Functioning of Drainage Basins
The Pakistan Floods of 2010
Pakistan Flood 2010 - Case Study
Could The Floods Have Been Prevented?
Objective: To analyse the functioning of a drainage basin as an open system with inputs, outputs, transfers, stores and feedback loops.
Starter: Watch the video to the right hand side and make notes on the following:
i. What is a watershed/drainage basin
ii. main functions of a watershed/drainage basin
iii. how human action can influence the functioning of the system.
Task 1 - Study the embedded presentation from geographyalltheway carefully. Make a copy of the two diagrams showing open and closed systems. Try to label the open system diagram with some of the features you noted in the starter activity.
Further higher level definitions of systems can be found here
Task 2 - Study the definitions below carefully and then copy and paste them into the correct column in the worksheet below. This site may help you (but try not to use it unless you are stuck!)
Task 3 - Click this link to access the GATW page for drainage basins.
i. Make notes on positive and negative feedback loops and try to give your own examples for each.
ii. Complete the worksheet on the drainage basin system and then apply your knowledge to match up feedback loops with different drainage basin conditions.
IB Style Question: Draw a labelled diagram to show the inputs, stores, processes and outputs of a drainage basin system. [4 marks]
Definitions for Task 2
All rivers receive a water supply and the area of land this comes from is known as a drainage basin. The boundaries of the basin are known as the watershed and will usually be marked by areas of higher land.
Percolation: When water travels from unsaturated ground into saturated ground.
Interception: When an object (building, tree) stops precipitation reaching the ground beneath.
Transpiration: Liquid water evaporating from vegetation.
River discharge: Eventually most rivers enter the sea and discharge the river's flow into the sea.
Groundwater flow: The movement of water through saturated ground.
Channel flow: Water that is travelling in rivers or streams.
Surface run-off (overland flow): When water travels across the surface of the earth
Stem flow: When intercepted water then travels down the branches and trunks of vegetation.
Evaporation: Liquid water from surface stores and rivers turning into water vapour (gas).
Surface storage: Any water that is held on the surface of the earth e.g. lake or pond. Some surface stores like puddles may only be temporary.
Infiltration: When water travels from the surface of the earth into the ground beneath.
Groundwater storage: Water that is stored in saturated ground.
Throughflow: The movement of water through unsaturated ground.
Canopy drip: Intercepted water dripping off vegetation onto the ground.
Soil-moisture storage: Water that is stored below the surface in unsaturated ground.
Precipitation: Any moisture that falls from the sky e.g. rain or snow.
Objective: Define stream discharge. Examine its relationship to stream flow and channel shape.
I use Integrated Approach (Waugh) for this section of the work page 68-70.
Starter: Watch the video to the right hand side and take notes from the sections on velocity, slope and discharge.
a - Outline the role played by friction in the flow of water
b - Define the two types of water flow.
c - Copy figure 3.15 (laminar flow) into your work book and label appropriately.
d - What is the relationship between velocity and turbulence?
e - Copy figure 3.15 (turbulent flow) into your work book and label appropriately.
f - Outline and sketch the three main influences on river velocity. This will include diagrams 3.16, 3.17, 3.19 & 3.20.
Task 2 - Copy & Complete the following sentences, deleting as appropriate
•A river in a deep, broad channel, often with a gentle gradient and small bedload, will have a greater/lesser velocity than a river in a shallow, narrow, rock filled channel – even if the gradient of the latter is steeper.
•The velocity of the river increases/decreases as it nears the sea
•The velocity increases/decreases as the depth, width and discharge of a river all increase.
•As the roughness increases/decreases, so does turbulance and the ability of a river to pick up and transport sediment.
Bed: The bottom of the river channel
Bank: The sides of the river channel.
Channel: The confines of the river, encompassing the bed and two banks.
Wetted Perimeter: The total length of the bed and the banks in contact with the river.
Cross-sectional area: The width of the river multiplied by the depth of the river. Because the depth of the river will vary across its width, an average depth reading is normally taken. The cross sectional area is normally given in m2.
Velocity: This is the speed that the water in a river is travelling at. The unit of measurement is normally metres a second (m/s). River velocity can be measured using a flowmeter (pictured right), or more commonly by timing a floating object over a set distance (pictured left). Velocity is then calculated by dividing the time (seconds) by the distance (metres).
Discharge: This is the amount of water in a river at a given point. Discharge is normally measured in cumecs (cubic metres a second). It is calculated by multiplying the cross-sectional area by the velocity.
As you move from the source to the mouth, both the discharge and velocity of a river increases.
Make a copy of the Bradshaw Model as shown in the presentation to the left hand side. Mount it in the centre of a piece of A3 paper.
Annotate each of the factors to show why they increase or decrease as you move from the upper to lower course of the river.
Exam question (10 minutes)
Explain the relationship between stream discharge and channel shape. (5)
Objective: To describe the characteristics of a hydrograph. Analyse the reasons for spatial and temporal (short term and long term) variations in hydrographs.
Starter: Watch the first video to the right hand side. What factors affect the shape of the yellow graph?
Background: There are eight key controls that affect drainage basins and ultimately the shape of the resultant flood hydrograph:
1. Basin size, shape and relief
2. Types of precipitation
4. Land use
6. Soil Type
7. Drainage Density
8. Tides and storm surges
These eight controls should not be viewed in isolation to one another. It is rare that just one of these control mechanisms would contribute towards a major event in a drainage basin. They should therefore be viewed as an interacting set of factors that contribute towards the behaviour of the basin.
