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Environment and development in coastal regions and in small islands |
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Environment and development in coastal regions and in small islands |
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Coastal
management sourcebooks 1
Case 9
Stabilizing the river mouth or tidal inlet
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N V I R O N M E N T A L
B A C K G R O U N D
Any tidal inlet, through a shoreline leading to a port, harbour, lagoon, salt pond, wetland, estuary or river can affect beaches adjacent to the inlet.
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Rivers bring sand and other sediment to the beach and coastal system. |
Rivers are particularly important to many coastal areas because they transport large volumes of sand and stones from farther inland to the sea. Much of this material eventually ends up on the beaches. As was seen in Case 6, mining sand from the river removes material before it can reach the shoreline. Thus, river mining reduces the supply of sand to the beach.
In the Caribbean islands, many rivers are blocked from entering the sea by sand bars, which are formed by wave action moving sand onto and along the beach. The river is then only able cut a channel through the beach to the sea during a storm or heavy rainfall. After the rain stops, the river mouth is closed naturally by a sand bar. However, even then, freshwater may continue to enter the sea by flowing through the sand beach. Sometimes, a concentration of freshwater forms a large pond behind the sand bar.
These sand bars are often favoured places for sand extraction. The excuse is often given that a channel is required to improve drainage, but removing sand from the bar is the same as removing sand from the beach.
Sometimes, the river mouth is diverted by a spit. (See Figure 5, Case 1.) Consisting of sand and/or stones and formed by wave action, the spit is joined to the shore at one end only. It forms by the process of longshore transport. Waves approaching from a constant direction result in a longshore current that moves the sand or stones along the beachface in one direction. (See Figure 16, Case 4.) The sand spit may grow across the mouth of the river, thereby diverting its path to the sea.
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Spits are coastal features formed by accretion. They may divert river channels. |
On many of the smaller islands, rivers may be dry for much of the year. Although the channel remains, there is no visible water flowing into it. These streams or rivers are often known locally as guts or ghuts. However, during heavy rains, these dry channels may become the paths of roaring torrents of water. Whether large or small, the river mouth is an area of constant change.
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Dry river channels, often known as ghuts or guts, may become roaring torrents of water during and after heavy rains. |
Water from wetlands and swamps may also break through the beach during heavy rainfall. Sometimes, such processes are aided by man cutting channels to drain coastal swamps. While these channels may provide for improved drainage, they also result in large volumes of sediment-laden water flowing into the sea. Seagrass beds and coral reefs may be damaged if large amounts of sediment are deposited on top of them. (See also Case 10.)
River mouths are particularly dynamic areas. They are shaped by waves, tidal currents and ocean processes, and are also acted upon by river processes.
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River mouths are areas of continuous change because they are shaped by ocean and river processes. |
Rivers may be influenced by tidal flows. As the tide rises, salt water flows into the river and mixes with the freshwater. This brackish water mixture may extend some considerable distance upstream. As was seen in Case 1, tidal currents are particularly significant at river mouths and the passages between islands.
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Photograph 30.
Beach drainage channel, Cotton |
Many of the Caribbean islands are less than one kilometre apart, particularly in the Bahamas and the Turks and Caicos Islands. Tidal currents play an important role in the channels between these islands, moving and depositing sand. On the seaward side of the inlet (or channel), sand may accumulate on the sea-bottom to form an ebb-tide delta. (See Figure 26.) Similarly, on the landward side of the inlet, sand accumulates to form a flood-tide delta. At some sites, sand being moved along the coast by the process of longshore transport becomes trapped in these deltas, which may extend hundreds of metres out into the sea.
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Shorelines bordering inlets are very dynamic areas. They may erode or accrete ten times faster than shorelines farther away from the inlet. |
These inlets are extremely dynamic and their position often changes over a few years. The sand deposits associated with inlets, particularly the ebb-tide delta, may change the wave energy in the local area. Coastal changes may be several times greater near an inlet than at a beach only one kilometre away from the inlet.
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R A C T I C A L
R E S P O N S E S
Do not build near a river mouth or inlet.
Do not build in the immediate vicinity of rivers or inlets. Look for alternative sites.
Rivers and inlets are very dynamic areas. One heavy rainstorm may completely change the shape of the river mouth, turning what was once land into a river channel. Rivers and inlets should be left to fluctuate naturally and no building should be undertaken in their immediate vicinity.
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Do not be deceived by a dry river channel. Always remember that the channel was shaped by water. |
Use historical aerial photographs to determine whether a site is a ‘safe’ distance from a river mouth or inlet.
There is no magic distance from a river mouth that can be used to define the safety of a site. Local conditions result in variations from island to island and from site to site. Particularly in the case of tidal inlets, coastal areas within 1–5 kilometres (0.6–3 miles) of the inlet can be expected to show high rates of change over a decade. Historical aerial photographs can be used to show how a tidal inlet or river mouth has changed its position over the years. These photographs exist in most Caribbean islands for as far back as the 1960s. They are usually stored at the lands and surveys departments.
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Do not build in the immediate vicinity of rivers or inlets. Look for alternative sites. |
Leave rivers and inlets to change naturally.
River mouths and inlets should be left to change naturally. Structures like jetties may change the velocity of a river and in turn the amount of deposited sand. The river mouth is an area where sand from farther inland is deposited. This sand then becomes part of the beach. Similarly, when inlets are stabilized with jetties, adjacent beaches are impacted; some will accrete, others will show dramatic erosion.
Conduct a full environmental impact assessment before any river or inlet stabilization structures are built.
It is sometimes necessary to stabilize a river mouth or inlet for navigational or other purposes. An environmental impact assessment is a planning tool that assesses the likely impacts of the stabilizing structures and designs ways and means of reducing negative impacts like beach erosion.
Carefully consider the impact of draining coastal swamps.
Mangrove swamps and wetlands serve many purposes. One of these is to filter sediment and debris from the land runoff before it reaches the sea. Thus, cutting a channel through the beach to drain the wetland will allow the sediment and debris to flow out into the sea. This could result in the nearshore waters becoming a dirty brown colour for several days. Furthermore, the sediment may cover adjacent seagrass beds and coral reefs. In extreme cases, accumulated sediment can result in their death. Coastal flooding may sometimes make it necessary to drain coastal wetlands. However, in each case, the advantages of draining must be weighed carefully against the disadvantages of damaging nearby seagrass beds and coral reefs.
Do not mine sand from river mouth bars.
If it proves necessary to drain a coastal area and cut a channel through the beach, even after taking into account the above responses, do not remove the sand from the beach system. Instead, place it on the beach downdrift of the river mouth.
