Ashish J. Mehta

Anthropogenic Impacts on Sedimentary Sources and Processes in a Small Urbanized Subtropical Estuary, Florida

Abstract The Loxahatchee River, a 500 km2 watershed on the southeast Florida coast, is a barrier-impounded drowned river valley experiencing rapid urbanization during the past 50 years. The estuary is currently composed of a sandy central basin with bifurcating forks accumulating organic-rich muddy sands. The Northwest Fork drains natural forest, residential, and agricultural catchments and has a much larger bayhead delta than the channelized Southwest Fork, which is only 50 years old and diverts much of the flow from the Northwest Fork. Sediment accumulation rates within muddy sand deposits are about 2–3 mm a−1, commensurate within error of the current rate of local sea-level rise. Previously established Holocene accumulation rates are close to relative sea-level rise, implying that sediment accumulation is in equilibrium with the creation of accommodation space. The main anthropogenic influences have been changes in surface sediment texture corresponding to dredging-induced modification of tidal currents and a localized >threefold increase in sediment accumulation rate supplied by sources local to the estuary. Late Holocene sedimentation has been episodic, and lithofacies vary from bioturbated transgressive sands at depth to peat and laminated fine-grained sediments then back to mottled muddy sands in the uppermost strata. This facies transition is attributed to a change of water column stratification conditions caused by decreased tidal flushing in the estuary possibly brought about by variation in sea level or episodic inlet closure by littoral sand transport. The magnitude of the anthropogenic changes in sedimentation is roughly equal with natural changes in lithofacies formation.

In Memoriam: Michel K. Ochi

Dr. Michel K. Ochi, Professor Emeritus in the Department of Civil and Coastal Engineering at the University of Florida, Gainesville, passed on April 13, 2013, at age 92. Born in Japan, he moved permanently to the United States in 1958. He acquired two doctorate degrees, one in naval architecture from Osaka Imperial University and another in mathematical statistics from American University in Washington, DC. Professor Ochi’s international academic renown had much to do with his superb work on the prediction of extreme waves critical to the design of ocean structures, including oil and gas facilities. He was the recipient of a number of scholastic awards, including the Davidson Medal and the Linnard Prize, both from the Society of Naval Architects and Marine Engineers (SNAME), and the Navy Superior Civilian Service Award. He also received an honorary doctorate in engineering from the University of Genoa, was a Fellow of SNAME and the Royal Institute of Naval Architects, and a member of ASCE. During 1958–1979, Professor Ochi was a research scientist with the Naval Ship Research and Development Center near Washington, DC. In 1979, he joined the University of Florida as a professor in engineering and remained until his retirement. He was an excellent research guide and, most of all, greatly admired by students for his clear and precise teaching method in wave statistics and related subjects. An individual of upright character and professional honesty, his humor-filled companywill be sadlymissed by all who came to know him. He is survived by his wife of 48 years, Margaret Duke Ochi.

Modulation of constituent release across the mud-water interface by water waves

Mechanisms that contribute to the modulation of the release flux of conservative constituents from bottom mud by water waves have been investigated. Laboratory flume tests were carried out by initially inoculating fluid-like clayey muds with dyes used as generic constituent surrogates. For each combination of dye and mud, dissolved and particle-bound release fluxes were determined as functions of wave and mud properties by numerically modeling constituent and sediment transport. In the absence of sediment entrainment the dissolved constituent flux in the interfacial diffusive sublayer is mainly governed by the rate of diffusion in mud heaving under wave action and by the rate of mass transfer across the sublayer. The diffusion coefficient in mud appears to be related to the rate of wave energy dissipation by internal friction, and the sublayer diffusion coefficient is found to follow a Chilton-Colburn type relationship for boundary layer mass transfer. When entrainment of sediment and associated pore water occurred, particle-bound constituent release became important. However, partial return of the entrained sediment and water due to redeposition of mud floes caused the net rate of constituent release to the ambient water column to be a comparatively small fraction of the total rate of release. This observation implies that transport models for nutrients and contaminants must accurately account for the two-way interfacial exchange of sediment and water to avoid a possible overprediction of constituent release at the mud boundary.

Fluidized Mud-Wave Interaction under Regular and Irregular Waves

Abstract Although a variety of cross-shore wave transformation models have been developed during recent decades, there is a lack of suitable models that can be applied to soft mud beds offshore. To address this issue, an integrated cross-shore model is introduced. This model combines the effects of wave shoaling, energy dissipation within fluid mud, and wave breaking. The constitutive equations for a viscoelastic model are adopted for the rheological behavior of fluid mud, whereas stationary mud below fluid mud is assumed to be elastic. As part of the integrated model, submodels for mud fluidization and wave transformation are developed for regular waves and then extended to irregular waves. Application of the integrated model at selected sites using available data reveals that it can reasonably predict mud fluidization and associated wave height transformation.