Skills Flashcards
Cranedale aim
To investigate downstream changes in the river channel on the River Derwent to see if they fit Bradshaw’s model
Crandale hypothesis
Load particle size decreases with distance downstream
Helwath Beck
110m
3km from source
Drainage basin 10km2
2nd order stream
Jugger Howe Beck
100m
5km from source
Drainage basin 28km2
3rd order stream
Broxa
60m
12km from source
4th order stream
Cranedale location
North York Moors, W of Scarborough
9km stretch dropping 50m in height
High moorland plateaus in the north, coniferous forest in the east and grassland in the south
Deep V-shaped valleys
Very little human interference
Permeable sandstone overlain by impermeable peat
Drainage basin has a high rainfall - river flows all year round > large
Risk assessment > safe and accessible
Landowner’s permission to access
Can all be visited in one day
Graded profile shows…
How a river changes downstream
Height/distance from source
Bradshaw’s model
All rivers follow this model
Based on a few USA rivers
River variables in proportion > we are testing this
Bed load size decreases downstream and becomes more rounded due to erosion - abrasion and attrition
Velocity increases downstream as tributaries added and less friction with bed and banks - greater efficiency and reduced channel bed roughness
Data collection method and sampling
Sites chosen using satisfied sampling - situated progressively downstream from source > OS map used, stream orders
Cross-section sampled using systematic point sampling
1. Peg and line to work out width - 90 degrees and 30m tape measure
2. Width/10 to give 11 sample points across river (including bank)
3. Metre ruler placed down at each sample point and stone touching bottom of ruler picked up and measured along its B-axis using a mm ruler
4. Stone placed back - repeated at all three sites
Data collection strengths and weaknesses
STRENGTHS
Systematic point sampling - representative of entire channel so bias reduced
Easy to replicate (reproducible) so can be directly compared > greater accuracy, conclusions more reliable
Easy and quick - all 3 sites in one day
11 points - sufficient for Mann-Whitney U stat test
WEAKNESSES
Difficult to measure B-axis accurately with a rule > sides not straight
Easier to pick up larger stones that hit ruler - skew in favour of larger stones
Not representative of smaller stones underneath
Data presentation method
Pebble size at sample points bar chart
Mean pebbles size on bar chart
Located bar chart - on cross-section/wetted perimeter
Located proportional circles of velocity (radius)
Data presentation strengths and weaknesses and justification
STRENGTHS
All in one clear graph - patterns can be made out and links between different variables
Ease of comparison between each site
Too much data for scatter graph etc.
Valid conclusions can be drawn
Suggests link with pebble size/ velocity and cross section
WEAKNESSES
Difficult to extract data, especially circles
3 needed
Not comparing directly with different sites
Can’t compare values easily - objective
JUSTIFICATION
Relate to aim, graph shows, illustrates conclusion clearly
Better than a single graph - one axes/comparisons between variables
Better than a scatter/line graph as more than one data points per sample point
Better than a pie chart - only averages and 3 sites
Better than a dispersion graph as difficult to read and only shows one variable
Data analysis method
Mann Whitney U test
Null hypothesis and research hypothesis
Rank and calculate U values and critical value
U1 = 49.5
U2 = 71.5
CV = 30
If either U value higher than CV then reject RH and accept NH - no significant difference at 0.05 significance level
Less than 5% probability that this happened due to chance
Data analysis strengths, weaknesses and justification
STRENGTHS
Can be used on different sized data sets
Determines significant difference and chance
Quantifies a perceived relationship
WEAKNESSES
Calculation long - prone to human errors
Does not explain why difference occurred
Cannot be applied to categorised data
JUSTIFICATION
Determines whether two sets of data of the same variable are significantly different
Tests between medians - outliers not used
Test proved no significant difference as U values higher than CV
Subjective view on data
Adds statistical validity to my investigation
Appropriate as measured in mm - continuous - can be ranked
Cranedale Conclusions
No significant difference between stone size at Helwath Beck and Broxa
How could your results be of use to geographers?
