Coastal management
Coastal management is defence against flooding and erosion, and techniques that stop erosion to claim lands. Protection against rising sea levels in the 21st century is crucial, as sea level rise accelerates due to climate change. Changes in sea level damage beaches and coastal systems are expected to rise at an increasing rate throughout the century, causing coastal sediments to be disturbed by tidal energy.
Coastal zones occupy less than 15% of the Earth's land area, while they host more than 40% of the world population. Nearly 1.2 billion people live within of a coastline and of sea level, with an average density three times higher than the global average for population. With three-quarters of the world population expected to reside in the coastal zone by 2025, human activities originating from this small land area will impose heavy pressure on coasts. Coastal zones contain rich resources to produce goods and services and are home to most commercial and industrial activities.
History
of harbours began with the origin of maritime traffic, perhaps before 3500 B.C. Docks, breakwaters and other harbour works were built by hand, often in a grand scale.The Romans introduced many innovations in harbour design. They built walls underwater and constructed solid breakwaters. These structures were made using Roman concrete. Vitruvius described three methods for building port structures. Other types of port structure such as rubble mounds and arched breakwaters built by means of timber floating caissons were used also.
Romans were the first dredgers in the Netherlands to maintain the harbour at Velsen. Silting problems there were solved when the previously sealed solid piers were replaced with new "open"-piled jetties.
Ancient harbour works are still visible, but most of them disappeared following the fall of the Western Roman Empire even if submerged remains are sometimes still visible under water.
Although most coastal efforts were directed to port structures, Venice and its lagoon is an example of measures not related to ports. Protection of the shore in Italy, England and the Netherlands began in the 6th century or earlier.
Middle Ages
Attack from the sea caused many coastal towns and their harbours to be abandoned. Other harbours were lost due to natural causes such as rapid silting, shoreline advance or retreat, etc. The Venetian Lagoon was one of the few populated coastal areas with continuous prosperity and development where written reports document the evolution of coastal protection works. In other words, this is one of the first accounts of the use of a sea wall to protect a coastal settlement.Modern Age
Little improvement took place beyond the Roman approach to harbour construction after the Renaissance. Then in the early 19th century, the advent of the steam engine, the search for new lands and trade routes, the expansion of the British Empire through her colonies, and other influences, all contributed to the revitalization of sea trade and a renewed interest in port works.Twentieth century
Prior to the 1950s, the general practice was to use hard structures to protect against beach erosion or storm damages. These structures included seawalls and revetments or sand-trapping structures such as groynes. During the 1920s and '30s, private or local community interests protected many coastal areas using these techniques on an ad hoc basis. In certain resort areas, structures proliferated to such an extent that the protection impeded recreational uses. Erosion continued, but the structures remained, resulting in a loss of beach area.The obtrusiveness and cost of these structures led in the late 1940s and early 1950s, to a more dynamic approach. Projects attempted to replicate the protective characteristics of natural beach and dune systems. The resultant use of artificial beaches and stabilized dunes as an engineering approach was economically viable and more environmentally friendly.
Limited knowledge of coastal sediment transport processes often resulted in inappropriate measures of coastal erosion mitigation. In many cases, measures worked locally, but exacerbated problems at other locations -up to tens of kilometers away- or generated other environmental problems.
European Code of Conduct
The essential source on coastal engineering is the European Code of Conduct for Coastal Zones issued by the European Council in 1999. This document was prepared by the Group of Specialists on Coastal Protection and underlies national legislation and practice.The Group of Specialists originated in 1995, pursuant to a decision by the Committee of Ministers of the Council of Europe. It emphasized the need for integrated management and planning, but that coastal areas continued to deteriorate. The Group claimed that this was due to difficulties in implementing the concept of "integrated management". The Group proposed that the Council of Europe, cooperate with the Coastal & Marine Union and United Nations Environment Programme.
Planning approaches
Five generic strategies are involved in coastal defense:- Abandonment
- Managed retreat or realignment, which plans for retreat and adopts engineering solutions that accommodate natural processes of adjustment
- Armoring by constructing seawalls and other hard structures
- Construct defenses seaward of the coast
- Adapting vertically by elevating land and buildings
Alternatively, integrated coastal zone management approaches may be used to prevent development in erosion- or flood-prone areas, reducing the need to address the changes. Growth management can be a challenge for local authorities who must provide the infrastructure required by new residents.
Managed retreat
Managed retreat is an alternative to constructing or maintaining coastal structures. Managed retreat allows an area to erode. Managed retreat is often a response to a change in sediment budget or to sea level rise. The technique is used when the land adjacent to the sea is low in value. A decision is made to allow the land to erode and flood, creating new shoreline habitats. This process may continue over many years.The earliest managed retreat in the UK was an area of 0.8 ha at Northey Island flooded in 1991. This was followed by Tollesbury and Orplands in Essex, where the sea walls were breached in 1995. In the Ebro Delta coastal authorities planned a managed retreat.
The main cost is generally the purchase of land to be abandoned. Relocation compensation may be needed. Human-made structures that will be engulfed by the sea may need to be removed. In some cases, armouring is used to protect land beyond the area to be flooded. Costs may be lowest if existing defences are left to fail naturally, but the realignment project may be more actively managed, for example by creating an artificial breach in existing defences to allow the sea in at a particular place in a controlled fashion, or by pre-forming drainage channels for created salt-marsh.
Managed retreat has become more of a necessary strategy due to climate change, as adaptation strategies can only do so much to stop sea level rise.
Hold the line
Holding the line typically involves shoreline hardening techniques, e.g., using permanent concrete and rock constructions. These techniques--seawalls, groynes, detached breakwaters, and revetments—represent more than 70% of protected shorelines in Europe.Alternatively, soft engineering techniques supporting natural processes and relying on natural elements such as dunes and vegetation can prevent erosive forces from reaching the back-shore. These techniques include beach nourishment and sand dune stabilization.
Historically coastal strategies were heavily based on static structures, while coastal areas otherwise reflect a dynamic equilibrium. Armouring often has the unintended consequence of moving the problem to another part of the coast. Soft options such as beach nourishment protect coastlines and help to restore the natural dynamism, although they require repeated applications. Maintenance costs can eventually require a strategy change.
Move seaward
In some cases a seaward strategy can be adopted. Examples from erosion include: Koge Bay, Western Scheldt estuary, Chatelaillon and Ebro delta.There is an obvious downside to this strategy. Coastal erosion is already widespread, and there are many coasts where exceptional high tides or storm surges result in encroachment on the shore, impinging on human activity. If the sea rises, many coasts that are developed with infrastructure along or close to the shoreline will be unable to accommodate erosion. They will experience a so-called "coastal squeeze" whereby ecological or geomorphological zones that would normally retreat landwards encounter solid structures and can migrate no further. Wetlands, salt marshes, mangroves and adjacent fresh water wetlands are particularly vulnerable to such a squeeze.
An upside to the strategy is that moving seaward can create land of high value which can bring investment.