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How do changes to the load size and shape of the River Lyn compare with the Bradshaw model?

Consolidating your thinking – fieldwork to calculate changes in the size and shape of bed load of the River Lyn

Bradshaw said in his model that a river’s load size and shape (average size and shape of material transported) will also become smaller and rounder with distance from the source.  This is because the further downstream material is carried then the greater the time available for it to be eroded by attrition and abrasion which makes rocks and stones smaller and rounder.  Also, during periods of low flow on the river e.g. during the summer only smaller particles can be carried downstream with all of the remaining material being deposited that are then vulnerable to being broken up by weathering such as freeze – thaw processes.

 

To test this assumption against the real world example of the River Lyn measurements can be taken of the average size and shape of the river’s bed load at sample points along the course of the river and then tested statistically to ascertain whether the changes identified are significant.   It is not possible of course to measure the size and shape of every stone and pebble in a river so it will be important to design a simple sampling method to provide a representative sample which can be replicated easily at each sampling location.  

The film and narration in the video on this page shows clearly how to sample and measure bed load size and shape of and further guidance is available at http://www.geography-fieldwork.org/rivers/river-variables/2-fieldwork.aspx and also http://www.earthstudies.co.uk/Geography/Individual%20Research%20in%20Geography%20G3/Powers%20Scale%20of%20Roundness.html and http://www.rgs.org/OurWork/Schools/Fieldwork+and+local+learning/Fieldwork+techniques/Rivers.htm

If at least 8 measurements of the wetted perimeter are made along the upper course of the River Lyn and a minimum of 8 in its middle course and a further 8 in the lower course then the Mann Whitney U test can be used to assess whether the difference between the medians of two sets of data e.g. Upper and Middle; Middle and Lower and Middle and Upper and Lower are significantly different assuming a null hypothesis of no significant difference existing between the locations. 

The following sites along the river are suggested as possible locations to take the sets of wetted perimeter measurements required:

Assessing risk and identifying control measures to manage risk

Please refer back to guidance on Pages 7 – 8 and use the template with regard to fieldwork above.

Secondary data sets

If access to these locations is not possible for primary data collection then the following tables provide sets of secondary data obtained from 11 sites along the length of the river system  for the students to test statistically:

 Site 1Site 2Site 3Site 4Site 5Site 6Site 7Site 8Site 9Site 10Site 11
Very Angular2          
Angular22212  1 3 
Sub Angular564351244 2
Sub Rounded12452755341
Rounded   1121 335
Very Rounded      1   1
            
Clast Size (cm) 111.613.810.8412.8271013.113.59.19.5
Clast Size (cm) 217.316.186.19.116.81016.11514.26.5
Clast Size (cm) 317.115.4134.57.85.110.21915.57.37.4
Clast Size (cm) 41910.19.812.86.56.97.116.42813.125
Clast Size (cm) 52518.111.210.34.45.36.62013.811.29
Clast Size (cm) 68.424.56.5411.315.58.224.6224.27.5
Clast Size (cm) 721.110.219.118.511.595.411.12111.218.4
Clast Size (cm) 81211.48.36.86.95.37.240236.313.5
Clast Size (cm) 9241711.711.66.42.86.31316.520.210
Clast Size (cm) 107.73216.39.84.128.4141718.22
Average Clast Size (cm)1814.710.568.848.089.577.9418.7318.5311.510.88

Now go to Investigation 8  -How do changes in the velocity of the River Lyn compare with the Bradshaw model?