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CHAPTER 4
PROTECTION FROM COASTAL EROSION
Thematic paper: The role of coastal forests and trees in
protecting against coastal erosion
Gegar Prasetya
1

1 Introduction
Shoreline changes induced by erosion and accretion are natural processes that take place over a
range of time scales. They may occur in response to smaller-scale (short-term) events, such as
storms, regular wave action, tides and winds, or in response to large-scale (long-term) events such
as glaciation or orogenic cycles that may significantly alter sea levels (rise/fall) and tectonic
activities that cause coastal land subsidence or emergence. Hence, most coastlines are naturally
dynamic, and cycles of erosion are often an important feature of their ecological character. Wind,
waves and currents are natural forces that easily move the unconsolidated sand and soils in the
coastal area, resulting in rapid changes in the position of the shoreline.

Excluding the impact of human activity, these processes are simply natural evolutionary
phenonema. Human activities along the coast (land reclamation, port development, shrimp
farming), within river catchments and watersheds (river damming and diversion) and offshore
(dredging, sand mining) in combination with these natural forces often exacerbate coastal erosion
in many places and jeopardize opportunities for coasts to fulfill their socio-economic and
ecological roles in the long term at a reasonable societal cost.

Development within coastal areas has increased interest in erosion problems; it has led to major
efforts to manage coastal erosion problems and to restore coastal capacity to accommodate short-
and long-term changes induced by human activities, extreme events and sea level rise. The erosion
problem becomes worse whenever the countermeasures (i.e. hard or soft structural options) applied
are inappropriate, improperly designed, built, or maintained and if the effects on adjacent shores
are not carefully evaluated. Often erosion is addressed locally at specific places or at regional or
jurisdictional boundaries instead of at system boundaries that reflect natural processes. This
anomaly is mostly attributable to insufficient knowledge of coastal processes and the protective
function of coastal systems.

The costs of installing hard structures for coastal protection are very high; strong negative public
reaction to rock emplacements along the coast often aggravate the problem (Bray et al., 1995;
Black, 1999; Clark, 1995; van der Weide, 2001). This has led to uncertainty among managers and
local government authorities on how to treat shoreline erosion. It has become an issue for serious
debate for politicians, coastal managers, land- and property owners, lawyers, bankers, insurers and
fisherfolk, especially in areas of intensive use and rapidly rising coastal land value. Many of these
stakeholders are resorting to planned retreat where houses or hotels are simply removed and the
coast is left to erode. However, planned retreat can be expensive, unnecessary and sometimes
impossible, especially in highly modified environments.

Increased interest in soft structures for coastal protection (including increased forest cover) and a
combination of hard and soft structures is predominating and is consonant with advanced
knowledge on coastal processes and natural protective functions. There is evidence that coastal


1
Agency for the Assessment and Application of Technology, Indonesia.


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forests and trees provide some coastal protection and that the clearing of coastal forests and trees
has increased the vulnerability of coasts to erosion (Figure 4.1) such as in Viet Nam (Mazda et
al., 1997; Cat et al., 2006), Malaysia (Othman, 1994), Indonesia (Bird and Ongkosongo, 1980;
Nurkin, 1994; Tjardana, 1995), Sri Lanka (Samarayanke, 2003), India (Malini and Rao, 2004;
Gopinath and Seralathan, 2005) China (Bilan, 1993) and Thailand (Thampanya et al., 2006). This
paper will elaborate on and discuss the causes of coastal erosion induced by human activities;
erosion management options; and the role of coastal forests and trees in protecting coastal areas
against coastal erosion, as well as their socio-economic and environmental considerations.


Figure 4.1 Coastal erosion sites reported in Asian and Indian Ocean countries; the
inset indicates how clearing of coastal forest such as mangroves has increased
the vulnerability of coasts to erosion (base map source from ITDB, 2004)
2 Coastal erosion: Extent and causes
Coastal erosion and accretion are natural processes; however, they have become anomalous and
widespread in the coastal zone of Asia and other countries in the Indian Ocean owing to
combinations of various natural forces, population growth and unmanaged economic development
along the coast, within river catchments and offshore. This type of erosion has been reported in
China, J apan, India, Indonesia, Viet Nam, Sri Lanka, Thailand, Bangladesh and Malaysia.
2.1 The extent of the coastal erosion problem in Asia
Bilan (1993) reported that the erosion rate in the northern part of J iangsu Province in China is
serious and as high as 85 metres/year; in Hangzhou Bay the rate is 40 metres/year, while in Tianjin
it is 1656 metres/year. Erosion persists even where preventive measures such as sea dykes are
constructed. Beach scour has been found along coasts with sea-dyke protection. This erosion is
attributable to many factors such as river damming and diversion, that leads to less sediment
supply to the coast, and the clearing of mangrove forests, which makes coastal areas more
susceptible to the hazard. J uxtaposing these phenomena, the intensification of typhoons and storm
surges during the 42-year period between 1949 and 1990 has meant that storm surges with
increasing tidal levels exceeding one and two metres have occurred 260 and 48 times respectively,
thus exacerbating the erosion problem. Most of the sediment taken offshore by the storm waves has
been returned in minimal quantities to the coast during normal conditions owing to the frequent
storm intensity.
According to Othman (1994), nearly 30 percent of the Malaysian coastline is undergoing erosion.
Many of these areas are coastal mudflats, fringed by mangroves. Behind the mangroves there are
usually agricultural fields protected from tidal inundation by bunds (dykes). Locally, mangroves


105
are known to reduce wave energy as waves travel through them; thus, the Department of Irrigation
and Drainage has ruled that at least 200 metres of mangrove belts must be kept between the bunds
and the sea to protect the bunds from eroding. However, the mangroves themselves are susceptible
to erosion when the soil under their root systems is undermined by wave action that mostly occurs
during periods of lower water level or low tide. The value of intact mangrove swamps for storm
protection and flood control alone in Malaysia is approximately US$300 000/kilometre
(http://ramsar.org).

In Viet Nam, most of the coastline in the south that is located in a wide and flat alluvial fan and
bordered by tidal rivers fringed by wide mangrove swamps, has been eroded continuously at a rate
of approximately 50 metres/year since the early twentieth century (Mazda et al., 1997; Cat et al.,
2006). This massive erosion mostly due to wave and current action and vanishing mangrove
vegetation is attributable to the long-term impacts of human activities since the late nineteenth
century and also human-induced change within watersheds (dam construction that has reduced the
sediment supply to the shore). Erosion still occurs in the central coastal zone of Viet Nam and
preventive measures such as sea dykes, revetments, and tree plantations have been implemented in
many coastal areas; however, in the southern coastal zone, mangrove plantations have mitigated
wave action and prevented further erosion (Cat et al., 2006).

The rapid erosion of the coast of Sagar Island in West Bengal, India, is caused by several processes
that act in concert; these are natural processes that occur frequently (cyclones, waves and tides that
can reach six metres in height) and anthropogenic activities such as human settlement and
aquaculture that remove mangroves and other coastal vegetation. The erosion rate from 1996 to
1999 was calculated to be 5.47 square kilometres/year (Gopinath and Seralathan, 2005). The areas
that are severely affected by erosion are the northeastern, southwestern and southeastern faces of
the island. Malini and Rao (2004) reported coastal erosion and habitat loss along the Godavari
Delta front owing to the combination of the dam construction across the Godavari and its
tributaries that diminishes sediment supply to the coast and continued coastal land subsidence.

Sri Lankas experience with coastal erosion dates back to 1920 (Swan, 1974; 1984). It has become
more serious because mangroves are being eradicated by encroachment (human settlement),
fuelwood cutting and the clearing of coastal areas for intensive shrimp culture. Mangrove forest
cover was estimated to be approximately 12 000 hectares in 1986; this dwindled to 8 687 hectares
in 1993 and was estimated to be only 6 000 hectares in 2000 (Samrayangke, 2003). Approximately
US$30 million has already been spent on breakwaters and other construction to combat coastal
erosion on southern and western coasts (UNEP, 2006); however, coastal erosion still persists in
some coastal areas.