Task 1 - Use page 62-63 of Waugh – Integrated Approach to make notes on the 8 key DB factors. Where possible, try to find an example (photo, graph or quote) to further illustrate the point.
Task 2 - Go to the GATW site by clicking here and complete the living graph activity.
Hydrograph Time Lapse Video
Task 3 - Open Geo Factsheet n.o 83 from January 2000 and complete the activities on the last page (subscription required).
Objective: To be able to discuss the natural and human causes and consequences of a specific river flood.
Task 1 - You are going to be completing a case study of the Pakistan Floods in 2010. To do this, we will be using the information on GATW. Click here to access the page directly.
You will need to fill in this two sided recording sheet.
Watch the videos to the right hand side and below. The second economic video - first 10 mins only.
River discharge is the volume of water flowing through a river channel. This is the total volume of water flowing through a channel at any given point and is measured in cubic metres per second (cumecs). The discharge from a drainage basin depends on precipitation, evapotranspiration and storage factors. Drainage basin discharge = precipitation – evapotranspiration +/- changes in storage.
Hydrographs can be used to illustrate discharge. These can be used to show annual discharge patters of flow in relation to climate. Over the short term a flood or storm hydrograph (figure 1.) can be used to show short term variations. They cover a relatively short time period, usually hours or days rather than weeks or months. Storm hydrographs allow us to investigate the relationship between a rainfall event and discharge.
Figure 1. A storm hydrograph
The starting and finishing level show the base flow of a river. The base flow is the water that reaches the channel through slow throughflow and permeable rock below the water table. As storm water enters the drainage basin the discharge rates increase. This is shown in the rising limb. The highest flow in the channel is known as the peak discharge. The fall in discharge back to base level is shown in the receding limb. The lag time is the delay between the maximum rainfall amount and the peak discharge.
The shape of a hydrograph varies in each river basin and each individual storm event. The hydrographs below show two contrasting environments.
Figure 2. Hydrograph before urbanisation
Rural areas with predominantly permeable rock increases infiltration and decreases surface runoff. This increases lag time. The peak discharge is also lower as it takes water longer to reach the river channel.
Figure 3. Hydrograph following urbanisation
Urbanisation is the main human impact on a storm hydrograph. Surface runoff increases when areas are urbanised due to the removal of top soil and vegetation. As roads, pavements and buildings are constructed the surface becomes impermeable. Laying drains leads to the rapid transportation of water to river channels which reduces the lag time.
Physical factors affecting storm hydrographs
There are a range of physical factors that affect the shape of a storm hydrograph. These include:
1. Large drainage basins catch more precipitation so have a higher peak discharge compared to smaller basins. Smaller basins generally have shorter lag times because precipitation does not have as far to travel. The shape of the drainage basin also affects runoff and discharge. Drainage basins that are more circular in shape lead to shorter lag times and a higher peak discharge than those that are long and thin because water has a shorter distance to travel to reach a river.
2. Drainage basins with steep sides tend to have shorter lag times than shallower basins. This is because water flows more quickly on the steep slopes down to the river.
3. Basins that have many streams (high drainage density) drain more quickly so have a shorter lag time.
4. If the drainage basin is already saturated then surface runoff increases due to the reduction in infiltration. Rainwater enters the river quicker, reducing lag times, as surface runoff is faster than baseflow or through flow.
5. if the rock type within the river basin is impermeable surface runoff will be higher, throughflow and infiltration will also be reduced meaning a reduction in lag time and an increase in peak discharge.
6. If a drainage basin has a significant amount of vegetation this will have a significant affect on a storm hydrograph. Vegetation intercepts precipitation and slows the movement of water into river channels. This increases lag time. Water is also lost due to evaporation and transpiration from the vegetation. This reduces the peak discharge of a river.
7. The amount precipitation can have an affect on the storm hydrograph. Heavy storms result in more water entering the drainage basin which results in a higher discharge. The type of precipitation can also have an impact. The lag time is likely to be greater if the precipitation is snow rather than rain. This is because snow takes time to melt before the water enters the river channel. When there is rapid melting of snow the peak discharge could be high.
Human factors affecting storm hydrographs
There are a range of human factors that affect the shape of a storm hydrograph. These include:
1. Drainage systems that have been created by humans lead to a short lag time and high peak discharge as water cannot evaporate or infiltrate into the soil.
2. Area that have been urbanised result in an in crease in the use of impermeable building materials. This means infiltration levels decrease and surface runnoff increases. This leads to a short lag time and an increase in peak discharge.
Hydrograph – a graph that shows river discharge and rainfall over time.
Flood – when the capacity of a river to transport water is exceeded and water flows over it’s banks.
Base flow – The baseflow of the river represents the normal day to day discharge of the river and is the consequence of groundwater seeping into the river channel.
Storm flow – storm runoff resulting from storm precipitation involving both surface and throughflow.
Bankfull discharge – the maximum discharge that a particular river channel is capable of carrying without flooding.
Peak discharge – the point on a flood hydrograph when river discharge is at its greatest.
Peak rainfall – the point on a flood hydrograph when rainfall is at its greatest.
Lag time – period of time between the peak rainfall and peak discharge.