DISCHARGE AND FLOOD HYDROGRAPHS
1 Discharge is the velocity of the river times its volume. It is the amount of water in the river passing a given point at a given time, measured in cumecs (cubic metres per second). It depends upon the river's velocity and volume.
2 Velocity is the speed of the river. It is measured in metres per second.
3 Volume is the amount of water in the river. It is the cross-sectional area of the river's channel measured in square metres.
A FLOOD HYDROGRAPH
In some drainage basins, discharge
and river levels rise very quickly after a storm - they are described
as having a "flashy" response to inputs of precipitation.
5 This can cause frequent, and occasionally serious, flooding. Following a storm in these basins, both discharge and river levels fall almost as rapidly, and after dry spells, become very low. In other drainage basins, rivers seem to maintain a more even flow
.6 The flood (storm) hydrograph is a graph showing the discharge of a river at a given point (e.g. at a gauging station) over a period of time.
7 A flood or storm hydrograph shows how a river responds to one particular storm.
8 When a storm begins, discharge does not increase immediately as only a little of the rain will fall directly into the channel. The first water to reach the river will come from surface run-off, and this will later be supplemented by water from throughflow. The increase in discharge is shown by the rising limb.
9 The gap between the time or peak rainfall and peak discharge (highest river level) is called lag time. A river with a short lag time and a high discharge is more likely to flood than a river with a lengthy lag time and a low discharge. TOP OF PAGE
FACTORS AFFECTING THE RESPOSIVENESS OF A RIVER
(STORM HYDROGRAPH SHAPE)
RIVER TEES - A CASE STUDY
[See "Understanding GCSE Geography on pages 40-43 and 172-173] UPPER COURSE
1 The River Tees is located in north-east England.
2 Its source area is high in the Pennines in the west and the river flows eastwards into the North Sea.
3 The source of the River Tees lies on Cross Fell in the Pennines, 893m above sea level, where rainfall is over 2000mm a year.
4 Run-off is high because of the impermeable rocks and the steep slopes.
5 The valley cross-section is steep sided and V-shaped and the long profile has a steep gradient.
6 The river occupies the whole of the valley floor.
7 The river is turbulent and clear, although often stained brown by the peat (semi-decomposed organic matter - too wet/acidic/cold for bacteria to break down) which covers much of the moorlands.
8 The river bed is rocky and there are many rapids and a waterfall at High Force: q the highest waterfall in England, with a very deep plunge pool at its base , the cap rock is made of a very resistant igneous rock called whinstone, -below the whinstone there are bands or sandstone and shales as well as some very chin coal seams, these rocks are less resistant and erode more easily creating an overhang in the cap rock. -over many thousands of years the waterfall at High Force has retreated upstream, creating an impressive gorge of recession.
9 Settlement and Communications:- The steep slopes and lack of flat land in the valley floor make the building of communications and settlements very difficult, and employment opportunities are limited, as result the population density is very low. -On the bleaker moorlands there is no settlement at all, but more farms and small villages are sited along the B6277 road which follows the more sheltered land in the Tees valley
10 Water supply -In the Tees Valley the water quality is very high which makes it suitable for use as a water supply for homes and industry. -The high rainfall also gives a reliable water supply. -There are several reservoirs in the river basin, the highest of these is Cow Green reservoir. -The reservoirs also help in flood control. The River Tees is 'flashy', meaning that watrer levels can rise very quickly following a rain storm. -Storing water in the reservoir helps to reduce flooding in the lower course.
11 Sheep farming - One of the major land uses in the upper Tees is rough grazing for sheep. The land is too steep to use machinery, the growing season too short and the soils too thin and acidic for growing crops. Above about 400m is found open moorland where the sheep roam freely in the summer . Below 400m there are fields bounded by traditional dry-stone walls. Only where the slopes are less steep is the pasture improved by fertilizers, the occasional crop of hay or barley is grown and a small herd of dairy cattle is grazed.
12 Tourism -The open moorland, High Force, the nature trail at Widdybank Fell, the shooting estates. the Pennine Way (a long-distance footpath) and the atttractive villages and market towns such as Middleton-ln-Teesdale, all attract visitors to Upper Teesdale. The visitors provide much-needed employment for local people in the hotels and restaurants, as car park attendants, visitor centre staff and shop assistants. However, tourism also brings traffic congestion, litter and overcrowding.
13 Industry --Upper Teesdale once had a thriving lead mining industry. Today the disused lead mines are attractive to visitors. There has been little other large-scale industry to replace lead mining. There is some limited employment in forestry, and in various craft industries, e g artists and textile workers. Whinstone is quarried at Holwick and used for roadstone because of its great resistance. The first really large employer in the valley is Glaxo-Wellcome, a chemical and pharmaceuticals company located on the outskirts of Barnard Castle.
1 Moving downstream the valley begins to widen and the river starts to meander .
2 There are more bridging points and larger villages and towns, such as Yarm.
3 Nearing the river mouth the river meanders in large loops across its flat tood plain (it is 30km as the crow flies from Darlington to Teesmouth but the river travels 75km)
4 It used to be longer but several of the meanders were cut off in the nineteenth century to shorten the journey for boats navigating the river up to Stockton and Yarm.
