Coastal Management Final
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Transcript of Coastal Management Final
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8/4/2019 Coastal Management Final
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Katarina Chow (33) 3R
Introduction
The increasing attacks of the natural disasters and drastic change of the climate in
the recent decade has resulted in massive amount of coastal and soil erosion along
the coastal regions. While the rising economic concerns and stakeholder pressure
on environmental sustainability of coastal and marine structural materials is in favour
of a comprehensive and integrated approach. Current coastal management has
focused on egocentric vision, recognizing the ecological uniqueness and particular
value of the coastal zone. Coastal zones contain rich resources and are home to
most commercial and industrial activities. In the European Union, almost half of the
population now lives within 50 kilometres of the sea and coastal zone resources
produce much of the economic wealth. Coastal protection takes consideration the
balance of economic, ecological and social factors when measures are being
implemented.
In the past, the general practice was to use hard structures to protect the coastline.
These structures include sea walls, revetments and groynes etc. (Please refer to
next page for more details ). In certain resort areas, structures had proliferated to
such an extent that the protection actually impeded the recreational use of the
beaches. Erosion of the sand continued, but the fixed back-beach line remained,
resulting in a loss of beach area.
As new techniques were developed, the use of artificial beaches and stabilized
dunes as an engineering approach was an economically viable and more
environmentally friendly means for dissipating wave energy and protecting coastal
developments.
When the high flood tides and surges were absorbed by salt marshes along the
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coast, these areas (e.g. in the Fens, Romney Marsh and Pett Level, Thames
Esturary) were reclaimed to create fertile farmland over the years. From time to time
these areas are flooded by the sea, and have been protected by higher and higher
sea walls. As sea level rises owing of the Greenhouse effect it will cost increasing
amounts to protect this land. In addition, changes on sea level have a direct
adaptative response from beaches and coastal systems. When the sea level rises,
coastal sediments are in part pushed up by wave and tide energy, so sea-level rise
processes have a component of sediment transport landwards. This results in a
dynamic model of rise effects with a continuous sediment displacement.
Over the past hundred years the limited knowledge of coastal sediment transport
processes at the local authority level has often resulted in inappropriate measures of
coastal erosion mitigation. In many cases, measures may have solved coastal
erosion locally but have exacerbated coastal erosion problems at other locations -up
to tens of kilometers away- or have generated other environmental problems.
In brief there are two types of coastal management- the hard engineering and soft
engineering. We will discuss how each measure was in protecting the coastline in
their respective places.
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Seawalls
Seawalls are built along the coast to protect valuable or resources. There are vertical
Sea wall and Curved Sea wall. They absorb the energy of the waves before they
can erode away loose materials. It can be made of concrete, rocks or wood. They
can be found along the coasts of Penang , Malacca and Singapore. However, the
energy of the waves would be redirected downwards to the base of the seawall that
may cause a strong backwash. This backwash would further wear away the base of
the seawall over years, causing it to weaken and eventually collapse if it is not
carefully maintained. Seawall are costly to build and maintain as there constant
repairs to be carried out throughout the years. As the strait of the seawall can be
very long along the coastline, it can serve as a recreational purpose. Stanley Park
Seawall in Vancouver is a good example.
Stanley Park Seawall
Vertical seawall
Vertical seawalls are generally constructed in very exposed areas. They reflect
waves. In severe storm situations, the walls can cause standing waves to develop.
The seawall is the 22km pathway in Vancouver's waterfront from the convention
centre on Burrard Inlet, around Stanley Park and False Creek, past Granville Island
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and ending at Kitsilano Beach Park. It is the most popular recreational facility in
Vancouver and is also a tourist attraction. The seawall is managed by the Vancouver
Park Board together with City of Vancouver Engineering Services.
The seawall is divided into two sections, one for walkers and joggers (closest to the
water) and another for cyclists and inline skaters (inside path). Signs indicate use
and warn of congested areas: Bikes must be walked in three areas in Stanley Park
due to congestion.
The seawall passes through 16 parks and past four community centres and nine
concessions. Construction of this seawall began in Stanley Park in 1917 and not
until 1980, the entire seawall loop around Stanley Park was declared officially
completed with the final paving between Third Beach and Second Beach. Since 1980
the seawall has been extended outside of Stanley Park.
In 2010/2011, two portions of the seawall, Stanley Park (near Second Beach) and
English Bay (near Sunset Beach) were renewed to address ongoing concerns with
erosion. With deep foundations and renewed surfacing, the new seawall is built to
withstand the tides for many years to come.
