The Hydrological Water Cycle Drainage Basin The continuous
The Hydrological (Water) Cycle: Drainage Basin – The continuous circling of water between the land, sea and atmosphere An area of land drained by a river and its tributaries Watershed – Tributary – Confluence – Source – Mouth – How does the Water Cycle Work? Solar Radiation: is the driving force behind the cycle as it enables e______ to take place from the land & sea Condensation of the water vapour leads to P_______. Most of the water that falls on the land is then returned to the sea through River Run-Off and Groundwater Flow Key Terms: Evaporation – Transpiration – The Drainage Basin As A System Key Terms Inputs ~ Stores ~ ~ Transfers ~ ~ ~ Condensation – ~ ~ ~ Precipitation – Outputs ~ Drainage Basin Features
Discharge: With increased distance downstream: • • • Discharge increases Depth increase Width increases Bedload increases Particle size decrease A: V- Shaped Valley B: U-Shaped Valley C: Floodplain VELOCITY × VOLUME = DISCHARGE What factors affect drainage? Discharge varies downstream with changes in: Urban Area Middle Course Upper Course The upper course is the highest section which is found in the mountains Here the river erodes a vshaped valley, the path is fairly straight and it flows downhill steeply. In the middle course the angle that the river flows down is less steep The river begins to meander and the valley sides are also less steep. Lower Course Rural / Forested Areas River Processes Transportation Erosion involves the wearing away of rock and soil found along the river bed and banks. There are 4 main forms of river erosion: • Hydraulic action • Abrasion • Attrition • Solution Rivers pick up and carry material as they flow downstream. A river may transport material in four different ways: • Solution • Suspension • Saltation • Traction The lower course has the gentlest slopes. This almost flat land is known as the flood plain. The river may have very large meanders and ox-bow lakes before it enters the sea. Deposition When a river loses energy, it will drop or deposit some of the material it is carrying.
River Meander Waterfall In the middle course the river has more energy and a high volume of water. The gradient here is gentle and lateral (sideways) erosion has widened the river channel. The river channel has also deepened. A larger river channel means there is less friction, so the water flows faster: The formation of meanders is due to both deposition and erosion and meanders gradually migrate downstream. The force of the water erodes and undercuts the river bank on the outside of the bend where water flow has most energy due to decreased friction. On the inside of the bend, where the river flow is slower, material is deposited, as there is more friction. When a river runs over alternating layers of hard and soft rock, rapids and waterfalls may form. When the river courses over resistant rock, erosion happens slowly, while downstream the erosion occurs more rapidly Often over time, the waterfall will recede back to form a canyon or gorge downstream as it recedes upstream, and it will carve deeper into the ridge above it. The rate of retreat for a waterfall can be as high as one and half metres per year. Causes of flooding River Features Floodplain Case Study- Flood River Derwent. March 1999 A floodplain is the area around a river that is covered in times of flood. A floodplain is a very fertile area due to the rich alluvium deposited by floodwaters. This makes floodplains a good place for agriculture. A build up of alluvium on the banks of a river can create levees Levees are natural embankments of silt along the banks of a river, which are often several metres higher than the flood plain. A flood occurs when a river bursts its banks and the water spills onto the floodplain. Physical Causes Human Causes • Heavy rainfall over North York Moors, 250 mm in 12 days, as much as usually falls in 3 -4 mths • Ground in Derwent drainage basin was already saturated by previous rain so it could hold no more • The peat of the North York Moors used to store water like a sponge for days before releasing it to rivers, but people had cut and removed lots of peat • Houses had been built on the fooldplain at Malton making the ground impermeable Impacts of Flooding
Sustainable River Management Soft Engineering Strategies River Management: Maximising the potential benefits of a river. Transport, Power, Recreation Whilst minimising the potential threat to human life & property • Waslands • Land-use zoning Hard Engineering Strategies • Dams • Afforestation • Levees, Embankments & Floodwalls • Dredging • Straightening • Storage Areas River Management Case Study- River Management River Nene Location The River Nene is the tenth longest river in the United Kingdom. It has a catchment area of 631 square miles (1, 630 km 2) and a mean flow of 328 cubic feet (9. 3 m 3) per second. Flood Risk The River Nene used to flood frequently at Northampton Heavy rains on 9 th April 1998 caused the River Nene and Grand Union Canal to overflow, flooding parts of the town and leading to two people’s deaths Flood Management Strategies in Northampton By 2002 The Environment Agency had completed a £ 6. 8 million project to protect areas of Northampton hardest hit by past flooding. In 2003 the warning system was upgraded The areas at risk of flooding are covered by the Environment Agency Flood Warning Service where they aim to give two hours’ notice of the possible onset of flooding At foot meadow close to the railway station 4 m high floodwalls have been installed to protect housing, industry and the Castle inn. This has created an open area of land which the river can flood onto giving protection to the railway station. Flood Management In Upton A new housing estate called Upton Square has been built just above the level of the floodplain. The river channel capacity at Upton was increased by building earth embankments set back 10 m from the river which have footpaths along the tops. The area has been visually improved by planting trees. cost £ 8 million. Washland created where water can be diverted from the River Nene in a controlled way.
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