In Indonesia, coastal erosion started in the northern coast of J ava Island in the 1970s when most of
the mangrove forest had been converted to shrimp ponds and other aquaculture activities, and the
area was also subjected to unmanaged coastal development, diversion of upland freshwater and
river damming. Coastal erosion is prevalent throughout many provinces (Bird and Ongkosongo,
1980; Syamsudin et al., 2000; Tjardana, 1995) such as Lampung, Northeast Sumatra, Kalimantan,
West Sumatra (Padang), Nusa Tenggara, Papua, South Sulawesi (Nurkin, 1994) and Bali (Prasetya
and Black, 2003). US$79.667 million was provided by the Indonesian Government to combat
coastal erosion from 1996 to 2004, but only for Bali Island in order to protect this valuable coastal
tourism base (Indonesia water resource donor database:http://donorair.bappenas.go.id). A
combination of hard structures and engineering approaches (breakwaters/jetties/revetments) of
different shapes that fused functional design and aesthetic values, and soft structures and
engineering approaches (beach nourishment) was used. They succeeded in stopping coastal erosion
on Sanur, Nusa Dua and Tanjong Benoa beaches, but were neither cost effective nor efficient,
because during low tide all of the coastal area was exposed up to 300 metres offshore; thus, these
huge structures were revealed and became eyesores.



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In Thailand, intensification of coastal erosion came to notice during the past decade (Thampanya et
al., 2006). Overall, the net erosion is approximately 1.3 to 1.7 metres/year along the southern
Thailand coastline. Total area losses amount to 0.91 square kilometres/year for the Gulf coast and
0.25 square kilometres/year for the western coast. Most of the eroded areas increase with larger
areas of shrimp farms, less mangrove forest area, and when dams reduce riverine inputs and coastal
land subsidence transpires. In areas where erosion has prevailed, the presence of mangroves has
reduced erosion rates. Mangroves dominating coastal locations exhibit less erosion than areas with
non-vegetated land or former mangrove areas.

Such examples indicate that there is a strong relation between major coastal erosion problems
throughout the region and degradation of the protective function of coastal systems such as coastal
forest and trees particularly mangrove forest. Artificial and natural agents that induce mangrove
loss and make coastal areas more susceptible to coastal erosion include anthropogenic factors such
as excessive logging, direct land reclamation for agriculture, aquaculture, salt ponds, urban
development and settlement, and to a lesser extent fires, storms, hurricanes, tidal waves and
erosion cycles owing to changing sea levels (Kovacs, 2000). More scientific investigation and
quantification of the physical processes and dynamic interaction of the system is needed to
understand how and under what circumstances mangrove forests and other coastal vegetation
effectively protect the shoreline from erosion. A number of efforts have focused on field
observations, laboratory and numerical model experiments and theoretical analysis (Wolanski
1992; Mazda et al., 1997; Massel et al., 1999).






Plate 4.1 Different types of coastal protection structures in Tanjong Benoa and
Sanur Bali for protecting the valuable tourism base. Clockwise: Satellite images of
the offshore breakwater and artificial headland, groynes and beach nourishment
(Google maps); headland and beach nourishment with coconuts; loc cit waru trees;
revetment protection using limestone in combination with waru trees
(note the dangerous placing of the boats)



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2.2 The causes of coastal erosion
Coastal erosion and accretion are complex processes that need to be investigated from the angles of
sediment motion under wind, wave and tidal current action; beach dynamics within a
sediment/littoral cell; and human activities along the coast, within river catchments and watersheds
and offshore, both at spatial and temporal scales. In terms of temporal scales, the issue of sea-level
rise is complex and produces a range of environmental problems. As the sea level rises, the water
depth increases and the wave base becomes deeper; waves reaching the coast have more energy
and therefore can erode and transport greater quantities of sediment. Thus, the coast starts to adjust
to the new sea level to maintain a dynamic equilibrium. Figure 4.2 lists the processes of coastal
erosion and accretion, as well as natural factors and human activities.

Figure 4.2 The complex processes of coastal erosion and accretion


The key physical parameters that need to be understood to identify coastal erosion as a problem in
the coastal zone are:
Coastal geomorphology: Coastline type and sensitivity to coastal processes.
Wind: The main force in wave generation; under the right environmental conditions, wind
may transfer sediment from the beach environment landward on all open coastlines.
Waves: They are the most important forces for sediment erosion and transport to the coastal
zone. They introduce energy to the coast and also a series of currents that move sediment
along the shore (longshore drift) and normal to the shore (cross-shore transport). It is
important to understand the movement of wave forms as well as water particles and their
interaction with seabed material; also how the waves determine whether the coasts are
erosive or accretional.
Tides: They are influential in beach morphodynamics. They modulate wave action,
controlling energy arriving on the coast and drive groundwater fluctuation and tidal currents.


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The interaction of groundwater with tides in the coastal forest environment is crucial in
understanding why coastal forest clearance causes intensive coastal erosion in particular
environments.
Vegetation: Important for improving slope stability, consolidating sediments and providing
some shoreline protection.

Equally significant human activities that must be considered over the range of spatial and time
scales are:
Activities along the coast: Building houses via land reclamation or within sand dune areas
and port/harbour development has a long-term impact on shoreline change; protective
seawalls lead to erosion at the end of the structures, generate beach scouring at the toe of
seawall and shorten the beach face. This can occur in the short term (less than five years) or
the long term (more than five years). Other structures such as groynes and jetties typically
cause erosion down-drift of the structure within a short period of time (between five and ten
years). Removal of dune vegetation and mangroves will expose low energy shorelines to
increased energy and reduced sediment stability, causing erosion within five to ten years
Activities within river catchments/watersheds: Dam construction and river diversion cause
reduction of sediment supply to the coast that contributes to coastal erosion. The effects of
dam and river diversion in terms of coastal erosion are not straightforward, but there are mid-
to long-term impacts (20 to 100 years) with spatial scales approximately from one to 100
kilometres.
Onshore and offshore activities: Sand and coral mining and dredging may affect coastal
processes in various ways such as contributing to sediment deficit in the coastal system and
modifying water depth that leads to altered wave refraction and longshore drift. The impact
of these activities will be obvious within a short period of time (one to ten years).

Understanding the key processes of coastal dynamics and how the coasts function both in spatial
and temporal time scales (short and long term), as well as human activities along the coast, within
the river watershed and offshore is essential for managing coastal erosion because it may occur
without reason. A quantitative understanding of changes in spatial and short- and long-term time
scales is indispensable for the establishment of rational policies to regulate development in the
coastal zone (NRC, 1990). Table 4.1 summarizes possible natural factors and human activities that
affect shoreline change over a range of time scales, leading to coastal erosion.



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Table 4.1 Possible natural factors and human activities that affect shoreline change



3 The coastal type and protective function of the coastal system
Coastlines comprise the natural boundary zone between the land and the ocean. Their natural
features depend on the type of rocks exposed along the coastline, the action of natural processes
and the work of vegetation and animals. The intensity of natural processes formed their origin
either as erosional or depositional features. The geological composition of a coastal region
determines the stability of the soil, as well as the degree of rocky materials and their breakdown
and removal.
3.1 Coastal types
3.1.1 Cliff coast
Cliff coast can be classified as hard coast as it was formed from resistant materials such as
sedimentary or volcanic rocks. This type of coast typically has a short shore platform that is
usually exposed during low tide. Natural erosion is attributable to slope instability, weathering and
wave action and leads to regression of the shoreline. As illustrated in Figure 4.3, extreme wave
conditions such as storm waves and tsunamis will have a less erosive effect on this type of coast;
traces of tsunami wave height can be found on cliffs as a trim line where trees or shrubs on the cliff
had been erased.



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Figure 4.3 Cliff coast (modified from ARC [2000] and French [2001])
3.1.2 Clayey bank coast
This type of coast can be classified as a semi-hard coast, consisting of cohesive soils; it is
common on estuarine coastlines and often has nearly vertical banks ranging from one to five
metres in height. The rate of erosion is relatively high compared to the hard coast because it is
composed of weaker and less resistant material. Erosion is mostly due to coastal processes,
weathering and loss of vegetation cover (ARC, 2000). For extreme events such storms and
tsunami, as illustrated in Figure 4.4, vegetation cover plays a significant role in protecting the coast
from flooding and inundation by reducing wave height and energy and decelerating tsunami flow
speed; hence, erosive forces and inundation distance are decreased.


