Hydrological processes and
drainage basins:
Case study of the River Souteyran
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1
Hydrological Processes and Drainage Basins: A Case Study Of The Souteyran River
Teachers’ notes
Channel processes and landforms come under close scrutiny in this unit, as does river morphology. River
morphology can usually be considered in three dimensions; long profile, cross-sectional shape and planform. This unit concentrates on the changes which the Souteyran exhibits on its journey along the southern
flank of Mont Lozere to its junction with the Tarn at Pont de Montvert.
The work for this unit is carried out from the Eagle's Nest without the need for transport except for the return
journey from Le Pont de Montvert. Students walk the entire length of the River Souteyran (approx. 10km)
over rough ground and along the Stevenson’s Trail. Most groups measure between 4 and 10 sites at a
variety of locations. Liaise with your group leader before you set out on the number of sites and the
locations that you wish to study.
This study gives many opportunities for students to produce coursework based upon the data collected. A
list of possible project titles is included with this unit.
Many groups spend a good deal of time in the evening drawing out cross-sectional areas.
We have
software that processes the student’s data quickly and efficiently, producing a cross-sectional diagram and
calculating discharge. This leaves groups with time for discussion and more sophisticated of the results that
they have collected. If you would like to use the software, please ask your group leader.
Some groups may choose to study variations in water quality. Because of the organic nature of the farming
in the valley, there is very little nitrate pollution and the acidic nature of the granite bedrock eliminates most
meaningful variations in pH. The most successful way of testing for changes in water quality is to look for
biological indicator species. However the ‘clean’ nature of the river means that little variation is apparent
between sites. For a more detailed investigation the Freshwater Pollution unit based upon the River Lot
should be considered.
On the walk down it is easy to see evidence of rejuvenation in the valley. This may be a useful teaching
point.
Key Specification Areas:
•
The drainage basin, hydrological cycle: the water balance;
•
Factors affecting river discharge: the storm hydrograph;
•
The long profile – changing processes: types of erosion, transportation and deposition, types of load,
the Hjulstrom curve;
•
Valley profiles – long profile and changing cross-profile downstream, graded profile, potential and
kinetic energy;
•
Changing channel characteristics – cross-profile, wetted perimeter, hydraulic radius, roughness,
efficiency, and links to velocity and discharge;
2
•
Landforms of fluvial erosion and deposition – potholes, rapids, waterfalls, meanders, braiding, flood
plains;
•
Process and impact of rejuvenation – knick-points, waterfalls, river terraces;
•
Physical and human causes of flooding – location of areas of high risk in an MEDC;
•
Impact of flooding – case study of the river Souteyran;
•
Flood management strategies – to include hard engineering and soft engineering.
Reference Texts
Barker, A., Redfern, D and Skinner, M. (2008) AQA AS Geography. Phillip Allen Updates.
Knighton D (1984) “Fluvial Forms and Processes”
Knill, R. and Smith, J. (2008) AQA AS Geography.
Lenon & Cleves (1994) “Fieldwork Techniques and Projects in Geography” Collins
Miller (2000) “Fieldwork Ideas in Action” Hodder & Stoughton
3
Introduction
General Information
The Massif Central, a roughly triangular upland area covering one-sixth of France, contains a landscape of
enormous variety characterised by a number of distinctive landforms. Water is an important and dynamic
component of this landscape system. The striking gorges of the limestone Causses reflect the erosive power
of the Massif's principal rivers. Today these waters have become a major recreational attraction and have
brought an important source of revenue into the region.
Elsewhere, water plays a vital role in agricultural activity and many rural valley communities rely upon the
flow of rivers for their continued prosperity. The dynamic nature of the river system has a direct impact on
man. Heavy rain in October 1987 and September 1994 brought severe flooding to many areas; the droughts
of 1989 and 1990 have reduced river flow and this, together with increased deposition, has made it almost
impossible for some of the canoeing and rafting businesses to operate profitably.
People also have an impact on rivers: pollution is becoming a major problem as more farmers turn to
chemicals to increase yields and as domestic waste tips build up outside the major settlements. River
channels have been straightened and banks protected in urban areas and in the rural sector channelisation
has been used to re-direct flow onto agricultural land.
The physical geography of the Massif Central means that the region forms a vast watershed from which
rivers flow to the Mediterranean, and the Atlantic coasts. These rivers are not important for navigation but
their valleys provide route-ways for road and rail transport away from the mountain mass to other parts of
France. The water input into these river systems comes principally from the Atlantic depressions.
