The goal of my master’s thesis is to understand the characteristics and mobility of coarse-grained sediment bodies in Portsmouth Harbor, New Hampshire. My research takes advantage of a couple of ways to (a) characterize these features, and (b) figure out if they’re actually moving, and how. But first…
Why do I care about sediment mobility in Portsmouth Harbor? The reasons are plentiful. The security and sustainability of Portsmouth Harbor, its navigable channels and infrastructure, depends in large part on understanding sediment dynamics. Movement of sediments within the harbor by tidal and wind- and wave-forced currents (tidal currents alone can reach speeds of 1.5 m/s under spring conditions) may cause scour and/or burial of infrastructure such as submarine cables, piers, and bridge supports. Particularly in the form of migrating submarine ripples and dunes, moving sediments may obscure targets and/or oceanographic sensors placed on the seafloor, posing a significant problem to target (e.g. mine) detection and data acquisition within the harbor. Above all, the economic viability of Portsmouth Harbor demands an understanding of sediment mobility, as maintaining navigable waterways requires the periodic removal of excess sediment – which doesn’t come cheap.
Of course, researchers have been concerned with sediment mobility for a plenty long time, and a variety of methods have been developed for studying sediment transport in shallow marine environments (e.g. estuaries and coastal seas). Some researchers have looked at bedform migration as a proxy for coarse-grained sediment transport; some have looked at local- and regional- scale trends in grain size statistics; and others have used acoustic backscatter to measure transport of fine- and coarse-grained sediments. Yet other researchers have opted to go the modeling route, and have adapted numerical models from flume experiments to study sediment transport in quasi-steady flow under current and wave conditions. Many more studies combine one or more of these approaches.
My thesis makes use of this combined approach for studying sediment mobility. Specifically, I’m interested in two things: (a) migration of bedforms within the study area, as determined from repeated multibeam echosounder (MBES) surveys over the seabed; and (b) determining approximate bed-load transport rates using a numerical model (van Rijn 1984, 1993, 2007) and available environmental data, including grain size and time-series of bottom currents. The idea is to use the numerical model as a “sanity check” on the bedform migration study.. as it can be difficult to assess bedform migration on short time-scales (weeks) due to the resolution and positioning uncertainty of the MBES system.
Now, for each of these approaches, there are a number of assumptions to be made. For the bedform migration study, we have to assume that the distance the bedforms are moving over the survey time-scale is larger than the positioning uncertainty of the MBES system (~0.5m is a safe estimate, since we’re using RTK GPS). For the numerical model, we must assume that bottom texture is adequately represented by certain grain size statistics (e.g. median grain size) ; that entrainment of sediment is primarily caused by grain-related bed shear stress, and not by form drag generated by bedforms; that bottom flow conditions are suffuciently represented by currents measured at a distance of 1.0m off the bottom; and that other empirical constants and relationships (e.g. the Nikuradse roughness length) apply.
