To what extent has longshore drift influenced the development of depositional features in East Africa?

 

Candidates are expected to define the term longshore drift,  talk about constructive waves, give the landforms  that result from longshore  drift then bring out other factors.

• Longshore drift is the movement of sediments/eroded materials along a coast by waves that approach at an angle to the shore but then the swash recedes directly away from it
• Waves approach the coast at an angle. Swash carries sediment up the beach at an angle. Backwash carries sediment down the beach with gravity – at right angles to the beach.

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Depositional landforms formed by longshore drift include

• Beach is a gently sloping strip of land along the coastline covered products of weathering and erosion such as sand, pebbles, rock and seashell fragments. The different types of beaches include
• Barrier beach is a narrow and elongated ridge of sand deposits approximately parallel to the shoreline separated from it by a lagoon. It is formed on gently sloping Coastlines by longshore drift waves. Materials are deposited under water as off shore sand bars appear above high tide, wave action gradually moves the deposited material to the mainland as barrier
• Beach Series of small horn-shaped projections separated by shallow indentations that Point Sea wards. Formed by eddies of powerful swash that scours coarser materials in the depression.
• A Bayhead beachis a type of beach that is crescent shaped and has developed at the head or back of a bay between two headlands by constructive waves e.g. Lido beach at Entebbe
• Beach rockis a friable to well-cemented sedimentary rock that consists of a variable mixture of gravel, sand and silt-sized sediment that is cemented with carbonate minerals and has formed along a shorelineg. found at Lutembe beach along L. Victoria
• Beach Berms is ridge like features formed by larger material that accumulates at furthest limit of Swash action. Develop on beaches when Swash is stronger than back wash e.g. Lutembe Beach, Lido etc.
• Beach Bars: Ridges of sand, Mud, gravel and shingle deposited off shore parallel to the coast formed on gently sloping coast/irregular shoreline.

Formation is attributed to either waves which drift materials along the shore or backwash combing materials directly down the beach.

 

Similarly the long breaking waves cause sand grains to more sea ward resulting into accumulation of material on the submerged line known a break Point bar. Repeated processes form a bar behind which develops a lagoon, mud flats & marshes.

 

Types of Bars include:

• Off Shore Bars: These are long ridges of sand and shingle deposited along a coast separated from the coast by a lagoon formed by waves breaking some distance offshore where the continental shelf is shallow leading to formation of a submarine bar that is slowly built upwards through continued deposition of
• Fore shore Sand Bar: A bar formed by constant accumulation of sand causing offshore bars to rise above the water ,
• Bay Bar: A bar which extends across a bay forms when a spit continues to grow length wise from one headland towards another. Link the two headlands enclosing a lagoon e.g. at Nabugabo bay.
• Barrier Island: A bar which gradually moves inland by wave attack and encloses the area of shallow water (sounds) and have no connection to the mainland.

 

Spits

Spit is a long narrow ridge of sand, shingle or pebbles in a linear form joined to the land at one end with the other end projecting into the sea or across the estuary.

It grows out from the coastline due to the effect of the long shore drift, often at a location where the line of the coast changes direction, usually at the mouth of an estuary or delta e.g. Kaiso spit and Tonya spit on L Albert.

 

Types of Spits include:

• Hooked Spit is a long narrow ridge of sand, shingle or pebbles joined to the land on one side with hooked end projecting into the sea or estuary. Formed by waves moving obliquely to the shore tending to swing around the end of the spit or waves approaching the shore from several directions force the open part of the spit to bend or The deep water off the spit allows wave refraction to curve hence forming a hooked spit e.g. at Kibanga.

• Cuspate Spit applies to two spit converging It is also a recurving of a simple spit until it becomes attached to the shore at both ends.

• Cuspate foreland is a triangular shaped deposit of sand, shingle projecting sea wards formed by convergence to one apex of two separate curved spits broadly at right angles or by two sets of constructive waves, the enclosed water is filled with deposits then colonized by vegetation leading to a cuspate foreland, e.g at Tonya point.
• Winged headlands: These are spits attached at both sides of the headland. They develop when spits develop at both sides of the headland. This occurs when waves / longshore drift reach the headland from opposite directions.

• Tombolo is a ridge/ bar of sand or shingle joining an island to the mainland or joining an island to an When the longshore drift operates between an island and the mainland, sediments may gradually be laid down in that in that zone. Deposition may start on one end of the mainland linking the two up. E.g. Bukakata-Lambu islands.

• Mudflats: Platforms of mud, silt and other forms of alluvium along gentle is especially in bays & estuaries. They develop when rivers & waves deposit material along gentle coasts especially in bays and estuaries between high and low tides e.g. near Tanga, mouth of R. Rufigi.

Candidates are expected to make an evaluation and bring other factors like:

• Wind direction which should be on shore such that material is carried and deposited by on- shore wind along the shoreline.
• Availability of weathered and eroded materials that are transported by constructive waves and deposited at the coast to form various landforms e.g. beaches.
• Presence of a relative shallow continental shelf to form the base upon which material is
• Presence of organic deposits/corals due to deposition and accumulation of dead
• Nature of the coastline i.e. gently sloping allows deposition & accumulation of material leading to formation of depositional coastal features.
• Human activities e.g. dumping of industrial wastes at the coast/ shoreline, soil and rock boulders to construct piers etc.
• River deposition at the mouth forming deltas.

 

NB: Conclusion should be drawn

Diagrams should be drawn where applicable.