Figure 4.4 Clayey bank type coast (modified from ARC [2000] and French [2001])



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3.1.3 Intertidal/muddy coast
This type of coast is characterized by fine-grained sedimentary deposits, predominantly silt and
clay that come from rivers; it can be classified as a soft coast. It has a broad gentle seaward
slope, known as an intertidal mud flat where mangrove forest, saltmarshes, shrubs and other trees
are found. Most erosion is generated by river damming that reduces sediment supply, diminishes
vegetation cover (usually mangroves and saltmarshes) and exposes vegetation roots by lowering
the mud flat (Figure 4.5) that leads to their final collapse. During storms, healthy and dense
vegetation/coastal forest and trees can serve as barriers and reduce storm wave height, as well as
affording some protection to the area behind them. In the case of a tsunami, coastal forest and trees
can decrease wave height and tsunami flow speed to some extent if the forest is dense and wide
enough. Both extreme events can cause severe erosion and scouring on the coast and at the river
mouth.

Figure 4.5 Intertidal/muddy coast (modified from ARC [2000] and French [2001])
3.1.4 Sand dune coast
This type of coast consists of unconsolidated material, mainly sand, some pebbles and shells; it can
be classified as a soft coast. It has a gentle seaward slope known as dissipative beaches that
have broad fine sand and gradually steep slopes at the backshore/foredunes. Its profile depends on
wave form and energy and wind direction; hence, profiles can be adjusted to provide the most
efficient means of dissipating incoming wave energy. This type of coast experiences short-term
fluctuation or cyclic erosion accretion and long-term assessment is needed to identify erosion as
a problem here. Often accretion and dune rebuilding take much longer than erosional events and
the beach has insufficient time to rebuild before the next erosive event occurs. Erosional features
are a lowered beach face slope and the absence of a nearshore bar, berm and erosional scarps along
the foredune. Generally, erosion is a problem when the sand dunes completely lose their vegetation
cover that traps wind-borne sediment during rebuilding, improves slope stability and consolidates
the sand. During extreme events such as storms and tsunamis (Figure 4.6), this type of coast can
act as a barrier for the area behind the dunes. Sand dunes and their vegetation cover are the best
natural protective measures against coastal flooding and tsunami inundation.



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Figure 4.6 Sand dune coast (modified from ARC [2000] and French [2001])
3.1.5 Sandy coast
This type of coast consists of unconsolidated material mainly sand from rivers and eroded
headlands, broken coral branches (coralline sand) and shells from the fringing reefs. It can be
classified as a soft coast with reef protection offshore. The beach slope varies from gentle to steep
slopes depending on the intensity of natural forces (mainly waves) acting on them. Coconut trees,
waru (Hibiscus tiliaceus), Casuarina catappa, pandanus, pine trees and other beach woodland
trees are common here. Most erosion is caused by loss of (1) the protective function of the coastal
habitat, especially coral reefs (where they are found) that protect the coast from wave action; and
(2) coastal trees that protect the coast from strong winds. During extreme events (Figure 4.7),
healthy coral reefs and trees protect coasts to some extent by reducing wave height and energy as
well as severe coastal erosion.

Figure 4.7 Sandy coast (modified from ARC [2000] and French [2001])



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3.2 The protective function of coastal systems
Coastal areas with natural protective features can reestablish themselves after natural traumas or
long-term changes such as sea-level rise. The protective features of the coastal system vary (Figure
4.8). The role of coral reefs in coastal protection has been studied for some time and recent efforts
have focused on the role of coastal vegetation, especially mangrove forest and saltmarshes in this
context.

Figure 4.8 The natural protective features of coastal systems
3.3 Scientific findings on the protective functions of coastal forests and trees
Scientific investigations on how coastal vegetation provides a measure of shoreline protection have
been conducted (Sale, 1985; Kobayashi et al., 1993; Mazda et al., 1997; Massel et al., 1999;
French, 2001; Blasco et al., 1994; Moller et al. 1999; Wu et al. 2001; Baas 2002; J arvela 2002;
Mendez and Losada 2004; Lee, 2005; Dean and Bender, 2006; Daidu et al., 2006; Moller, 2006;
Turker et al., 2006). These field, laboratory and numerical studies show that mangrove forest and
other coastal vegetation of certain density can reduce wave height considerably and protect the
coast from erosion, as well as effectively prevent coastal sand dune movement during strong
winds. Healthy coastal forests such as mangroves and saltmarshes can serve as a coastal defence
system where they grow in equilibrium with erosion and accretion processes generated by waves,
winds and other natural actions.
3.4 Field studies
The coastal areas around the Bay of Bengal are vulnerable to strong winds, storm surges, tectonic
movement, oversedimentation, rapid coastal erosion, fluctuating water and soil salinity and long
periods of constant flooding. Based on their scientific investigations in the Bay of Bengal, Blasco
et al. (1994) reported that the mangrove areas in India and Bangladesh, especially at the mouth of
the Ganges (known as the Sunderbans the largest natural mangrove area of the region found in
one block) were able to heal cyclonic wounds and maintain the extent of their total area through
natural succession without human interference. Mangroves in these regions have withstood highly
adverse environmental conditions such as muddy soils with high salt and water content, destructive
tidal effects and strong winds over the flat areas where they have grown in geological terms since
the Tertiary (lower Miocene) Period. Via their root systems, mangroves can stabilize the substrate
where they occur. According to studies, most erosion is caused by diversion of river flow to coastal
areas and mangrove regression due to human activities that convert them for agriculture or
aquaculture purposes.

Fan et al. (2006) analysed cross-shore variations in the morphodynamic processes of an open coast
mudflat in Changjiang Delta, where waves play a dominant role in shaping the tidal-flat profile
during typhoons. Each year, roughly seven out of 16 typhoons directly strike Chinas coast with a
95 percent probability of hitting southwards and the coast of the Chanjiang Delta; they generate


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waves up to 6.2 metres in height. One-third of the mudflat is colonized by Spartina alterniflora,
followed by scirpus (Scirpus mariquete and Scirpus triquiter), and then gradually transits into a
less-vegetated pioneer zone behind the bar mudflat. The boundary between the mature marsh and
the pioneer marsh is located near the mean neap high water mark and single Scirpus stems can
survive near the neap lower water elevation. The site-specific erosion rate is related to the local
water depth, sediment properties, vegetation and exposure time per semi-diurnal tidal cycle. The
area below mean sea level (MSL) at the intertidal mudflat is characterized by dynamic changes in
erosion and accretion phases; meanwhile, the higher section is dominated by continuous accretion
due to the abundant sediment supply. The area where the Spartina-dominated marshes are found
has continuous accretion without significant erosion owing to the protection afforded by high and
dense vegetation.

Moller et al. (1999) studied wave transformation over saltmarshes through field and numerical
modelling studies. There is quantitative evidence of the effectiveness of a meso- to macrotidal
open coast saltmarsh in attenuating incoming waves over a range of tidal and meteorological
conditions. Field measurements indicated that wave energy dissipation rates over the saltmarshes
were significantly higher (an average of 82 percent) than over the sand flat (an average of 29
percent); this is due to the saltmarshes having greater surface friction compared to the sand flat.
Based on the numerical model, the surface friction factors are of the order 0.4. The results of this
study provide empirical support to maintaining saltmarshes in front of existing coastal defence
structures and for creating new saltmarshes as part of coastal set-back/shoreline re-alignment
schemes, as well as reduction of design criteria for flood defence embankments that are fronted by
saltmarshes.