The Massif is well watered. Lozère has more than 2700 km of water courses and 230,000 hectares of forest
land – it constitutes “Water Tower” and green lungs of Languedoc-Rousillion to which it belongs.
Precipitation is high with more than 1,200 mm per annum being recorded in the highest areas. The whole of
the Massif has more than 75 days of frost each year and large areas of ground are snow covered for more
than two months of the year, rising to six months in the high mountains. The prevailing soil and geological
conditions favour rapid run-off and overland flow tends to be the dominant process during periods of heavy
rain (See Figure 1). This is accentuated where deforestation has occurred on a large scale. There are
many natural and artificial water stores that may help to regulate river-flow, such as Lac Villefort at Villefort
and the Reservoir de Cambous between Florac and Alès. These are used for the supply of water to
surrounding towns and for recreational purposes.
River management is an important aspect of water control and conservation in the Massif. Much of the
management in the Cévennes is entrusted to the National Park which is primarily responsible for maintaining
the landscape. Elsewhere Regional River Authorities have implemented various river management
schemes. Good management can only be achieved by gaining a full understanding of the physical
processes acting within river channels - the processes at work today, the erosional history, and the physical
controls of the environment (See Figure 2).
4
Although total yearly precipitation is high in the region, it is as always distribution that is the problem. During
the long, dry summer, water supply may be barely sufficient to meet demand. With an ever-increasing
number of tourists in the region, the problem is becoming more acute.
The Departement of Lozère has
become sufficiently concerned to fund research by the University of Alès into the hydrology of the region. It is
hoped that a more efficient way of managing the available water will be found.
Specific Information
The Souteyran Valley lies along the southern slopes of Mont Lozère. It contains two rivers, the Souteyran
and Rieumalet, which is a tributary of the former. Both these flow into The Tarn at Le Pont de Montvert. The
rivers are characteristic of upland streams which drain a granite and peat area. The soils are generally
acidic and very thin on the slopes, but are deeper in the valley bottoms where periodic flooding has provided
some input of alluvium. Both rivers are important to the valley communities: The Eagle's Nest relies upon
them for its water supply as do Finiels and Prat Souteyran. In the latter two villages the water from these
rivers has been carefully diverted along small drainage ditches or 'beals' which sometimes appear to be
flowing uphill! Some farmers feed the water into ponds where trout are bred for sale in local markets and to
the restaurant in Le Pont de Montvert. Both rivers flow continuously and much of this flow can be attributed
to a slow release from stores held high up on Mont Lozère. These are peat bogs that play an important role
in the basin hydrological cycle.
Discharge, precipitation and temperature data have been collected daily, from a site close to the Eagles
Nest, since December 1997. This data is available at the Centre or from our Web pages.
River Souteyran Storm Hydrographs
In early October 2001, there was a short violent storm around the Eagles Nest. The following data was
collected and may be used to produce a storm hydrograph. The rainfall information was collected by the
centre’s automatic station and the river was measured at centre’s water intake.
Time
Rainfall (mm)
River Height (m)
Discharge (m3/s)
09-00 (03-10-01)
0
0.34
0.041
19-00 (03-10-01)
0
0.34
0.041
20-00 (03-10-01)
0.5
0.38
0.051
21-00 (03-10-01)
6.5
0.39
0.058
22-00 (03-10-01)
1.5
0.43
0.206
23-00 (03-10-01)
0
0.41
0.131
24-00 (03-10-01)
0
0.40
0.068
09-00 (04-10-01)
1 (over 24 hrs)
0.38
0.053
09-00 (05-10-01)
0
0.38
0.051
09-00 (06-10-01)
0
0.35
0.045
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Later in the month a more prolonged rainfall event occurred, although of similar intensity at times and the
following results were recorded. This provides a useful contrast to the previous shorter event. Photographs
of the River Tarn in Pont de Montvert were taken during this event and are quite dramatic.
