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Department of Irrigation and Drainage
Ministry of Natural Resources and Environment
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Coastal Management

Groynes

Groynes are built to stabilize a stretch of natural or artificially nourished beach against erosion that is due to primarily to a net longshore loss of beach material. Groynes function only when longshore transport occurs. Groynes are narrow structures, usually straight and perpendicular to the preproject shoreline. The effect of a single groyne is accretion of beach material on the updrift side and erosion on the downdrift side; both effects extend some distance from the structure. Consequently, a groyne system (series of groynes) results in a saw-tooth-shaped shoreline within the groyne field and a differential in beach level on either side of the groynes.

Single groyne at Wilayah Persekutuan Labuan

Labuan Blocks

Labuan Blocks are concrete mass blocks. They are cast insitu, often with locally available material and therefore have a relatively low capital cost. Because they are portable and removable they can be easily transported to any places, reducing the cost of coastal erosion control project. Labuan Blocks can be placed as sloping “mattresses” or as near vertical cubic blocks. The latter are intended for bank or cliff stabilisation and are not normally suitable for use in shoreline situations.

 
Labuan Blocks as sloping “mattresses”-
Taman Robina, Seberang Perai Utara, Pulau Pinang

 

Labuan Blocks (backshore) are generally preferred to rock revetment (foreshore) in coastal environments being less reflective of wave energy and more stable. Sand is also better able to accumulate on Labuan Blocks, potentially softening their appearance.

 
Labuan Blocks as vertical cubic blocks-Pancur Hitam, Wilayah Persekutuan Labuan


 

Function

The purpose of Labuan Blocks is to provide short term (3-5 years) protection from backshore erosion by absorbing wave energy along the dune face. Their application is restricted to the upper part of sandy beaches, since they are not sufficiently durable to withstand regular direct wave action. As they are concrete structures placed at backshore, they will tend to trap wind blown sand and sediment transport and allow the growth of vegetation under favourable conditions. This only applies to sloping structures.

Labuan Blocks provide a short term alternative to rock armour structures in areas where large rocks are not available at an acceptable cost, or where long term protection is not appropriate.

Description Of Drawings

 

 

Casting In-situ

 

The moulds for the Labuan Blocks are made locally. Ready-mixed concrete is then delivered to site. The ready-mixed concrete is then discharge from the mixer-truck and placed inside the moulds. The next process is the vibration of concrete. This is essential to eliminate the entrapped air and forcing the particles into a closer configuration. Internal vibrator like poker vibrator is used. Some admixtures can also be used to accelerate the hardening or the development of early strength of concrete.

The most common cement used in this case is the Ordinary Portland (Type I) cement with a Grade of 35. As to avoid sulphate attack (sea water attack), Sulphate-Resisting (Type V) cement is recommeded.

The next step in this process is curing at normal temperature; that is to keep concrete saturated, or as nearly saturated as possible, until the originally water filled space in the fresh cement paste has been occupied to the desired extent by the products of hydration of cement. Curing may be aided by wetting the moulds before casting. The blocks must be wetted during hardening. The concrete should be sprayed with water or covered with suitable covering to avoid drying out.

Methods Of Placement

Concrete Blocks should be placed as a sloping revetment as shown in pictures. Near vertical walls are more likely to suffer toe scour and structural collapse as they are less able to dissipate wave energy during storm wave attack. They are also much more obtrusive to the dune landscape and will not become buried by new foredunes. 

Structure face slopes are a compromise between flatter faces that absorb more wave energy, and therefore suffer less toe scour, and steeper faces that give the structure a smaller footprint. A slope of 1:1.5 is a reasonable compromise, and is in keeping with natural dune slopes. 

The approximate limits of wave run-up can be established by observing and recording the location of the strand line over Spring tide periods during both monsoon and more normal wave conditions. The toe of a freshly eroded dune face is normally just below the run-up limit of the most recent severe sea.

 


During placement, regrading of the beach/dunes may have to be carried out so as to adequately bed the blocks. A suitable geotextile should be used to prevent the underlying sand from being washed out through the blocks. Edge details should be carefully addressed to prevent exposure of unsightly lengths of textile. Landward edges can be buried to fix the geotextile during Labuan Blocks placement. Seaward edges should be trimmed or firmly secured. 
The placing device for all the concrete blocks shall be lowered to rest on the ground before releasing the concrete blocks and no blocks shall be dropped from height greater than 0.5 m.

