Mud-capped dredge pits: An experiment of opportunity for characterizing cohesive sediment transport and slope stability in the northern Gulf of Mexico

Abstract

Mud-capped dredge pits (MCDPs), defined as sand borrow sites from paleo river channels with cohesive sediment-capped walls, are used for northern Gulf of Mexico (NGoM) coastal restoration projects due to lower cost than more distal restoration resources. MCDP evolution, including infilling and pit wall slope stability, is poorly understood in relation to sandy borrow areas. We present and interpret bathymetric data from Sandy Point MCDP, which was constructed in 2012 about 20 km northwest of Southwest Pass, the most active Mississippi River distributary, in water depths of ∼11 m. Repeat bathymetric surveys show the pit is infilling ∼54 cm (or volumetrically 200,000 m3) per year, which is on the same order as seasonal-scale and an order of magnitude higher than the decadal-scale vertical accretion rates observed seaward of Southwest Pass. This infilling rate is higher than predicted based solely on excavated volume when compared with other dredge pits in sandy substrate, sediment-starved settings. MCDP infill is volumetrically dominated by far-field (here defined as river plume or shelf bed resuspension) sediment, with only ∼9% of pit infill from pit wall failure. Geomorphic comparison of Sandy Point MCDP walls with Southwest Pass mudflow gully walls shows that the MCDP is oversteepened relative to natural depression features in this setting; the convergence of MCDP and mudflow gully gradients towards a common value (4–5°) may represent an “angle of repose” in lieu of decadal-scale forcings. Findings from this experiment of opportunity show that MCDPs (1) are effective sediment traps, (2) have steep but relatively laterally stable walls (3) do not represent significant risk to infrastructure in a 3-year time scale, and (4) are predicted to infill within about 2 decades provided a proximal sediment source exists.