Water on the land

Waterfalls and Gorges

A waterfall is created by erosion. It is a step in the smooth course of the river. As the water flows over the hard resistant rock such as whinstone, hydraulic action and abbrasion occur. It erodes away the soft rock such as shale or sandstone underneath the cap rock and is undercut. As this continues to erode, the overhang of the hard resistant rock collapses due to lack of support. The bed of the river below a waterfall contains boulders, from the collapse of the overhang, which is eroded by splashback from behind the waterfall. This creates a plunge pool at the bottom of the waterfall. This is due to the sheer force of water hitting eroding the soft rock. The the large rocks from the overhang collapsing rub against the bed eroding it deeper (corrasion). The collapse of the overhang causes the waterfall to retreat upstream which creates a steep-sided gorge of recession. The process continues in a cycle.

Process of waterfall formation (diagram)

Ox-bow lakes and Meanders

An ox-bow lake develops from a meander. The strongest current is on the outer bend and causes intrusive erosion whilst the slowest current is on the inner bend and causes deposition. As the outer banks of a meander continue to be eroded through processes such as hydraulic action, the neck of the meander becomes narrower and narrower. The meander therefore grows and the two outer bends get closer together until a narrow piece of land called the swan's neck develops between the two river cliffs. If there is heavy rainfall and there is a flood the river is able to get rid of excess water quickly by taking a shortcut from one side of the bend to the other leaving a horse shoe shaped channel of water which was the old meander. Material is deposited blocking off the old meander forming an ox-bow lake.

Ox-bow lakes and Meanders (diagram)

Plan of the meander (diagram)

Cross section of a meander (diagram)

Formation of Flood plains and Levees

The flood plain is wide, flat area of land either side of the river in it's lower course. The flood plain is formed by both erosion and deposition. Lateral erosion is caused by meanders and slow migration downstream to widen the flood plain and create a flat valley floor. Sediment which is deposited on the slip-off slopes start to build up the valley floor. This is added to by sediment deposited when the river overflows onto the flat valley floor. The reaon it is deposited is that there is greater friction here than in the river channel and the river's velocity therfore falls. Everythime the river floods another layer of alluvium builds up until it is quite thick. Levees are natural embankments of sediment along the banks of a river in it's lower course abd are often several metres above the flood plain. Levees are formed along rivers that flow slowly, carry a large load and periodically flood.

Cross section showing how flood plains are formed

Meander migration (diagram)

How levees are formed (diagram)

Upper course

During the upper course of a river the nature and velocity of the flow is very turbulent and appears fast but is surprisingly slow due to friction from the rough bed and banks. The river channel is narrow and shallow and the valley is v-shaped. The dominant processes that occur in the upper course is vertical erosion. The size of the load is large boulders and the shape in angular. The composition is boulders of the bedload. There are many land forms that form in the upper course of the river such as: waterfalls and gorges.

Middle course

During the middle course of a river the nature and velocity of the flow is somewhat turbulent and faster due to decreased friction. The river channel is wider and deeper and the valley is bowl shaped. The dominant processes that occur in the middle course are transportation, lateral erosion and some deposition. The size and shape of the load is smaller boulders with a sub-angular shape. The composition is made up of pebbles. The land forms that are created are meanders or ox-bow lakes.

Lower course

During the lower course of the river the nature and velocity of the flow is fairly smooth and faster still in spite of the gentle gradient. The river channel has broad and gentle sides with a valley that is more bowl shaped. The dominant process in the lower course is deposition. The load size and shape is very small pebbles that are rounded. The composition is made up of tiny sand particles. The land forms that are created are flood plains, levees and meander scars.

Precipitation/Temperature

Short intense rainstorms can produce rapid overland dlow and steep rising limb. If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run-off. Snow on the ground can act as a store producing a long lag time and shallow rising limb. Once a thaw sets in, the rising limb will become steep.

Recent weather conditions

If the ground is already saturated then flooding is likely.

Land use

Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the lag time.

Afforestation - incepts the precipitation, creating a shallow rising limb and lengthening the lag time. Defforestation is the opposite.

Rock and soil type

Permeable rocks mean rapid infiltration and little overland flow therefore shallow rising limb.

Infiltration is generally greater on thick soil although less porous soil e.g clay acts as a permeable layers. The more infiltration occurs the longer the lag time anf shallower the rising limb.

Relief

Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time.

Area

Large precipitation means more surface run-off and lag time increases.

Physical long term causes

• Most of the country is the huge flood plain and delta of the River Ganges and Brahmaputra.
• 70 per cent of the total area is less than 1 metre above sea level.
• Rivers, lakes and swamps cover 10 per cent of the land area.

Physical short term causes

• Tropical cyclones and storm waves cause heavy rain and coastal flooding.
• the snow melts in the Himalayas in the late spring and summer.
• Heavy monsoon rainfalls especially over the uplands including the Himalayas.
• Urbanisation on the flood plain in recent times.
• Since 1950 most of the forest cover in the drainage basin of the River Ganges have been cut down.

Human long term causes

• Some people blamed global warming for the increased flooding. A rise in sea level is blamed for the duration of 56 days of the 1998 floods and the warmer global temperature is blamed for the especially high rain fall in the Himalayas.

Human short term causes

• Large areas of forest have been cut down to prvide fuel, timber and grazing land. This is called deforestation.
• The removal forest cover has increased landslides, soil erosion and overland flow. This creates strong surface run-off. The soil is deposited in the river channels causing the raising of the river beds. This means the channels cannot carry as much water and so flooding is made worse.

Environmental

• The short term environmental effect was that vegetation was underwater.
• A medium term effect was that over 57% of the land area was flooded.
• Another medium term effect was that large amounts of land used for housing and farming were eroded away.
• Another medium term effect was that diseases spread and crops were ruined so people went hungry.
• A long term effect was that as the water drained away, brown fields of rotting crop, villages buried in sand and wrecked roads and bridges were left behind.

Economic

• A medium term economic effect was that in Naibari district 240 villages were submerged, crops were underwater.
• A long term effect was that the floods of 1998 cost the country almost $1 billion.
• Another long term effect was that as the water drained away, brown fields of rotting crop, villages buried in sand and wrecked roads and bridges were left behind.

Social

• A short term social effect was that diseases spread such as bronchitis and diarrhoea.
• A long term effect was that ovr 1000 people were killed and millions made homeless.
• Another long term effect was that in Assam 1 million people lost their homes.

Political

• A medium term political effect was that supplies of drinking water and dry food ran low.
• A long term effect was that the government would be expected to do something.

Advantages of inputing 3500km of earth embankments

• It is a form of soft engineering so it is cheaper than building concrete embankments.

Disadvantages of inputing 3500km of earth embankments

• It is quite expensive and very weak as they are built by hand and it would have the risk of breaking. It would also take a long time to build.

Advantages of building seven huge dams

• It is a form of hard engineering so it would be very efficient and would keep water out.

Disadvantages of building seven huge dams

• They would be quite expensive and they would have to be built outside of Bangladesh because of the political issues. It would take a long time to build and for it to work.