“ .. If we are not careful, future wars are going to be about water and not about oil.”
15 th March 2001
Hydrological Cycle Water is vital for all life. It occurs as a liquid (rivers, seas and oceans), as gas (water vapour) and as solid (glacier and ice-caps). This is the movement of water and its transportation between gaseous, liquid and solid form.
Transpiration is the process of water loss from plants. Transpiration takes place when the vapour pressure in the air is less than that in the leaf cells, i.e. transpiration is nil when the relative humidity of the air is 100%.
The flood hydrograph indicates whether a particular river has a high or low flood risk, by showing the relationship between precipitation and run-off.
Make your own hydrograph with the following information:
The discharge starts at around 8 cumecs.
Peak rainfall is 65mm.
It reaches this at 10.00am.
The lag time is 9 hours.
Therefore the peak discharge is at …
The Peak discharge is 60 cumecs.
Add on your axes first.
You will need a bar and a line graph.
Label correctly (eg rising limb, falling limb), or add a key.
Add on throughflow, groundwater flow and overland flow ( surface run-off) .
Remember to show the steepness of the line graph.
When you are finished, write a description of what your hydrograph is showing.
Some hydrographs cover a full year. Precipitation and discharge cover the full year.
Other hydrographs show a few days only. These ones show only one period of precipitation and river discharge which results.
Basic terms: Run off : the sum of all the rainwater that flows over the surface of the river basin (stream flow and overland flow) Through flow : the downslope movement of water through soil towards streams and rivers. Base flow : groundwater movement, which often lags behind precipitation by weeks, months or even years. The usual, reliable, background level of a river, maintained generally by seepage from ground water storage Stormflow : discharge that is not baseflow.
Approach segment: the discharge of a river before the storm. Some words to do with hydrographs:
Lag time: the time delay between max rainfall and max discharge. Lag time varies according to the type of drainage basin.
Rising limb: part of the graph which shows increasing discharge
Falling limb (recession limb): part of the graph which shows decreasing discharge
Bankful discharge: the point at which a river is full. If the level increases any more, the river floods.
Precipitation may be intercepted by plants. Large areas of dense vegetation will intercept large amounts of water. One way of reducing the impact of flood in an areas is to plant trees and slow down the runoff. Deforestation, on the other hand, will increase the risk of flooding.
A drainage basin is the _________area of a river and its_________. The boundary of the catchment area is called the_______. A drainage basin can be viewed as a ______ with inputs such as _________and solar energy and ______such as evapo-transpiration and _____.
catchment tributaries watershed system precipitation outputs runoff
A drainage basin is the catchment area of a river and its tributaries . The boundary of the catchment area is called the watershed . A drainage basin can be viewed as a system with inputs such as precipitation and solar energy and outputs such as evapo-transpiration and runoff .
River Basin Sea Source of River Confluence of River and tributary Tributary of River Flood plain Mouth of River Estuary of River Watershed of River (boundary between basins)
The Course Of A River Characteristics Upper Course Middle Course Lower Course Slope Width Depth Straightness Load Main work Valley width Type of load usually steep narrow shallow winding little large/small angular erosion transportation quite steep quite wide quite deep meandering some medium/small rounded transportation Narrow -V gentle quite wide -U wide deep big meanders lots small+ rounded transportation deposition wide
A small stream flows from an upland source to the mouth where it enters the sea. The river channel widens as it follows its course to the sea, and the amount of water it carries increases as other streams and rivers join it.
The river’s energy is linked to its velocity (speed). High velocity means high energy. River with lots of energy wear away the channel banks producing the load sand, stone, pebbles, boulders…
When a river has little energy, the load is deposited on the bed and banks of the river.
Levees are basically caused by floods. In times of low flow any deposition takes place on the river bed and this raises the height of the river bed.
In times of flood the water leaves the channel. As it does so it loses energy and the courser and heavier material is deposited near to the river on the banks.
Finer material is carried further onto the flood plain.
After many floods the river builds a bank on either side of the river and this can lead to catastrophes.
As the river has been raised above the flood plain and the banks are often higher still, when the river floods the water can no longer drain back into the river channel and often has to form a new one.
River Rejuvenation All your knowledge of rivers should help you to understand this final process which forms a unique set of landforms.
An Ait (or Eyot ) is a riverine island in England typically formed by the deposition of sediment building up over a period of time. Aits usually have a bar shape and may become permanent islands; sometimes an ait is eroded however, with the resulting sediment deposited further downstream that may create another ait. A channel with numerous aits is called a braided channel .
There is further evidence of meandering (833524) before it flows into the Severn.
At 760545, the River Teme is flowing in an Easterly direction. It is approximately 100m wide. It will be slow flowing, as it is in its lower course and the land either side is flat. The river is also a tributary of the River Severn, with the confluence at 850522. It is definitely natural, as it meanders often (779548). There is also an ox-bow lake at 775548. It is joined at a confluence by the Leigh Brook at 785537. It continues to meander its way towards the Severn, and there appears to be an eyot at 806533, due to the deposition of sediment. There is further evidence of meandering (833524) before it flows into the Severn.
The Valley surrounding the river is flat, indicating a flood plain, although it does have some steep sides a little further out, (20m at 795539). It is approximately 1km at its widest (7953 and 8252).
The River Gowy between 467670 and 431775 is in its lower course. It is flowing in a northerly direction. At the early part of this section, it is fairly narrow but becomes wider, about 80m (460685). It is only about 12m above sea level at this stage. It is on flat land as there are few contours, and it meanders (458698) its way towards the sea, showing it is natural. It may have been artificially straightened between 431738 and 430760 however. The flood plain becomes larger (4372) and there are embankments built here to prevent flooding. It will have a fairly high discharge due to tributaries flowing into it (4372). By 431759 the river becomes tidal, before it enters the large Stanlow Banks (mudflats) at 431775. Its valley is wide, with a flat valley floor, and is therefore likely to be U-Shaped.