Suggests Bradshaw’s model is not applicable to this/British rivers
Derwent is glaciated/rejuvenated
Secondary data used
From previous fieldwork by other groups
Weather reports
OS maps
Bradshaw’s model
Evaluation: Improvements and extensions
Use callipers instead of a rule - more accurate measure of B-axis
More sites measured for more representative data = more reliable conclusions
More points along river width - reduce error margin = reliable
Measure A,B and C axes and work out VOLUME of each stone - more accurate
Power’s’ scale of Roughness to look at pebble shape downstream (chi squared)
Compare at different rivers or in different weather conditions/seasons for comparison/different order streams - same conclusions?
Other variables measured eg velocity
Land use compared
Cranedale success
Found no sig difference
May be due to rejuvenation of River Derwent or redirection after glaciation period
Other variables may influence
Understanding has further developed - links between variables etc
How accurate/reliable?
Scatter graph uses and examples
Used when investigating the relationship between 2 continuous variables
Used in combination with Spearman’s rank
Velocity/dist from source = positive
Pebble size/ dist from source = negative
Scatter graph advantages
Clearly show positive, negation or no correlation - line of best fit
Shows strength of correlation - distance from line
Useful when carrying out Spearman’s Rank - gives a more precise and objective expression of the strength and reliability of the relationship
Graph allows analysis on which conclusions can be based - line of best fit
Lots of points in a small space
Patterns identified quickly and easily and anomalies identified
Relatively easy to construct
Shows data spread clearly
Scatter graph disadvantages
Line of best fit is a subjective judgement and can give a subjective judgement
Only 2 variables can be displayed
Too few data points can produce skewed results - incorrect graph analysis
Impossible to label data points - hard to ascertain exact values
Too many data points can quickly make graph unreasonable
Bar graph uses and example
Used to display categoric or grouped data
Absolute values and contrasts between areas and places
Simple show a single series of data eg temp/month
Comparative show 2 or more sets of dad side by side eg temp/month different locations
Compound show how the total in each bar is divided up into a number of subtotals eg traffic - cars/vans/lorries
Divergent where data is spread on either side of x or y axis eg population pyramids (y axis)
Histograms show frequency of occurrence of data eg pebble sizes in a river/frequency
Pictographs show data in the form of pictures (key) eg world pop/yrs
Bar graph advantages
Relationships can be easily perceived and compared
Original data can be easily extracted
Many potential uses - versatile
Can show pos and neg values
Pictographs make it very easy to see data
Good visual representation of statistical data - general trend
Simple to construct and easy to understand
Clear to see anomalies
Bar graphs disadvantages
Histograms can be confusing as area of bar represents data - cacluclations needed
Pictogrpahs have no actual scale - not very accurate and only a limited amount of data can be dispalcyes
Only small data sets can be plotted or patterns can be missed
Wide range of data - loses impact/difficult to read
Graph categories can be reordered to emphasise certain effects
Use only with discrete data
Limited space for labelling with vertical bar graphs
Reading accurately can be difficult
Time consuming
Line graph uses and examples
Use continuous data
Simple show a single series of data eg rainfall
Comparative show 2 or more sets of data on the same graph using the same scale eg DTM
Compound have several different components eg worl d popn/ time split into continents
Line graph advantages
Can suggest trends and estimate future patterns eg 1941 no UK census, but using 1931 and 1951 popn data, 1941 popn can be estimated (interim data can be inferred)
Can be combined with bar graphs to show more info eg climate graphs, storm hydrographs
Log and semi log scales can be used
Can compare multiple continuous data sets easily
Line graph disadvantages
Too many lines can be confusing
Only continuous data
Triangular graph uses and examples
Scatter graphs showing 3 sets of variables to see interrelationships
3 variables that each total 100 eg employment structures where employment is divided into primary, secondary and tertiary sectors as a percentage of working population
Triangular graphs advantages
Varying proportions can be seen - relative importance
Dominant component can be identified
Shows clusters