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Introduction to Coastal Engineering Dr Introduction to Coastal Engineering Dr. R N Sankhua Introduction The development of the human activities in the coastal zone is strongly related to the development of coastal protection and the construction of flood protection structures, e.g. dwelling mounds, dykes and dyke openings. All those constructions were designed and built up with the support of now called coastal engineers. With the by and by change in the use of the coasts, the demand for an overall management of the coastal zone arose, including administrative-, socio-economic, biological, regional development, regional planning, civil engineering- and coastal engineering aspects. Since the functional and constructional design of all coastal structures are performed by coastal engineers and the assessment of the possible development of the coast is impossible without the knowledge and work of coastal engineers, coastal engineering plays an important role for the management of coastal stretches. It has been claimed that half the population of the industrialized world lives within one kilometer of the coast and that roughly 75% of the world's population lives within 60 km of the coastline. Coasts are by their nature dynamic areas, with hot spots prone to particularly rapid changes due to interrelated natural and physical causes. Such hot spots include sandy coasts (beaches and dunes), coastal wetlands, coastal cliffs composed of weak, easily eroded sediments, and coral reefs. Coastal change costs money. Coastal change also constitutes a direct hazard to human populations. As sea-level rise induced by global warming increases the threat to the environment, it is likely that coastal protection and management will become an increasingly important challenge for many societies. Coastal change is caused by natural and/or human factors. The types of change most threatening to human use and enjoyment of the coastal zone are erosion and deposition of sediment, deterioration in coastal ecosystems (e.g., loss of species), and pollution of coastal land and waters. It is becoming increasingly apparent that successful management of these coastal problems must be integrated that is, all aspects of often multiple problems must be __________________________________ 1 CoastalEngg - Dr R N S, Director, NWA addressed, and the problems themselves must be seen in a wider context of other linked environments. Protection against the sea level rise in the 21st century will be especially important, as sea level rise is currently accelerating. This will be a challenge to coastal management, since seawalls and breakwaters are generally expensive to construct, and the costs to build protection in the face of sea-level rise would be enormous. Changes on sea level have a direct adaptive response from beaches and coastal systems, as we can see in the succession of a lowering sea level. When the sea level rises, coastal sediments are in part pushed up by wave and tide energy, so sea-level rise processes have a component of sediment transport landwards. This results in a dynamic model of rise effects with a continuous sediment displacement that is not compatible with static models where coastline change is only based on topographic data. Coastal Zone Definitions and Subdivisions Many coastal zone features and subdivisions are difficult to define because temporal variability or gradational changes between features obscure precise boundaries. In addition, nomenclature is not standardized, and various authors describe the same features using different names. If the same name is used, the intended boundaries may differ greatly. This ambiguity is especially evident in the terminology and zonation of shore and littoral areas. In the absence of a widely accepted standard nomenclature, coastal researchers would do well to accompany reports and publications with diagrams and definitions to ensure that readers will fully understand the authors’ use of terms. Fig 1 depicts some coastal terminology. Coastal zone is defined as the transition zone where the land meets water, the region that is directly influenced by marine or lacustrine hydrodynamic processes. The coastal zone extends offshore to the continental shelf break and onshore to the first major change in topography above the reach of major storm waves. The coastal zone is divided into four sub-zones, ♦ Coast. ♦ Shore. __________________________________ 2 CoastalEngg - Dr R N S, Director, NWA ♦ Shoreface. ♦ Continental shelf Surf zone Plunge line Fig: 1 Coastal terminology The coast is a strip of land of indefinite width (12 nautical miles) that extends from the coastline inland as far as the first major change in topography. Cliffs, frontal dunes, or a line of permanent vegetation usually mark this inland boundary. On barrier coasts, the distinctive back barrier lagoon/marsh/tidal creek complex is considered part of the coast. The seaward boundary of the coast, the coastline , is the maximum reach of storm waves. The shore extends from the low-water line to the normal landward limit of storm wave effects, i.e. the coastline. Where beaches occur, the shore can be divided into two zones: backshore (or berm) and foreshore (beach face). The foreshore extends from the low-water line to the limit of wave uprush at high tide. The backshore is horizontal while the foreshore slopes seaward. This distinctive change in slope, which marks the juncture of the foreshore and backshore, is called the beach or berm crest. The shoreface is the seaward-dipping zone that extends from the low water line offshore to a gradual change to a flatter slope denoting the beginning of the __________________________________ 3 CoastalEngg - Dr R N S, Director, NWA continental shelf. The continental shelf transition is the toe of the shoreface . Its location can only be approximately marked due to the gradual slope change. Although, the shoreface is a common feature, it is not found in all coastal zones, especially along low-energy coasts or those consisting of consolidated material. The shoreface, especially the upper part, is the zone of most frequent and vigorous sediment transport. The continental shelf is the shallow seafloor that borders most continents. The shelf floor extends from the toe of the shoreface to the shelf break where the steeply inclined continental slope begins. It has been common practice to subdivide the shelf into inner, mid, and outer zones, although there are no regularly occurring geomorphic features on most shelves that suggest a basis for these subdivisions. Although the term inner shelf has been widely used, it is seldom qualified beyond arbitrary depth or distance boundaries. Site-specific shelf zonation can be based on project requirements and local geologic conditions. Cuspate Foreland: A triangular accumulation of sand/gravel located along the coastline. This feature is formed by the joining of two spits. Planning Approaches __________________________________ 4 CoastalEngg - Dr R N S, Director, NWA There are five generic strategies for coastal defence. The decision to choose a strategy is site-specific; depending on pattern of relative sea-level change, geomorphological setting, sediment availability and erosion, as well a series of social, economic and political factors. Alternatively, integrated coastal zone management approaches may be used to prevent development in erosion or flood-prone areas to begin with. Growth management can be a challenge for coastal local governments /local authorities who often struggle to provide the infrastructure required by new residents seeking sea-change. Sustainable transport investment to reduce the average ecological footprint/footprint of coastal visitors is often a good way out of coastal gridlock. Do nothing- no protection, leading to eventual abandonment – the ’do nothing’ option, involving no protection, is a cheap and expedient way to let the coast take care of itself. It involves the abandonment of coastal facilities when they are subject to coastal erosion, and either gradually landward retreat or evacuation and resettlement elsewhere. This option is very environmental friendly and the only pollution produced is from the resettlement process. However it does mean losing a lot of land to the sea and people will lose their houses and their homes. Managed retreat or realignment- which plans for retreat and adopts engineering solutions that recognize natural processes of adjustment, and identifying a new line of defence where to construct new defences. Managed retreat is an alternative to constructing or maintaining coastal structures. Managed retreat allows an area that was not previously exposed to flooding by the sea to become flooded. This process is usually in low lying estuarine or deltaic areas and almost always involves flooding of land that has at some point in the past been reclaimed from the sea. 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 sea, inter-tidal and salt-marsh habitats. This process may continue over many years and natural stabilization will occur. The main cost is generally the purchase of land to be flooded. Housings compensation for relocation of residents may be needed. Any other human made structure which will be engulfed by the sea may need to be safely dismantled to __________________________________ 5 CoastalEngg - Dr R N S, Director, NWA prevent sea pollution. In some cases, a retaining wall or bund must be constructed inland in order to protect land beyond the area to be flooded, although such structures can generally be lower than would be needed on the existing coast. Monitoring of the evolution of the flooded area is another cost. Costs may be lowest if existing defenses are left to fail naturally, but often the realignment project will 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.
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