Hard coastal protection structures

Hard coastal protection structures are common worldwide. For example, the percentage of armoured coastline is about 16% along the North Sea coasts and more than 8% along the Mediterranean coasts. About 14% of the total US coastline is protected by hard structures, and coastal structures exist along 9.4% of the coastline in the Great Barrier Reef World Heritage Area (Australia). Seawalls currently protect 7% of the coast of India, with another 7% planned or under construction. Mainland China has with 60% the greatest percentage of hardened coastline.

This article provides an introduction to hard coastal protection structures. An overview is given of structures that are often used in practice. The conditions for applying hard structures are discussed together with their advantages and disadvantages. A list of authoritative literature references is provided and Coastal Wiki articles are indicated where a more detailed discussion of hard coastal structures can be found. Large parts of this article are taken from the coastal engineering handbook of Reeve, Chadwick and Fleming (2018).

Introduction

Figure 1: Common types of coastal protection structures.

There is a wide range of coastal works that might be employed to tackle a particular situation, each of which may perform a number of different functions. They will also have differing engineering lifespans as well as different capital and maintenance cost streams. The potential economic benefits will also have a strong influence on the final solution that might be adopted whilst still conforming to the objectives and policies developed through the Shoreline Management Plan and Strategy Study. Figure 1 shows some of the more common types of coastal works that are often used and include artificial headlands, groynes, offshore breakwaters, beach nourishment and seawalls. The characteristics of the coasts for which they are suited, the basic advantages and disadvantages are listed in this figure. CIRIA Report 153 also provides a useful summary guideline for the application of hard construction works as given in Table 1. The comments provided are mainly of a technical nature; note that there are a number of wider environmental considerations that need to be considered in the context of a full appraisal (see the article Shoreline management).

Modern design practice places much emphasis on attempting to hold a healthy beach on the shoreline as the primary means of protection. A sufficiently substantial beach can accommodate the dynamic changes that are the result of differing climatic conditions. These so-called ‘soft’ solutions are generally considered to be more environmentally friendly than traditional ‘hard’ protection works. However, where human life may be at risk and high density, high value conurbations exist, the use of hard elements of a defence may be unavoidable.

Conditions for applying hard coastal structures

A good understanding of the coastal environment at a site under consideration is an essential prerequisite to assessing the ability of a coastal defence option to perform as it is intended. A complex interaction exists between the various elements defining the coastal environment. The introduction of coastal protection works will invariably modify nearshore processes in some way and it is important to account for that feedback effect. Coastal morphology at any location is the result of erosion and accretion patterns which, in turn, depend on the interaction of wave climate, currents and tides with this morphology. The causes and effects of coastal features must always be considered when dealing with works which affect littoral movement. The origin of beach material can be from inland sources brought to the coast by rivers or from the erosion of cliffs in the immediate or adjacent coastlines. Sometimes there can be shoreward pathways of sediment from offshore sources. In some cases, these processes may no longer be active and the beach may consist of relic materials.

Knowledge of the geology underlying the nearshore zone is important because a stratum that is different from the surface material can affect the way in which a beach behaves. A thin veneer of loose material on an erodible platform can act as an abrasive and accelerate erosion, whilst its existence on an impermeable base will be inherently unstable and more mobile than an equivalent deep beach. These factors are also material to the design of foundations of coastal structures.

Table 1. Guidelines for the application of hard coastal structures .

Structure Type	Characteristics coast	Advantages	Disadvantages
Groynes	Shingle beach: any tidal range; Sand: micro-tidal only; Low net littoral drift (gross drift may be high); Low structures suitable for low wave energy; Large mound type structures suitable for high wave energy.	Allows for variable levels of protection along frontage schemes to put coastal policies into practice.	Can induce rip currents which increase erosion, particularly on sand beaches; Vertical structures potentially unstable in case of large cross-structure beach profile differences; Requires recharge to avoid downdrift erosion.
Detached breakwaters	Shingle beach: any tidal range; Sand: micro-tidal only; Dominant net drift direction; Constant wave climate, not storm dominated; Creation of amenity pocket beaches or salients.	Allows for variable levels of protection along frontage.	Large visual impact particularly with macro-tides; May cause downdrift beach erosion and leeward deposition of fine sediment and flotsam; Inshore tidal currents may be intensified; May cause hazardous rip currents; Difficult to construct due to cross-shore location; Difficult to balance impact under storms and long-term conditions; Difficult to balance impact on both shingle and sand transport.
Shore-connected breakwaters	Bay-type shoreline, Shingle beach: any tidal range; Sand: limited effect with macro-tides; Dominant net drift direction; Any wave climate; Strong shoreline tidal currents ("fishtails" only); Creation of amenity pocket beaches.	Allows for variable levels of protection along frontage; Can be used to create amenity features; Longshore and cross-shore control.	May cause leeward deposition of fines and flotsam; Little design guidance at present.
Seawall/Revetments	Sand or shingle beach: any tidal range, any wave climate; Low gross drift rate; Provides secondary line of defence where beach cannot be designed to absorb all wave energy during extreme events.	Well developed design methods; Provides equal protection along frontage; Can be designed to support a sea front development.	No drift control; Beach lowering; May become unstable if erosion continues.
Sills	Shingle or sand beach; Low wave energy; Low and variable drift; Submerged with micro-tides, regularly exposed with macro-tides.	Well developed design methods; Provides equal protection along frontage; Can be designed to support a sea front development.	Storms may remove beach irreversibly; Level of protection reduced during storm surge events.
Beach drainage systems	Sand beaches, normally up to the high-water line; Any tidal range; Any wave climate or drift rate.	Responds to beach developments.	Limited experience of use; Effectiveness not clear; Long-term maintenance may be expensive; Risk of failure during short duration, extreme storms.

