River Case Studies

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Mississippi-Missouri

The drainage basin is 3,221,000 km2 placing it fifth in the world.

Combined the Mississippi-Missouri are 6,020 km making it the world's third longest river

The river discharges 584 million tonnes of sediment per year

Drains 40% of the USA

Moves around 13% of the USA's international freight and communities live on the flood plains

Mississippi flows through to states and has 100 tributaries including the Missouri

Upper course is in NW Minnesota and lower course discharges into the Gulf of Mexico

Basin takes in large areas of agricultural land in mid-western USA

Upper catchment area lies across parts of the Rocky and Appalachian Mountains.

Valley bottoms have been urbanised

Floodplain is 200km across and used to be made up of fertile silt deposited by floods.

Land forms include: meanders, natural levees, point bars, braiding and pool and riffle systems

The riverscape has 300 dams, storage reservoirs, flood walls, diversion, channels and artificial levees.

Until 1993 the river appeared controlled.

In 1993 the Mississippi flooded; it was a 1:500 year event.

The flood wall in St. Louis suffered a leak of 150,000 litres per minute. Although it was stopped with sandbags and 5,400 kg of stone.

70% of the levees suffered damage and were breached in the upper course of the river.

Substantial damage in the upper course including:

50 people dead

72,000 homes flooded

50,000 people evacuated

crop losses of $2.6 billion

26 million sandbags used

2.6 million km2 flooded

River traffic halted for several months

Disruption to trade on a massive scale

85% of levees did their job

31 levees overtopped

8 were over topped

3 were breached

Of the non-federal levees 800 failed.

Causes:

Double the average amount of precipitation in June and July

A wet April meant that the drainage basin stores were filled causing a steep rising limb

Concentration of rainfall

Confluence of two huge rivers

Large area of drainage basin

Management focus was on prevention rather than control

Development on floodplain sites

Poorly built levees

Unexpected impacts of dam building

Negative impacts of channelisation

Lack of understanding of river processes

Responses:

Relocation

Natural swamps or wetlands encouraged

Cutting back on the amount of urban land

Relocate levees away from the river

River allowed to flood some places

River Rhine Flood- January 1995

Causes:

Melting snow and heavy rain which was 3.5 times the average

Saturated ground

300% more urban areas that 900 years previous

Increase in arable farming so compacted soil

Narrowing of the river

Construction

Shortening and straightening of the river so it slows faster and steeper so higher peak downstream

Effects:

250,000 evacuated

River 10m higher than usual in Cologne

50,000 evacuated from Well

Cologne was 6 foot deep

Big economical costs

Saturated dykes

Reduction in farm produce

Responses:

Deliberate flooding to equalize pressure

Tiled downstairs so easier to clean in Wells

Traditional houses are raised due to having cellars

Temporary flood barriers in Cologne

Organisation of shelter and compulsory evacuation in Well

Building and strengthening of dykes

Evacuation of farm animals

Island villages

Street embankments

Compulsory rehousing to allow land to flood

Lower flood plain by 5-6m

Stop regeneration of wood which could reduce capacity

Retention basins built upstream on floodplain to reduce flow downstream allow the low lying low value land to flood protecting the more valuable land this increases storage capacity in order to reduce chances of flooding and reduce the peak of the hydrograph

Land use:

Intensive agriculture

Settlements

Commercial traffic on river

Industrial

The Kano River Project

Climate:

800 m precipitation June-August

Savannah

3 months rain

9 months drought

The project and aims:

3 dams: the Tiga, Challawa and Kafian Zaki

A system of canals for domestic and industrial use, to irrigate farmland and provide HEP

Short growing season and wasted water

Who benefits:

Farmers production as crops can grow all year round so 2-3 yields a year instead of just one.

More people working more often so a larger income for those who would usually get less

More land usage means maximum benefit to farmers as big farms can afford to access the water.

Migrant workers can move to where the crops are

Positive multiplier effect as more wages means that the economy can be stimulated

Variation in diet as a variety of crops can be grown

Fishing industries due to the reservoirs.

Who loses:

Small farm owners as they can't afford the infrastructure to irrigate

The water that is released by the dam is unpredictable so crops are more likely to be lost

Nomads have no rights to the land so are squeezed out and this creates a loss of culture.

The rice is water deprived as traditional farmers can't use floods to there benefit

Lake Chad fishing industry as the lake is shrinking

Other Positives:

Two crops per year instead of just one

Reliable water supplies

Higher crop yield (2-3 tonnes per hectare)

Drinking water for Kano

Other Negatives:

Lake Chad is shrinking

Higher cost than expected

Lower yields due to lack of experience and maintenance

Over production causes reduction in price

Fish population has fallen

Less food for birds

Groundwater levels fall and wells dry up

30-50% less water downstream

Keswick Floods 1985

Social Impacts:

Homes flooded up to 5 inches

Damage done to cars

Rugby club under four to five inches of water

Christmas disrupted

People had to be evacuated

Communications interrupted

Environmental:

Diesel ended up in the water, thus contaminating it.

Dirty water due to sewage

Sewers ended up backing up

Economic:

High costs in damage done to homes and cars

Closure of roads

Increased costs for businesses and in fuel prices

Insurance costs around £30,000 per house

Blame:

People blamed the North West Water authority because they allowed the Thirlmere Reservoir to remain too full allowing the River Greta to flood.

Causes:

High amount of rainfall in December at 348.2 mm

Soil was saturated by August

December so possible snowfall but unlikely

Lynmouth August Flooding 1952

Physical characteristics:

Raining on Exmoor so ground was saturated meaning the water stayed on the surface or ran off into rivers.

River Lyn had a steep hydrograph so short time lag and high peak discharge

Impermeable rock

River Lyn has a small drainage basin with steep slopes

Human developments:

Diverted into the town

Flowed through a narrow culvert so reduction in width

Two small bridges over the river

Specific events:

Heavy thunderstorms on the 15th brought 200/300 mm of rain in a 14 hour period

Rained for 12 of the first 14 days of August

Ground was saturated

Effects:

19 boats were destroyed/disappeared

1000 people were left homeless due to flood water damage

Death toll of 34

130 cars were carried away by the river

Buildings were torn down due to how unstable the flood had left them

Management:

Tree-planting scheme to increase infiltration

Straightened river

Flood embankments to increase flood capacity

Restricted rebuilding

Bridges were made bigger

The river was restored to original channel