Download coastal management issues regional vegetation structure beach

Text and figures: Dr Ian Eliot, University of Western Australia
Images:
Dept of Land Administration; University of Western Australia;
South West Development Commission; Coastwest/Coastcare;
Dept of Conservation and Land Management
LOW ENERGY BEACHES
HIGH ENERGY BEACHES
COASTAL SAND DUNES
Low energy beaches are characteristic of sheltered locations protected from the direct effects of swell.
These occur where there are reefs, deep bays and estuaries. The beaches may be exposed to the full
effects of tides and other fluctuations in sea level. Breaking waves are predominantly less than 0.5 metres
in height, often less than 0.25 metres, and seldom exceed 1.0 metre except during extreme weather
conditions. The form of the beach profile is largely related to the size of its sediments, and its scale in
relation to the range of tide and sea level fluctuation. Seaweed is deposited in distinct lines along the
beaches, and some beaches may be covered with large masses of seaweed.
High energy beaches occur on open coast locations exposed to the full effects of sea, swell, tides and other
fluctuations in sea level. Sand moves from a berm (on the beach) to a bar (in the water) when wave energy
changes from low to high. The scale of the beach depends on extreme high-wave conditions and tidal range,
it can vary from tens of metres to kilometres.
Coastal sand dunes form wherever there is a supply of sandy sediment and onshore winds are of sufficient
strength to blow the sand up from the open beach. Sources of sand include material transported to the coast
by streams and rivers, material eroded from rocky coasts, reworking of sediment from the continental shelf,
and skeletal material derived from marine organisms. Once the sand has accumulated on a beach it may be
shifted by onshore winds to be trapped by vegetation and form dunes. The size of the dunes is dependent on
availability of sand, wind strength and the stability of the beach.
REFLECTIVE BEACH
FOREDUNE RIDGES
F L AT B E A C H
HWL
MWL
LWL
Erosional
Fine silt & sand
subtidal
intertidal
upper beach
Garden Island, Cockburn Sound
SEGMENTED BEACH
Storm Ridge
Flat Facet
Erosional
HWL
MWL
LWL
Bi-modal grain size
upper beach
intertidal
subtidal
TRANSITIONAL BEACH
Berm
Step
HWL
MWL
LWL
Erosional
Medium/coarse sand
subtidal
intertidal
upper beach
BEACH CHANGES
Sandy beaches are dynamic. Their form changes with variation in the weather and associated changes in
sea level and wave conditions. Dramatic changes in beach form occur in response to storm activity,
particularly when high sea levels (storm surges) coincide with high tides. Subtle, but substantial change
occurs seasonally where the coast is subject to calm and stormy seasons. Such variation is apparent
between summer and winter in the south west, as well as between the dry (winter) and wet (summer)
seasons in north Western Australia. The seasonal change occasionally is referred to as a ‘sand cycle’
although it is not truly cyclic. The size of changes alters from year to year in response to variation in the
frequency, intensity and duration of the stormy season.
STORM SURGE
Storm surge is a change in water level
brought about by atmospheric forces
associated with storm activity. It has three
major components: lowering of air pressure;
increased wind shear on the water surface;
and high breaking waves. These combine
to cause the water level to exceed the level
predicted for tidal conditions. Conversely,
in calm nearshore conditions rising air
pressure, strong offshore winds and low
wave activity cause water levels to fall
below those predicted for tidal conditions.
The photographs show Trigg Island during
calm and stormy conditions.
Both photographs were taken at a similar
time when the tide was low. Beaches tend
to erode under high water level conditions
and accrete during low water level
conditions.
calm conditions
Trigg Island
stormy conditions
ACCRETION and EROSION
Accretion is the accumulation of sand on
beaches whereas erosion refers to its loss.
The balance between the two is constantly
changing in response to changing wind,
wave, current and tidal conditions. The
main mechanism for sediment exchange
between the beach and the ocean involves
interaction between waves, the tide and
groundwater level. Wet beaches with high
groundwater levels enhance erosion.
On high energy sections of coast, accreting
beaches are commonly wide and contribute
sand to the formation and growth of
foredunes. In contrast, eroding beaches
tend to be narrow and the foredunes are
either absent or cliffed.
The photographs show the same beach
near Siesta Park at Busselton. The beach
was eroded in 1978 but has accreted
during the past two decades.
eroded
Scout Camp,
Busselton
The removal of part of a foredune’s
vegetation cover may cause it to be
destabilised. This results in the exposed
parts of the ridge being eroded by winds
blowing onshore. The windblown sands are
redeposited in dunes with a U-shape
(blowouts). Elongated forms of blowout are
referred to as hairpin or parabolic dunes.
A blowout continues to form for as long as
it is supplied with sand or until the dune
slack (the hollow between the arms of the
U), is eroded down to groundwater level or
bedrock. When the latter occurs foredune
vegetation becomes re-established,
the foredune ridge begins to reform, and
the sand supply to the blowout ceases or
is severely reduced.
Trigg Beach,
Perth metropolitan region
Lake Preston
D I S S I PAT I V E B E A C H
Rounded beaches, like those along
the south east of Cockburn Sound,
at Exmouth and at Busselton, are
comprised of medium grained sand.
