Understanding Beach Dynamics
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Understanding Beach Dynamics


Beaches are buffer zones, preventing coastal erosion, coastal retreat and salt water intrusion.

Sedimentary budget (volume of sand) of beaches is controlled by the interaction of sources (when sand is added) and sinks (when sand is taken away). This is called a dynamic equilibrium.

Conditions vary from beach to beach depending on location, coastal topography, offshore bathymetry (wave and swell patterns) and inflow of rivers.

Sources

Rivers - coarse sand
Cliffs - longshore transport
Dunes - wind transport
Artificial Nourishment - importation by man

Sinks

Longshore currents
Dredging
Mining
Wind erosion
Over wash

When the sources are in excess of the sinks we have accretion. More sinks than sources - erosion.

Seasonal changes in currents and wave action produce a cycle of erosion and accretion. The level of sand on a particular beach may rise and fall by as much as a metre. Heavy wave action in the rainy season removes sand from the beach and calm, steady wave action in the dry season builds it back up. If a beach is mined while the level of sand is high, a subsequent storm or heavy wave action will erode the beach and threaten the coastline. Sand removed from a beach will be replaced by sand from offshore banks, depleting the offshore banks, again resulting in loss of protection from storm activity.

From the 1980-82 Institute of Marine Affairs (IMA) report by Cicely Georges:

"Mining results in an almost permanent loss of sediment. It creates conditions similar to those prevailing during the storm season, i.e. lower beach slopes and reduced berms without the protection of offshore bars. As a result, though the beach is naturally replenished during months when lower energy conditions prevail there is no sediment to rebuild the berm to its former shape and height."

Tobago's external sources of sand via rivers and offshore deposits are limited. Removal of sand will lead to little or very slow replenishment, although again, conditions vary from beach to beach. An IMA study of the mining of Richmond beach showed significant loss of aesthetic value but recuperation in one year due to inflow from Richmond river. The study showed no such recuperation at Goldsborough Bay and Great Courland Bay.
 
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