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Old 7th June 2009, 04:11 PM   #1 (permalink)
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Geography A-level: Deltas and Estuaries and Changes to Coastal Areas

Source taken from:http://www.s-cool.co.uk/alevel/geogr...tal-areas.html

Geography A-level: Deltas and Estuaries and Changes to Coastal Areas

Introduction

Deltas are features found where rivers that contain much sediment flow into the sea; velocity is reduced and deposition of material occurs. They are 3-dimensional features with most deposition occurring below the water surface.

Estuaries are where fresh water (rivers) and salt water (sea) interact. They attract industry and settlement due to their provision of:

Flat land.
Water supply.
Deep water.
Food.
Transport.
For example, Avonmouth on the Severn estuary.

Delta form and formation


Not all rivers have deltas at their mouths and they vary greatly on size and shape. According to geomorphologists, there are 3 main estuary shapes:

Accurate: Rounded, convex edges. For example, the Nile.
Cuspate: Material evenly spread from the river.
Birds foot: Spread over a wide area with many 'distributaries' extending out to sea. For example, Mississippi.
Deltas build up over time and on the diagram, 3 beds are visible:

Bottom set: Have finest material, carried furthest in suspension and sinks to sea bed.
Foreset: More readily deposited, coarser material. Builds out to sea.
Topset: Deposits from river with the most coarse material.
Factors influencing formation of deltas:

Amount and type of sediment available.
Variations in volume of water discharged from river.
Aspect and geometry of coast.
Coastal processes in operation, for example, wave action.
Changes in coast level.
Impacts of climate on growth of vegetation and marine organisms.
Deltas are especially favourable for agriculture, due to the deposition of fine sediment, but at the same time are places of high flood risk as shown by the Ganges Delta.

Estuaries

Essentially where rivers and coasts meet. They are often important wildlife habitats, but equally attract human settlement: in the UK, approximately 20 million people live near to estuaries, due to their flat building land. They exist where either a coastline has been raised - meaning that the lower area of the river is drowned or the coastline has subsided. The fact that salt marshes can develop distinguishes then from Rias.

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Old 7th June 2009, 04:12 PM   #2 (permalink)
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Re: Geography A-level: Deltas and Estuaries and Changes to Coastal Areas

Introduction

Sea levels have varied considerably over time - especially during the Pleistocene (approximately 1 million years ago).

Present situation was reached around 7000 years ago.

Rise in sea level resulted in rias, drowned glaciated river valleys (fjords), and submerged forests. Most estuaries are a result of postglacial sea level rise.

Earth crust movement and/or tectonic activity can further complicate the situation.

Land surfaces are rising relative to sea level (isosatic uplift/recoil). Rate is related to the thickness of ice that existed.

Coastal landforms need to be considered with regard to both world-wide (eustatic) sea level rise and rising land.

Isostatic uplift is usually faster than eustatic sea level rise.

Negative change: A fall in sea level in relation to the land.

Positive change: Sea level rises in relation to the land (or land sinking in relation to the sea).

Changes

Isostatic change

Local changes in sea level, due to ice weight depressing earth's crust lying beneath it.

Eustatic change

Large scale/worldwide changes. In times of maximum glaciations, 3 to 4 times more water was stored on land than it is today. Global change in the hydrological cycle resulted and a worldwide eustatic fall in sea level of up to 150m.

Sequence of changes

Ice sheets and glaciers form, eustatic fall in sea level, and negative change in base level.
Ice sheets continue to grow. Land is depressed by weight, isostatic change, positive change in base level.
Ice sheets begin to melt, eustatic sea level rise, positive change in base level.
Continued decline of glaciers, isostatic uplift of land, positive change in base level.
Submerged coastlines

Rias

Drowned river valleys, such as Milford haven in South Wales. In the ice age when rivers still flowed they cut down to the new lower sea level. With the rise in temperatures and release of water from land, they became flooded.

Fjords

Flooded U-shaped valleys. Glaciers form U-shaped valleys and during the ice age they eroded to below sea level. With ice melt the valleys were flooded, creating deep, narrow inlets, common in Norway.

