3. What are the factors that affect river regimes and hydrographs?

1.3A_Annual_hydrograph.pngWe need to understand what is happening in a river’s catchment area, so that we can plan for problems, such as floods. How much water a river is carrying will vary from month to month. This is called an annual hydrograph.
This graph shows how the river discharge changes throughout the year This pattern of flow is called the river regime It is much lower in the summer. The rainfall is not significantly less but the warmer summer temperatures means that more of the water is evaporated up stream from Bewdley (which is about halfway down the River Severn) and so the discharge is much less because of it. In addition, in Summer there is more vegetation to intercept the water and stop it reaching the river in the first place. Other issues that alter the average monthly discharge are man-made, in the form of reservoirs with dams, and abstraction (the withdrawal of water from the river for industrial and domestic uses) that can control the amount of1.3B_Lake_Clywedog.png water in a river channel. An example of a reservoir and dam on the River Severn which was put in especially to help control flooding down stream is at Lake Clywedog ( see the picture on the right).

Storm Hydrographs:

A couple of terms:
The discharge of a river is the volume of water per second passing a particular point and is measured in cumecs (cubic metres per second)
To an estimate of this – estimate the cross-sectional area and multiply by the average velocity.
[More about the practical aspects of this Section 10 of the unit: What can we measure in rivers and how can we do it?]
1.3C_Storm_hydrograph1.pngStorm hydrographs (are often referred to as just hydrographs, confusing I know) is a diagram that shows how a river responds to heavy rainfall at a particular point. It is these can help us assess what needs to be done to overcome problems that can be caused when these infrequent events occur.
The horizontal axes is the time in hours. There are usually 2 vertically axes one on each side – but not always, so take care!
The rainfall is always in bars while the change in discharge is a smoothed curve. As you see the rain falls and there is quite a long gap between the rain falling and the discharge reaching its maximum. So we have labels for storm hydrographs that help us to discuss differences between one and another

As you can see 4 terms:1.3D_Storm_hydrograph.png
There is a level of water that is coming from the source and stays fairly level – the basal flow.
Then there is a point at which the rain is heaviest – the peak rainfall.
There is a time at which the discharge is greatest, when it is at the peak discharge.
The difference in time between these 2 is called the lag time.
These storm hydrographs will vary from place to place.
Let’s think about 2 drainage basins that are similar in every way, except that one has steeper sides than the other. You have identical storms hitting both of them. How would one storm hydrograph be different from the other?
Starting with the shallow sloped one – the rain will hit the earth and quite a bit will soak in but some will run down the slope to the bottom, but it will go quite slowly and take a while to reach the river – so the lag time will definitely be longer and the peak discharge may be a little less than in the case of the steep sided basin, where it will run down quickly and little will have an opportunity to soak in.
I hope you can see which is which? The one on the left has a shorter lag time and a slightly greater peak discharge, so it must be the steep sided one.
Here are some other scenarios
I hope you can see that the lag time will be shorter for Basin A in 1, 2, 3, 4, 5 and 7 and the peak flow will be greater in 1, 2, 3, 4, 5, 6, 7, 8. I will leave you to work out what happens in the other cases (You can always ask if you don’t know).
What is different
Basin A
Basin B
Faster run-off on steeper slope
Slower run-off on gentler slopes
2.Rock type
More surface run-off on impermeable rocks
Through flow and groundwater flow and water infiltrates and percolates through permeable rock
3. Soil
Very thin soil so less infiltration
Deep soil so more infiltration
4. Natural vegetation
Thin grass and rocky slope – less interception
Forest – more interception; stemflow slows up the rate of heavy rain reaching the ground so more will infiltrate
5. Land-use
Urban environment- concrete/tarmac stop infiltration – water into drains and enters the river quickly
Rural – few roads/drains and fields to allow infiltration
6. Use of river
Very little
Water extracted for industry and domestic use; dams help control water down stream
7. Drainage density
High – more streams to collect the water quickly
Low – fewer streams to collect water so it will take longer to reach the channel network
8. Basin size
Large basin – the water will take longer to reach the river so the lag time will be longer, but the peak flow will be much greater
Small basin – the lag time will be shorter but the peak discharge will be less

Go to 1.11 General revision to find an animation that looks at both hydrographs and flood defences

Things to do
http://www.uky.edu/AS/Geology/howell/goodies/elearning/module12swf.swf although for AS level, it looks at all the aspects you need to cover – may well end up being a homework!!
http://techalive.mtu.edu/meec/module01/Landuseimpacts.htm goes into hydrographs a bit
http://www.bbc.co.uk/scotland/education/int/geog/rivers/flash.shtml choose hydrographs one after reading this page
Question sheet to be added for homework

Key Terms Check:
Discharge - this is the amount of water in a river at any given point and time. Discharge is measured in cumecs (cubic metres per second)
Velocity - speed of a river (measured in metres per second)
Hydrograph - a graph showing changes in river discharge over time in response to a rainfall event.
Lag time - the time taken between peak rainfall and peak discharge
Rising Limb - shows the increase in discharge on a hydrograph
Falling Limb - shows the return of discharge to normal / base flow on a hydrograph
Peak Rainfall - maximum rainfall (mm)
Peak Discharge - maximum discharge (cumecs)

What do you need to remember?

  • Know the meaning of all the terms on BOLD
  • The difference between an annual hydrograph and a storm hydrograph
  • All the reasons why the annual hydrograph is likely to be lower in summer than in winter
  • To be able to draw and label a storm hydrograph and explain the terms
  • To be able to identify what features of the landscape affect the storm hydrograph in what way