Mazda et al. (1997) observed the physical processes in fringe forest in coastal areas of Thuy Hai
and Thuy Truong in Thai Thuy District, Thai Binh Province, Viet Nam, in a delta of the Gulf of
Tonkin. This study elaborated the characteristics of water elevation and water flow in these areas
and demonstrated wave reduction by mangroves in the tidal flat off the coast of Thuy Hai where
Kandelia candel has been planted for several years. Based on these field studies, the wave and
current characteristics of propagation through the mangrove forest area are as follows:
Tidal elevation rises faster at the early stage of flood tide and falls more slowly at the later
stage of ebb tide owing to the effects of flow resistance by mangrove vegetation and the
bottom mud. In comparison with changes in mangrove swamps dominated by Rhizophora
spp. and Bruguiera spp., changes effected by Kandelia candel are considerably smaller
because Rhizopora spp. and Bruguiera spp. have intricate and large prop roots or numerous
pneumatophores compared to Kandelia candel. These facts suggest that the effect of the drag
force on Kandelia candel on long-period waves, such as tidal waves, is weak compared to
those of Rhizophora spp. and Bruguiera spp.
The wave height of the swell increases with increasing tidal level, and decreases with
increasing proximity to the coast, which suggests wave energy loss caused by bottom friction
and resistance to flow by the mangrove vegetation.
Wave size decreases considerably through denser mangrove areas; therefore, in well-grown
and healthy mangrove areas, the effects on wave reduction do not decrease with increasing
water depth, which has important practical implications.
According to the research, the effectiveness of mangroves with Kandelia candel of sufficient
height (three to four years old) in reducing wave height per 100 metres was as high as 20
percent and increased to 95 percent when the trees were six years old. At this age, one metre
wave height on the open coast will be reduced to 0.05 metre at the coast compared to 0.75
metre without mangroves. Vegetation height and density and the width of the area to be
planted are important factors in reducing wave height and protecting the coast from erosion.
The effect of wave reduction was considerable even when water depth increased due to the
high density of vegetation.



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Mazda et al. (2002) also analysed coastal erosion caused by tidal forces at Loang Hoa, South Viet
Nam, which is located in a wide, flat alluvial fan and lies between two major tidal rivers the Mui
Nai River and the Nga Bay River. Based on field and numerical studies, they found that
degradation of mangroves along the tidal rivers led to intensification of tidal currents at the mouths
of the rivers and erosion on the coast. This study reached the same conclusion as a study carried
out by Wu et al. (2001) for Merbok Estuary, Malaysia.

Othman (1994) observed that nearly 30 percent of the coastline of Malaysia is undergoing erosion.
Many of these areas are coastal mudflats, fringed by mangroves. He found that instead of erosion
due to clearing of the mangrove area for development projects, conditions in the west coast of
Peninsular Malaysia suggest that mudflats undergo a cycle of accretion and erosion such as found
in Sungai Burong, Pulau Pinang, where this cycle is about 20 years. Based on his observations in
Sungai Besar Selangor, a 50-metre-wide belt of Avicennia is sufficient to reduce waves of one
metre to a height less than 0.3 metre. However, these mangroves are also susceptible to erosion
generated by the lowering of mudflats in front of the mangroves that leads to waves agitating the
mud base below the root system and causing trees to collapse. Avicennia is a pioneer species that
decelerates currents via its root system, and together with its trunk, attenuates wave height. The
closer the trees are to each other, the greater the wave energy will be reduced. A five-year new
growth of Avicennia can serve as an efficient wave attenuator. In the studied area behind the
Avicennia zone, Rhizophora and Bruguiera zones were found, which contribute to reduced wave
height and velocity.
3.5 Laboratory model experiments
Among the different coastal protection techniques and procedures, the protective capacity of
coastal vegetation has yet to be investigated and analysed in detail (Turker et al., 2006).
Knowledge on the interaction between vegetation and incident waves creates a better
understanding of ecological and geomorphological processes in coastal waters, with particular
respect to coastal defence management by vegetation. Important developments in understanding
the effects of vegetation on coastal bed morphology and the interaction between waves, sediment
transport and the vegetated area can be achieved through extensive studies in controlled laboratory
conditions. The controlled laboratory environment will allow measurement of wave parameters
that are not easily measurable in natural conditions. Coops et al. (1996) conducted an experimental
study in a wave tank to assess the interaction between waves, bank erosion and emergent
vegetation. They used two helophyte species, Phragmites australis (Cav.) Trin. ex Steudel and
Scirpus lacustris L., and two types of sediment (sand and silty sand) in a wave tank. Their findings
showed that emergent vegetation influenced the erosive impact of waves by both sediment re-
inforcement and wave attenuation. A smaller amount of net erosion was measured in the wave-
exposed area covered by vegetation than in the area where there was no vegetation. Most of the
erosion of the soil occurred due to the uprooting of rhizome parts, and in this case it happened to
Scirpus lacustris but not to Phragmites australis. The greatest wave attenuation was measured in
fully developed vegetation of both species.

Struve et al. (2003) combined laboratory experiments (in wave flume) and a two-dimensional (2-
D) numerical model to investigate additional roughness owing to vegetation as an important factor
for influencing water velocities and levels in a mangrove estuary. The effect of varying stem
diameter and density was tested and staggered and linear models of tree distribution were also
analysed. The smallest dowels used in some experiments were fitted with bent extensions similar
to the stilt roots of Rhizophora. The results of the study showed that the effect of stilt roots was
similar to stems, despite their different shape, and the change in velocity along the flume was
related to the surface area of the model trees. A comparison between staggered and linear model
tree distribution indicated that hydraulics shading had an effect based on the size and interference
of the wave.



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Turker et al. (2006) examined a changing beach profile under the protection of emergent
vegetation (Phragmites australis without foliage) through various wave conditions in a laboratory;
the most important governing parameters of coastal erosion in term of waves and sand properties
under the protection of emergent vegetation were evaluated and defined. The findings showed that
emergent vegetation is a relevant element for beach protection that tends to minimize erosion and
even leads to zero erosion (Turker et al., 2006). The vegetated area absorbs substantial wave
energy due to friction and drag force; the experimental analysis showed that the amount of erosion
is directly proportional to wave height and inversely proportional to sediment particles and the
density of vegetation.
3.6 Numerical model and analytical studies
Massel et al. (1999) used a theoretical approach to predict the attenuation of wind-induced random
surface waves in mangrove forest. The geometry of mangrove trunks and their locations were
taken into account and the interaction between mangrove trunks and roots was introduced through
the modifications of the drag coefficients. Examples of numerical calculations based on
observations of wave attenuation through mangrove forest at Townsville, Australia, and Iriomote
Island, J apan, showed that the resulting rate of wave energy attenuation depends strongly on the
density of the mangrove forest, the diameter of mangrove roots and trunks, and on the spectral
characteristic of the incident waves. Computation results for very dense mangrove forest (width
unfortunately not defined), indicated that waves attenuate very quickly with distance from the
mangrove front, and in the area behind the mangroves they are negligible. With very low density of
mangrove forest and the same wave characteristics, wave energy is transmitted relatively easily
through the mangrove forest; however, approximately 86 percent of the energy is still dissipated by
the mangroves. The field observations (Figure 4.9) show almost the same results on how the
mangrove forest can attenuate waves significantly over a relatively short distance. Wave energy is
reduced by 75 percent in the wave passage through 250 metres of mangroves.

An interesting result recently revealed by Dean and Bender (2006) in relation to the effects of wave
damping by vegetation and bottom friction on the static wave set-up during a severe storm is in
line with studies to establish hazard zones associated with 100-year storm events along the
shoreline of the United States; it can be used to explain the phenomena investigated by Blasco et
al. (1994) for the Bay of Bengal area. Based on these studies, using various wave characteristics,
the effect of vegetation and bed friction on both internal and bottom energy losses, and associated
forces, resulted in a net wave set-down rather than wave set-up, which decreased wave impact on
the shoreline behind the vegetated/coastal forest area.

More research is still needed for other coastal forests and trees such as Pandanus, Casuarina and
pine, which do not directly interact with waves during normal conditions, but do so during extreme
events such as major storms and tsunamis. Current knowledge is adequate to derive a general
guideline on the protective role of the coastal forest in combating coastal erosion. However, it
should be borne in mind that the effectiveness of each species in protecting the coast from erosion
is very site-specific.



117


Figure 4.9 Wave attenuation by mangrove forest (Rhizophora sp., Aegiceras sp.,
Ceriops sp.) at Cocoa Creek, Australia is obvious; measurements at sites 25 show
the decline in wave energy transmission through the mangrove forest.
The incoming wave was measured at site 1 (Massel et al.,1999)
4 Managing the coastal erosion problem: Options for coastal protection
Assessing coastal erosion can be done by visual observation and through discussions with
inhabitants to ascertain its degree and when it started. Common visual indicators to identify erosion
problems are summarized in Table 4.2. However, determining the causes of coastal erosion and
which coastal protection options should be used requires a comprehensive study of coastal
processes that work on a regional scale (not only on sites) through every season.

