WAEC SSCE - Geography - 2018 (Practical and Physical Geography)

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A geographical account of flooding in urban areas

Flooding in urban areas occurs when the volume of water on the land surface exceeds the capacity of the drainage system to carry it away, so that streets, compounds and buildings become submerged. The account below is written under the three required headings.

(a) Four causes of urban flooding

1. Excessive or prolonged rainfall. Heavy downpours release more water than the drains can discharge within a short time, so the excess spills onto roads and into buildings.
2. Dumping of refuse into drains and channels. Solid waste, silt and vegetation block gutters and culverts, obstructing the free flow of water so that it backs up and overflows.
3. Poor engineering and construction of the drainage system. Undersized, badly aligned or unlined drains cannot cope with peak run-off, and poor town planning allows settlements to be built on flood plains and across natural water channels.
4. Removal of vegetation and sealing of the ground with concrete and tarmac. Paving reduces infiltration and greatly increases surface run-off, sending a larger volume of water more quickly into the streets.

(b) Three effects of urban flooding

1. Loss of lives and properties. People are drowned or swept away, while buildings, roads, bridges, vehicles and household goods are destroyed or damaged.
2. Spread of water-borne diseases and health hazards. Flood water mixes with sewage and refuse, contaminating drinking-water sources and causing outbreaks of cholera, typhoid and other water-borne diseases.
3. Disruption of movement and economic activity. Submerged and eroded roads make transport difficult, businesses, schools and offices close, and people are displaced and rendered homeless.

(c) Four measures that can be used to control urban flooding

1. Provide adequate, well-engineered drainage and desilt it regularly. Construct wide, properly graded drains and culverts and clear them of silt and refuse before and during the rainy season.
2. Enforce efficient urban planning. Prohibit building on flood plains and across natural water channels, and reserve these areas as green or open space.
3. Ensure proper waste disposal and public enlightenment. Stop the dumping of refuse into drains and rivers, and educate residents on keeping gutters clear.
4. Construct dams, embankments and reservoirs and dredge rivers. These works store or divert excess water and increase channel capacity, controlling the overflow of rivers into the built-up area.

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(a) Gorge

Characteristics: A gorge is a narrow, steep-sided (almost vertical) and deep river valley. It is usually cut in hard, resistant rock, has a fast-flowing river occupying the whole floor, and often occurs below a waterfall or in a limestone/arid region.

Mode of formation: A gorge is formed mainly by rapid vertical (downward) erosion by a river where the rock is hard and the sides do not weather back quickly. It may form by the retreat of a waterfall upstream, by river rejuvenation, or by the collapse of the roof of an underground cavern in a limestone area.

Annotated diagram: Draw a cross-section of a very narrow, deep valley with near-vertical walls and a river filling the floor; label "vertical/steep sides", "hard resistant rock", and "river occupying floor".

(b) Seif dune

Characteristics: A seif (longitudinal) dune is a long, narrow ridge of sand lying parallel to the prevailing wind direction. It has a sharp crest, may run for many kilometres, and occurs in groups separated by wind-swept corridors of bare rock.

Mode of formation: Seif dunes form in deserts where a strong, steady prevailing wind blows sand along, and occasional cross-winds from a slightly different direction sweep the sand into long parallel ridges aligned with the dominant wind. Continuous saltation drags the sand grains forward along the ridge.

Annotated diagram: Draw a plan/oblique view of long, straight, parallel sand ridges; add a large arrow showing the prevailing wind blowing along the length of the ridges, and label "crest", "parallel to wind", and "corridor of bare rock between dunes".

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Conditions under which the following features are formed

(a) River capture

River capture (river piracy) occurs when a more powerful stream diverts and takes over the headwaters of a weaker, neighbouring stream. The conditions include:

• Two rivers flowing in adjacent valleys separated by a narrow watershed, where one river lies at a lower level than the other.
• One river being more powerful (with a steeper gradient and greater erosive power) so that it can cut its valley faster, especially by headward (backward) erosion.
• Rapid headward erosion by the more powerful stream until it cuts back through the watershed and reaches the valley of the weaker stream, diverting its upper course. The point of diversion is the elbow of capture, the beheaded river below it is the misfit stream, and the dry valley left behind is the wind gap.

(See diagram: two parallel rivers, the captor cutting back to the elbow of capture, with wind gap and misfit stream marked.)

(b) Waterfall

A waterfall is a sudden vertical fall of a river over a steep rock face along its course. It forms where:

• A band of hard (resistant) rock lies across the river's course over softer rock; the soft rock downstream is worn away faster, leaving a step over which the water falls.
• There is a sudden change of gradient or a fall in the level of the land, for example where a river crosses a fault line, a plateau edge or an escarpment.
• The river plunges over a steep cliff or edge, for example at the edge of a hanging valley, a coastal cliff, or where a hard rock ledge (like a lip) causes a vertical drop, e.g. Farin Ruwa or the Zambezi at Victoria Falls.

(See diagram: river crossing a hard rock band over softer rock, with the fall, plunge pool and undercutting shown.)

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(a) Eclipse of the Moon (lunar eclipse)

A lunar eclipse occurs when the Earth lies directly between the Sun and the Moon. The Earth blocks sunlight and casts its shadow on the Moon. This can occur only at full moon, when the Sun, Earth and Moon are aligned.

If the whole Moon passes into the dark central part of the Earth’s shadow, called the umbra, there is a total lunar eclipse. If only part of the Moon enters the umbra, the eclipse is partial. The Moon may appear reddish because some sunlight is bent through the Earth’s atmosphere into the shadow.

(b) Characteristics of great circles

• A great circle is formed by a plane passing through the centre of the Earth; it divides the Earth into two equal halves, called hemispheres.
• Great circles are limitless in number because they can be drawn in many different directions through the Earth’s centre.
• They are the largest possible circles on a globe. The Equator is a great circle; a complete circle formed by a meridian and its opposite meridian is also a great circle.

(c) Uses of great circles

• They help to locate places on the Earth’s surface, especially through the Equator and the system of latitude and longitude.
• They are used to determine directions and to plan routes in navigation.
• They show the shortest route between two places on the Earth’s curved surface, helping aircraft and ships save travelling time and fuel.

Examination reminder: For a lunar eclipse, remember the order Sun – Earth – Moon. For great circles, link their shape through the Earth’s centre to their navigation use: they provide shortest-distance routes.

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(A)(i) Three landforms found in a limestone (karst) region

• Doline: a circular or funnel-shaped depression formed where water dissolves limestone or sinks into the ground.
• Uvala: a larger, irregular depression formed when two or more dolines join together.
• Stalagmite: an upward-growing limestone deposit on the floor of a cave, formed when lime-rich water drips from the roof.

(A)(ii) Underground features of a limestone region

(B) Three ways limestone is beneficial to people

• It is used as a building material and crushed to make aggregate for roads and concrete.
• It is an important raw material in the manufacture of cement.
• It is used in the manufacture of glass.