Coastal Management

I

Introduction

Coastal Management, the control of natural processes in the coastal zone, often to protect against erosion and to alleviate flood risk. The coastal zone is naturally very dynamic: coastal processes erode and deposit sediment, which creates a variety of coastal features and a constantly changing coastline. Coastal zones also provide a range of important habitats, such as sand dunes and salt marshes. Coasts are also typically highly populated areas, accommodating 50 per cent of the world’s population, which has resulted in the development of many coastal areas.

Natural processes operating in coastal zones can create hazards for people who live in such areas. Coastal erosion, caused by erosion from waves and wind, can result in a loss of land and property. Rapid rates of coastal erosion have been recorded along the east Essex coastline, with a loss of 66 cm (26 in) in one night at Fairlight Cove following a storm in December 1979.

Flooding of low-lying coastal zones is also a common hazard, particularly in areas where land has been reclaimed from the sea. In 1953 a storm surge in the North Sea caused severe coastal flooding in England, the Netherlands, and Belgium. To protect against such hazards, a range of different techniques has been used to manage coastal processes and protect coastal zones. Traditionally, hard engineering structures have been implemented which maintain a static coastline and defend coastal zones from erosion and flooding. However, more recently, soft engineering approaches, which work in conjunction with coastal processes, have been adopted. See also Flood Control.

II

Hard Engineering Techniques

Traditionally, coastal management has involved the construction of “hard” structures that provide a physical barrier between the land and the sea. These are typically expensive to construct and maintain and do not consider natural coastal processes. The design varies greatly, either to reflect or absorb wave energy.

Sea walls are a solid barrier between the sea and the coast that protect land from wave erosion and from flooding. Sea walls are usually constructed from masonry, concrete, or rock and are constructed at the base of cliffs or at the back of beaches. Although sea walls can effectively protect land behind the sea wall, they are expensive and can cause a number of environmental problems. Sea walls only deflect wave energy, which can result in the depletion of the foreshore, and they prevent interaction between the sea and the land behind the sea wall. This can cause problems as the coastline cannot respond to natural changes and the sediment supply is affected, which can cause depletion of sediment. Examples of sea walls can be found in Bognor Regis and Blackpool, where their construction has been responsible for beach loss.

Revetments, made from rock or timber, are placed parallel to the sea and are used to protect cliffs or dunes by dissipating wave energy. Revetments are cheaper than sea walls and are used where the full cost of a sea wall is not feasible.

Gabions—boulders or rocks placed in wire cages—are another traditional engineering structure used to prevent erosion. Gabions absorb some wave energy and are flexible and porous to sea water, which reduces problems of scour associated with impermeable sea walls. However, gabions are prone to damage from debris and from storm scour, and are therefore only used as a short-term solution. Gabions have been used to stabilize sand dunes between Hunstanton and Holme on the Norfolk coastline and can also function as groynes.

Groynes—wooden, concrete, or rock structures placed perpendicularly to the coast—are one of the most common coastal management techniques in use. Groynes intercept sediment moving along the shore, known as longshore drift, which encourages sediment accumulation and protects the coastal area. As groynes interrupt longshore drift, they often cause depletion of sediment levels. This is a widespread problem, with notable examples along the Hampshire and Sussex coast and along the north-west coast of Portugal, where groynes have accelerated cliff and sand dune erosion, threatening local housing developments. Jetties are a similar management structure to groynes: single, larger structures designed to stop longshore drift completely for the management of ports or harbours.

Offshore breakwaters reduce problems of coastal erosion by dissipating the energy of the waves hitting the coast. This is achieved by either increasing the friction of the seabed or creating a permeable barrier to the waves. This often results in sediment deposition in the lee of the barrier, which provides further protection but may cause problems from sediment depletion. Offshore breakwaters, consisting of old barges, have been used along the Essex coastline to reduce salt marsh erosion and encourage mudflat accumulation.

The creation of artificial bays can reduce coastal erosion. Bays lengthen the coastline by reducing the amount of wave energy per unit area and thereby trapping sediment, which creates beaches. Artificial bays are formed by constructing “hardpoints” that are resistant to erosion along the coastline, causing the coastline between these points to erode and form bays. Artificial bays are not as stable as natural bays as they are not formed by natural wave refraction. They also have a large impact on the environment, as the coastal morphology and supporting habitats are fundamentally altered. For these reasons they are rarely used.