Overall, the Vancouver Seawall is a prime example of how seawalls can
simultaneously provide shoreline protection and a source of recreation which
enhances human enjoyment of the coastal environment. It also illustrates that
although shoreline erosion is a natural process, human activities, interactions with
the coast and poorly planned shoreline development projects can accelerate natural
erosion rates.
Sea walls are probably the second most traditional method used in coastal
management. Modern seawalls aim to re-direct most of the incident energy,
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resulting in low reflected waves and much reduced turbulence and thus take the form
of sloping revetments. The most extreme example is that Sea walls at Kamaishi City
where the world's largest sea wall at a cost of 1.5 billion dollars were built. However,
after the attack of the recent tsunamis, much of Kamaishi City is now destroyed.
A major tsunami can have a period of 10 minutes or more and a wavelength of
100 miles or more, it may pile up and wash over seawalls
The tsunami creates a strong downpour like flood waters rushing over a dam.
The seawalls failed catastrophically to protect people and properties. The lesson to
be learned is that sea walls and engineered structures can protect key facilities if
they are built high enough and strongly enough, but they cannot be depended on to
protect large areas in the largest tsunamis or the strongest hurricanes. Water will
simply pass by well-designed structures that protect key facilities, leaving them
undamaged. However, large tsunamis and hurricane storm surges can pile up water
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in front of large sea walls then rush over them like water over a dam.
When they are needed the most, building larger and stronger sea walls to protect
large areas would have been too costly to be effective. In the case of the ongoing
crisis at the nuclear power plants, higher and stronger sea walls should have been
built if power plants were to be built at that site. However, the Japanese engineers
made a fundamental design error. They built walls for a far smaller design basis
earthquake than actually occurred. They built the reactor and sea walls for a
maximum of M8.2 when this earthquake was M8.9.
Curved seawalls
Curved seawalls are designed to enable waves to break to dissipate wave energy
and to repel waves back to the sea. The curve can also prevent the wave
overtopping the wall and provides additional protection for the toe of the wall.
-Concave structure introduces a dissipative element. Curved seawalls aim to re-
direct most of the incident energy, resulting in low reflected waves and much reduced
turbulence. The design and construction of curved seawalls are more complicated.
The deflected waves can scour material at the base of the wall causing them to
become undermined.
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Though sea walls are considered strong in holding back waves, they are not always
reassuring. At Torcross, a new curved seawall was built after an attack of enormous
storm in 1979. Unfortunately, the following year, another bad storm caused the loss
of up to five metres of the beachhead along a stretch of beach about 1000 metres in
length. Part of the motor way ( A379 road) along Slapton Sands near the village was
also destroyed. The maintenance of the road is imperative to Torcross as it is the
main access route for the villagers and the local businesses. The South Hams
District Council tried to keep the A379 from being eroded away by road realignment
and the importation of shingle from parts of Slapton Sands.
A study by Natural England after the 2001 storm confirmed that Slapton Sands is and
will continue to retreat backwards, due to the reduction in the amount of shingle
available, increasing frequency of storms and a rise in sea level over the next 50
years.
Another unsuccessful example happened at the Sandsend, North Yorkshire coast.
The seawalls were built to redirect the wave energy, however, they failed to do this.
Instead, energy was absorbed, causing them to brake and damage badly.
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Generally speaking, seawalls are generally a successful way to control coastal
erosion but only if they are constructed well with materials that are able to withstand
the force of ongoing wave energy. Seawalls are considered useful as their usage
expectancy is much longer than other soft engineering options, and they can
simultaneously provide recreation opportunities and protection from not everyday
erosion but that of extreme events. Evaluating the successes and shortcomings of
seawalls after major natural events may enhance our understandings on their
weaknesses for future improvement and reassessment.
Revetment
Revetments are always made as sloping structures and are very often constructed
as permeable structures using natural stones or concrete blocks, thereby enhancing
wave energy absorption and minimising reflection and wave run-up. They may be
watertight, covering the slope completely or porous to allow water to filter through
after the wave energy has been dissipated. Revetments can also consist of sand-
filled geotextile fabric bags, mattresses and tubes. Such structures must be protected
against UV-light to avoid weathering of the fabric. Sand-bagging is often used as
emergency protection. Geotextile fabric revetments are fragile against mechanical
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impact.
