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New South Wales Government
A variety of coastal protection measures can be used to enhance or preserve beach amenity and to protect coastal developments at risk of erosion or recession. These include seawalls, groynes, offshore breakwaters, artificial headlands, beach nourishment and dune rehabilitation and management. Structural works are also used to stabilize coastal entrances (training walls).
To be effective, the type of protection must be compatible with coastal processes at the site. Information is required concerning the magnitude and mechanisms of existing longshore sediment transport, together with likely long term shoreline changes from erosion, accretion or recession. A detailed understanding of coastal processes and hazards is essential to the successful design, construction and operation of coastal protection works.
Protection works have the potential to impact on areas outside those being protected. Therefore any proposal for protection works must take account of the wider implications and consider the impact in a whole embayment or region, as well as the marine environment.
A seawall is a structure built along the shoreline parallel to the beach. Its purpose is to impose a landward limit to coastal erosion and to provide protection to development behind the wall. Seawalls can be built of many materials including timber, steel, concrete, rock, gabions, bitumen, plastics, ceramics and specially designed armour units. Along the NSW coastline, seawalls are commonly constructed from dumped rock, concrete and gabions. The face of a seawall may be vertical, curved, stepped or sloping. Figures D6.1 shows various types of seawalls.
Seawalls are best designed as continuous structures over the full length of coastline to be protected. They are not well suited to the protection of isolated properties. In these circumstances, erosion around the ends of the wall can lead to collapse if this problem is not addressed in the design.
Whilst seawalls can be constructed anywhere up the beach profile, they are best located in the "higher" regions where only waves from extreme storm events can reach them. During normal conditions sand is returned to the beach in front of the seawall and recreational amenity of the beach is re-established.
Seawalls are commonly used in conjunction with other beach protection measures such as groynes and beach nourishment.
Types of Seawalls
Depending upon the type and materials of construction, seawalls can be classified as:
Rigid seawalls include gravity walls, sheet piling, caissons and concrete revetments. Advantages of rigid seawalls include their compact nature (minimum plan area) and their tendency not to harbour rubbish. However, they can be subject to catastrophic failure by freak waves or toe erosion.
Flexible seawalls are constructed from quarry rock, shingle and from specially manufactured concrete units. Whilst not as compact as rigid seawalls, flexible seawalls can sustain considerable deformation caused by erosion and settlement without total failure occurring. Because of the broken nature of their surface, flexible seawalls tend to harbour rubbish.
Semi-flexible seawalls are constructed from gabions, bitumen, grouted rock, specially designed units of concrete, ("SEABEES", "DOLOSSE", "TRIBARS", etc.), ceramics and geotextiles. They are more compact than flexible seawalls and may not be as susceptible to the catastrophic failure of rigid seawalls.
Whilst many rigid seawalls have been built along the NSW coastline in the past (often in an apparent attempt to recreate the Victorian beach promenades of England), there is now a general tendency away from this form of construction for the following reasons:
Because of their sensitivity to freak waves, more severe design wave conditions are adopted for rigid structures than for flexible and semi-flexible seawalls. The high reflectivity of rigid seawalls can result in accelerated sand loss in front of the wall during a storm, and delay beach rebuilding following a storm. Rock scour blankets, gabions, etc. can be used to protect the foundations of a rigid structure from undermining. Alternatively, this protection can be provided by founding such structures at depth on non-erodible materials.
The performance of rigid seawalls can be improved by incorporating various features such as a curved wave deflection barrier along the crest of the wall, which significantly reduces wave overtopping and enables the crest to be lowered (see Figure D6.2).
Flexible and Semi-Flexible Seawalls
In recent years, flexible and semi-flexible seawalls constructed from rock, shingle or proprietary concrete units have been the most common form of construction along the New South Wales coast. A typical conventional two layer armoured seawall is shown in Figure D6.3. Provided design wave conditions are not exceeded, this form of construction has the following advantages:
Note that some of the proprietary concrete units need to be formally interlocked to achieve their strength and protection potential, e.g. "SEABEES". In this case, the resulting seawall may behave more as a rigid than semi-rigid structure, and be subject to the same types of failure as the former.
Because of their permeable nature, flexible and semi-flexible seawalls are susceptible to scour behind the wall caused by wave overtopping or poor seepage control. If extreme, soil loss caused by this scour can lead to the landward collapse of the wall. The risk of scour by wave overtopping can be reduced by incorporating a relatively impermeable blanket of rock, clay, grass, etc. along the crest.
Regular maintenance of the flexible and semi-flexible seawalls is generally required to ensure their structural integrity.
The mass of the armour unit used to protect flexible structures is proportional to the cube of the design wave height. A doubling of the design wave height for long term coastal erosion, recession or increasing sea levels would require an eight fold increase in armour unit mass to provide the same level of protection. For this reason, careful consideration must be given to the effects of long term erosion or increases in sea level on design wave height.