Fjards

Glaciated lowland areas that have since been flooded.

Emerged coastlines


Raised beaches

Landforms that occur due to isolated uplift of the land in comparison to the sea, as the weight of ice is slowly removed. Old wave cut platforms and beaches are now above the impact of waves (as shown in the diagram above).

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Old 7th June 2009, 04:12 PM   #3 (permalink)
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Re: Geography A-level: Deltas and Estuaries and Changes to Coastal Areas

Humans are drawn to the coast, initially for ease of transport and industry, but increasingly for recreation purposes. The result is a need to manage and control both coastal erosion flooding which is difficult, given the dynamic nature of coastal environments.

Coastal management

This spans the vast array of measures, which are outlined below, and is subject to large amounts of controversy. In the UK, there are numerous organizations involved in the management of the coastline, ranging from the National Trust to the Department for Environment, Food & Rural Affairs (DEFRA).

Maritime and district councils (approximately 120) tend to coastal protection, whilst the Environment Agency is responsible for management of coastal flooding. As a result, approaches are rather disjointed. The best protection for a coast is always a beach.


Hard engineering

These are structures that try to limit the impact on waves and their energy on a stretch of coastline. For example, a sea wall rebounds a wave's energy back to sea, limiting the impact of the waves. They are contentious due to cost and impact on other areas of the coast but do allow a reduction of cliff erosion.


Hard engineering approaches are seen at the base of cliffs (sea walls and groynes), or on the cliff face (drainage and pinning).

Aims of approaches, advantages and disadvantages:

Approach: Aims: Advantages: Disadvantages:
Sea walls To limit erosion by creating a barrier. Effective defence against erosion and flooding. Reflected energy picks up sediment, which then is used in 'scouring' and can undermine the sea wall. Very expensive £2000+ per metre. Other areas left exposed.
Revetments and Rip Rap To dissipate (reduce) wave energy. Cheap and less unsightly than sea walls. Allows for more build up of sediment to protect the beach. If wooded have a short life span. Beach scouring still occurs.
Groynes Limit longshore drift, increase sedimentation rate, build up beaches. Very efficient in trapping sediment. Sediment input lower down coastline is reduced, limiting protection offered to cliffs lower down coast. Rapid cliff erosion can occur. For example, Barton on Sea.
Breakwaters To break waves further out to sea, reducing their energy at the coast. Effective and have less impact on other areas of the coast line. Costly as they need to be very strong. Can be short-lived. Appreciation of local tidal conditions vital for success.
Cliff modification To reduce cliff height and slope angle, reducing mass movements. Long term solution. Careful planning needed, and rock falls, for example, can still occur.
Cliff drainage To stabilize cliffs by reducing weight of water. Widely applicable in the UK. Alters cliff hydrology which impacts on ecology.
Gabions Stabilize foot of cliff and help drainage. Cheap and relatively quick to install. Unable to be used where they would be subject to wave erosion.
Soft engineering

Viewed as more environmentally friendly, as this approach tries to work with natural processes to reduce, rather than prevent erosion.

Approaches, aims, advantages and disadvantages:

Approach: Aim: Advantages: Disadvantages:
Beach replenishment To build up beach via dumping sand, creating a larger area over which energy is dissipated. For example, Sherringham Norfolk. No costly building involved, cheap and long term New material (sand) must match the original type. Offshore dredging alters wave approach and refraction. Materials from quarries are expensive.
Stable bays Bay created to trap sediment. Works with natural processes, less expensive than hard engineering. New approach, little known about its success.
Managed retreat

Sometimes seen as the 'do nothing' approach. Nature is allowed to take its course. In the UK arguments for this include the fact that coastal protection is costly and increasing. Managed retreat is cheaper and more environmentally friendly.

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Old 7th June 2009, 04:13 PM   #4 (permalink)
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Re: Geography A-level: Deltas and Estuaries and Changes to Coastal Areas

For missing illustrations, please refer to the source directly.

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