Coastal Environments

Introduction to Coastal Environments

Coastal environments are dynamic interfaces where the land, sea, and atmosphere meet. The primary agent of change in these environments is the action of waves, which are responsible for both eroding the coastline to create dramatic cliffs and headlands, and depositing sediment to form beaches and spits. Understanding these processes is crucial for managing coastal areas, especially in countries with extensive coastlines like Pakistan.

1. The Power of Waves

Waves are created by the transfer of energy from the wind blowing over the sea surface. The size and energy of a wave depend on three factors: the wind speed, the length of time it blows, and the fetch (the distance of open water over which the wind blows). Waves are crucial in shaping the coast and are categorised into two main types:

* Constructive Waves: These are low-energy waves with a low frequency (6–9 waves per minute). They have a long wavelength and a low height. The swash (the movement of water up the beach) is strong and carries sediment onto the shore. The backwash (the movement of water back down the beach) is weak and does not have enough energy to remove the sediment. Consequently, constructive waves build up beaches. They are common in summer and in sheltered bays.

* Destructive Waves: These are high-energy waves with a high frequency (10–15 waves per minute). They have a short wavelength and a great height, causing them to plunge downwards onto the beach. The backwash is stronger than the swash, resulting in the removal of sediment from the beach. Destructive waves are responsible for coastal erosion and are common during storms and along exposed coastlines.

2. Processes of Coastal Erosion

Coastal erosion is the wearing away of the land by the sea. The rate of erosion is influenced by rock type (geology), wave energy, and coastal management. There are four main processes:

1. Hydraulic Action: This is the sheer force of the water crashing against the coastline. When waves strike a cliff face, air is trapped and compressed in cracks and joints. As the wave recedes, the pressure is released explosively, which can weaken and break off pieces of rock over time.
2. Abrasion (or Corrasion): This is the 'sandpaper' effect. It occurs when waves use sediment (rocks, pebbles, sand) as tools to wear away the coastline. The waves pick up these materials and hurl them against the cliff face, chipping and grinding it away. This is often the most effective erosion process.
3. Attrition: This process does not directly erode the coast but affects the sediment. As rocks and pebbles are moved by the waves, they constantly collide with each other. This causes them to break down into smaller, smoother, and more rounded particles.

* Exam Trap: Do not confuse abrasion and attrition. Abrasion is the cliff face being worn down by sediment. Attrition is the sediment particles wearing each other down.

1. Solution (or Corrosion): This is a chemical process where certain types of rock are dissolved by the seawater. It is most effective on rocks like chalk and limestone, which are vulnerable to the mild acids in the water.

3. Landforms of Coastal Erosion

The constant action of waves creates distinct landforms.

* Headlands and Bays: These form on discordant coastlines, where bands of hard and soft rock are perpendicular to the coast. Softer, less resistant rock (like clay) erodes more quickly to form sheltered bays. The harder, more resistant rock (like chalk or granite) is left jutting out into the sea as a headland. The Makran coastline in Balochistan features prominent headlands and bays formed by this process of differential erosion.

* Cliffs and Wave-Cut Platforms:

1. Waves attack the base of a cliff through hydraulic action and abrasion, forming a wave-cut notch.
2. As the notch deepens, the overlying rock becomes unstable and collapses under gravity.
3. This process repeats, causing the cliff to retreat inland.
4. A gently sloping, rocky surface is left at the foot of the retreating cliff, known as a wave-cut platform, which is often only visible at low tide.

* Caves, Arches, Stacks, and Stumps: This is a sequence of erosion that occurs on a headland.

1. A weakness or fault in the headland is eroded by waves, forming a cave.
2. If the cave erodes through to the other side of the headland, or two caves erode back-to-back, an arch is formed.
3. Weathering attacks the top of the arch, while waves attack its base. Eventually, the arch roof collapses, leaving an isolated pillar of rock called a stack.
4. The stack is further eroded at its base and collapses, leaving behind a small, low-lying remnant called a stump, which may only be visible at low tide.

4. Landforms of Coastal Deposition

Deposition occurs when the sea loses energy and can no longer transport its sediment load. This usually happens in sheltered areas where constructive waves are dominant. The main transport mechanism for sediment is longshore drift.

* Longshore Drift (LSD): This is the process that moves sediment along the coastline.

1. The prevailing wind causes waves to approach the beach at an angle.
2. The swash carries material up the beach at this angle.
3. The backwash then pulls the material straight back down the beach due to gravity.
4. This zigzag movement results in the net transport of sediment along the coast.

* Beaches: The most common depositional landform, formed by the accumulation of sand and shingle in bays.

* Spits: A long, narrow ridge of sand extending from the land into the sea. Spits form where the coastline abruptly changes direction, but longshore drift continues to deposit material in the original direction. The end of the spit can be curved by secondary winds, forming a recurved spit. The area behind the spit is sheltered, allowing a salt marsh to develop.

* Bars: A spit that grows across a bay, joining two headlands. This traps water behind it, forming a freshwater lagoon.

* Tombolos: A spit that connects an island to the mainland.

5. Coastal Management in Pakistan

Managing coastal erosion is vital to protect homes, businesses, and infrastructure. In Karachi, the valuable land along Clifton Beach and the DHA seafront faces significant erosion. To combat this, authorities have built extensive sea walls (a form of hard engineering), which absorb and deflect wave energy. While effective at protecting the land behind them, sea walls can be expensive and can increase erosion further down the coast by reflecting wave energy.