2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 4
Presentation Time: 8:45 AM


JOHNSTON, Shelley and FITZGERALD, Duncan M., Dept of Earth Sciences, Boston Univ, 675 Commonwealth Ave, Boston, MA 02215, shelleyj@bu.edu

Sediment transport is a complex process that is difficult to quantify because it is controlled by a number of factors, including current velocity, water depth, sediment grain size and supply, antecedent morphology, and underlying geology. It is also difficult to measure sediment transport because instrumentation disturbs the water flow and therefore affects sediment transport. Tidal environments pose additional challenges to researchers when modeling sediment transport because tidal currents are unsteady and morphological features may not be in equilibrium with the current conditions. Despite difficulties, accurately quantifying sediment transport is essential to understanding the dynamics of tidal inlets.

In sandy inlets sand waves and megaripples are common and their migration can be a proxy for bedload. Because acoustic bathymetric measurements are highly accurate and do not interfere with the currents, this method can be applied more easily and over a larger area than direct measurements of sediment transport. Bedform migration is a meso-scale sediment transport process and needs to be quantified in order to link micro-scale processes explaining the movement of individual sand grains and the larger macro-scale processes that shape our shorelines.

Thirteen single-beam acoustic bathymetric surveys were collected during a 3-day period at Moriches Inlet, Long Island New York. The surveys were collected as little as 1-hour apart and therefore can be used to describe the detailed movement of the bedforms within a single tidal cycle. In addition, side scan, sediment data, and current data were gathered to describe the geometry and setting of the bedforms. The bedforms are ~0.3-0.6 m high with a spacing of ~15 m. Preliminary data show that some of the most prominent bedforms migrate up to 0.5 m/hr. The tidal currents are symmetrical with ebb and flood currents reaching 45 cm/sec.