Revetment failure resulting in the exposure of the underlying geotextile layer
( Portsmouth Harbour Coastal, UK August 2007)
A revetment is a passive structure, which protects against erosion caused by wave
action, storm surge and currents. The main difference in the function of a seawall
and a revetment is that a seawall protects against erosion and flooding, whereas a
revetment only protects against erosion. It is used at locations exposed to erosion or
as a supplement to seawalls or dikes where both erosion and flooding occur. Most
revetments do not interfere with transport of longshore drift. Since the wall absorbs
the energy instead of reflecting, erosion of the structure occurs from time to time.
Major maintenance is required after a period of time. Example of the coastal
revetments at Portsmouth Harbour (along side the motorway A27) illustrates
wearing would have occurred after the revetment was built.
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On site to the east of the A27 A2030 junction. Gaps in the block work
developed along the revetment where the toe support was insufficient
Groynes
Groyne is a physical barrier, is often built at right angles to stop sediment transport in
the direction of long shore transport. Groyne helps to conserve beach by delaying
export of material. This type of structure is cheaper as it is often constructed from a
variety of materials such as wood, rock or bamboo and is normally used on sandy
coasts. Protection of the shore by use of one groyne only is most often inefficient.
Therefore, shore protection by groynes is designed as a group comprising from a few
to tens of individual structures. The main disadvantage of Groynes is that it induces
local scour at the toes of the structures, causing erosion down drift. Moreover,
groynes do not protect the beach against storm-driven waves. Groynes are
considered as cost effective defence measure which requires little maintenance and
is easily repaired.
Long shore drift was causing problems at the Clifton Springs Boat Harbour, located
near Geelong in Victoria, Australia. The sand was following the natural coastal
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process and was filling up the harbour. This decreases the available drift for boats
and also increases the maintenance cost. To alleviate this, the City of Greater
Geelong proposed to construct a groyne, effectively blocking the sand from entering
the harbour.
Another successful example; the coastline along the East Coast Park of Singapore,
where groyne is constructed at the right angles to the beach to prevent material from
being carried away by longshore drift. It creates J-shaped beaches and thus
preventing sand from being washed away. This structure has been in place for over
years, providing protection for the harbour and improvement of beach amenity.
However, at Sandsend there are groynes made of wood. These groynes cannot
withstand the waves energy and get damage badly. They have to be changed every
single year. The waves from the sea would rise onto the groynes causing the wood
to gradually wear down until they were broken completely.
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Sea swirling around groynes at Sandsend on the North Yorkshire coast
Gabions
Gabion revetments (foregound) are generally preferred to gabion walls (background)
in coastal environments being less reflective of wave energy and more stable.
Gabions are wire cages filled with crushed rocks that are piled up along coast e.g.
gabions are found along the coast in North Norfolk, UK, to reduce coastal erosion.
However, gabions offer short-term protection because the wire cages are corroded
by sea water easily and need constant maintenance. It provides short term (5-10
years) protection from backshore erosion by absorbing wave energy along the dune
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face. They may be used to direct the force of a flow of flood water around a
vulnerable structure. Gabions are also used as fish barriers on small streams. Their
application is restricted to the upper part of sandy beaches, since they are not
sufficiently durable to withstand regular direct wave action. They should not be
installed on shingle beaches because wear and tear will rapidly cause damage to the
baskets. As they are porous structures they will tend to trap wind blown sand and
allow the growth of vegetation under favourable conditions. Gabions are also easily
destroyed by excessive trampling. Gabions ruin the natural beauty of coastline,
therefore gabions may not be appropriate in certain situations.
Overstrand is at the north coast of Norfolk in England. The gabion embankment
behind the promenade and retaining wall has been effective in stabilising one section
of the cliffs. Following the cliff failure at Clifton Way in May 1990, work was
undertaken to stabilise the site. As a result, surface and sub-surface drains were
constructed and a large amount of rock armour was used to hold them in place.
When the channels were first put in, they were put in the wrong places. This rock is
clearly visible and vegetation was added to the soil at Clifton Way, but the shrubs
died due to the bad soil condition.
Rip-Rap
Large boulders, of 10 tonnes or more, are piled up along the shoreline to form a type
of sea wall. The rocks are dumped on top of each other leaving gaps between them
that allow water through. This disperses the energy of the waves and reduces their
eroding power. The boulders must be large, strong and resistant to erosion. Granite
and basalt are often used. Small or weak rocks would not be able to withstand the
impact from the waves and would quickly be eroded. The cost of building this
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structure is relatively low.