Options for combating coastal erosion are traditionally twofold, namely hard structural/engineering
options and soft structural/engineering options. These solutions have at least two hydraulic
functions to control waves and littoral sediment transport (Kawata, 1989); in applying the
solutions, their underlying principles should be well-understood, otherwise they will fail. A
combination of hard and soft options has become more popular recently for optimum results
because they have weaknesses when used singlely. Many schemes have failed and resulted in
environmental and socio-economic problems owing to improper design, construction and
maintenance, and were often only implemented locally in specific places or at regional or
jurisdictional boundaries, rather than at system boundaries that reflect natural processes
(Kamphuis, 2002).




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Table 4.2 Common visual indicators for identifying erosion problems
All coastlines Cliff and platform
(hard coast)
Clayey banks
and muddy
coast
(semi-hard
coast)
Sandy coast
(soft coast)
Object (e.g. fence, shed or
tree) which falls into the
sea
Very steep cliff faces Tree angle Stable backdune
vegetation in the
active zone
Presence of existing
coastal erosion
management works
(particularly poor condition
of structures)
Shore platforms

Non-vegetated
clayey banks
Damaged vegetation
in the active zone
(exposed roots)
Sea caves, notches,
stacks
Slumping slopes Erosion scarps
Debris at toe of cliff

Dislodged
vegetation in the
coastal area
Discontinuous
vegetation cover on
foredunes
Tree angle at the top
of the cliff
Erosion scarps Irregular foredune
crest, blow outs
Very steep dune
formation
4.1 Hard structural/engineering options
Hard structural/engineering options use structures constructed on the beach (seawalls, groynes,
breakwaters/artificial headlands) or further offshore (offshore breakwaters). These options
influence coastal processes to stop or reduce the rate of coastal erosion.
4.1.1 Groyne
A coastal structure constructed perpendicular to the coastline from the shore into the sea to trap
longshore sediment transport or control longshore currents. This type of structure is easy to
construct from a variety of materials such as wood, rock or bamboo and is normally used on sandy
coasts. It has the following disadvantages:
Induces local scour at the toes of the structures.
Causes erosion downdrift; requires regular maintenance.
Typically more than one structure is required.
4.1.2 Seawall
A seawall is a structure constructed parallel to the coastline that shelters the shore from wave
action. This structure has many different designs; it can be used to protect a cliff from wave attack
and improve slope stability and it can also dissipate wave energy on sandy coasts. The
disadvantages of this structure are:
It creates wave reflections and promotes sediment transport offshore.
Scour occurs at the toes of eroded beaches.
It does not promote beach stability.
It should be constructed along the whole coastline; if not, erosion will occur on the adjacent
coastline.



119
4.1.3 Offshore breakwater
An offshore breakwater is a structure that parallels the shore (in the nearshore zone) and serves as a
wave absorber. It reduces wave energy in its lee and creates a salient or tombolo behind the
structure that influences longshore transport of sediment. More recently, most offshore breakwaters
have been of the submerged type; they become multipurpose artificial reefs where fish habitats
develop and enhance surf breaking for water sport activities. These structures are appropriate for
all coastlines. Their disadvantages are:
They are large structures and relatively difficult to build.
They need special design.
The structure is vulnerable to strong wave action.
4.1.4 Artificial headland
This structure is constructed to promote natural beaches because it acts as an artificial headland. It
is relatively easy to construct and little maintenance is required. The disadvantages are:
It is a relatively large structure.
It can cause erosion downdrift of the protected length of coastline.
Has poor stability against large waves.
4.2 Soft structural/engineering options
Soft structural/engineering options aim to dissipate wave energy by mirroring natural forces and
maintaining the natural topography of the coast. They include beach nourishment/feeding, dune
building, revegetation and other non-structural management options.
4.2.1 Beach nourishment
The aim of beach nourishment is to create a wider beach by artificially increasing the quantity of
sediment on a beach experiencing sediment loss, improving the amenity and recreational value of
the coast and replicating the way that natural beaches dissipate wave energy. Offshore sediment
can be sourced and is typically obtained from dredging operations; landward sources are an
alternative, but are not as effective as their marine equivalents because the sediment has not been
subject to marine sorting.

This method requires regular maintenance with a constant source of sediment and is unlikely to be
economical in severe wave climates or where sediment transport is rapid.

It has been used in conjunction with hard structural/engineering options, i.e. offshore breakwaters,
headlands and groynes to improve efficiency.
4.2.2 Dune building/reconstruction
Sand dunes are unique among other coastal landforms as they are formed by wind rather than
moving waters; they represent a store of sand above the landward limits of normal high tides where
their vegetation is not dependent on the inundation of seawater for stability (French, 2001). They
provide an ideal coastal defence system; vegetation is vital for the survival of dunes because their
root systems bind sediment and facilitate the build-up of dune sediment via wind baffle. During a
storm, waves can reach the dune front and draw the sand onto the beach to form a storm beach
profile; in normal seasons the wind blows the sand back to the dunes. In dune building or
reconstruction, sand fences and mesh matting in combination with vegetation planting have
successfully rgenerated dunes via sediment entrapment and vegetation colonization. The vegetation
used should be governed by species already present, such as marram, sand couch grass and lyme
grass.


120
4.2.3 Coastal revegetation
Based on studies and scientific results, the presence of vegetation in coastal areas improves slope
stability, consolidates sediment and reduces wave energy moving onshore; therefore, it protects the
shoreline from erosion. However, its site-specificity means that it may be successful in estuarine
conditions (low energy environment), but not on the open coast (high energy environment). In
some cases, revegetation fails because environmental conditions do not favour the growth of
species at the particular site or there is ignorance as to how to plant properly given the same
conditions. It is also possible that anthropogenic influences have completely altered the natural
processes in the area. The most obvious indicator of site suitability is the presence of vegetation
already growing. This can be extended by other factors such as the slope, elevation, tidal range,
salinity, substrate and hydrology (Clark, 1995; French, 2001).
4.2.3.1 Coastal revegetation in muddy coastline environments (tidal zones of
estuaries)
In muddy coastal environments or within the tidal zones of estuaries, mangrove forest and other
indigenous shrub species are commonly found. Most erosion in these zones is attributable to the
removal of the mangroves and other trees. To overcome this problem, replanting is necessary
because these trees can regenerate and serve as coastal defence structures.

Planting vegetation species relative to their correct elevation in mudflat environments is important.
At low- and subtidal deltas below the high water mark, saltmarsh species are recommended.
Saltmarshes are typically zoned according to elevation, the zones being controlled by the frequency
and duration of tidal inundation. Within this zone, Spartina as a pioneer species is tolerant of more
frequent inundation than higher marsh species, and as such, is often used because it can be planted
well down the intertidal zone (French, 2001). Other saltmarsh species that can be used are
helophyte species such as Phragmites australis (Cav.) Trin. ex Steudel and Scirpus lacustris L.
Within this area, mangroves are also recommended and can be planted easily. If the area already
has a serious erosion problem, then special seeding techniques are needed.

A combination of species is suggested to reduce pest damage; however, the choice should be well
planned in order to avoid competition. A number of publications provide planting/replanting
guidelines, for example Hanley (2006). The mangrove forest should have a minimal width of 300
metres, densities of at least 0.5 metre and be planted in staggered alignment.
4.2.3.2 Coastal revegetation on other coastal types
Sandy coast
Beaches composed of fine sand are usually broad and have a gentle seaward slope representing a
low energy environment; beaches with coarse sand, gravel, shells, or broken coral branches have
relatively steep slopes representing a high energy environment. Short-term fluctuations on these
coasts (if there is no human intervention) do not mean that an erosion problem exists; variations on
the beach face are the natural response of the beach to wave form and energy and also strong
winds. After extreme conditions, a naturally eroded beach, with features such as a lowered beach
face slope, the absence of berms and erosional scarps along the backshore/foredune will return to
normal during lower wave energy seasons when waves return sand to the beach and wind
transports it landwards to rebuild the upper beach and foredune. Therefore, long-term observations
are needed before deciding that the beach is being seriously eroded.