The beach varies from an upwardly
convex to concave shape in response
to changing wave and water level
conditions. The beach commonly
terminates with a step at the water line
and from that merges with a subtidal
terrace. Ripples characterise the seabed
in the near shore waters. The sand
moves between the upper and lower
parts of the beach between berm and
step in response to low and high energy
conditions.
West Busselton
BLOWOUTS
Transitional beaches form between
extreme low and high energy conditions
on high energy beaches. Longshore
currents, swift flowing rips and sand bars
are common, and the shoreline fluctuates
rhythmically along the beach.
Such beaches combine elements of the
reflective and dissipative beaches.
They frequently form at times of seasonal
change when the beaches become
dangerous for inexperienced swimmers.
Trigg and Scarborough beaches, provide
good examples of transitional beaches.
Roebuck Bay, Broome
Depostional state
Leeman,
Central Coast
Goode Beach, Albany
Segmented beaches, such as at Cooke
Point, Port Hedland; Karratha and
throughout the Kimberley, are largely
comprised of two distinctly different grain
sizes. Very fine sand or silty sediment
comprises the lower intertidal zone. In
contrast to this, very coarse sand, even
gravel and cobble, form the upper part of
the beach. Chenier beaches, formed
along a lens of sand perched on a muddy
base, fall into this category. The sand or
cobble component of the beach is a storm
ridge. Its sediments are deposited when
storm surges wash over the landform.
ROUNDED BEACH
A sandy coast is dynamic and constantly
changing in response to changes in the
weather and the local wave regime.
Extreme shoreline movement involves an
exchange of sediment between the frontal
dunes (those closest to the water’s edge)
and the near shore waters. When the
beach is wide or during a storm, sand is
blown landwards and trapped by small salttolerant plants. The trapped sand
accumulates in a distinct ridge. This is a
foredune ridge and sometimes is referred
to as beach ridge. Its form is determined
by the plant species, the rate of sediment
supply from the beach and the level of
energy to which the beach is exposed.
The highest ridges tend to be those on
the highest energy beaches.
A reflective beach is dry and wide, with
a steep beachface that ends with a step
into deep near shore water, with no bars.
Beach sediment tends to be coarse
grained. During low energy conditions
waves less than 1.0 metre high break at
the shore, surge up the beach and are
reflected seaward from a steep beachface.
The photograph shows Goode Beach near
Albany as an example, with its narrow surf
zone, waves breaking at the shore and a
steep beachface. A berm and cusps
(curves) have formed high on the beach.
Flat beaches occur on Garden Island and
at Lancelin and Coral Bay. They are made
up of silty sediment or fine sand, and have
very low gradient extending from the
foredunes to the near shore waters. The
near shore water levels will change in
response to low or high energy conditions.
Swash bars (ridges and runnels where the
water has washed over the beach)
migrate across the beach with changes in
wave and tide conditions. The upper
beach and the subtidal zone may be
vegetated, the latter with sea grasses.
SAND SHEETS
Individual blowouts, once detached from
the frontal dunes, may continue to migrate
inland until they run out of sand or
vegetation is re-established. However,
blowouts from the same foredune ridge
commonly join to form a broad sheet of
bare sand. Both forms are mobile sand
sheets. They tend to overwhelm landforms,
including older dunes, in their path as they
migrate. Where this happens the older
sands are incorporated in the migrating
sand sheet. Second order dunes form on
the surface of the sand sheets in response
to changing wind conditions. These may be
transverse, running at right angles to the
direction of the prevailing wind, U-shaped
or cresent shaped - with the arms of the U
projecting forward in the direction the dune
is travelling.
Dissipative beaches form where the
average height of breaking waves exceeds
2.0 metres. However, they may also prevail
with lower waves where the sediment
comprising the beach is very fine grained.
The photograph shows Yokinup Beach,
near Thomas River under dissipative
conditions. Large waves at Yokinup break
by continuously spilling, or reforming and
breaking against submerged bars, as they
travel across a wide surf zone. Their energy
is largely spent by the time they reach the
shore.
Yokinup Beach,
Thomas River,
Esperance
Geraldton
COASTAL MANAGEMENT ISSUES
REGIONAL VEGETATION STRUCTURE
Western Australia extends approximately from 14oS to 35oS latitude, ranging from the wet-dry tropics of the
north west, through the arid Pilbara, to the cool temperate south coast. The botanical provinces and districts
of the coast follow the climatic zones, albeit with great diversity of species composition and community
structure at a local level associated with variation in geology and soils. There is a wide variety of beach, dune,
tidal creek, and lagoon environments within inlets, gulfs, reef-protected coast and open-coast environments
along the mainland. As a result there is substantial variation in the distribution of vegetation communities at
any place.
Under natural conditions changes to the coast do not attract attention unless associated with unusual and/or
extreme circumstances and hence do not require management. Coastal management concerns
environmental, social and economic management of coastal resources for sustainable use by people.
The three components of management must be balanced to be most effective. This requires resource
identification, strategic and local planning, implementation of the plans, and ongoing monitoring to ensure
management goals are being achieved in a highly dynamic environment. Without such plans management
only responds to crisis situations.
