Sediment dynamics on a steep, megatidal, mixed sand–gravel–cobble beach

Abstract

Abstract.Results are presented from a pilot study of shore-face sediment dynamics on a steep, poorly sorted, coarse-grained, megatidal beach at the head of the Bay of Fundy, Nova Scotia, Canada. The experiment involved the first field deployment of a prototype wideband, pulse-coherent, bistatic acoustic Doppler profiling system. Measurements of the vertical structure of flow and turbulence above a sloping bed, as well as bed material velocity, demonstrate the capabilities of this instrument vis-à-vis studies of nearshore sediment dynamics at the field scale. The second focus of the paper is the unexpected observation that the surficial sediment median diameter, across the lower two-thirds of the intertidal zone, underwent a pronounced decrease when wave forcing was more energetic, compared to values observed during calmer conditions. The explanation for this result appears to involve the formation -in wave-dominated conditions -of metre-scale wavelength, 20 cm high ripples on the rising tide, which are then planed flat by the swash and/or the shore break on the subsequent ebb.

Vertical structure of pore pressure under surface gravity waves on a steep, megatidal, mixed sand‐gravel‐cobble beach

“…The mean peak period, computed for storms ( m) and intervals between storms, was 7.6 and 9.7 s, respectively. Mean peak wave periods measured during two previous studies were 5.5–6 s [e.g., Hay et al ., ].…”

“…Results from earlier field deployments (2012, 2013) at Advocate Beach are presented in Hay et al . [], Stark et al . [], Stark and Hay [], and Wilson et al .…”

“…The beach face is exposed to the full 500 km fetch of the Bay of Fundy and adjacent Gulf of Maine. At low tide, the beach face is uniformly planar and the intertidal zone up to 100 m wide during spring tides [see Taylor et al ., ; Hay et al ., ]. From the lower beach face to beneath lowest low water, the beach composition transitions to cobble and boulder‐sized material.…”

“…Well‐sorted beach cusps are often seen to form on the upper beach, with cusp horns typically composed of loose gravel and cobbles, and fine material in the embayments. Sediment sorting has also been observed to follow energetic wave events in the form of widespread fining of the surficial material [ Hay et al ., ]. During storms, peak incident wave periods typically fall within the wind‐wave band ( T peak = 5.5–6 s), with longer period swell usually limited to the weaker wave forcing conditions between storms.…”

“…During storms, peak incident wave periods typically fall within the wind‐wave band ( T peak = 5.5–6 s), with longer period swell usually limited to the weaker wave forcing conditions between storms. The combination of a steep beach slope and typically short period, wind‐generated incident waves in the Bay of Fundy result in a highly energetic shore break for significant wave heights of m and larger [ Hay et al ., ].…”