Storm Profile Seawalls
A recent development has been to construct seawalls from rocks of much smaller size than required for conventional design. Provided a sufficient volume of rock is placed, a stable "beach" profile is naturally developed during storm conditions. Physical model tests are usually required to determine this profile. The advantages of this form of construction include:
One disadvantage is that this form of protection may be aesthetically displeasing: the "beach" consists of a mixture of rocks and sand which may reduce amenity. The structure also occupies a larger space than conventional walls.
Groynes are coastal structures built approximately normal to the shoreline. Their purpose is to trap sand and thereby increase the width of the beach. Groynes can be constructed from a similar range of materials as seawalls. When used on the open coast, they must be strong enough to withstand substantial wave forces.
For groynes to be effective, there must be a supply of sand from either longshore transport or from beach nourishment. In a longshore transport situation, sand is trapped on the updrift side of the groyne. As the groyne embayment fills, the alignment of the shoreline changes to become more normal to the wave direction. During this filling process, there is a consequent reduction in sand supply downdrift of the groyne. This results in shoreline erosion at downdrift locations. Dune management measures may be required both up drift and down drift to accommodate changes in the beach and dune systems.
Where groynes are used, it is essential that their effect on the downdrift coastline and the consequences of a changed shoreline alignment be closely examined.
Downdrift erosion can be reduced by artificially filling the groyne embayment under a beach nourishment program. This minimises disruption to the longshore transport process as the embayment fills
Groynes do not significantly affect onshore/offshore movement during storms and are therefore not usually effective as a means of managing short term erosion.
Where there is insufficient sand on a beach to meet storm erosion or long term sediment loss, additional sand can be placed by mechanical means. This is referred to as beach nourishment. It is a favoured means of beach protection for resort and high amenity beaches because it promotes amenity and unlike some other structural measures, does not have adverse effects on adjacent areas of the coastline.
Provided sufficient sand is used, beach nourishment can provide total protection. However, it may be an expensive means of control, and it is often used in conjunction with other control measures such as seawalls and groynes. Dune management measures would be needed to accommodate the increased sand volume.
To prevent excessive offshore losses of the placed material, the nourishment sand should be similar in size or preferably slightly coarser than the natural beach material. Common sources of nourishment sand include dunes, coastal inlets and offshore areas. When the source material is borrowed from offshore areas it is important to ensure that the dredged area does not alter the existing wave refraction patterns to the detriment of the adjacent coastline. Many potential nourishment sources are being removed or sterilised through coastal development and dredging operations, which will hinder future nourishment programs should they be necessary. Local authorities should consider setting aside reserves for future use. Some local authorities require sand excavated during the construction of coastal developments to be returned to the beach. Where there is long term sediment loss it is desirable that the source of material is outside that particular active beach system.
In a beach nourishment program, the volume and frequency of placement of sand depend upon the rates of offshore and onshore losses. Offshore loss depends upon the wave exposure of the site and the size of the sand. Onshore loss is by sand drift.
"Sand bypassing" is a special form of beach nourishment used to alleviate the downdrift erosion caused by training walls. Training walls are typically constructed at the entrances of coastal inlets for flood mitigation purposes or to improve navigation. They can act as groynes, trapping sand on the updrift side and causing shoreline erosion on the downdrift side. Training walls often project much further out to sea than ordinary groynes. Hence, the associated downdrift erosion can be extensive. To limit this erosion, sand can be pumped from the updrift embayment or from other sources to the downdrift shoreline, thereby bypassing the training walls. Sand bypassing, like beach nourishment, is a relatively expensive and continuing operation.
Offshore breakwaters are structures built approximately parallel to the beach but some distance offshore. They may protrude above water level or be submerged; they may be continuous or consist of a series of segments. The purpose of offshore breakwaters is to reduce the intensity of wave action in inshore waters and thereby reduce coastal erosion. Offshore breakwaters are normally constructed from the same materials as seawalls. A particular form of the offshore breakwater is the "T-groyne" in which the offshore structure is connected to the shore for ease of construction, maintenance or for subsequent use.
Fully submerged breakwaters consisting of underwater mounds or artificial reefs of sand and small rocks have been used for coastal protection purposes overseas, e.g. at Durban in South Africa. Under normal conditions, waves pass over the mound or reef with little modification. Under storm conditions, the larger waves break on the mound thereby dissipating energy and reducing shoreline erosion.
Unlike groynes, offshore breakwaters can be used to reduce erosion at a beach which has no net longshore transport. However, if longshore transport exists, an offshore breakwater will act like a groyne and cause downdrift erosion.
Offshore breakwaters are not a common form of coastal protection along the shoreline of New South Wales. They are costly to construct because of the prevailing wave climate and their use is generally limited to the protection of sheltered areas not exposed to full wave attack.
The natural headlands of a pocket beach restrict longshore sand transport. Such headlands act as groynes, but on a much larger scale. Artificial headlands can be constructed to achieve a similar effect, e.g. large groynes that extend into deep water, or offshore breakwaters connected to shore (T-groynes). On the open coast, this form of protection requires large and expensive structures. Consequently, their use has been restricted to more protected shallow areas with less severe wave conditions.