Building rip-rap walls at Aldeburgh is another approach to dealing with erosion along
the coastline. These aim to lessen the force of the destructive waves. As the waves
break on the shore they fail against large boulders or concrete blocks. The many
gaps in between the blocks help to absorb the energy of the waves.
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Rrip-rap adjacent to a concrete wall on the Cornish coast runs below a road and
protects the soft cliff rocks from the sea where the waves hit it at an angle. As the
coast bends, the wall is replaced by rip-rap that is capable of withstanding the direct
impact from the waves. The rip-rap is also a less expensive alternative to building a
longer wall.
Offshore Breakwaters
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Offshore breakwaters reduce the intensity of wave action in inshore waters and
thereby reduce the damage of the coastal erosion. They protect the coast and
harbor by reducing force of high energy waves before they reach the shore.
Breakwaters can be built with one end attached to coast or away from coast. The
breakwaters may be small structures, placed one to three hundred feet offshore in
relatively shallow water, designed to protect a gently sloping beach. e.g. breakwaters
can be found along beaches at East Coast Park as well as Siloso Beach on Sentosa
in Singapore. They will serve to protect the beach there, as erosion has been
accelerated by the wash from the high-speed ferries plying between the nearby
World Trade Centre and the Riau islands of Indonesia. However, breakwaters are
unable to provide complete protection so unprotected parts of the beach will still be
prone to erosion. therefore breakwaters are not always effective
The breakwater is placed offshore from and usually parallel
to the shoreline to protect a shore area from waves
In 1994 a breakwater was constructed as part of a coast defence scheme at
Sidmouth in Devon. Sidmouth is a popular holiday destination which depends on it
shingle beach for both protection from storms and as an am entity for tourists. The
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severe storms in 1989 and 1990 caused substantial depletion of the beach and
exposure of the foundations of the seawall.
A coastal protection scheme comprising of an offshore rock breakwater, rock
groynes and beach replenishment scheme. The construction works were carried out
in the winter months to minimize disruption.
The breakwaters at Sidmouth offer protection from the prevailing south-
westerly seas
Beach Nourishment
This measure requires adding large amount of sand to the beah to replace the ones
eroded away. This leads to the improvement of the beach quality and storm
protection. However, this measure is expensive to transport large quantity of sand to
fill up the beach as sand is continually getting eroded away. Coral reefs also get
destroyed as the extra sand washed out to sea and covers the corals, depriving them
of the light which they need to survive.
Nourishment gained popularity because it preserved beach resources and avoided
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the negative effects of hard structures. Instead, nourishment creates a soft (i.e.,
non-permanent) structure by creating a larger sand reservoir, pushing the shoreline
seaward.
It is important to note that beach nourishment does not halt erosion, but simply
provides sediment from an external source, upon which eroding forces will continue
to act. In this regard, beach nourishment provides a sacrificial, rather than a fixed
barrier against coastal erosion. Shoreline erosion will continue to occur, but the
widened and deepened beach will provide a buffer to protect coastal infrastructure
and other assets from the effects of coastal erosion and storm damage.
Gold Coast beaches in Queensland Australia have experienced periods of severe
erosion. In 1967 a series of 11 cyclones removed most of the sand from Gold Coast
beaches. The Government of Queensland engaged engineers from Netherlands to
advise them. The recommendations include beach nourishment and an artificial reef.
It was required to dredge 3,500,000 cubic metres (4,580,000 cu yd) of compatible
sand from the Gold Coast Broadwater which is delivered through a pipeline to
nourish 5 kilometers (3.1 mi) of beach between Surfers Paradise and Main Beach.
The new sand was stabilized by an artificial reef constructed at Narrowneck out of
huge geotextile sand bags. The new reef was designed to improve wave conditions
for surfing.
The cost/benefit ratio was conservatively estimated at 75:1 for a AUS$10million
investment into beach replenishment. Coastal tourism heavily depends on sun, sea
and sand. Beach nourishment has the potential to promote recreation and tourism
through beach widening (Nicholls et al., 2007b). This may serve to enhance pre-
existing tourism or may serve to attract tourists to the area, thus encouraging
development. This project is considered as successful as it improves beach amenity,
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allowing more open space with improved fishing, diving and surfing conditions.
These help to flourish the tourism market, in return bring along the economic
benefits.
