General Coastal Notes + Landforms! 1 Types of Coastlines: Type Description Primary Coast which is essentially in the same condition when sea level stabilized Coastline after the last ice age, “younger”. They are created by erosion (the wearing away of soil or rock), deposition (the buildup of sediment or sand) or tectonic activity (changes in the structure of the rock and soil because of earthquakes). River deltas are an example of a primary coast. They form where a river deposits soil and other material as it enters the sea. Secondary Coastline shaped primarily by marine forces or marine organisms, and wave Coastline energy is the main agent shaping the coastline. Emergent Emergent coastlines are stretches along the coast that have been exposed by Coastline the sea due to a relative fall in sea levels. Submergent Submergent coastlines are stretches along the coast that have been Coastline inundated by the sea due to a relative rise in sea levels. Concordant A coastline consisting of bands of rock, which run parallel to the coast. Coastline Discordant A discordant coastline occurs where bands of differing rock type run Coastline perpendicular to the coast. Factors affecting Wave Energy: Factor: Description Wind Speed Wind speed will in turn affect wind energy, which determines the size of the waves. Higher wind speed = Higher wind energy = Larger waves Fetch Fetch is the distance of the ocean which the wind blows over, Longer distance = Larger waves Wind How long the wind has blown in the same direction (steady winds at slower Duration speeds can create higher and more powerful waves than those resulting from brief, strong gusts) Wave Terminology: Term Explanation Crest The very top of the wave Trough The hollow between two crests Wave height The vertical distance between the top of one wave crest and the bottom of the next trough Wave Length The horizontal distance between any one point on one wave and the corresponding point on the next Wave The ratio of height to length steepness Wave speed The velocity in which waves travel Deep water Waves that are in water that is deeper than half their wavelength waves Shallow water Waves that are in water that is shallower than 1/20 their wavelength (the waves important difference on these last two is whether or not the sea floor influences the motion of the wave) Swash When a wave breaks and the water travels up the beach this is called the swash. Swash pushes sediment up the beach away from the sea. Backwash When the water from the waves starts to run back down the beach it is called the backwash. Backwash pulls beach material towards the sea. Constructive and destructive waves: Constructive Destructive Long and low waves Short and high waves Long wavelength Short wavelength Low wave frequency High wave frequency Calm conditions Stormy conditions Waves slide forward in shallow water Waves curve downward in deep water due to steep due to gentle offshore slope (Spill waves) offshore slope (Plunge waves) Swash > Backwash, hence deposits Backwash > swash, hence erodes Coastal Landforms: EROSION-DOMINATED Headlands & Bays Headlands and bays are formed along discordant coastlines, which have alternate bands of resistant and less resistant rocks. Differential erosion results, and the softer rocks are eroded more quickly than that of the harder rocks, giving rise to an irregular coastline of headlands and bays. Wave refraction, the bending of wave fronts so as to break almost parallel with the shore, enhances their formation. As the waves approach shallower waters near the headlands, they encounter the retarding influence of the sea floor due to frictional drag. They increase in height, decrease in wavelength, and slow down. This drag causes the waves to slow down and bend inwards at the headlands. Thus, since wave energy is proportional to wave height, the wave energy is concentrated and directed at the headlands. Those waves have a stronger backwash than swash, and hence carry out erosion at the headlands. However, waves approaching the bays continue unimpeded and so move ahead of the wave segment off the headlands. Wave height is decreased since the energy of waves approaching the shallower waters is spread out and diffracted. Such low-energy waves have a stronger swash than backwash, hence depositing sediments, and enhancing the formation of bays. General Coastal Notes + Landforms! 