Meanders - the River Avon, Hampshire
The Hampshire Avon rises on the Marlborough Downs and flows south to its mouth at Christchurch.
In the lower course, south of Ringwood, there is a series of meanders and oxbow lakes as well as visible meander scars.
The meanders have a typical wavelength of 50-100m and the river itself is 5-10m wide.
Waterfalls - High Force in upper Teesdale
The River Tees rises in the Pennines and flows eastwards to its mouth at Middlesborough.
In upper Teesdale an outcrop of igneous rock called the Whin Sill causes the formation of the High Force waterfall.
The Whin Sill is the resistant cap rock which overlies softer sandstone, limestone,shales and coal seams. These are eroded more quickly, leaving the overhang of High Force.
The waterfall created is 22m high - the tallest in England. Ahead of it lies a gorge stretching over 500m downstream.
Causes of flooding - Boscastle, Cornwall 2004
Boscastle is a small coastal village with a harbour on the north coast of Cornwall, near Bude.
On Monday 16th August 2004 it experienced a dramatic flash flood event in the middle of the afternoon.
Over 100 cars were washed out to sea, trees were uprooted and several buildings, such as Clovelly Clothing and Harbour Lights, were destroyed. Local residents and tourists had to be rescued from rooftops by helicoptor.
The flooding resulted from a combination of factors:
- Village sits at output point point of small drainage basin so it reacts quickly to inputs often giving flashy storm hydrographs.
- Two rivers converge in the village- the Jordan and the Valancy.
- Village sits in the base of a very steep valley with heights dropping over 300m in 6km so surface run-off occurs.
- Very narrow valley floor with no floodplain
- Higher then average rainfall in July and August so water table was high anyway, and soil nearly saturated. River levels rose over 2m in 1hr.
- 130mm rain fell between noon and 6pm compared to the total average rainfall in August being 70-90mm
- Estimated 1 million tonnes of water rushed through the village
- Intense rainfall caused by two warm, moist airstreams converging and rising rapidly. Clouds formed up to a height of 40 000ft
- High tide prevented water from easily leaving at mouth
- Small landslide in upper basin temporarily blocked basin leading to build up of water which suddenly burst free.
- Tarmac car park at top of villge caused rapid surface run-off
Environment Agency report early in 2005 concluded that the floods in Boscastle were a 1 in 400yr event.
Major MEDC flood - southern Britain, July 2007
Weather conditions throughout the summer of 2007 were exceptional. The jet stream, which influences the path taken by low-pressure weather systems in the north Atlantic had followed an abnomally southerly track. This meant the usual anticyclonic weather conditions influenced by the high pressure cell in the Azores did not materialise.
Rainfall totals May-July 2007 were highest on record since 1766, with many areas having twice the long term average.
On 20th July there was very high levels of rainfall in southern Britain, for example, 145mm in Worcestershire and 120mm in Oxfordshire. On the basis of historical data such a storm would only be expected to occur once in several hundred years.
Flood risk in the summer is usually reduced by dry soil conditions.
MEDC flood cont.
However, in this case there had been early summer rainfall so soils were already close to saturation. Groundwater levels were also much higher than normal, so there was little infiltration capacity.
Intense storms on 20th July resulted in severe flash floods in urban areas, including Stratford-upon-Avon and Buckingham. The rainfall quickly overwhelmed urban drainage systems and the emergency services were sent out to rescue stranded people and orgnise evacuations.
Transport networks were disrupted, and as this was the last day of school for many students, the volume of holiday traffic was high. Thousands of motorists were stranded on the M5 motorway.
As the water began to move downstream many floodplains were inundated, causing catastrophic flooding and extensive crop damage.
Worst affected areas were lower parts of the Severn and Warickshire Avon basins, and some upper reaches of the Thames, including Oxford.
3 people died as a direct result of the flood and at one point 45 000 houses were without power. Some 350 000 houses had no running water after the water works was flooded and 140 000 were still affected more than a week later.
Economic costs included:
- £25million damage to Gloucestershire's roads
- Farmers in affected areas lost up to 50% of their crops leading to shortages and price rises the following autumn.
- £3billion flood damage covered by insurance
- £1billion cost to the water industry
Major LEDC flood - Bangladesh, 2004
Bangladesh is a low-lying country most of whilch lies on the delta land of three major rivers, the Ganges, Brahmaputra and Meghna. The sources of these rivers are in the Himalayas so snowmelt adds to their discharge in spring.
At times during the rainy season, Bangladesh suffers from tropical revolving storms that bring exceptional winds, intense rainfall and storm surges. Such conditions severely affect the discharge of the three rivers and their distributaries, causing regular floods.
Human factors have played an increasing role in the severity of floods in recent decades. As in more developed countries, urbanisation has occurred and the capital city Dhaka now has a population of more than 1 million. In addition to this, rapid deforestation in the Himalayas has had a negative effect on the rates of interception and evapotranspiration, resulting in more water reaching the rivers.