5 At Teesmouth the nver flows into an estuary where there are huge areas of mud flats such as Seal Sands. These are important wildlife areas for migratory birds and seals.
6 The lower course is much more urbanized and industrialized than the upper sections, with large towns such as Stockton and Middlesbrough and the vase port of Teesside.
7 Industries such as chemicals, ships, steel-making and engineering developed during the Industrial Revolution.
8 Today, Teesside is a major centre for the ICl petrochemicals industry based ac Billingham and Wilton, and is well placed to receive oil and gas from the fields in the North Sea.
9 Shipbuilding has been replaced by oil platform construction and there is a huge modern integrated steelworks (raw materials are converted to finished goods in one factory) at Redcar.
10 The heavy industries (bulky/large raw materials and finished products have taken advantage of the flat and relatively cheap expanses of mud flats in the estuary.
11 The land is easily reclaimed and there is a nearby labour supply and good transport: networks.
12 The port also provides a sheltered harbour for the import of raw materials and the export of finished goods.
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RIVER BASIN MANAGEMENT
1 The River Tees has a long history of flooding.
2 The first documented flood was at Croft on the lower Tees in 1356.
3 The Tees valley is also home to a large population and many industries, all requiring a reliable water supply.
4 The river is managed to provide a water supply and to control flooding.
5 In recent years there have also been developments to increase its potential for recreation and tourism.
6 Cow Green reservoir was built in 1970 to provide water for the growing industries on Teesside.
7 It is a regulating reservoir, storing water in times of plenty and releasing enough for the needs of industry in times of low flow.
8 In times of severe summer droughts water can be added to the River Tees via a tunnel which connects it to the River Tyne and Kielder reservoir.
MANAGEMENT IN THE LOWER TEES VALLEY
1 The Tees Barrage (a man made barrier across a river) -The aim of the Tees Barrage was to improve the water quality and recreational value of 22km of the lower Tees. The barrage was completed in 1995 and cost £54 million. The 22km stretch of river between Yarm and Stockton is now kept permanently at high tide. The water is fresher and cleaner as it does not mix with the tidal salt water in the lower estuary. The barrage also reduces the risk of flooding at very high tides or during a storm surge. The barrage has acted as a catalyst for £500 million of investment in offices, housing, educational, leisure and shopping facilities.
2 Dredging The lower stretches of the Tees estuary are dredged (sediment is extracted) periodically to improve navigation by maintaining a deep-water channel. There has also been some dredging in the upper parts of the estuary to reduce the flood risk - it increased the capacity of the channel.
3 Cutting of meanders In 1810, the Tees Navigation Company cut across the neck of the Mandale Loop, a large meander near Stockton. The new route shortened the river by 4km. Other screeches of the river have been artificially scraightened. This allows the water to move faster along the channel (less energy lost with the river banks), reducing the flood risk.
4 Yarm's flood defence scheme Yarm, a historic market town and once an inland port, is located on the inside bend of a large meander. Yarm is particularly prone to flooding. The most recent serious flood was in January 1995. Since then a new flood defence scheme costing £2.1 million has been built which:
§ reinforced concrete walls with flood gates for access by people and vehicles.
§ earth embankments to contain the river.
§ gabions (baskets filled with stones) to protect the walls and embankments from erosion.
The scheme also incorporates features designed to reduce the visual impact of the walls and to enhance the environment. There are fishing platforms, new screet lighting and a comprehensive planting scheme. English Heritage approved all building materials used so that they were in keeping with the existing architecture of the town.
5 Improved flood warning systems. These have better liaison with the Meteorological Office, police and other emergency services. 6 New development discouraged Building on low-lying and flood-prone land is discouraged - an example of land-use zonation (land is used for activities not damaged by inundation e.g. playing fields, parks, urban forests/walks etc)
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FLOOD DEFENCE/PREVENTION SYSTEMS
1 Diversionary Spillways - these are overflow channels which can take surplus water during times of flood to spreading grounds (land which is set aside - usually for recreational purposes) into which the floodwater slowly settles.
2 Re-routing of rivers - dig new channels and divert river flow around large settlements/valuable land uses.
3 Embankments/Revetments - increase the carrying capacity (cross-sectional area) of river channels and strengthen the banks - rivers are often above the height of the surrounding land - any breach in levees/embankments can cause accelerated destruction (worse than would have occurred before the intervention by mankind in order to try to the manage river systems)
4 Dams and Reservoirs - see previous notes.
5 Afforestation of the catchment - see previous notes.
6 Restricted use of flood-plains - legislation, higher selective insurance premiums/refusal to insure particular locations.
7 Co-ordinated flood warning and emergence reaction procedures e.g. by the US Corps of Engineers
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|Text by MRoden. Storm Hydrograph by RShah. Web Design by KPhipps. Copyright ©2000|
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