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Beach Erosion Update
By: Dylan Anderson
Latest Information
on Beach Erosion:
The pictures and points
below indicate the latest
situation on the St Francis
Beach:
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Ralph Road: 5pm
19 March |
Ralph Road: 10am
20 March |
Ralph Road: 10am
20 March |
Detailed surveys have been
undertaken of the full
length of St. Francis Bay
beach on a monthly basis,
starting from October 2006.
These surveys include
walking the low and high
water marks as well as
additional lines along the
centre of the beach.
Measurements are taken at 4m
intervals in order to
provide an accurate
assessment of losses and
gains from the beach over
the periods shown below.
All the readings are taken
using a precision GPS system
allowing accurate
calculation of volume gains
and losses over the entire
beach. The beach has been
divided into four main areas
covering the four sections
to be protected by the
artificial reefs. These are
as follows:
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Losses for the
full six month
period October,
2006 to March,
2007 amounts to
90 317 m³
with 100 379 m³
having been
lost in the two
month period
February to
March, 2007.
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It is also
important to
note that the
following six
month period
April -
September. 2007
covers three of
the winter
months where the
most erosion can
be anticipated.
It is possible
that the losses
off the beach
for the full
year will be in
the range 150
000 - 200 000 m³
which is
significantly
more than
previous
estimates.
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It is also
important to
note that Sector
4 of the beach
from Aldabara
Run to the
Estuary mouth
has lost over 61
363 or 67.94% of
the total loss
over this
period. This is
the
section including
the barrier dune
between the
beach and the
Ski Canal and
confirms the
forecast by ASR
that this could
be breached in
the next 10 - 15
years depending
on the type of
wave events
experienced and
the average rate
of erosion. In
the case of
abnormal storm
events being
experienced the
barrier dune
could even be
breached earlier
than the
forecast which
would threaten
the viability of
the total canal
system.
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Area 1: Nevil
Road to Anne Avenue 645m |
Area 2: Anne
Avenue to Peter Crescent
690m |
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Area 3: Peter
Crescent to Aldabara Run
600m |
Area 4: Aldabara
Run to the Kromme River
Mouth 960m |
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Total
Length of Beach
2.895 km.
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Figure 1.
The graph above
shows the volume
of sand lost and
gained over the
different areas
of the beach
since October
2006.
Note:
The volumes
indicated are
conservative.
For further
information
contact
the Beach Trust
|
Month |
Area 1 |
Area 2 |
Area 3 |
Area 4 |
Total |
|
CM³ |
Percent |
CM³ |
Percent |
CM³ |
Percent |
CM³ |
Percent |
CM³ |
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Oct 06 |
-2392 |
-16.30% |
-1703 |
-11.60% |
-2076 |
-14.14% |
-8507 |
-57.96% |
-14678 |
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Nov 06 |
1050 |
29.93% |
-488 |
-13.91% |
192 |
5.47% |
-4262 |
-121.49% |
-3508 |
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Dec 06 |
4974 |
35.01% |
3677 |
25.88% |
1057 |
7.44% |
4499 |
31.67% |
14207 |
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Jan 07 |
1569 |
11.17% |
5414 |
38.56% |
3759 |
26.77% |
3299 |
23.50% |
14041 |
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Feb 07 |
-7470 |
-11.10% |
-17395 |
-25.86% |
-11403 |
-16.95% |
-31009 |
-46.09% |
-67277 |
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Mar 07 |
-1005 |
-3.04% |
-5657 |
-17.09% |
-1057 |
-3.19% |
-25383 |
-76.68% |
-33102 |
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Apr 07 |
5203 |
24.64% |
2961 |
14.02% |
3078 |
14.58% |
9874 |
46.76% |
21116 |
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May 07 |
-6129 |
-486.04% |
3876 |
307.38% |
-2167 |
-171.85% |
3159 |
250.52% |
-1261 |
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Jun 07 |
228 |
11.92% |
1376 |
71.93% |
-238 |
-12.44% |
547 |
28.59% |
1913 |
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Jul 07 |
2441 |
28.31% |
1677 |
19.45% |
-2528 |
-29.32% |
7031 |
81.56% |
8621 |
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Aug 07 |
-2472 |
-15.88% |
2543 |
16.34% |
5028 |
32.31% |
10463 |
67.23% |
15562 |
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Sep 07 |
1089 |
30.57% |
-3021 |
-84.81% |
4930 |
138.41% |
564 |
15.83% |
3562 |
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Net (+/-) |
-2914 |
-7.14% |
-6740 |
-16.52% |
-1425 |
-3.49% |
-29725 |
-72.85% |
-40804 |
Table 1.