In Netherlands, more than one-quarter of the land is below sea level and about 80%
of the coast consists ofsand dune or beach. The shoreline is closely monitored by
yearly recording of the cross section at points 250 meters (820 ft) apart, to ensure
adequate protection. Where long-term erosion is identified, beach nourishment using
high-capacity suction dredgers is deployed
Natural Beach (Do nothing)
In some areas, it is accepted that nothing can be done, and the local authority may
draw a line to alert the community that building is not allowed and no protection will
occur. It is a cheap and expedient strategy, which is also considered as very
environmental friendly, the only pollution produced is from the resettlement process.
A good example is at Dunwich (South west coastline of England), where low cliffs
made of soft sands are being eroded, however, there has been no attempt to stop
this. Many people believe that the only effective way to stop erosion is to allow the
waves to create a new beach. This would mean losing some of the shore and
perhaps the village at Dunwich itself. Thus, this Do Nothing approach is not well
received socially in the local community.
Another example on Do Nothing approach is from the angle of the ecological and
wildlife value such as those at Keyhaven and Lymington. The area is low-lying with
the threat of flooding at times of highest tides or during severe storms. There is a
growing body of opinion which believes that flooding of certain coastal areas should
be allowed as a natural event (which would preserve natural habitats) rather than
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protecting them by expensive schemes. Attempts have been made to protect wildlife
habitats by creating Nature Reserve and a bird sanctuary, and to encourage
sustainable tourism by establishing country parks. This is the approach that the New
Forest Council has explored.
Managed Retreat
An alternative option is to move structures and infrastructure inland as the shoreline
erodes. Retreat is more often chosen in areas of rapid erosion and in the presence of
little or obsolete development. It is cost effective. It preserves natural coastline and
probably saves lives. There are no direct costs apart from that of removing any
defences already in place and maintenance costs are very low.
Sediment flow is also restored to its natural state, beaches can be naturally
replenished due to erosion of the coast, providing protection and the balance of the
coastline returns.
To the west of the beach at St Margarets at Cliffe, in Kent UK, is a headland made
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from chalk. Longshore drift is moving sediment from the west towards the east, so
the pebbles on St Margarets beach will have come from the far side of this cliff.
The headland projects further out to sea than the beach at St Margarets and thus
protects the beach to some extent. However, because the headland is subjected to
the full force of the waves moving in from the west, it is vulnerable to erosion.
Stabilizing tall cliffs such as these can very difficult and expensive. Preventing these
cliffs from eroding could also cut off the supply of sediment that is needed to maintain
the beach.
This is an example of Managed Retreat. Nature is taking its course and a recent
rock fall can be seen at the base of the cliff. This approach saves money and helps
to ensure a supply of sediment for St Margarets beach, but at the cost of losing the
buildings on the cliff top. Compensation may have to be paid for the loss of the
buildings.
Freiston Shore and Abbott's Hall Farm, at Great Wigborough in the Blackwater
Estuary, it is one of the largest Managed Retreat schemes in Europe. It covers
nearly 280 hectares of land on the north side of the estuary. As monitoring of the
managed retreat scheme is lacking, very few sites are effectively monitored and
evaluated. This hampers the future managed realignment projects.
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Freiston Shore Managed Realignment site, Lincolnshire
Conclusions
Over the years various measures have been developed to defense against flooding
and erosion. Traditional defences include the building of concrete sea-walls and the
construction of groynes. Although their main purpose is to prevent material moving
along the beach, they can help to reduce the effects of breaking waves and by
widening the beach. However, it is now realized that concrete sea walls, apart from
being eyesores and at variance with local habitats, absorb rather than reflect wave
energy. Without constant maintenance and expense, they can be breached. The
modern approach is to work with nature, rather than against it and to implement
schemes that are more cost effective, which retain wildlife and which enhance the
environment. Combining hard and soft engineering measures is sometimes
necessary to improve the efficiency and provide an environmentally and
economically acceptable coastal protection system. Hard engineering measures are
known to be relatively costly and spoil the aesthetic aspect of the beaches or
coastline. Meanwhile, soft engineering can take time to become effective, and the
protection may only last for 5 to 10 years, which is rather short term. To optimize the
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long term positive impact, an integrated approach is preferred (i.e combining hard
and soft engineering measures). For example, combining beach nourishment and
groynes ; and rock revetments and groynes. There is evidence that coastal forest
and trees can play a protective role in coastal erosion. Their clearance has
increased the vulnerability of coasts to erosion. The vegetation can improve slop
stability, consolidate sediment and diminish the amount of wave energy moving
onshore, which should be encouraged.
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