately greater suspended sediments and sediment transport than steady-state flows, however slough velocities and slough sediment transport tended to increase substantially over 10 weeks. Sediment traps, sediment biomarker analyses, and acoustic Doppler velocimeters (ADVs) deployed showed that high flows and sediment transport were limited to within 500-m of the culverts. As flows typically moved in the “wrong” direction (east instead of south), active management approaches such as hydrologically reconnecting and expanding remnant sloughs may be needed to maximize sheetflow and move water in the historic direction. Preliminary evidence suggests canal backfilling improves habitat for bass populations and reduced mobilization of high-nutrient sediments; however, this phenomenon also requires additional monitoring to verify the long-term effect of backfilling.