3 Sea Cliffs When high-energy waves reach land with steep slopes, they erode the weaker parts of the steep slopes to produce a notch. Continued erosion and undercutting enlarges the notch to form a steep rock face called a cliff. After a period of time, the cliff becomes steeper and retreats further landwards. Wave-cut Undercutting at the base of the cliff, together with the removal of eroded platforms material, causes the cliff to retreat landwards, exposing a flat terrace at the foot of the cliff called a wave-cut platform. After a period of time, the wave- cut platform becomes wider, and when it is being buried by deposits with cause a belt of shallow water that decreases the wave energy, erosion of the platform ceases. Offshore Terraces They are formed from eroded materials, which are transported away and deposited in the offshore zone. Sea Caves Due to wave refraction, waves approaching the shore converge at the sides of the headlands, and the energy of the waves is directed there. They often attack and widen lines of weakness into hollows, which form the sea caves. Sea Arch Sea arches are formed when two caves on opposite sides of the headland join to form a complete opening. Stacks Stacks are formed from arches that are further eroded. The arch collapses from the erosion, leaving behind the seaward pillar of the rock, known as the stack. Stumps Further erosion reduces stacks to stumps that are only visible at low tide. Blowholes A blowhole is a near-vertical cleft (cleft: a fissure/split) or cylindrical tunnel leading from the rear top if a sea cave upward to the land surface above. Due to the presence of near vertical lines of weakness, waves surging into the sea cave during high tides tend to force and General Coastal Notes + Landforms! 5 compress the air into the lines of weakness. When the waves retreat suddenly, the compressed air is set free, expanding explosively. The rocks are shattered, resulting in the opening and widening of the lines of weakness along the cave. Ultimately, part of the roof of the cave collapses, producing a blowhole. Geos A geo is a narrow, long and steep-sided inlet, which was originally a sea cave. It can be formed in two ways, such as when erosion extends a sea cave landwards, causing the cliff to be undercut by waves. The top portion of the cave may collapse, resulting in a geo. Geos can also be formed from blowholes, when continued wave erosion widens the blowhole till the entire roof of the sea cave collapses, hence becoming a geo. FORMED BY DEPOSITION BEACHES: A gently sloping platform formed by the accumulation of material (e.g. mud, sand, pebbles, cobbles) deposited by constructive waves on/along the shore, between the highest and lowest water levels. Bay-head beach Formed at the head of a bay between two headlands, usually made of finer particles. Bay-side Beach Formed at sides of bay, composed of coarser sand particles, or a mixture of sand, gravel and pebbles. Bay-mouth beach Formed at the tip of a headland, consists of coarser particles, gravel and boulders. Spit A narrow ridge of sand or shingle deposited by longshore drift at a/an sharp or abrupt turn of the coastline or across the mouth of a river. The longshore drift, running parallel to a relatively straight coast, causes beach materials to be laterally transferred even at the turn of the coastline. As slack water occurs at the turn, the waves lose energy and deposit the sand or shingle in the form of a ridge, forming a spit. Recurved Spit One end of the spit is attached to the mainland while the other end is free, and projects into the sea. As the spit grows and extends into deeper water, wave action causes the free end to be curved towards the land, enclosing a water body called a lagoon, forming a recurved spit. General Coastal Notes + Landforms! 7 Tombolo A tombolo is a narrow piece of land, such as a spit or a bar, extending from a shore to an island, formed through deposition of sediments. BARS: Bars are narrow ridges of sand and/or gravel deposited by waves across a bay, usually in a direction parallel to the shore. When a bar is first formed, both ends are free (not attached to the land). Continued deposition may cause its ends to extend and join the land. Bay-mouth bar A spit grows from a headland and joins up with the headland on the opposite end, forming a bay-mouth bar. OR two spits extending from The bar encloses a body of water in the bay, called a lagoon. Bay-head bar Wave refraction on bay shores sweeps materials towards the head of a bay to form a ridge of accumulated material rising from the sea floor, above the low tide level. (Location: closer to beach) Offshore bar Develops where waves break offshore, due to the gentle slopes under water. Deposition occurs, forming the offshore bar. It is free at both ends and runs parallel to the shore. Cuspate bar A cuspate bar is a crescent-shaped bar uniting with the shore at each end. It Cuspate Foreland may be formed by a single spit growing from shore and then turning back to again meet the shore, or by two spits growing from the shore and uniting to form a bar of sharply cuspate (a pointed end where two curves meet) form.