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| Estuaries | • | Saltmarshes | • | Sea cliffs | • | Rocky shores | • | Beaches | • | Lagoons |
Estuaries are semi-enclosed bodies of water, which have a free connection with the open sea. They differ from other coastal inlets in that sea water is measurably diluted by inputs of freshwater and this, combined with tidal movement, means that salinity is permanently variable. The mixing of two very different water masses gives rise to complex sedimentological and biological processes and patterns.
Estuaries come in all shapes and sizes and are called many different names, e.g. bays, lagoons, harbours, inlets or sounds, and the upper part of rias. An estuary is a body of water formed where freshwater from the rivers and streams flows into the ocean, thus mixing with seawater. They are places of transition from land to the sea and from freshwater to saltwater. Estuaries are influenced by the tide and can be microtidal (small tidal range), mesotidal (moderate tidal range) or macrotidal (large tidal range). They are complex ecosystems, interlinking the terrestrial environment with the aquatic environment. Estuaries act as nature's water treatment facilities, flood control areas, buffers against storm damage, and protectors against coastline erosion.
Four main types of estuary can be identified:
- Coastal plain estuaries
- These occur where pre_existing valleys were flooded at the end of the last glaciation.
- They are less than 30 m deep.
- Extensive mudflats and saltings often occur.
- Bar-built estuaries
- There is a sediment bar across their mouths.
- They are partially drowned river valleys.
- They are associated with depositional coasts.
- They are only a few metres deep.
- Complex estuaries
- These river estuaries are formed by a variety of physical influences, including glaciation, river-erosion, sea-level change and geological constraints from hard rock outcrops.
- Few exist.
- Ria estuaries
- These are drowned non-glaciated river valleys.
- The estuarine part is restricted to the upper reaches.
- The predominant substrate is mostly bedrock.
- Outer parts are little diluted by freshwater and are defined as shallow inlets or bays e.g. Bantry Bay in Co. Cork and Kenmare in Co. Kerry.
- Away from the open sea there are usually soft sediments.
 Killary Harbour, Ireland's only fjord is a complex estuary formed by glacial erosion. |
Saltmarshes
Saltmarshes are flat, poorly drained areas of land in intertidal regions subject to periodic flooding by tides. They are typically found between the upper limits of the the very lowest (neap) and very highest (spring) tides in protected bays, estuaries, and other sections of sheltered low energy coastal environments with medium or large tidal ranges (>3m) where sediment becomes deposited on intertidal mudflats under slack water conditions. Salt marshes may also develop on coastal peat deposits. These are mainly confined to the west coast of Ireland and often occur in areas where blanket bog has been encroached by the sea as a result of rising sea levels over long periods of time, frequently combined with the more recent cutting of peat.
Usually the upper parts of the marsh are more elevated so they are only
totally inundated at the highest tides. The lower zones are inundated more
frequently, especially where channels dissect the marsh. Tidal saltmarshes
in temperate regions are characterized by the presence of a branching network
of tidal creeks. Saltmarsh creeks are often extensions of systems that occur
on the intertidal mud and sand flats that exist seaward of the marsh and
while most terminate on the marsh, some may extend beyond the landward margin.
These networks provide a pathway for the flow of water, sediments and nutrients
into and out of the salt marsh. The height and reach of the tide is an important
factor controlling the channeling, branching network and water movement
in marshes and tidal creeks.Mud banks are less stable than saltmarshes as a result of the higher levels of physical disturbance arising from the displacement of sediment and erosion by tides. Mud flats and vegetated marsh areas are separated from one another around the mean high water neap.
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Sea cliffs
Sea Cliffs and rocky coasts occur in both high and low relief areas where the underlying geological structures are relatively resistant to the erosive forces of the sea, rain and wind. They are formed as waves undercut a steep shoreline slope. Along a submerging, steep, rocky coastline irregularities in the shoreline will cause waves to refract and focus at shoreline promontories or headlands. They are mainly found along the Atlantic coast. In areas of high relief the landscape is often characterised by steep cliffs, rocky outcrops and deep, usually clear offshore marine waters with small embayments. Although more mobile sedimentary systems (such as pocket beaches, dunes and shingle shores) may be present they are seldom extensive.
 The Cliffs of Moher, Co. Clare, made of sedimentary rock, stand almost 200 m high. |
The platform is often termed a wave-cut or an abrasion platform. More resistant parts of coastal rock formations stick out as headlands, or rock stacks or islands offshore and the weaker rocks are cut back into coves and bays. Solid and massive formations are generally eroded more slowly than formations that disintegrate easily e.g. friable sandstones or rocks with joints and bedding planes. Weathering and erosion attack these lines of weakness, digging out caves and coves. Natural arches can form where less resistant rock has been eroded from a headland or island. Where the forces produced by the hydraulic action of waves and the compression of trapped air puncture the roof of a cave, water and spray are driven up through blowholes.