Time
Rainfall (mm)
River Height (m)
Discharge (m3/s)
17-10-01
1
0.35
0.045
18-10-01
30
0.37
0.048
19-10-01
19
0.37
0.049
20-10-01
79
0.52
0.687
21-10-01
3
0.55
0.886
22-10-01
0
0.55
0.759
23-10-01
0
0.55
0.734
24-10-01
0
0.54
0.531
25-10-01
0
0.53
0.466
26-10-01
0
0.52
0.437
27-10-01
0
0.51
0.428
28-10-01
0
0.51
0.428
29-10-01
0
0.50
0.361
30-10-01
0
0.48
0.307
01-11-01
1
0.44
0.176
FIGURE 1: The Drainage Basin System
Transpiration
Evaporation
Precipitation
Interception
Throughfall
Surface Storage
Surface
Runoff
Infiltration
Vegetation
Storage
Soil water storage
Throughflow
Percolation
Groundwater Storage
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Baseflow
Channel
F low
FIGURE 2:
Schematic Diagram of the Relative Rates of Downstream Change in Channel Form
Discharge
Sediment Load
Bed Material Size
Average Size
Valley Slope angle
Width
Depth
Channel Slope gradient
Velocity
Hydraulic radius
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Aims
•
To investigate downstream changes within the river channel and to account for the changes which
take place.
•
To Investigate if the River Souteyran is a ‘model’ river
•
To investigate the potential for flooding in the Souteyran valley and identify flood risk in le Pont de
Montvert.
Hypotheses
•
The cross-sectional area increases downstream.
•
Bed-load size decreases downstream.
•
Bed material will become more rounded downstream.
•
Channel efficiency will increase downstream.
•
The channel is more efficient at bank-full levels.
•
Mean velocity and discharge increase downstream.
•
Channel gradient will decrease downstream.
•
The Souteyran valley has a low flood risk, but contributes to a high potential flood risk in the river
Tarn, further down the valley, due to a range of factors (vegetation, geology, relief, etc.)
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Data Collection Sites
Most groups measure between 5 and 10 sites at a variety of locations over a 10km stretch. Please liase with
your group leader over the number and location of the sites that you wish to study. If a large number of sites
are to be visited then considerable planning is necessary. This may involve the splitting of groups to visit
different sites or the allocation of tasks.
Under high flow conditions it may be impossible to sample at some of the more downstream sites.
Equipment
Ranging Poles
Tape Measure
Chain
Metre Rules
Callipers
Flow Metres
Stopwatches
Clinometers
pH meter
Conductivity meter
Power’s scale of roundness
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Method and Organisation of Study
Task One – Cross Sectional Area and Bedload
(Hypotheses 1, 2 and 3)
Channel form is more a measurement of bank-full discharge than any other factor. Discharges lower than
bank-full are likely to have a smaller proportion of excess energy to mould the channel, and although flood
discharges may cause damage and channel change, their effects will be neutralised by the next bank-full
state (once every 1-3 years).
i)
Most natural channels are irregular. Where is the 'bank-full' cross section to be measured from?
ii)
Climatic conditions have changed dramatically over the last 10,000 years. Is the present river
responsible for the channel erosion?
Method
1.
Present flow Cross-section
Measure water surface width and divide into 10 equal intervals – i.e. divide the width by 10. This will mean
that you take 11 depth readings. Your first measurement should be taken directly against one bank and your
last measurement should be taken against the other bank.
At each of the 11 sites measure stream.
Calculate the mean depth.
Cross-sectional area is calculated by multiplying the width by the mean depth.
2.
Bank-full Cross-section
Measure the width of the channel.
Measure the distance from the tape to the water surface. This value is added onto the present flow depth
reading (normally once the mean has been calculated). The tape must be held taut and horizontally at bankfull level.
Cross-sectional area is calculated by multiplying the width by the mean depth.
3.
Bedload
At 11 equally spaced points across the stream (ideally the same points that you took your depth readings
from) measure the ‘b axis’ of the FIRST PEBBLE that your finger touches. The ‘b axis’ or width is used as
this is considered to be the most representative axis of most clasts in relation to their size.
Calculate the average size of sediment for the station.
The shape (roundness) of the clast is compared to a Power’s roundness scale.
Task Two – Channel Efficiency
(Hypotheses 4 and 5)
Hydraulic Radius
The efficiency of a channel is controlled amount of contact between the bed and banks and the flowing
water. High degrees of contact give high levels of friction and thus make an inefficient channel. The most
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commonly used method for expressing channel efficiency is through the hydraulic radius. This is the ratio of
cross-sectional area (CSA) to wetted perimeter (wp).
Hydraulic radius = CSA/wp
Hydraulic radius is not expressed in any unit, but the higher the figure then the more efficient the channel.