Severe erosion problems on these types of beaches are usually due to human activities such as dam
building that decreases the river sediment supply to the coast, vegetation clearance on dunes and in
beach woodlands, offshore mining, and building inappropriate coastal structures. In terms of
erosion generated by vegetation clearance, revegetation of the area using indigenous vegetation is
the only option. Other coastal protection options should be considered in combination with
revegetation if the erosion problem is attributable to multiple factors.


121
Cliff and platform structures
Erosion of cliff and platform structures where there is no beach during high tide is due to complex
processes and no single process predominates. These include gradual changes to cliff morphology
owing to weathering and wave action at the base of the cliff, and slope instability due to episodic
failure of the cliff. Planting shrubs and trees will improve slope stability, for example with belukar
(dense thickets possibly dominated by isolated trees tangled with lianas); however, other coastal
protection options should be considered in combination with revegetation.
4.3 Combinations of options
As mentioned already, combining hard and soft solutions is sometimes necessary to improve the
efficiency of the options and provide an environmentally and economically acceptable coastal
protection system. Hard solutions are known to:
cause erosion and unnecessary accretion;
be expensive and often further aggravate the problem; and
spoil the aesthetic aspect of the beaches or coastlines they seek to protect, hence decreasing
their economic value, especially for tourism purposes.

Meanwhile, many soft solutions can:
take time to become effective (not overnight or quick-fix solutions), which generates
negative public response; and
be effective solutions only in medium- to long-term perspectives (five to ten years).
A planned retreat where the coast is left to erode can be expensive, unnecessary and sometimes
impossible, especially in highly modified environments such as tourism areas and waterfront cities.
To optimize the long-term positive impact of soft solutions, many combinations with hard
solutions can be selected; combining beach nourishment and artificial headlands/groynes and
revegetation and temporary offshore breakwaters/artificial reefs that act as interim hard structures
is the most common approach.
4.3.1 Beach nourishment and artificial headlands/groynes
To reduce the frequency of renourishment and downdrift erosion in beach nourishment options,
artificial headlands or groynes are often used as they can trap the downdrift movement of sediment.
4.3.2 Revegetation and temporary offshore breakwaters/artificial reefs
In some cases, revegetation in a low energy environment is required because deforestation of the
coastal forest has led to direct exposure to wave action. There is also a need to establish offshore
breakwaters/artificial reefs as temporary wave protection structures for mangroves and
saltmarshes; otherwise, seawalls/revetments for vegetation that grows above the highest water
mark such as waru, Casuarina, pine and palm trees can be built. Once the plants have established
themselves, the structures may be removed.
5 Social and environmental implications
Social and environmental problems caused by coastal erosion are relevant and easy to observe.
Losses in aquaculture (fish or shrimp ponds) due to erosion diminish fishery productivity and
increase the number of unemployed people. Erosion has the same impact on tourism and urban
areas where decreasing property values are a major problem. The problem is exacerbated when
coastal protection measures have been improperly designed, constructed and maintained, or when
they stop locally at specific places such as in front of hotels or properties that can afford to protect
their own beaches, but the adjacent coast is left to erode (Plate 4.2). Or they may stop based on
ownership (high public vs. private values) or at jurisdictional or administrative boundaries rather


122
than at system boundaries that reflect natural processes. These solutions create more problems
than answers. Coastal erosion cannot be resolved in a piecemeal fashion; protective measures
should be integrated, consider socio-economic conditions and reflect the natural processes that
work in the region.


Plate 4.2 Coastal protection efforts to protect a valuable tourism base; meanwhile,
the adjacent shore with less economic value has minimal and improper protection.
Even revegetation with waru to replicate planting at the neighbouring resort failed;
the coast was then abandoned and left to erode

Hard and soft options have positive and negative aspects. Most hard options are effective solutions
in the short term but create domino effects. They stop local erosion in order to protect the asset, but
then contribute to erosion in adjacent areas. In the long term their effectiveness is mostly
unsatisfactory. Meanwhile, soft options are effective solutions in medium- to long-term
perspectives, but the main issue (French, 2001; Eurosion, 2004) is raising awareness among the
public so that they provide effective protection for their homes and businesses; the public
perception of security is critical. During the planning process, it is quite common for many people
to admit feeling much more secure behind a concrete wall than behind a beach and forest. In this
context, a combination of hard and soft structures is advisable.

Public or community involvement in using coastal forest and trees as coastal protection measures is
very important during planning, implementation and monitoring; it will raise awareness on these
solutions and the concomitant benefits for the ecosystem and all stakeholders. A good example
comes from Bangladesh (Clark, 1995). The coastal green belt that had been incorporated with
coastal embankments used a variety of trees that afforded not only protection from hazards, but
also offered various benefits from the output of the green belt such as fruits, nuts, thatch, coconuts,
fuel and poles.

Nature has not only demonstrated how to erode, but also how to protect, and there is probably no
protective measure initiated by human beings that has not been originally developed by nature
(Bruun, 1972; Bache and MacaSkill, 1981).
6 General guidelines on managing coastal erosion and their options
Understanding the key processes of coastal dynamics and how coasts developed in the past and
present, as well as over the short and long term, is very important for managing coastal erosion
problems because coastal erosion may occur without cause for concern. This can be very complex
and possibly controversial where many conflicts of interests exist within the coastal environment.
The main underlying principles for coastal erosion management are as follows (NRC, 1990; ARC,
2000):
Identify and confirm coastal erosion as a problem.



123
Identify, confirm and quantify the cause of the problem and ensure that any management
option is well thought out before implementing coastal erosion measures.
Understand the key processes and characteristics of coastal dynamics and system boundaries
that reflect the natural processes of the erosion problem.
Determine the coastal erosion measure options and implement them using proper design,
construction and maintenance with careful evaluation of the effects on adjacent shores.
Consider the balance of the options costs and their associated benefits.

A flowchart of this general guideline is given in Figure 4.10.


124


Figure 4.10 Flowchart of the guidelines for managing coastal erosion
(modified from ARC, 2000)
Site identification
Confirming that coastal erosion is a problem
Identifying, confirming and quantifying the cause of the
problem and understanding the environmental context
Understanding the key processes
Determining the coastal erosion measure options
Are non-structural/soft
structure options viable?
Are hard structural options
viable?
Are combined options
viable?
Assess effects, preferred option and public
consultation
Are costs and associated
effects acceptable?
Proceed to detailed design, construction, maintenance
and monitoring
yes
No
yes
No
yes
yes
No
yes


125
Site identification: Simple site identification of coastal erosion can be done visually and through
discussion with local inhabitants to give an indication of what is happening on the coast and when
it started.

Confirming coastal erosion is a problem: As the coast has transient features, it is natural for it to
erode or accrete sediment in response to changing forces; therefore, confirming that coastal erosion
is a problem is very fundamental. Generally, coastal erosion is problematic in tourism areas,
waterfront cities/residential blocks and aquaculture sites (shrimp ponds). Any legal issues pertinent
to coastal management and development should be considered and discussed.

Identifying, confirming and quantifying the cause of the problem: Identifying the cause of the
problem requires analysis that should consider any possible sources both natural and
anthropogenic confirm them, and quantify the scale and intensity of the source and impact in
the past, present and possibly in the future. Field observations and data collection of not only
physical, but also socio-economic (including historical if available) data of the region are required
and crucial at this stage. This will give an idea of what kind of options could be implemented,
related to any legal or policy framework on coastal erosion management for the region.

Understanding the key processes: Understanding the key processes of coastal dynamics and how
the coasts are functioning in areas where coastal erosion is a problem is essential to determine the
system boundary that reflects natural processes. Many mathematical/numerical and physical
models of coastal systems have been developed as tools to understand the behaviour of coastal
systems. These tools can predict coastline evolution and interaction with the source of the problem
and possible options to be implemented in the short and long term. These tools thus contribute to
countermeasure planning and design as well as coastal erosion management.

Determining options for addressing coastal erosion: Choosing the optimum option must involve
the public or community affected by the erosion. Discuss the available options and provide
technical information such as design of the options, materials to be used, construction methods and
maintenance and costbenefit analysis in a wider context to consider the balance of the cost and
associated benefits.
6.1 Set up a green belt/buffer zone
The purposes of setting up coastal green belts must not be solely for preventing coastal erosion and
mitigating other natural hazards, but also for addressing the socio-economic status of the local
communities as well as ecological sustainability.

