|Environment and development
in coastal regions and in small islands
CSI info 4
SETBACK CALCULATION FOR A SAMPLE BEACH
Basic formula for setback calculation :
setback = (p) + (h) + (s)
|where,||(p)||is the change in coastline position,|
|(h)||is the change in coastline position
likely to result from a
category 4 hurricane,
|(s)||is the predicted coastline retreat
resulting from sea level
Changes in coastline position (p)
This parameter is based on historical and recent changes.
Historical changes in the position of the offshore step between 1968 and 1991 were:
East section = -0.7 m/yr
West section = +0.1 m/yr
Mean = -0.3 m/yr
Recent changes in beach width based on the beach profile data between 1992 and 1994 were +2.6 m/yr
Based on local information and the existence of beachrock ledges (an indicator of longterm erosion), the historical trend takes precedence at this beach. Therefore the projected change over the next 30 years = (-0.3) x 30 = -9.0m
Changes in coastline position likely to result from a major (category 4) hurricane (h)
The average dune retreat at this beach during Hurricane Luis was -18 m.
Predicted coastline retreat resulting from sea level rise up to the year 2030 (s)
The predicted coastline retreat resulting from sea level rise up to the year 2030 is -10 m.
This bay is fronted by seagrass beds; however, there were reefs composed of dead Acropora palmata at the western headland. These were largely reduced to rubble by Hurricane Luis and thus the western section of the beach in particular may be vulnerable to increased wave action. The dunes at this beach were heavily mined in the 1980's.
Setback Calculation (p) + (h) + (s) = (-9) + (-18) + (-10) = -37 m
Local factors such as the loss of the Acropora palmata reef and the history of dune mining make this beach especially vulnerable to future erosion. The calculated setback was therefore increased by 20%.
Average setback for the entire beach = 44 m landward of the "permanent" vegetation line.