I N F R A S T R U C T U R E AT R I S K
A mangrove community at the mouth of the Mitchell
River in the Kimberley. Sixteen species of mangrove
are found on the Kimberley coast. The occurance of
species is based on tidal inundation, soil type, soil
salinity, drainage, plant interactions and animal
activity.
Sparse woody vegetation on the orange soil of the
Peron Peninsula at Shark Bay. There is a notable
lack of a foredune on the white sands of the beach
due to the eroding beach. The erosion, together with
pedestrian traffic in the area, has caused a loss of
vegetation from the old dune surface.
Coastal infrastructure is generally placed at risk
due to a lack of information for planning and
management and failure to adopt the
precautionary principle. Risk arises as a
result of inadequate environmental
assessment, poor planning, bad engineering
design, inappropriate management and design
policy, inadequate maintenance, and attempts
to extend facility use beyond its practical
structural life. A retreating shoreline on an
eroding coast, and sand drifts on an accreting
coast are problems. The photographs show
a beach access ramp and stormwater
discharge pipe exposed during severe erosion
on North Cottesloe Beach and the Onslow
foreshore after Cyclone Vance.
Cottesloe SLSC, Perth metropolitan area
A low foredune ridge north of Gum Tree Bay on the
central west coast. The ridge is vegetated by Cakile
maritima, Atriplex cinerea, Atriplex isatidea and other
species. At the back of the foredune are perennial,
woody vegetation that includes species such as
Olearia axillaris, Myoporum insulare and Acacia
rostellifera.
Onslow
U N C O N T R O L L E D C O A S TA L A C C E S S
Uncontrolled vehicle and pedestrian access
to the coast causes a loss of vegetation from
dunes. This leads to blowouts and mobile
sand sheets. Similarly, unmanaged pedestrian
tracks from developed areas, such as car
parks and residential dwellings, can lead to
destabilisation of dunes and cause problems
with sand drift. In addition to physical damage
and loss of biodiversity resulting from excessive
vehicle and pedestrian traffic, there is a
significant reduction in the aesthetic value of
coastal landscapes and high maintenance
costs. Random vehicle tracks with many paths
leading to the same destination characterise
some of the Western Australian coast such as
Wedge Island and Waychinicup, shown in the
photographs.
A low-lying foredune ridge east of Port Geographe,
Busselton. The foredune is vegetated by Euphorbia
paralias, a toxic weed. A new foredune is starting to
form immediately landward of the beach as a result
of frequent easterly wind in Geographe Bay and
sand trapped by structures at the marina visible in
the background.
vehicle track network,
Wedge Island,
central coast
Foredune and cliff face vegetation at Cowaramup
Bay in the south west. The foredune vegetation of
the region is similar to that found in adjacent regions.
However foredune development is limited by a
restricted sand supply from the reworking of beach
sediments and the erosion of cliffs.
Waychinicup Back Beach,
Albany
accreted
C O N F L I C T I N G C O A S TA L U S E R S
SEASONAL CHANGE
summer
Back Beach,
Bunbury
winter
Storm activity is likely to be more frequent in
one season in some parts of Western
Australia, although storms may occur
throughout the year. It also varies markedly
from year to year. The only constant is that
sandy beaches adjust to wave and weather
conditions by accreting or eroding. Where
inter-seasonal variation is large, as it is in the
southern part of the State, beaches tend to
be eroded during the stormy season and
accrete under calmer seasons. However,
this does not wholly conform to intuition.
For example, the coast from Geraldton to
Busselton experiences two stormy ‘seasons’
per year. The first, the season of highest
energy, is associated with winter storms and
the second with strong sea breezes and
frontal activity in summer. Calmer conditions
prevail in autumn and spring in the south
west.
Potential conflicts between users arise where
the coast is intensively used. Coastal activities
along the central west coast are illustrated in
the photographs. At Wedge Island, vehicles
and boats compromise the safety of swimmers,
disturb local fauna and threaten the stability of
surrounding dunes. Illegal squatter shacks can
compromise the ecological values of the area
and restrict coastal access for other uses.
The variety of activities and the intensity of
coastal use are potentially hazardous. Careful
planning and judicious management are
required to minimise the risks in these
circumstances.
A high foredune ridge at Four-Mile Beach,
Esperance. Vegetation cover on the foredune
includes Spinifex hirsutus, Arctotheca populifolia and
Cakile maritima, as well as woody plants such as
Olearia axillaris and Acacia cyclops. High foredune
ridges are common on high-energy dissipative
beaches such as Four-Mile Beach.
View east along a well-developed foredune ridge at
Kanidal Beach near the Eyre Bird Sanctuary, south
of Cocklebiddy. Here there is a wide variety of
vegetation, with Spinifex hirsutus, Arctotheca
populifolia and Euphorbia paralias closest to the
shore. Olearia axillaris and Acacia cyclops are also
visible. The photograph shows a full sequence of
dune development from foredune ridge to the bare
sand sheets in the background.
Wedge Island
illegal Squatters Shacks