The purposes of coastal green belts/buffer zones must serve to:
control and stabilize the shoreline by holding and trapping sediments and consolidate land
for areas such as intertidal mudflats with mangrove green belts and sandy coasts with
Casuarina, pine trees or coconuts and palm trees;
attenuate the force of devastating storm surges and waves that accompany cyclones and
tsunamis;
provide an amenity and a source of food, materials and income for local communities; and
benefit biodiversity and create habitat corridors for wildlife that can be used for conservation
activities and ecotourism development.

In general, the underlying concepts of setting up green belt/buffer zones (Clark, 1996) are:
Social forestry: This should not be considered as a source of government or private sector
revenue, but to support sustainable livelihood development among the coastal community.


126
Ecodevelopment: This is beneficial for conservation activities, educational and recreational
opportunities.
Participatory planning, implementation and monitoring: The indigenous knowledge of local
communities should be used in decision-making so they receive benefits directly.

Selection of the vegetation for setting up the green belt/buffer zone should take into account the
natural protective function of the coastal system as illustrated in Figure 4.11:
Start with vegetation at the waters edge and gradually proceed to hydric species inland.
Select water-edge vegetation that is found locally on each type of coast. In most cases the
width of the buffer zone for the intertidal delta ranges between 300 and 500 metres,
depending on the slope of the region.
Select beach vegetation that is found locally on each type of coast. The width of the buffer
zone should be a minimum of 100 metres for the flat area, even with sand dunes or coastal
embankments.
7 Conclusions
Coastal erosion and accretion are natural processes; however, they may become a problem when
exacerbated by human activities or natural disasters. They are widespread in the coastal zone of
Asia and other countries in the Indian Ocean owing to a combination of various natural forces,
population growth and unmanaged economic development along the coast, within river catchments
and offshore. This has led to major efforts to manage the situation and to restore the ability of the
coast to accommodate short- and long-term changes induced by human activities, extreme events
and sea-level rise. Understanding the key processes of coastal dynamics and how coasts are
functioning both in spatial and temporal time scales (short and long term), in juxtaposition with
human activities along the coast, within river watersheds and offshore is crucial for managing
coastal erosion problems. Three main conclusions can be drawn on the roles that coastal forest and
trees can play in combating coastal erosion:

1) There is evidence that they provide some coastal protection and their clearance has
increased the vulnerability of coasts to erosion. Based on scientific findings, the presence
of vegetation in coastal areas will improve slope stability, consolidate sediment and
diminish the amount of wave energy moving onshore, therefore protecting the shoreline
from erosion.
2) Increased interest in soft options (in this case the use of coastal forest and trees) for coastal
protection is becoming predominant and is in line with advanced knowledge on coastal
processes and the natural protective function of the coastal system. This is because hard
options are mostly satisfactory in the short term, while soft options are effective in medium
to long-term perspectives (five to ten years).
3) A combination of hard and soft solutions is sometimes necessary to improve the efficiency
of the options and to provide an environmentally and economically acceptable coastal
protection system.



127

Figure 4.11 The natural protective functions of coastal forest and trees, starting with
water-edge vegetation on intertidal deltas, rising to hydric species on higher soils
or land. The green belt model below with coastal embankments
(in Bangladesh) combines different type of trees, including fruit trees,
which benefit local communities (Clark, 1995)
Acknowledgements
The author wishes to thank FAO reviewers for their input during manuscript preparation.


128
Bibliography
ARC. 2000. Auckland regional council, 2000. Technical Publication No. 130. Coastal Erosion
Management Manual.
Baas, A.C.W. 2002. Chaos, fractals and self-organization in coastal geomorphology:
simulating dune landscapes in vegetated environments. Geomorphology, 48: 309328.
Bache D.H. & I.S. Macaskill. 1981. Vegetation in coastal and stream-bank protection.
Landscape Planning, 8: 363385.
Bird, E.C.F. & O.S.R. Ongkosongo. 1980. Environmental changes on the coast of Indonesia.
NRTS-12/UNUP-197. Tokyo J apan, the United Nation University. 55 pp.
Black K.P. 1999. Submerged structures for coastal protection: A short summary of what they
are, why we need them and how they work. Hamilton, New Zealand, Artificial Reefs
Program. Centre of Excellence in Coastal Oceanography and Marine Geology Department of
Earh Sciences, University of Waikato and National Institute of Water and Atmospheric
Research. 9 pp.
Bilan, D. 1993. The preliminary vulnerability assessment of the Chinese coastal zone due to
sea level rise. Proceedings of the IPCC eastern hemisphere workshop, Tsukuba, J apan 36
August 1993.
Blasco, F., J anodet, E. & M.F. Bellan. 1994.Natural hazards and mangroves in the Bay of
Bengal. Journal of Coastal Research, Special Issue No.12: 277288.
Bray, J .M., Carter, D.J . & J .M. Hooke. 1995. Littoral cell definition and budgets for central
southern England. Journal of Coastal Research, 11(2): 381400.
Bruun, P. 1972. The history and philosophy of coastal protection. Proc. 13
th
Coastal
Engineering Conf. ASCE.New York. NY (1). pp 3374.
Cat, N.N., Tien, P.H., Sam, D.D. & N.N. Bien. 2006. Status of coastal erosion of Viet Nam and
proposed measures for protection. This volume (abstract).
Clark, J .R. 1995. Coastal zone management handbook. Lewis. 695 pp.
Chong, J . 2005. Protective values of mangroves and coral ecosystem: A review of methods and
evidence. IUCN. G.V.
Coops, H., Geilen, N., Verheij, H.J ., Boeters & van der Velde. 1996. Interaction between waves,
bank erosion and emergent vegetation: an experimental study in a wave tank. Aquatic
Botany, 53: 187198.
Crowly, G.M. & M.K. Gagan. 1995. Holocene evolution of coastal wetland in wet-tropical
northeastern Australia. Holocene, 5(4): 385399.
Daidu, F., Guo, Y., Wang, P. & J .Z. Shi. 2006. Cross-shore variations in morphodynamic
processes of an open-coast mudflat in the Changjiang Delta, China: with an emphasis on
storm impacts. Continental Shelf Research, 26: 517538.
Dean, R.G. & C.J . Bender. 2006. Static wave setup with emphasis on damping effects by
vegetation and bottom friction. Coastal engineering, 53: 149156.
Eurosion. 2004. Living with coastal erosion in Europe: Sediment and space for sustainability.
Part IV a guide to coastal erosion management practives in Europe: lessons learned.
27 pp.
Fan, D., Guo, Y., Wang, P. & J .Z. Shi. 2006. Cross-shore variations in morphodynamic
processes of an open-coast mudflat in the Chanjiang Delta, China: with an emphasis on
storm impacts. Continental Shelf Research,26: 517538.
FAO. 1998. Integrated coastal area management and agriculture, forestry and fisheries. FAO
document repository, www.fao.org.PART C.
French, P.W. 2001. Coastal defences: processes, problems & solutions. Florence, KY, USA,
Routledge. http://site.ebrary.com/
Gopinath, G. & P. Seralathan. 2005. Rapid erosion of the coast of Sagar island, West Bengal
India. Environment Geology, 48: 10581067.
Huq, S., Karim, Z., Asaduzzaman, M. & F. Mahtab (Eds.) 1999. Vulnerability and adaptation to
climate change in Bangladesh. Dordrecht, the Netherlands, J . Kluwer Academic Publishers.
147 pp.