Method
In the same location as the cross-section, run the chain along the bank and bed of the stream following all
the contours of the channel. This gives the wetted perimeter at present flow levels. Measure the length
using the tape measure. Note that the wetted perimeter is always greater than the width!
To find the bank-full simply measure the two sections of bank from the surface to the bank-full level and add
this onto the previous reading. The hydraulic radius can now be calculated.
Task Three – Gradient and Average Velocity
(Hypotheses 6 and 7)
Method
1.
Gradient
Place the ranging poles at either end of a measured 10m stretch of stream.
Using the clinometer, measure and record the gradient of the stream.
2.
Using a Flow Metre/Impellor assembly
Measure the width of the stream and divide into 4 equal intervals (this will provide 3 measuring points – ¼,
½, & ¾ across the channel).
At each of these 3 points measure the velocity –set the impellor at ½ of the water depth and ensure that it is
pointing upstream. Make sure that you stand downstream of the impellor when taking readings!
Record the time taken for the impeller to move from start position to finish position.
Calculate the velocity using the formula or chart.
Calculate the mean velocity for the station.
(For hydroprop flow meter chart, see appendix 2.)
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Analysis
Cross Sectional Area (Hypothesis 1)
Using the Centre’s Software:
Use the computer program to plot the cross-sections of the river sites. The program will automatically
calculate wetted perimeter, wetted area, hydraulic radius, mean velocity and discharge (cumecs) once all the
relevant data has been inputted. Ask your group leader if you require assistance.
By Hand:
Draw a cross-section of the stream at each survey point. A cross-section is drawn using the width and depth
measurements. Draw a straight line in proportion to the width of the stream channel. Work out a vertical
scale for the depth measurements, this should include the bank-full depth. Calculate the difference in depth
between bank-full and current stream level. At this calculated distance below the bank-full level, draw a
straight line to represent the water surface. This line should be in proportion to the water surface width.
Below the line representing the water surface, mark the 10 depth measurements across the stream. Join up
the points representing the depth of the stream. Mark on the extent of the floodplain on each side of the
channel. Try to use the same scale for all survey points as this will make it easier to compare the differences
downstream.
Ensure the cross-section is fully labelled. Describe any changes in cross-sectional shape and area
downstream. Is there any pattern? Try to give reasons for any trends shown. Does on or more of the sites
not fit the pattern? Use your site descriptions to explain why this might be so.
Bedload Size and Shape (Hypothesis 2 and 3)
Tabulate the results that you obtained for each site. Calculate a mean bedload size for each site.
Refer to your site cross-sections and try to explain the changes in bedload across the stream channel.
Calculate the standard deviation and interquartile range of bedload sizes at each site. Remember this is a
mountain stream prone to flooding. Why might the standard deviation and interquartile range be quite large?
Draw a scatter graph to show changes in bedload downstream. Do you notice a trend? Why?
Channel Efficiency (Hypothesis 4 and 5)
Calculate the hydraulic radius for each site at both present flow and bank-full levels.
How do these change downstream and relate to each other?
How does hydraulic radius relate to other factors measured, especially discharge and velocity?
Mean Velocity and Discharge (Hypothesis 6)
Tabulate your velocity readings and calculate a mean velocity for each site. Describe any patterns in
changes in velocity across the stream channel. Can you account for these patterns? Draw a scatter graph
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to show how average velocity varies downstream? Does this show the result that you expected? If not, why
might this be so?
The discharge is the cross-sectional area (m2) multiplied by the average velocity (m/s) and is measured in
cubic metres per second (cumecs). If you have used the Centre’s software, you can tabulate you discharge
directly from your printouts. If not, first calculate the cross-sectional area as described above.
A third method of obtaining discharge measurements for each site is to use Manning’s ‘n’. This can be
calculated using the following formula:
R 0.67S 0.5
n
Q=A x
Where
Q = Discharge
A = Cross Sectional Area
R = Hydraulic Radius (Area x Wetted Perimeter)
S = Channel Gradient (This must be a tangent, so convert from degree)
n = Manning’s ‘n’. (This is a constant – you will need to select the appropriate constant from
the table below.)
