Geoscience Reference
In-Depth Information
Storm of 1962 had little effect upon long-term erosion
over this 42-year period, with its impact embedded in
the overall storm wave climatology.
In contrast, normally low-energy coastlines affected
periodically by big storms respond differently. Here,
rare but big storms generate waves with enough energy
to exceed a threshold for complete beach erosion.
Beaches undergoing this type of change occur in the
Gulf of St Lawrence and the Gulf of Mexico where
tropical storms are the main cause of shoreline retreat
of barrier islands. Along the Texas coast of the Gulf of
Mexico, tropical storms frequently affect the coastline.
For example, between 1994 and 1998, eight tropical
storms struck the north-western Gulf. However, only
tropical storm Josephine in October 1996 generated
waves that exceeded the threshold for significant
erosion by overwashing vegetated dunes. In this low-
energy environment, this threshold is exceeded by any
storm generating a storm surge of 0.9 m and waves in
excess of 3 m lasting more than 12 hours. Smaller
storms, even though they can persist five times longer,
do not cross this threshold. In the Gulf of St Lawrence,
episodic
events that change the barrier coastline are
even rarer because this region lies at the northern
boundary of hurricanes, which here occur as infre-
quently as once every twenty-five years. Barrier
shorelines remain spatially stable for years only to
undergo landward retreat by tens of meters during
these events. More dramatic is the puncturing of
barriers to form temporary tidal inlets that then infill
slowly over the following decade.
40
y = -46.8x + 38.1
R
2
= 0.07
20
0
-20
A
-40
-60
0.6
0.7
0.8
0.9
1.0
Monthly sea level (m)
40
y = -0.016x + 6.68
20
R
2
= 0.09
0
-20
-40
B
-60
0
200
400
600
800
1000
1200
Monthly rainfall (mm)
40
y = -0.33x + 7.82
R
2
= 0.12
20
0
-20
-40
C
-60
0
20
40
60
80
Previous year's wave height (m)
Linear regression lines (with 95 per cent confidence
limits) between average deviation from the mean high
tide position on Stanwell Park Beach, Australia and
A) monthly sea level, B) monthly rainfall, and C) height
of previous year's storm waves (Bryant 1985, 1988).
Graphs A) and B) are for the period 1895-1980, while
graph C) is for the period 1943-1980.
Fig. 8.14
CONCLUDING COMMENTS
While sea level rise may be responsible for recent
sandy beach erosion (the Bruun Rule), this factor may
be subservient to the role played by rainfall and
storms. Rainfall controls beach position through its
influence on the watertable - as does rising sea level.
Storms are a continuous process operating over the
time span of decades or longer. Individual storms
are not necessarily responsible for beach erosion.
However, if storms occur in clusters, then beaches
erode because there is less time for beach recovery
before the next storm. Three variables: rainfall, sea
level and storms, form a suite of interrelated variables.
Analysis of data for Stanwell Park Beach illustrates this
aspect. Groups of environmental parameters that
individually can be linked to erosion, collectively and
logically, operate together. Furthermore, changes in
highest historical rates of erosion produced by lesser
events. Beaches along this section of coastline are
eroding on average 1.5 m yr
-1
. Islands with more
southerly exposure have slower rates of erosion, a fact
implying that the direction of storm tracking and wave
approach are important. Along this coastline between
1945 and 1985, waves exceeded 3.4 m in height an
average of 64 hours annually. East-coast lows and strong
onshore winds generated by anticyclones accounted for
13 and 24 per cent, respectively, of this total. Most
storm waves were produced by storms tracking along
the polar front eastward across the continent, especially
south of Cape Hatteras. The Great Ash Wednesday
