1.2- Flood Hydrographs Flashcards
River flow is studied by measuring
The discharge of a river
Define discharge
Volume of water passing a measuring point in a given time
What is the equation to work out discharge?
Discharge ( m^3 per seconds) = cross-sectional area (m^2) X velocity (metres per second)
What is a storm hydrograph?
A graph that shows how river discharge changes as a result of a period of heavy rain
When it rains, only a very small proportion of the precipitation falls directly into the river channel- most of the rain falls on the valley sides and takes time to reach the river = the water makes its way downslope and the river rises as shown by
The rising limb of the hydrograph
Flow increases to the point of peak discharge; from that time the amount of water reaching the river as a result of the rainstorm will begin to
Decrease (receding or falling limb)
The starting and finishing level show
The base flow fed by groundwater
The time delay between peak rainfall and peak discharge is known as
Lag time
Rivers that have a rapid rise in discharge over a short period of time are said to have
Flash responses and produce a ‘peaky’ graph
Why do some rivers produce a flash response and thus peaky graph?
Water is mainly entering the river via surface run-off
Other rivers will have a more benign response to the same rainfall event- their discharge will not rise so high and the lag will be extended over a longer period of time = graph much flatter. This is because
More water is entering the river via throughflow and groundwater flow and less via surface-run-off- this has the effect of spreading spreading out the flow over an extended period of time; hence the graph is flatter (the reasons for this can be explained by the characteristics of the drainage basin itself)
The way a river responds to a rainfall event has significant implications for its management- flash responses lead to a very abrupt rise in discharge over a very short period of time which can lead to
More frequent flood events- the characteristics of the drainage basin will explain how a river responds to a rainfall event
Water enters the river through 3 main routes:
Surface run-off, throughflow and groundwater flow (very little precipitation can fall directly into the channel so this can be discounted)
_______ ___-___ is by far the fastest route to the river, therefore drainage basins that encourage this and limit infiltration are likely to lead to a more ‘peaky’ hydrograph
Surface run-off
How does vegetation restrict surface run-off?
- deciduous forest in summer increases rates of transpiration by as much as 40%
- this is achieved through interception
- water doesn’t even reach the surface
- vegetation therefore restricts surface run-off by reducing the quantity of water reaching the surface
- speed at which precipitation reaches the surface is slowed down by the forest canopy
Rainfall reaches the surface via the canopy in a process known as
Throughfall
What is throughfall likely to lead to?
- this slow transfer of water to the surface is more likely to lead to infiltration and percolation rather than surface run-off
Any form of human intervention or development that removes dense forest vegetation is likely to increase
Surface run-off
What is a river regime?
The variability in its discharge throughout the course of a year in response to precipitation, temperature, evapotranspiration and drainage basin characteristics
There are almost ____ river systems in the UK and they vary greatly, being extremely sensitive to variations in climate, land-use changes and water abstraction
1500
River systems are also influenced by
The landscape through which they flow- mountain rivers fed by high rainfall will differ from less flashy groundwater fed streams in drier areas of southeast England
UK rainfall is fairly evenly distributed throughout the year with a slight autumn/winter maximum. By contrast, seasonal variations in temperature and sunshine intensity ensure that
- evaporation loses are heavily concentrated in the summer half-year (April- September)
- this results in a marked seasonality on river flows with maximum flows normally in the winter and minimum flows normally occurring in the summer or autumn
- however urban catchments may not conform to this pattern
In addition to climatic conditions, explain how geological characteristics of individual catchments, their permeability especially, has a major influence on river flow patterns
- take example of neighbouring catchments in Thames basin
- although they experience almost identical climatic conditions, the flow regime for the Lambourn, which is fed by outflows of from the underlying chalk, is markedly more stable than that for the Ock which drains a largely impermeable clay catchment, allowing a more immediate response to rainfall events