Gains/losses
and
percentages over the
four
different areas,
combined total for each
month in far right column
and net
gains/losses
and
percentages for each
area in bottom row. Extreme
losses
over February/March, 2007
are evident from the survey results.
Update 4
October, 2007
In looking at
the total beach
area by area it
is clear that a
number of key
observations can
be made over the
full survey
period from the
start of
October,
2006. These are:
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Area
1:
Nevil Road
to Anne
Avenue:
Only limited
erosion over
the period
due to
partial
protection
from
revetments,
Santareme
and Cape St
Francis on
the West.
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Area
2:
Anne Avenue
to Peter
Crescent:
Erosion is
vertical
where there
are rock
revetments
(Ralph Road)
and
landwards
where there
is
no protection.
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Area
3:
Peter
Crescent to
Aldabara
Run: Erosion
in the
vertical
plane due to
the rock
revetments
in this
area. Very
limited
erosion
overall.
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Area
4:
Aldabara Run
to the
Kromme
River:
Unprotected
stretch with
the bulk of
the erosion
here being
landward
threatening
the barrier
dune.
It is clear from
the 12 month
figures that the
further north on
the beach one
progresses the
greater the
erosion on an
exponential
scale with
almost 73% or 29
725 cu m of the
total erosion of
40 804 cu m
being in Area 4
(barrier dune)
Area 3 is the
one with full
rock revetments
protecting the
houses on the
beachfront and
shows the least
erosion. Here
the erosion is
in the vertical
plane as it is
impossible for
there to be
landward erosion
of any
magnitude. This
is resulting in
more beach being
lost in front of
the revetments
as has already
taken place in
front of Ralph
Road (Area 2).
In terms of the
various
stories that the
beach is
returning, all
that is
happening is as
the top of the
beach is eroded
it drops the
sand on to the
beach where it
may appear
that accretion
is taking
place. This is
currently
evident in
the Peter
Crescent /
George Road area
as well as at
the mouth of the
Kromme. For real
accretion to
take place there
needs to be a
sand source as
was previously
available in the
Santareme dune
field prior to
it being
developed for
housing.
The good news is
that we have
not experienced
strong Easterly
swells in September,
2007 though
these could
still be coming
in October /
November,
2007. These
swells cause
much of the
beach erosion as
experienced in
February /
March, 2007 (see
note above).
This has
resulted
in lower than
anticipated
erosion over the
full 12 month
period from
October, 2006.
Note: We
will shortly be
in a position to
include a full
review of the 12
month survey
period from
October, 2006
showing both
landward and
vertical erosion
rates for each
section of beach.
Figure 2. Shows a
summary of net gains or
losses for the different
areas including a total,
over the past since October
2006, with
greatest losses experienced
in area 4 from Aldabara run
to the mouth.
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Left:
Storm seas over
the 3rd to the
5th of March
2007 resulted in
acute erosion
leading to the
undermining and
partial collapse
of this section
of Ralph Road.
See old
revetments in
foreground. |
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Right:
Old revetments
placed on the
beach in 1995
have been
re-exposed by
recent erosion,
the beach has
retreated an
average of 20m
along this
stretch of
beach over the
last twelve
years.
In some areas
close to the Ski
Canal there has
been close to
this amount of
erosion since
October, 2006.
See
latest
recommendations from
Dr Shaw Mead of
ASR below.
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