Transportation systems and shore protection structures

Transportation systems along shorelines affect natural habitats and ecosystem functioning. Intertidal habitats such as coral reefs, mangroves, saltmarshes, and beaches are important and sensitive ecosystems are often the first ones to be obliterated when shoreline development begins. Revetments, protective structures consisting of sloping rock walls, are often built in conjunction with transportation system development to dissipate wave energy and minimise wave reflection. Such structures tend to be less damaging than vertical seawalls but sand transported by reflected wave might undermine revetments. Seawalls consist of vertical walls protecting shorelines from the impact of waves and prevent inland flooding. Though relatively inexpensive they lead to severe beach erosion as reflected wave energy is maximised. This in turn often leads to the collapse of the wall as the entire structure is undermined. Groins are shore protection structures that extend out to sea from the shoreline. They interrupt longshore transport of sand trapping sand on the updrift side of the groin, stabilising or widening the beach on that side of the wall. On the downdrift side on the other hand groins cause shoreline erosion - thus causing as many problems as they solve. Offshore breakwaters are aligned parallel to the coast and, as the name suggests, offshore. They are usually constructed to stabilise inlets and/or maintain beaches.

South Carolina, USA: Seabrook Island's shoreline is strongly influenced by surrounding tidal inlets and their associated tidal delta which trap sand and influence beach accretion and erosion. Developments were undertaken at Seabrook Island in the early 1070's. Followed a period of natural beach accretion. However, soon after it was followed by period of rapid erosion, placing many properties at risk. In an attempt to control beach erosion and protect properties a number of breakwaters, seawalls, groins etc were developed along the shoreline, literally consolidating 80% of the beach. However, erosion continued, and in 1979 500 m of the structures collapsed during Hurricane David. In the 1980s a project relocated the main inlet, restoring the beach by freeing sand that had been trapped in the abandoned tidal delta.

Moen Island, Federated States of Micronesia: Causeway construction impeded water inflow into and proper circulation within the bay leading to poor water quality and contaminating Moen Island's shellfish resources. To remedy the situation, large culverts were installed in order to allow for increased tidal exchange and thus reduce pollution. Palmyra atoll: consists of a large reef line encircling three lagons: West Lagoon, Central Lagoon and East lagoon. Construction of causeways on the atoll prior to World War II completely blocked water circulation within the east lagoon, leading to serious water quality problems and collapse of resident coral communities. Reefs in the Central and West lagoons were destroyed by the dredging of a channel through the western reef and between the two lagoons. The channel also altered water circulation patterns, causing west- drifting sediments to smother coral communities off the western end of the atoll.