Channel Type
Manning’s n
Earth canal, straight
0.020
Artificial channel – shuttered concrete
0.014
Winding natural river
0.035
Natural channel <30m wide, sluggish weedy pools
0.070
Mountain rivers, cobbles and boulders
0.050
Major rivers >30m wide, clean regular
0.025
This method is particularly useful for calculating discharge at bank-full levels since it is not usually possible
to measure velocity under these conditions. By removing the area from the formula it is possible to estimate
a bank-full velocity.
Plot a scatter graph to show how discharge varies downstream. Does it show the pattern you expected? If
not, why might this be so?
Discharge is very closely linked with hydraulic radius. This is the cross-sectional area (m2) divided by the
wetted perimeter (m). The hydraulic radius for each site is given on your computer print out, or can be
calculated by hand. It is an efficiency ratio. The higher the number, the greater the efficiency of the stream
channel. What might the relationship be between discharge and hydraulic radius? You could test for a
relationship by using Spearman’s Rank or Pearson’s Product.
Channel Gradient (Hypotheses 7)
Compare the gradient readings at each of the sites. How do these change downstream?
Most theories relating to the long profile of a stream suggest that gradient should decrease. Why may this
not have happened on the Souteyran?
Is there any relationship between gradient and velocity?
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Points For Discussion
Comment on trends and anomalies of graphs and calculations. Particular points to consider might be:
Problems encountered by the group in carrying out the planned investigations.
Limitations of the study – length of river, number of samples, methods of sampling etc.
Ways of improving the accuracy of the study. Which methods are the liable to contain the most errors and
can they be improved upon?
What else may affect the results, for example how do humans impact on the river?
How would different weather conditions change the results?
What changes have taken place in pH and dissolved materials (if measured)? How do these relate to other
factors?
Is the Souteyran a typical river? Can it be related to river models?
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Appendix 1:
Site Of Study Stations
Key:
N
-
Study
Site
River
Site
Distance
Altitude
No.
Downstream
(m)
(km)
1
0
1440
2
0.8
1340
3
1.05
1310
4
1.95
1220
5
2.2
1200
6
3.75
1060
7
3.8
1050
8
4.45
1020
9
5.75
960
10
6.95
880
Scale
0
1000m
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Downstream Rivers Recording Sheet
Station
Present Flow
Station
Bank full
Present flow
Station
Bank full
Present Flow
Bank full
Width (w)
Wetted
Perimeter(w)
Measure
Depth (m)
Measure
from
Measure
from
tape to water
tape to water
tape to water
surface
surface
surface
……….
……….
……….
Add to present
Add to present
Add to present
flow
flow
flow
mean
mean
depth
depth
depth
Mean:
Sediment:
Size
from
Shape
Size
Shape
Mean Size &
Shape
Time Taken
Mean Time
Velocity(m/s)
Gradient
pH
Conductivity
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Size
Shape
mean
RIVER INVESTIGATION RESULTS SHEET
It is recommended that this sheet be filled in with pencil so that any errors can be easily changed.
Station
Station
Station
Present flow
Bank full
Present flow
Bank full
Present flow
Bank full
m
m
m
m
m
m
m
m
m
m
m
m
m2
m2
m2
m2
m
m
m
W (w)
Depth (d)
Area (A)
(w x d)
m2
m2
Wetted
Perimeter (P)
m
Hydraulic
Radius(A/P)
Present flow only
Present flow only
Present flow only
m/s
m/s
m/s
m3/s
m3/s
m3/s
ppm
ppm
ppm
cm
cm
cm
Velocity (v)
Discharge (Q)
Conductivity
pH
Gradient (S)
Sediment
Shape
Sediment Size
17
m
Possible Project Titles
The following are some possible ideas for projects which students can undertake for coursework using data
collected in this unit.
•
The Souteyran River exhibits the classical model of development forwarded by W.M. Davis.
•
How do channel variables change downstream?
•
To examine the relationships between width and depth, and velocity and discharge along a river
channel.
•
Do streams of different orders exhibit different characteristics?
•
The Souteyran River becomes more efficient downstream.
•
Velocity is related more to channel efficiency than to gradient.
•
Sediment size and shape are related to velocity.
•
Sediment size and shape will change downstream.
•
Channels with small, rounded bed-load are more efficient than those characterised by large, coarse
poorly sorted materials.
•
Does velocity change with water depth?
•
Abstraction of water has an impact on channel variables within the River Souteyran.
•
A comparison of a river channel at bank-full and normal flow conditions.
•
Dissolved load and pH are related to geology and land-use.
•
Stream channel size and discharge is related to the size of basin area drained.