Configuration dredging is dredging to a pattern such that wave refraction limits the effects of wave action on a stretch of coastline. Its usefulness on the open coast is restricted by the variety of wave directions possible and the scale and cost of works required. It is more applicable to sheltered bays and could only be considered in an environment where there was great confidence in the understanding of coastal processes.
Design Wave Height
Coastal protection structures are generally located within the surf zone and are therefore subject to forces associated with "depth limited broken waves". The design wave height for such structures can be expressed as follows:
H=a . d
|where H||is the design wave height,|
|d is the water depth which controls breaking|
|a is a coefficient referred to as the "Breaker Index"|
The governing water depth is the difference between SWL and the seabed level to the seaward end of the structure. The SWL used for design purposes should reflect the effects of astronomical tide, storm surge and wave set up (see Appendix B4). The seabed level used for design purposes must take into account likely scour during design storm conditions and include an allowance for any long term erosion or accretion resulting from an imbalance in the sediment budget. Beach profiles taken immediately after a storm provide the best data to assess scour and seabed levels for design purposes. However, such data are only available at a limited number of sites where local authorities have had the foresight to make such measurements. In most cases estimates of scour level are determined from aerial photography, often taken some time after a storm event.
The breaker index is a function of several variables which include wave period, bed slope and wave grouping. Its value is presently poorly defined. A value commonly used in practice has been 0.8, which is reasonable for flat slopes and long period waves, but underestimates the maximum wave height for relatively steep beach slopes and short period waves. For the latter conditions, model tests and field data indicate values between 0.8 and 1.2.
In choosing a design value for the breaker index, consideration should be given to the susceptibility of the structure to failure or damage from a group of three of four extreme waves. Rigid structures are more prone to failure under these conditions, and a correspondingly higher value of the breaker index should be adopted. Flexible structures such as rubble mounds may be damaged, but rarely fail from a group of high waves. Consequently, lower values can be adopted. For high cost structures, or structures where the consequence of failure is significant, wave model testing should be used to verify the design.
In selecting the foundation level of a coastal structure, consideration must be given to the possibility of local scour at the toe. This may result in the failure of rigid structures; flexible structures can tolerate settlement without failure. Incorporation of a scour blanket to protect against toe erosion is commonly used as a safeguard against failure or damage from this cause.
The crest level adopted in the design of coastal protection structures needs special attention. A crest level which is never overtopped will significantly increase the cost of the structure. As the crest level is reduced, so the probability of failure caused by overtopping is increased.
Most structures are designed to allow for some overtopping. For seawalls, scour blankets landward of the crest and/or wave reflecting walls can be used to protect the structure from damage and to limit overwash. For breakwaters and groynes, a significant increase in strength of the crest can be achieved by careful placement of the armour units to ensure good interlocking and by the use of concrete or bitumen grouting. If grouting is used it is essential that good drainage is provided to prevent the excessive uplift pressures. To achieve this purpose, intermittent rather than continuous grouting is generally used.
Soil and Drainage Considerations
Whilst coastal structures are designed primarily to resist forces from waves and currents, they also act as retaining structures for the material behind them. Thus, they are subject to soil and water pressures in the same way as any other retaining structure and must be designed to resist these forces.
The drainage of coastal structures needs careful consideration. Many impermeable structures have failed by collapse in the seaward direction caused by soil and water pressures behind them. For permeable structures, seepage may result in the removal of material from behind the structure and eventual failure by collapse landwards. This can be avoided by correctly designed soil filters or geotextiles. Where geotextiles are used, protection against light is essential.
Availability of Materials
All coastal protective works require materials of one sort or another. The availability of material and its suitability for coastal protection will largely determine the overall costs of a project. The coastal engineer would be greatly assisted if this knowledge was readily available.
Special attention needs to be given to the availability of sand suitable for beach nourishment programs. The major sources of sand are:
The last source is expensive as compared to land based sources. However, many of the land sources are presently becoming unavailable as a result of removal for the construction industry, commercial and private development of the land and a general trend for environmental protection irrespective of the consequences of that protection. It would be desirable that sources of beach nourishment sand be identified, and where appropriate, be reserved for that purpose.
Bruun, P. "Design and Construction of Mounds for Breakwaters and Coastal Protection", Elsevier Press.
Coastal Engineering Research Center, (1984). "Shore Protection Manual". Fourth Edition. Waterways Experiment Station. U.S. Army Corps of Engineers. U.S. Government Printing Office, Washington D.C., 20404.
Institution of Engineers, Australia. Proceedings of various Australasian Conferences on Coastal and Ocean Engineering.
The Coastal Engineering Research Council of the American Society of Civil Engineers. Various Proceedings of International Conferences on Coastal Engineering.
Silvester, R. "Coastal Engineering", Elsevier Press.