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The Water Cycle
Can you define the words in bold?
The Water Cycle is a closed system. This means that water continually cycled and is not lost or gained. Water is evaporated from the land and oceans by the sun’s energy. As the water vapour rises in the atmosphere, it cools and condenses into clouds. As these clouds rise over the land they cool further and the water droplets become heavier than the air and fall as precipitation.
The precipitation that falls within a watershed will eventually reach the river channel. It can do this as overland runoff. Some water will infiltrate into the soil and travel as throughflow, some water will percolate into rocks and travel as groundwater flow. Some water will be intercepted by vegetation or be storedin lakes, reservoirs or aquifers.
Typically, rivers have a concave ‘longitudinal profile’. The characteristics and processes occurring in the river change downstream.
Upper course -Rivers usually start in the uplands. Because the landscape is steep, streams have a lot of energy and flow quickly. The stream erodes vertically creating deep, narrow channels in v-shaped valleys. Channels are usually straight and often have a step-pool pattern with small cascades (‘steps’) over boulders and cobbles alternating with deeper areas.
Middle course – As the gradient of the river decreases the river begins to erode laterally. Small meanders form and the river is surrounded by a flat floodplain.
Lower course – The low gradient of the river in its lower course means it has little energy and material is deposited. Lateral erosion occurs on the outside of meander bends.
Can you explain what each of the images below is showing?
Flooding occurs when the volume of water in a river exceeds the capacity of the channel. The water then overtops the banks and overflows onto the floodplain. Flooding is usually caused by prolonged rainfall. When the soil becomes saturated it cannot hold any more water and rainfall is quickly delivered to the channel leading to higher discharge and floods.
Short duration rainfall events can also result in flooding if they are very intense. The amount of rainfall can exceed the speed at which infiltration into the soil occurs. This is called a ‘flash flood’. Flooding can also be caused by snowmelt.
Recently there has been very bad flooding in the Yorkshire Dales that has damaged buildings, transport routes and crops. This is a result of very high rainfall. Scientists have suggested that the increasing amount and intensity of rainfall is a result of climate change. Other factors can contribute to flooding.
Can you explain how each of the following could contribute to flooding?
This photo from the Darlington and Stockton Times shows the amount of water at Hawes bridge in December 2015.
Sometimes, where lots of properties are at risk of flooding, it is necessary to use Hard-engineering solutions to protect them. These include:
Flood Barriers such as the Foss barrier in York
Enlarging the channel
Flood relief channels
Sometimes these structures are necessary to safeguard properties but they should not be overused. The do not work with the rivers natural processes and can often make problems worse elsewhere. For example, flood embankments prevent the river making use of its floodplain and increase the conveyance of the river. This means more water arrives downstream faster and can result in bigger floods.
Soft-engineering solutions try to work with the rivers natural processes and hold more water in areas where it cannot cause too much damage. This can be achieved by:
Land use zoning
Wetland and river bank conservation
Watch the video below to see how natural techniques have been used to ‘slow the flow’ in Gloucester.