•
Rejuvenation along the Souteyran has directly affected channel variables.
The following titles require more data to be collected on the Souteyran or elsewhere.
•
Do streams of the same order exhibit similar characteristics?
•
How does discharge relate to rainfall in the Souteyran basin? (Secondary data available in this unit
and at The Eagles Nest).
•
Comparison of an upland stream with a lowland stream.
•
Comparison of a stream on granite with a different rock type.
•
The River Souteyran can be considered to be clean and unpolluted along its course.
18
Investigating Flood potential in the souteyran Valley.
Aims
™ To investigate the main factors that influence flood response in a small catchment.
™ To study issues of flood risk in a catchment settlement
Hypotheses
™ Different land uses will have different infiltration rates.
™ Relief has an influence on infiltration rate.
™ Antecedent soil moisture will influence infiltration rate.
Data Collection Sites
Visit Roc Du Couillou for an overview of the Souteyran catchment. Use this location as the base for the land
use catchment mapping which may depend on weather conditions. Various sites along the course of the
river can be used for site specific evaluations with different land uses.
Equipment
Catchment map (Appendix 1)
Blank catchment mapping sheets (Appendix 2 & 3).
Pont de Montvert flood risk assessment worksheet (Appendix 5 & 6).
Results recording sheet (Appendix 4).
Pont de Montvert base map(Appendix 7).
Pont de Montvert flood risk assessment map (Appendix 8).
Clinometers
Ranging poles.
Infiltration ring
Mallet
Metre ruler.
Tape measure.
Soil texture key
Soil Moisture sample bags (Microwave and crucibles in lab)
Compass.
19
Method and Organisation of Study
The day will start with some land use mapping from one of the many vantage points close to the Eagles
Nest. The class will then follow the course of the river downstream stopping at specific sites for their
infiltration experiments on different land uses. The class will finish in Pont de Montvert for the final mapping
exercise
1.
Catchment Mapping
From Roc Du Couillou look out over the Souteyran valley and record on the blank base maps (Appendix 2 &
3) what the ¼km2 squares are dominated by, in terms of:
Land Use:
(1)
Deciduous woodland
(Dark green)
(2)
Coniferous woodland
(Light green)
(3)
Pasture/Meadow
(Yellow)
(4)
Broom scrub
(Orange)
(5)
Bare soil
(Brown)
(6)
Bare rock/Scree
(Grey)
(7)
Paved/Urban
(Black)
and Relief:
(1)
Flat
(2)
Gentle
(3)
Intermediate
(4)
Steep
(5)
Very steep
(Use one map for land use and a second map for relief. The colour designations are for the follow up when
producing neat land use maps)
2.
Site evaluation at various land use sites:
Please note that there may not be enough time to study all land uses in detail, it is best to concentrate on 3
in particular. On arrival at the centre discuss with the course leader what they would recommend. For each
land use type make sure you have six pieces of data (enough for a Mann Whitney U significance test),
depending on how many student groups there are some repeats may need to be done. At each site conduct
the following. See recording sheet (Appendix 4):
Infiltration rate
Carefully with the aid of the mallet, hammer the infiltration can into the ground by approximately 3-5cm. With
health & safety in mind, it is better for a member of staff or the course leader to perform this operation. Once
20
the infiltration can is firmly in the ground, pour water into the can to a level of 10cm. With the aid of a metre
ruler placed inside the can, record the level of the surface of the water level at 1 minute intervals over a total
period of 5 minutes (Note that if all water infiltrated before 5 minutes, record the exact time taken).
Angle of slope.
Using a clinometer and ranging poles, assess the angle of slope over a standardised distance, such as 10m
measured with a tape measure.
Soil compaction
Use a skewer to push down into the ground. Remove the spike from the ground and measure the depth of
penetration (i.e. how much of the spike stuck in the ground) with the aid of the meter ruler.
Soil texture test
With a soil sample, follow the instructions on the soil texture key to identify its texture.