Summary

Appropriate Locations

Sandy beach sites suffering periodic moderate to severe erosion where backshore assets are at risk. Useful for backshore protection.

Costs

Moderate, (RM1,500 per meter run).

Effectiveness

Well placed Labuan Blocks provide reasonable fixed defences, but have a limited life of 3-5 years due to deterioration of the concrete.

Benefits

Useful solution where armour rock is considered inappropriate or too costly. Various forms available. Can be buried by sand and vegetation. Permeable face absorbs wave energy and encourages upper beach stability


Beach Nourishment

Beach nourishment is also known as beach replenishment, beach feeding or beach recharge. Beach nourishment is a soft structure solution used for prevention of shoreline erosion. Material of preferably the same, or larger, grain size and density as the natural beach material is artificially placed on the eroded part of the beach to compensate for the lack of natural supply of beach material. The beach fill might protect not only the beach where it is placed, but also downdrift stretches by providing an updrift point source of sand.

It is entails finding a suitable source of material that is compatible with, but not necessarily identical to the material on the beach to be nourished. It is often the most satisfactory means of protecting a shoreline as it provided the necessary reservoir of material that allows a beach to respond to wave and achieved equilibrium. The period for re-nourishment of the beach is about 5-10 years.

BefireSand Nourishment, Teluk Cempedak, Kuantan, Pahang (2004)

 

 

After Sand Nourishment, Teluk Cempedak, Kuantan, Pahang (2003)

 

Breakwaters

Breakwaters are built to reduce wave action in an area in the lee of the structure. Wave action is reduced through a combination of reflection and dissipation of incoming wave energy. When used for harbors, breakwaters are constructed to create sufficiently calm waters for safe mooring and loading operations, handling of ships, and protection of harbor facilities. Breakwaters also built to improve maneuvering conditions at river mouth entrances and to help regulate sedimentation by directing currents and by creating areas with different levels of wave disturbance.

Breakwaters can be classified into two main types: sloping-front and vertical-front structures. Sloping-front structures are in most cases rubble-mound structures armored with rock or concrete armor units, with or without wavewall superstructures. Vertical-front structures are in most cases constructed of either sandfilled concrete caissons or stacked massive concrete blocks placed on a rubble stone bedding layer. In deep water, concrete caissons are often placed on a high mound of quarry rock for economical reasons.

 

Breakwaters at Sungai Marang rivermouth, Terengganu

 

Geotube

Geotube is define as a large tube (greater than 2.5m in circumference) fabricated from high strength woven geotextile in lengths greater than 6m, used in coastal and river applications and typically filled hydraulically with a slurry of sand and water. An apron of geotextile is also designed together with geotube to protect the foundation of the main geotube from the undermining effects of scour. Scour can be present at the base of the geotube due to wave and current action. Scour aprons are typically anchored by a small tube at the water’s edge or by sandbags attached to the apron.

Geotube breakwaters are soft structures that are better able to conform to a mangrove forest environment. In the unlikely event that some small boats hitting the Geotube, it would be like running aground on a sandbar thereby posing no dangers to the boat.

 

Geotube at Tanjung Piai, Pontian, Johor.

Revetments


Revetments are onshore structures with the principal function of protecting the shoreline from erosion. Revetment structures are flexible and typically consist of armor rock or cast concrete blocks. Revetments rest on the surface being protected and dependent on it for support. They are relatively light structures and are well suited to locations free of heavy wave attack. Properly designed and constructed revetments are long life structures and require little maintenance. Almost all concrete armor units revetment depend on their interlocking design for stability. Voids within the revetment permit quick drainage over the surface of the slope and hence reduces wave run-up. However, it has been observed recent years by DID, revetments cause wave reflection and flanking erosion, reducing the beach width and sandy shores.

 

SAUH Revetment, Haji Sirat, Selangor.
Basalton Revetment, Pulau Besar, Melaka


Flex-Slab Revetment, Pantai Klebang, Melaka
Rock Revetment, Kg Pasir Pandak, Kuching, Sarawak
 
Bun
Seawall


Last Updated 2017-05-18 13:23:55 by Administrator

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