WAEC SSCE - Geography - 2013 (Practical and Physical Geography)

Explanatory notes on flooding

(a) Areas of occurrence

• River flood plains and valleys such as those of the Niger and Benue in Nigeria.
• Low-lying coastal areas and deltas (the Niger Delta, Lagos and other coastal lowlands).
• Poorly drained towns and cities where drainage cannot cope with heavy rain, and areas below dams.

(b) Causes

1. Heavy and prolonged rainfall that makes rivers overflow their banks.
2. Poor or blocked drainage and the dumping of refuse in drains and channels.
3. Release of water from over-filled dams, and high sea tides or storm surges on the coast.
4. Building on flood plains, deforestation and silting of river channels, which reduce the land ability to absorb or carry water.

(c) Negative effects

• Loss of life and outbreak of water-borne diseases such as cholera.
• Destruction of houses, roads, bridges and other property, and displacement of people (homelessness).
• Destruction of farmland and crops, leading to food shortage, and pollution of water sources.

(d) Control

• Construction of drainage channels, dams and embankments (dykes) to hold and guide flood water.
• Dredging of rivers and regular clearing of drains and refuse to keep channels open.
• Afforestation and control of building so that people do not settle on flood plains, plus early warning and public education.

(a) Two types of delta

• Arcuate (fan-shaped) delta - for example the Nile delta.
• Bird foot (finger-shaped) delta - for example the Mississippi delta.

Other acceptable types: estuarine delta and cuspate (pointed) delta.

(b) Four conditions favourable for the formation of a delta

1. Large load of sediment: the river must carry a heavy load of sand, silt and mud to deposit at its mouth.
2. Gentle gradient and reduced velocity: near the mouth the land is nearly flat, so the river slows down and drops its load.
3. Sheltered, calm sea or lake with weak tides, waves and currents, so the deposited material is not swept away.
4. Shallow water at the river mouth that fills up quickly, and a coastline that is not sinking, allowing sediment to build up above water.

Also acceptable: absence of a strong offshore current and a fairly tideless sea.

(c) Three main characteristics of a delta

• It is a low, flat, triangular or fan-shaped tract of new land built of alluvium at the mouth of a river.
• The river splits into many branches called distributaries that cross the delta to the sea.
• It is made of fertile, fine deposits (silt and mud), is often swampy or marshy, and is enclosed by lagoons and sand bars.

(a) Definition of weathering

Weathering is the breaking down, disintegration and decomposition of rocks in situ (in place) by the action of the weather and other agents, without the broken material being carried away. It differs from erosion because no transport of the debris is involved.

(b) Four processes of chemical weathering

1. Solution: some minerals dissolve directly in water. Rain water, being slightly acidic, dissolves soluble rocks such as rock salt and limestone, removing them in solution.
2. Carbonation: rain water combines with carbon dioxide to form weak carbonic acid, which attacks limestone. The insoluble calcium carbonate is changed to soluble calcium bicarbonate and washed away, forming features such as caves and clints and grikes.
3. Oxidation: oxygen dissolved in water reacts with iron-bearing minerals in the rock to form iron oxides (rust). The rock is stained reddish-brown, weakened and crumbles.
4. Hydrolysis (and hydration): water reacts chemically with minerals such as the feldspar in granite, changing them into soft clay minerals; in hydration minerals absorb water, swell and flake away, so the rock decays.