In low-lying areas, barriers and barrages protect against coastal flooding and can also be used for power generation. Flood defence barriers, such as the Thames Barrier, London, prevent inland water movement and are only raised when floods are predicted. Permanent barriers can also be used for flood defence and power generation; examples are found in the estuaries of the Humber, Severn, and Mersey rivers. Both flood defence and permanent barriers are expensive and have significant environmental impacts, such as sediment depletion and changes in tidal flow in estuaries.

Hard engineering techniques can provide effective coastal defences for a limited time, but they maintain a static coastline and alter the arrangement of coastal sediment, which affects the coastline downdrift and can result in greater problems than those they were intended to solve. Hard engineering structures are costly to maintain, particularly with rising sea levels and increasingly these are being replaced with soft engineering solutions.

III

Soft Engineering Approaches

Soft engineering approaches attempt to work with natural coastal processes, providing a more sustainable and cost-effective management strategy. There are numerous different soft engineering techniques used, such as beach nourishment, sand dune stabilization, and managed realignment.

Beach nourishment, also called beach recharging or feeding, is a technique used to increase the quantity of sediment on a beach that is being eroded, so that the beach can dissipate wave energy and protect the coastline. This can range from a few truckloads to multi-million pound schemes requiring delivery of sand dredged from the seabed. It is important that the sediment is a comparable size and type to the beach, so that natural processes can operate. As this technique is conducted on eroding beaches, the process often needs to be repeated. Beach nourishment is practised at Bournemouth, where the beach is eroding due to sediment depletion from an updrift sea wall and groynes. This began in 1970 and is repeated every 8 to 10 years to maintain the beach.

An alternative way to encourage sediment accumulation in intertidal zones is to reduce the velocity of the water currents, resulting in natural sedimentation of mudflats. Water velocity may be reduced using brushwood groynes, which encourage the formation of ponds of water in mudflat areas, or by planting vegetation, which increases friction. Brushwood groynes have been used to encourage sedimentation in the intertidal zone of Wallasea Island, Essex.

Sand dunes are important for coastal management as they act as a natural sea wall, providing a barrier between the sea and the land. Changes to the amount of coastal sediment, often as a result of coastal engineering, sea level rise, and human activity all threaten sand dune systems. A number of techniques can be enforced to stabilize dune systems and thereby reduce erosion rates. One of the most frequently used techniques is the planting of vegetation, most often marram grass (Ammophila aremaria), which traps and stabilizes sand. Thatching is also used to stabilize exposed dunes and fencing can encourage deposition and protect vegetation. Sand recycling—the placement of sand from accreting zones to eroding zones—can also protect dune systems. In the Netherlands, dune systems are a fundamental sea defence and sand replenishment is frequently used to maintain dune height.

Managed realignment, or managed retreat, is the landward setback of sea defences to give land back to the sea and encourage the creation of natural intertidal habitats, such as salt marsh, which act as a natural coastal defence. As sea levels rise, intertidal habitats would naturally migrate inland, but they are unable to do so because of coastal defences and development, a situation known as “coastal squeeze”. In response to this, managed realignment works with natural processes and creates space for coastal habitats to respond to sea level rise. This is usually in estuarine areas and often uses land that was previously claimed from the sea. Managed realignment is a cost-effective and sustainable management technique, but it is still experimental in nature. Managed realignment has been practised on the Blackwater estuary, Essex, where more than 100 hectares (247 acres) has been flooded to encourage the development of salt marsh.

IV

Challenges to Coastal Management

One of the greatest challenges to implementing effective coastal management schemes is sea level rise, currently at an accelerating rate of 1-2 mm per year and expected to rise to over 5 mm per year over the next 20 to 30 years. This is expected to cause accelerated rates of erosion and an increase in the frequency and intensity of storm surges and coastal flooding, for example in the English Channel and the Bay of Bengal. This will put increased pressure on existing engineering structures and, in Britain, many of these are at the end of their functional life. In response to this, there has been a movement away from traditional hard engineering structures to softer, more habitat-based approaches, which are more sustainable and cost-effective.

Increasingly, coastal managers are adopting the Integrated Coastal Zone Management (ICZM) framework. ICZM takes a holistic approach to coastal management, considering coastal processes, land use, development, nature conservation, and human activity. It encourages sustainable development, so that any human use or development of the coastal zone must consider the coastal sediment budget and coastal habitats. ICZM strategies have been implemented worldwide although debate remains about funding and management.