A more detailed discussion of the coastal structures of Table 1 can be found in the articles:

• Groynes and Groynes as shore protection
• Seawalls and revetments and Seawall
• Bulkheads
• Revetments
• Detached breakwaters, Detached shore parallel breakwaters and Applicability of detached breakwaters
• Floating breakwaters
• Coves - artificial formation and use (application of shore-connected breakwaters)
• Port breakwaters and coastal erosion
• Piers and trestles
• Beach drainage
• Stability of rubble mound breakwaters and shore revetments
• Ecological enhancement of coastal protection structures

Further reading

There are a number of publications and standards that deal with general facets of coastal structure design and include some excellent information and detailed guidance. These are:

• Coastal Engineering Manual. US Army Corps of Engineers (USACE), 2008
• The Rock Manual. The use of rock in hydraulic engineering (2nd edition).CIRIA. London, 2007
• A guide to managing coastal erosion in beach/dune systems. Scottish Natural Heritage, 2000. https://www.nature.scot/sites/default/files/2017-07/Publication%202000%20-%20Beach%20Dunes%20-%20a%20guide%20to%20managing%20coastal%20erosion%20in%20beach%20dune%20systems.pdf
• Rogers, J., Hamer, B., Brampton, A., Challinor, S., Glennerster, M., Brenton, P. and Bradbury, A. 2010. Beach Management Manual (second edition). CIRIA report C685.
• BS6349 (2013) 1 - 1. Maritime Works, General. Code of Practice for planning and design for operation. British Standards Institute (shopbsigroup.com)
• BS6349 (2016) 1 - 2. Maritime Works, General. Code of Practice for assessment of actions. British Standards Institute (shopbsigroup.com)
• BS6349 (2012) 1 - 3. Maritime Works, General. Code of Practice for geotechnical. British Standards Institute (shopbsigroup.com)
• BS6349 (2013) 1 - 4. Maritime Works, General. Code of Practice for planning and design of floating and mooring systems. British Standards Institute (shopbsigroup.com)
• BS6349 (2013) 5. Maritime Works. Code of Practice for dredging and reclamation. British Standards Institute (shopbsigroup.com)
• BS6349 (expected late 2016) 6. Maritime Works. Code of Practice for floating structures. British Standards Institute (shopbsigroup.com)
• BS6349 (1991). Maritime Structures - 7. Guide to the design and construction of breakwaters. British Standards Institute. Revision commenced 2016.
• Abbott, M.B. and Price, W.A. (eds.) 1994. Coastal, Estuarial and Harbour Engineers Reference Book. CRC Press.
• Pilarczyk, K.W. (ed.) 1990, Coastal Protection. A.A. Balkema (publisher), Rotterdam.
• Allen, R. T. L. 1998. Concrete in Coastal Structures. Thomas Telford.
• Fleming, C. A. 1990. Guide on the uses of groynes in coastal engineering. CIRIA report 119.
• PIANC 1992. Guidelines for the design and construction of flexible revetments incorporating geotextiles in marine environments. Committee II, Working Group No. 21. ISBN: 9782872230440.
• Van der Meer, J.W. 2018. EurOtop. Manual on Wave Overtopping of Sea Defences and Related Structures, www.overtopping-manual.com
• Brampton, A. (ed.) 2002. Design and Practice Guides, Coastal defences. Thomas Telford, London.
• Pilarczyk, K. W. and Van Overeem, J. 1987. Manual on artificial beach nourishment. CUR report, 130.
• Jensen, C. 2007. The rock manual - the use of rock in hydraulic engineering (2nd edition).CIRIA, CUR Centre For Civil Engineering and CETMEF.
• Besley, P. 1999. Wave Overtopping of Seawalls Design and Assessment Manual. HR Wallingford R&D Technical Report W178. Environment Agency.
• Bruun, P. 1989. Port Engineering .Gulf Publishing Company.
• McConnell , K. 1998. Revetment systems against wave attack: A design manual. Technical Report. Thomas Telford
• Thomas, R. S. and Hall, B. 1992. Seawall Design. Butterworth Heinemann/CIRIA, Oxford, UK
• Van der Meer, J.W. 2002.Technical Report Wave Run-up and Wave Overtopping at Dikes Publication. Technical Advisory Committee on Flood Defence (TAW), Delft
• Dean, R. and Dalrymple, R. 2010. Coastal processes with engineering applications, Cambridge University Press.

References

The authors of this article are Andrew Chadwick, Dominic Reeve and Christopher Fleming