129
J arvela, J . 2002. Flow resistance of flexible and stiff vegetation: a flume study with natural
plants. Journal of Hydrology, 269: 4454.
Kamphuis, J .W. 2002. Introduction to coastal engineering and management. Advance series in
ocean engineering. World Scientific. 437 pp.
Komar, P.D. 1998. Beach processes and sedimentation. Second edition. Prentice-Hall.
Kobayashi, N., Raichle, A.W. & T. Asano. 1993. Wave attenuation by vegetation. Journal of
Waterway, Port, Coastal, and Ocean Engineering, 119(1): 3048.
Kawata, Y. 1989. Methodology of beach erosion control and its application. Coastal
Engineering in Japan, 32(1): 113132.
Kovacs, J .M. 2000. Perceptions of environmental change in a tropical coastal wetland. Land
Degradation & Development, 11: 209220.
Lee, T.M. 2005. Monitoring the dynamics of coastal vegetation in Southwestern Taiwan.
Environmental Monitoring and Assessment, 111: 307323.
Malini, B.H. & K.N. Rao. 2004. Coastal erosion and habitat loss along the Godavari delta
front a fallout of dam construction (?). Current Science, 87 (9): 1232126.
Massel, S.R., Furukawa, K. & R.M. Brinkman. 1999. Surface wave propagation in mangrove
forests. Fluid Dynamics Research, 24: 219249.
Mazda, Y., Magi, M., Nanao, H., Kogo, M., Miyagi, T., Kanazawa, N. & D. Kobashi. 2002.
Coastal erosion due to long-term human impact on mangrove forests. Wetlands Ecology
and Management, 10: 19.
Mazda, Y., Wolanski, E., King, B., Sase, A., Ohtsuka, D. & M. Magi. 1997. Drag force due to
vegetation in mangrove swamps. Mangroves and Salt Marshes, 1: 19931999.
Mazda, Y., Magi, M., Kogo, M. & N.P. Hong. 1997. Mangroves as a coastal protection from
waves in the Tong King delta, Viet Nam. Mangroves and Salt Marshes, 1: 127135.
Mendez, F.J . I.J . & Losada. 2004. An empirical model to estimate the propagation of random
breaking and nonbreaking waves over vegetation fields. Coastal Engineering, 51: 103118.
Moller, I., Spencer, T., French, J .R., Leggett, D.J . & M. Dixon. 1999. Wave transformation over
salt marshes: A field and numerical modeling study from North Norfolk, England.
Estuarine, Coastal and Shelf Science, 49: 411426.
Moller, I. 2006. Quantifying saltmarsh vegetation and its effect on wave height dissipation;
results from a UK east coast saltmarsh. Estuarine, Coastal and Shelf Science, XX: 115.
NRC. 1990. Managing coastal erosion. Committee on Coastal Erosion Zone Management, Water
Science and Technology Board, Marine Board, Commission on Engineering and Technical
System, National Research Council. Washington, DC, National Academy Press. 182 pp.
Nurkin, B. 1994. Degradation of mangroves forest in South Sulawesi, Indonesia.
Hydrobiologia, 285: 271276.
Othman, M.A. 1994. Value of mangroves in coastal protection. Hydrobiologia, 285: 277282.
Prasetya, G.S. & K.P. Black. 2003. Sanur and Kuta Beaches in Bali case studies for replacing
traditional coastal protection with offshore reef. Proceedings of Artificial Surfing Reef,
2003. Raglan, NZ.
Sale, E.V. 1985. Forest on sand, the story of Aupouri state of forest. The New Zealand Forest
Service.
Samarayanke, R.A.D.B. 2003. Review of national fisheries situation in Sri Lanka. In: G.
Silvestre, L. Garces, I. Stobutzki, M. Ahed, R.A. Valmonte-Santos, C. Luna, L. Lachica-Alino,
P. Munro, V. Christense & D. Pauly (Eds.) Assessment, management and future direction of
coastal fisheries in Asian countries, pp. 9871012. WorldFish Center Conference Proceedings
67. 1120 pp.
Sato, Y. & N. Mimura. 1997. Environmental problems and current management issues in the
coastal zones of south and southeast Asian developing countries. Journal of Global
Environmental Engineering, 3: 163181.
Struve, J ., Falconer, R.A. & Y. Wu. 2003. Influence of model mangrove trees on the
hydrodynamics in a flume. Estuarine, Coastal and Shelf Science, 58: 163171.
Swan, S.B.Stc. 1974. The coast erosion hazards southwest Sri Lanka: an introductory survey.
University of New England. 182 pp.


130
Swan, S.B.Stc. 1984. The coastal geomorphology of Sri Lanka. New England Research in
Applied Geography, No. 40. 125 pp.
Syamsudin, K. & F. Riandini. 2000. Coastline evolution monitoring at up drift and downdrift
of some coastal structure in Indonesia. Proceedings Institut Tekonologi Bandung on Seminar
on Sediment Transport. Supplement 32, No.3. 2000, pp. 4554.
Tjardana, P. 1995. Indonesian mangroves forest. Duta Rimba, J akarta.
Thampanya, U., Vermaat, J .E., Sinsakul, S. & N. Panapitukkul. 2006. Coastal erosion and
mangrove progradation of Southern Thailand. Estuarine, Coastal and Shelf Science, 68:
7585.
Turker, U., Yagci, O. & M.S. Kabdasl. 2006. Analysis of coastal damage of beach profile under
the protection of emergent vegetation. Ocean Engineering, 33: 810828.
UNEPWCMC. 2006. In the front line: shoreline protection and other ecosystem services
from mangroves and coral reefs. Cambridge, UK, UNEP-WCMC. 33 pp.
Van der Weide, J ., de Vroeg, H. & F. Sanyang. 2001. Guidelines for coastal erosion
management. In: E. Ozhan, ed. Medcoast 01: proceedings of the fifth international conference
on Mediterranean coastal environment. Vol. 3, pp. 13991414. Ankara. Turkey.
Wolanski, E. 1992. Hydrodynamics of mangroves swamp and their coastal waters.
Hydrobiologia, 247: 141161.
Wu, Y., Falconer, R.A. & J . Struve. 2001. Mathematical modeling of tidal currents in
mangroves forests. Environmental Modelling & Software, 16: 1929.
Xia, D.X., Wang, W.H., Wu, G.Q., Cui, J .R. & F.L. Li. 1993. Coastal erosion in China. Acta
Geographica Sinica, 48(5): 468476 (English abstract).


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Field study presentation: Status of coastal erosion in Viet Nam and
proposed measures for protection
N.N. Cat, P.H. Tien, D.D. Sam and N.N. Bien, Centre for Training, Consultancy and
Technology Transfer, Viet Nam Academy of Science and Technology

The coastal zone of Viet Nam is 3 260 kilometres long, extending through the territories of 24
provinces and cities. Interaction between the land and the sea, between the dynamic forces of
rivers and the sea and between natural and human processes occurs in this zone. On average, river
mouths are found every 20 kilometres along the Viet Namese coast. Landforms are diverse and
natural calamities occur frequently, causing multidirectional impacts on natural and socio-
economic conditions. In particular, coastal erosion generates many difficulties for the coastal
population.

Some coastal processes (for example, wind, waves and tides) have been identified as key factors
vis--vis influencing coastal erosion. Human activities (inter alia vegetation clearance, harbour
development, land reclamation) may often result in sediment addition or reduction. During a
tsunami impact, coastal erosion is caused by the rundown of the water, which will remove a high
percentage of sediment and debris from the land. Modification of soil composition after such an
event should be taken into account during rehabilitation activities.

Mitigation measures currently in use are hard structures (for example walls, rip-raps), soft
structures (beach nourishment, mangrove plantation) and a combination of the two. The
effectiveness of each mitigation measure can only be analysed on a case-by-case basis.

Extreme erosion in Viet Nam depends on several factors that should be further investigated in
order to respond in an effective manner. Even though human pressure has been recorded as one of
the main causes, to date, the construction of the Dinh Vu Dam seems to have decreased erosion in
some regions of the country. Protective measures such as dykes and revetments and the plantation
of trees are used to control erosion. The Viet Namese Government is investing in research for
effective protective measures.

Key points and observations emphasized in the discussions

The main conclusions of the presentations and discussions were as follows:
1. Longshore erosion may cause worse problems than cross-shore erosion.
2. Economic, environmental, social and cultural values are all subjects of protection from
coastal erosion; national governments can determine priorities by taking these factors into
account, depending on the specific situation (often areas of high economic value are
protected with appropriate measures, while land with low economic value is left
unprotected).
3. Coastal forests (and forests in general) provide benefits to local populations (they provide a
wide range of wood and non-wood forest products) and to fauna.
4. Trees prevent coastal erosion and stabilize shoreline areas by consolidating sediment and
building up land.
5. The establishment of a green belt/buffer zone should always be done using participatory
planning in collaboration with local communities.
6. Sufficient time frames, local participation and space availability are key factors for
supporting reforestation options.