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Engineer Explains How City
Beach Was Eroded
It has been suggested
that a Beach Erosion
Board be formed in W.A.
similar to that in the
U.S.
The American B.E.B. is a
civil function of the Depart-
ment of the Army and has
several provinces, namely, (a)
carry out research and col-
lect data through the Corps
of Engineers for protection
of Federal land, State land
or other land in cooperation
with municipalities, -etc.;
(b) administer Government
funds for research done un-
der contract to the board by
several universities and re-
search institutions through-
out America; (c) set the proc-
gramme of research, collect
the results and publish re-
ports which are disseminated
all over the world. The Uni-
versity of W.A. receives
these publications.
It is preferable to carry out
such investigations in specialis-
ed institutions such as univer-
sities because trained personnel
are required as well as compre-
hensive libraries. Research in
this field was recently inaugur-
ated in the civil engineering de-
partment of the University of
W.A.
Premature
It appears that the formation
of a Beach Erosion Board in
this State at the moment is a
little premature. The university
should be able to fulfil the
functions of a necessary research
none Help
By B. SILVESTER
Lecturer in Civil En-
gineering (Hydraulics).
University of W.A.
body. Such work could be exe-
cuted on a post-graduate level
with material costs and salaries
supplied by local authorities
and the State Government in
combination. At the Queensland
University Engineering School
research into river erosion and
silting has been carried out for,
and financed by, local authori-
ties.
The study of waves, tides, cur-
rents and related phenomena
which is known as physical
oceanography is relatively new,
having come into its own dur-
ing World War II. Much more
information is required and al-
though accurate relationships
have not been found between
the various elements, several
observations can be made that
have reached the stage of being
general knowledge.
Wave Refraction
Waves are caused by wind. To
find the wind at any time a
synoptic weather chart is drawn
Such a chart for Australia at
noon each day is published in
The West Australian. Four
charts a day are drawn at the
Weather Bureau.
The spacing of the isobars
(lines joining points of equal
atmospheric pressure), gives the
wind strength; the closer the
isobars the stronger the wind
Knowing the speed of the wind
that blows over a giveh stretch
of ocean one can calculate the
height. length and speed of the
waves that will occur in deep
water.
As these waves approach the
shore the shallower water slows
them up, building up their
height until they break. When
they approach the coast at an
angle they swing round so that
on breaking they are almost
parallel to the shore. This
swinging round of the wave
crests is known as wave refrac-
tion and is a very important
factor in beach erosion, because
the slight angle (5 to 10 deg.)
to the coast that the waves break
causes a current to form paral-
lel to the shore known as a lit
toral current. This current
moves sand along the beach
known as littoral drift.
When waves are formed by a
strong wind close to the coast
(up to 500 miles) they are very
steep, that is, the ratio of their
height to their length is large
(See Figure 1). These steep
waves tend to erode beaches
and sweep the beach sand into
deep water. On the other hand,
waves that are formed 500 to
1.500 miles from the coast and
which pass through relatively
calm sea before breaking will
be long and low, known as
swell, and will,tend to build up
the beach.
Littoral Current
What are the most consistent
types of waves that approach
the coast of south-western Aus-
tralia? The final year civil en-
gineering students have just
commenced an analysis of wea-
ther charts of W.A. for the last
three years to ascertain this in-
formation for Bunbury, Fre-
mantle and Geraldton. The
magnitude of this task may be
gauged by the fact that there
are 4,380 charts to be examin-
ed.
Although a detailed picture
will not be obtained for some
time, it is safe to say, from
knowledge of the climate over
the years. that for most of the
year waves approach the coast
between Bunbury and Gerald-
ton from a southerly or south
westerly direction. It is only
during the winter months of
June, July and August. and very
occasionally in other months,
that waves come from a north
westerly direction. These waves
are generally steeper that the
swell from the S.W and so are
less effective in causing lit-
toral currents Thus for most
of the year the littoral current
nea- the W.A. coast is from
south or north.
Effect Of Groynes
A groyne will interrupt the
free movement of sand along a
beach. In Figure 2 it is shown
how the beach will be built up
on one side, probably to the
end of a groyne, while slight
erosion will take place on the
leeward side. No further move-
ment will take place when the
beach becomes parallel to
crests of the breaking waves as
seen in the figure. If the built up
beach reaches the cad of the
groyne before such equilibrium
is reached the sand will pass
around the groyne along the
coast.
The built up beach will have a
gentle slope that can withstand
erosion by steep waves better
than the steep grade of the
beach on the eroded side of the
groyne. If several groynes are
built they should be of such
length and spacing that the
equilibrium shore sections are
as illustrated in Figure 3. Con-
siderable thought must be given
to the procedure in erecting
such a field of-groynes because
adverse conditions during con-
struction could cause the loss
of one or more of them.
From the facts listed above
one is able to hazard a guess
as to the sequence of events
at City Beach that recently
led to the undermining of the
ocean beach road. The sum-
mer bad left a steep beach on
the north side of the groyne.
The cyclonic disturbance caus-
ing strong north-west winds was
located very near the coast
(its centre was about 300 miles
west of Cape Leenwin). Steep
waves were formed which ap-
proached City Beach from the
north-west. The beach north of
the groyne was easy prey to
such waves and was carried out
into deep water by the rip
which probably formed as
shown in Figure 4.
Had some swell from the
north-west occurred on one or
two occasions before this on
slaught the beach north of the
groyne may have been built up
to a gentler grade sufficient to
withstand the erosive power of
these steeper waves Possibly
the erosion that took place
would not occur again in 20
years.
Before a remedy should be
suggested for Cottesloe the fol-
owing work is required:
(a) Measure the waves ap-
proaching the beach, either by
a wave recorder of by a wea
ther chart analysis, for a
period of at least one year.
(b) Construct a wave refrac-
tion diagram from a contour
map of the ocean bed as far
west as Rottnest Island. This
would give the equilibrium
shape of the shore for the most
consistent waves arriving.
(c) Measure the amount and
direction of littoral drift, by
specially constructed samplers,
from Cottesloe to the North
Mole, both during N.W. and
S.W. winds.
(d) At this stage a suitable
solution may present itself.
Even so a model of the beach
may be necessary to test any
theories, especially in regard to
possible erosion north of Cot-
tesloe.
Two Remedies
A cursory glance, with the
above knowledge as a back
ground, would indicate that
Mudurup Rocks act as a
groyne, the north side of which
is Cottesloe with its steep beach
subject to erosion at any time
by steep waves. Two remedies
present themselves.
* Remove Mudurup Rocks to
sea bed level to give a straight
coast line at this point
* (ii) Construct a "second"
groyne on the northern side of
Cottesloe beach in order to re-
tain some of the south to north
littoral drift.
Such a suggestion should be
tested by all scientific means
economically possible before
any costly construction is
undertaken.
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