Soil Moisture
Collect soil samples from the field and place in a sealed container. When back in the lab, with the aid of
scales that can measure to within 0.01g. Measure and record the mass of an empty crucible. Place a
sample of the soil in the empty crucible, making sure not to over fill the crucible so there is danger of spillage
throughout the experiment, or to press the soil down and compact it which can impede the escape of
moisture by evaporation through the soil pores. Weigh the crucible with the soil sample and record the
result, then place in an oven or microwave to dry the soil (If using an oven do not have the thermostat set to
high since there is a danger of burning off the organic material which would affect the result. Have it set to
120°C which is sufficient. If using a microwave, make sure you also put a separate vessel of water in the
microwave as well, because if there is no moisture in the original soil sample, the microwave could be
damaged since it works by heating water molecules). After taking the sample out of the oven/microwave,
weigh the sample in the crucible then place back in the oven/microwave. Repeat this process till you reach a
constant mass to be sure that all moisture has evaporated.
3.
Settlement
In Pont de Montvert, Students will carry out a corridor survey along the Tarn with the aid of the base map
recording their observations with respect to flood risk assessment at the selected sites (Appendix 5 or 6).
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Analysis
1.
Catchment Mapping
Process the data by producing a neat land use map (with the colour key mentioned in the methodology) in
the class room, students will be able to assess what the catchment area is dominated by or if there are
distinctive sectors. It is possible to work out a percentage breakdown of the different categories. Also
produce a neat map based on the relief data, use different intensities of one colour (such as purple) to
represent the different grades of relief with the darkest being the steepest.
Flood Risk Score
For each of the squares calculate the flood risk score, which is the Land use score (1-7) multiplied by the
Relief score (1-5). The flood risk score will vary from 1 (flat deciduous woodland; low risk) to 35 (very steep
paved/urban surfaces; high risk). Produce a new map with these scores to highlight higher flood risk areas.
Possible extension; Calculate an overall average of all the flood risk scores to give a Catchment flood risk
score. This could be used as a measure of comparisons with other catchments.
2.
Land use evaluations
For each of the land uses calculate the:
Infiltration Rate
Divide the total drop of water in the infiltration can (convert to mm) by the time taken (in minutes). This will
give you a result of infiltration in mm/minute. Multiply the result by 60 so it is expressed in mm/hr. This is
then comparable to rain fall rates.
Soil moisture
Use the following equation to work out % soil moisture
(Mass of original soil sample in crucible) – (Mass of dried soil in crucible)
------------------------------------------------------------------------------------------------------ x 100
(Mass of original soil sample in Crucible) – (Mass of crucible)
i.e. (Mass lost due to evaporation)
---------------------------------------- x 100 = % Soil Moisture
(Mass of original soil sample)
Possible tasks;
Mann Whitney U test to see if there is a significant difference in infiltration between land uses (Hypothesis 1).
Spearmans Rank Correlation Coefficient between slope angle and infiltration (Hypothesis 2)
Spearmans Rank Correlation coefficient on soil moisture verses infiltration rate (Hypothesis 3).
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Considerations on stats tests;
Number 1 does not take into account the influence of the slope of the land, soil texture or the antecedent
moisture.
Number 2 does not take into account the land use, soil texture or antecedent moisture.
Number 3 does not take into account the land use, soil texture or angle of slope
3.
Settlement Evaluation
Looking at the corridor mapping from Pont de Monvert with the students’ observations. Discuss how at risk
the settlement is from flooding as a result from extreme weather and what is present to limit the damage of
flooding (Such as channel modifications, high bridge and wall by road, bottom floor used as celler). Produce
a neat flood assessment map using IGN Pont de Montvert base map (Appendix 7) based from observations
and compare to actual flood assessment map (Appendix 8). Work out what the impacts of potential flooding
would be.
Discuss the advantages and disadvantages of possible flood management techniques, with
reference to cost benefit analyses.
Possible methods of flood defence/flood risk reduction.
Channel modification.
Dam.
Afforestation.
Different farming techniques, contour ploughing.
Points For Discussion
Bring all three elements together in a class discussion.
Which parts of the catchment contribute most to flood risk?
Does precipitation exceed infiltration leading to surface run off which increase flood risk?
What techniques further management plans could be utilised to further lessen these potential flood risks?
Are these measures cost effective?
How do you think this catchment compares to a local example near you?
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Appendix 1:
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Appendix 2:
25
Appendix 3:
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Appendix 4: Site evaluation recording sheet
Land use
Angle of slope
Soil compaction
Soil Texture
Start
1 min
Infiltration rate, height of water
2 min
Time
3 min
4 min
5 min
if
all
water
infiltrated in less than 5
min
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Appendix 5:
Appendix 6:
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Appendix 1. Hydroprop Flow Meter Chart.
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