Short‐term changes in the plan shape of a sandy beach in response to sheltering by a nearshore mud bank, Cayenne, French Guiana

Abstract

Montjoly is a headland-bound embayed sandy beach in Cayenne, French Guiana, that shows long-term plan shape equilibrium in spite of periodic changes in accretion and erosion that alternately affect either end of the beach. These changes are caused by mud banks that move alongshore from the Amazon. The mechanisms involved in changes in the plan shape of the beach in response to the passage of one of these mud banks were monitored between 1997 and 2000 from airborne video imagery and field work. The beach longshore drift to the northwest, driven by the incident easterly to northeasterly swell usually affecting this coast, became temporarily reversed as the mud bank, migrating from east to west, initially sheltered the southeastern end of the beach. The difference in exposure to waves engendered a negative wave height gradient alongshore towards the southeast, resulting in the setting up of a cell circulation and counter-active longshore drift from the exposed northwestern sector to the southeast. Sand eroded from the exposed sector accumulated first in the southeastern, and then the central sectors of the beach. The effect of increasing beach sheltering by the mudbank moving west is highlighted on the videographs by an 'arrested' pattern of beach shoreline development. The videographs show hardly any changes in beach plan shape since January 1999, due to sheltering of the beach from wave attack by the mud bank. It is expected that the eroded sector will recover in the future as the mud bank passes, leading to re-establishment of the northwesterly sand drift. This temporally phased bi-directional drift within the confines of the bounding headlands results in a rare example of mud-bank-induced beach rotation, and probably explains the long-term equilibrium plan shape of Montjoly beach.

Monsoon-influenced variations in morphology and sediment of a mesotidal beach on the Mekong River delta coast

“…In the beach transects close to the southernmost distributary of the Mekong River, the removal during the winter monsoon of mud deposited on the outer trough and platform during the rainy season reflects this annual cycle of sediment transport. A similar alternation of mud coverage and removal was reported from a headland-bounded mesotidal beach in French Guiana that receives an abundant supply of mud from the Amazon and Orinoco rivers (Anthony et al, 2002;Dolique, 2004, 2006). For the beach in French Guiana, however, the cycle was irregular, ranging from months to years, and did not correlate with the seasonal fluctuations of deepwater wave direction and height.…”

Medium timescale beach rotation; gale climate and offshore island influences

“…These shifts have been associated with changing climatic conditions, for example, in a detailed study of Narrabeen Beach, New South Wales, Australia, Ranasinghe et al (2004), showed that variations during strong positive/negative Southern Oscillation Index phases (El Niño/La Niña) influenced beach rotation. Other variables also influence rotation, as shown by Anthony et al (2002) at Montjoly Beach, Cayenne, French Guiana, where patterns of nearshore mudbank migration induced rotation. Similarly, Thomas et al (2011b) linked beach diminution and migration, associated with spit collapse, to multi-century shoreline rotation within the current area of study.…”

Mesoscale Morphological Change, Beach Rotation and Storm Climate Influences along a Macrotidal Embayed Beach

“…Morphological responses of embayed beaches to storm and gale forcing have also been studied in the Northern [17][18][19] and Southern Hemispheres by amongst others [10,14]. The underlying causes of wave directional change have also been linked to subtidal mud bank and sandbank migration [20][21][22]. Unlike the macrotidal beach work carried out in this research, most rotation studies utilize variations in the location or volume of the subaerial zone to identify shoreline response; this was because almost all were studied in locations with microtidal or mesotidal ranges see for example [10,20,[23][24][25][26][27].…”