Certain types of rock result in characteristic cliff forms. The Giants Causeway in Co. Antrim, for example, has a distinct appearance formed from columnar basalts. In limestone areas, the sea may penetrate and widen fissures that originated as underground solution passages. On soft clay formations, cliffs are subject to reoccurring slumping, especially after wet weather.
Rocky shores
Rocky shores are subdivided into three main categories on the basis of wave exposure since the degree of wave action on a shore determines, to a large extent, what animal and plant communities are present. On the most exposed shores, the effects of sea spray and wave splash can be far-reaching. Low rocky cliffs (<5 m in height), overhangs, rocky headlands and rock pools are subject to regular wetting by wave splash or sea spray, and terrestrial vascular plant communities do not exceed 50%.
 Waves breaking over a rocky shore in Doolin, Co. Clare. |
Beaches and sand dunes
Beaches bordering continental areas in temperate climatic regions tend to be formed of pale yellow to brown quartz-rich sands, the most common solid products of weathering and erosion. Pebble or shingle beaches also occur, in which the pebbles are usually derived from some fairly local source such as an adjacent line of cliffs. Changes in sea level during the Pleistocene Ice Age have largely determined the present distribution of beach sediments. Sands of many present-day beaches migrated away from the shelf edge as sea level rose following the last glaciation.
No two beaches are the same, and beach widths can range from a few tens to many hundreds of metres, depending on factors such as slope, tidal range, prevailing winds, sediment type and so on.
Beaches are accumulations of loose sand or pebbles. Much of the energy of waves breaking upon the shore is dissipated in moving sediment, though some is always reflected back out to sea, the proportion depending on the beach slope; beach profiles can change quite rapidly in response to changes in wave energy.
On most beaches, three zones of wave action can generally be identified. As waves approach the shore, they become unstable and break. This is the breaker zone and it can be quite wide. Landward of the breaker zone is the surf zone and this is generated by the breaking waves. Finally there is the swash zone, where smaller waves are pushed up the beach slope, which is alternately covered by the upsurge of water (swash) and the return of water to the sea (backwash). The sloping part of the beach between high and low water is sometimes called the beach face.
Shallow sloping beaches may be characterized by a series of low broad sandy ridges or bars separated by runnels. Sediment movement forms these features. Beaches with moderate to steep slopes and relatively narrow intertidal zones are often characterized by berms, which are flat-topped ridges at the upper limit of wave action.
 Sandy beach backed by sand dunes in Fanore, Co. Clare. |
Lagoons
The meaning of a “lagoon” varies greatly around the world, as the word is generally used to identify different water bodies, including saline lakes, tidal bays and freshwater ponds. Most definitions are based on their morphology, their environmental conditions, indicator species or a combination of the three. The latter is the case of the European Union’s Habitats Directive and its Interpretation Manual which define lagoons as “expanses of shallow coastal salt water, of varying salinity or water volume, wholly or partially separated from the sea by sand banks or shingle, or less frequently, by rocks (morphology). Salinity may vary from brackish water to hyper salinity depending on rainfall, evaporation and through the addition of fresh seawater from storms, temporary flooding by the sea in winter or tidal exchange (environmental conditions)”.
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- Sedimentary Lagoons: the only ones considered real lagoons by the narrowest definitions i.e. enclosed by a barrier of sand or shingle. In Ireland, like the rest of Northwest European lagoons, they usually lack inlets, and the interchange with the sea is by percolation through the barrier.
- Rock lagoons: these have barriers composed mainly of rocks. The seawater usually penetrates through fissures in the rock or inlets, although some of the lagoons that fall in this category are shore lagoons, eventually reached by the sea during high spring tides and storms. These inputs of salt water seem to be enough to develop and maintain brackish conditions. These are mainly found in Counties Clare and Galway.
Saline lake lagoons: these are separated from the sea by a strip of
land composed of earth or peat, not by a barrier of marine origin. They
can be isolated from the sea at low tide (silled lagoons) or remain
in permanent contact with it (inlet lagoons). They are concentrated
in Counties Galway and Donegal, where wide extensions of coastal blanket
bogs and fens can be found.- Artificial lagoons: these are the result of human action and have artificial barriers. These barriers were built for different purposes i.e. to reclaim land from the sea, as coastal defences or to serve as causeways to roads and railroads. Most of them have outlets, which range from simple channels to sluices, with or without pumps. They occur all around the coast, but particularly on the east coast and in Co. Cork.