River management is much harder to implement in less developed countries like Bangladesh. The country is one of the poorest in the world, with average GDP per capita at around $300. Most of the population rely on subsistance agriculture to survive, growing rice on rented land.
This means taxation revenue is limited and Bangladesh relies heavily on foreign aid to finance large-scale development projects which might help to prevent floods. In 2004, the monsoon season brought more rainfall than usual. From late june through to september the three main rivers burst their banks, resulting in widespread flooding.
- During July and August approx 38% of land was flooded including 800 000ha of agricultural land and the capital city, Dhaka
- Nationwide, 36million people were made homeless
- By mid-september the death toll had risen to 800, mainly from water bourne diseases due to lack of clean water
- Flood caused serious damage to infrastructure. Boats were afloat on the main runway at Sylhet airport and all domestic and internal flights had to be suspended during July
- Value of damage was estimated to be at $2.2billion or 4% of the total GDP for 2004
- Poor were less able to withstand effects so landless labourers and small farmers were the most severely affected in rural areas. In urban areas it was typically the slum dwellers, on poorly drained land.
- Environmental impacts: riverbank erosion, soil erosion, water-logging and water contamination
Short term responses included the government working with NGOs providing emergency relief in the form of rice, clothing, medicines, blankets and towels. The UN activated a disaster management team to coordinate the activities of the various UN agencies. They supplied critical emergency supplies and conducted a damage and needs assessment in affected areas. Bilateral aid from individual countries was directed to the UN team. Self-help schemes where locals work together to rebuild properties are also common.
Long term responses:
- Largely dependent on foreign aid. In the past river management schemes have been funded but have proved inadequate. Such schemes paid little attention to local knowledge of the rivers so failed
- More recently, small-scale community based projects have resulted in lives being saved. Flood shelters and early-warning systems have successfully been put in place.
- Following the 2004 flood additional financial aid was granted for a period of 5 years. This was mainly in the form of a loan from the World Bank to pay for repairs to infrastructure, water resource management and education.
- Disaster-preparedness is a key priority for the future. this includes flood management and improved water resources. It is also planned that, in future, flood-resistant designs should be used in all social and economic infrastructure projects.
Hard Engineering - Three Gorges dam, China
It is the largest hard-engineering project ever undertaken on a river. The project, due for completion in 2009, is located on the Yangze River in China. The river has a long history of flooding and the dam will be used to generate electricity from HEP for central and eastern China. This includes the city of Shanghai which has a population of more than 13 million people.
The dam will also reduce the flood risk for 15 million people and improve navigation along the river. A huge reservoir, some 600km long and 1km wide has been created behind the dam. The dam itself, completed in 2006, is 2.3km long and 180m high. According to official figures, it has cost somewhere in the region of £25billion to build.
- Forced resettlement of 1.2million people from several cities, 11 centres and over 100 villages to newly built settlements.
Hard Engineering cont.
- The area of resettlement runs along the Yangtze River and most people have been located as near as possible to their former homes.
- Many cultural monuments have been lost as a result of the flooding, including the Zhang Fei temple.
- Between 1998-2004 the amount of sediment transported by the river below the dam fell by over 50% resulting in increased rates of erosion downstream. Sediment will accumulate behind the dam which will require dredging
- Afforestation is needed on the slopes in the drainage basin to reduce the amount of sediment washed into the river and the reservoir behind the dam
- The dam will have negative environmental and ecological effects. There is an increased likelihood of landslides in the immediate area around the reservoir and many species, such as the white flag dolphin, will have their habitats disrupted
Soft Engineering - River Quaggy restoration scheme
The River Quaggy runs through southeast London. Since the 1960s it has been havily managed and artificial channels and culverts were built to divert it beneath the ground surface as it passed through Greenwich.
As a result of increased flood risk due to continued urban development in Lewisham and Greenwich, more needed to be done to protect the area from flooding. Hard engineering methods, such as further widening and deepening of the channel were considered, but instead the Environment Agency chose a more sustainable soft approach.
This solution was proposed by local residents who formed the Quaggy Waterways Action Group to campaign for a sustainable approach which would improve the local environment.
Soft Engineering cont.
The plan was to bring the river back above ground once again, cutting a new channel for it through Sutcliffe Park, and creating a multi-functional open space.
In this way flood management and the quality of the park would be improved. Although a culvert remained to take some water underground during flood conditions, a new lake was created to take over when this grew full.
The park itself was lowered and shaped to create a floodplain where water could collect naturally instead of rushing downstream through artificial channels to flood Lewisham town centre. The parks flood storage capacity of 85 000m cubed of water, equivalent to 35 Olympic swimming pools, has reduced the risk of flooding for 600 homes and businesses in Greenwich and Lewisham, and has created a diverse environment for wildlife.
Soft Engineering cont.
By redirecting the river to a more natural course and including a flood storage area, the scheme has created a wetland environment with reedbeds, wild flower meadows and trees.
The scheme won the Natural Environment category in the 2007 Waterways Renaissance Awards and the Living Wetland Award