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The following points highlight the five main types of coasts. The types are: 1. Cliff Coast 2. Clayey Bank Coast 3. Intertidal/Muddy Coast 4. Sand Dune Coast 5. Sandy Coast.
Type # 1. Cliff Coast:
Cliff coast can be classified as “hard” coast as it was formed from resistant materials such as sedimentary or volcanic rocks. This type of coast typically has a short shore platform that is usually exposed during low tide. Natural erosion is attributable to slope instability, weathering and wave action and leads to regression of the shoreline.
As illustrated in Figure 3.7, extreme wave conditions such as storm waves and tsunamis will have a less erosive effect on this type of coast; traces of tsunami wave height can be found on cliffs as a trim line where trees or shrubs on the cliff had been erased.
Type # 2. Clayey Bank Coast:
This type of coast can be classified as a “semi-hard” coast, consisting of cohesive soils; it is common on estuarine coastlines and often has nearly vertical banks ranging from one to five metres in height. The rate of erosion is relatively high compared to the hard coast because it is composed of weaker and less resistant material. Erosion is mostly due to coastal processes, weathering and loss of vegetation cover.
For extreme events such storms and tsunami, as illustrated in Figure 3.8, vegetation cover plays a significant role in protecting the coast from flooding and inundation by reducing wave height and energy and decelerating tsunami flow speed; hence, erosive forces and inundation distance are decreased.
Type # 3. Intertidal/Muddy Coast:
This type of coast is characterized by fine-grained sedimentary deposits, predominantly silt and clay that come from rivers; it can be classified as a “soft” coast. It has a broad gentle seaward slope, known as an intertidal mud flat where mangrove forest, saltmarshes, shrubs and other trees are found.
Most erosion is generated by river damming that reduces sediment supply, diminishes vegetation cover (usually mangroves and saltmarshes) and exposes vegetation roots by lowering the mud flat (Fig. 3.9) that leads to their final collapse. During storms, healthy and dense vegetation/coastal forest and trees can serve as barriers and reduce storm wave height, as well as affording some protection to the area behind them.
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In the case of a tsunami, coastal forest and trees can decrease wave height and tsunami flow speed to some extent if the forest is dense and wide enough. Both extreme events can cause severe erosion and scouring on the coast and at the river mouth.
Type # 4. Sand Dune Coast:
This type of coast consists of unconsolidated material, mainly sand, some pebbles and shells; it can be classified as a soft coast. It has a gentle seaward slope —known as dissipative beaches that have broad fine sand and gradually steep slopes at the backshore/foredunes.
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Its profile depends on wave form and energy and wind direction; hence, profiles can be adjusted to provide the most efficient means of dissipating incoming wave energy. This type of coast experiences short-term fluctuation or cyclic erosion —accretion and long-term assessment is needed to identify erosion as a problem here.
Often accretion and dune rebuilding take much longer than erosional events and the beach has insufficient time to rebuild before the next erosive event occurs. Erosional features are a lowered beach face slope and the absence of a nearshore bar, berm and erosional scarps along the foredune.
Generally, erosion is a problem when the sand dunes completely lose their vegetation cover that traps wind-borne sediment during rebuilding, improves slope stability and consolidates the sand. During extreme events such as storms and tsunamis (Fig. 3.10), this type of coast can act as a barrier for the area behind the dunes. Sand dunes and their vegetation cover are the best natural protective measures against coastal flooding and tsunami inundation.
Type # 5. Sandy Coast:
This type of coast consists of unconsolidated material — mainly sand from rivers and eroded headlands, broken coral branches (coralline sand) and shells from the fringing reefs. It can be classified as a soft coast with reef protection offshore. The beach slope varies from gentle to steep slopes depending on the intensity of natural forces (mainly waves) acting on them.
Coconut trees, waru (Hibiscus tiliaceus), Casuarina catappa, pandanus, pine trees and other beach woodland trees are common here.
Most erosion is caused by loss of:
(i) The protective function of the coastal habitat, especially coral reefs (where they are found) that protect the coast from wave action; and
(ii) Coastal trees that protect the coast from strong winds. During extreme events (Fig. 3.11), healthy coral reefs and trees protect coasts to some extent by reducing wave height and energy as well as severe coastal erosion.
