- Created by: Megan Phoenix
- Created on: 12-05-15 16:36
Aim and hypothesis
- To investigate the fluvial downstream changes in channel characteristics, especially that affecting discharge, ar Burbage Brook, Derbyshire, in light of the Bradshaw model.
- Null: There will be no increase in discharge with distance downstream at Burbage Brook.
- Alternative: There will be an increase in discharge with distance downstream at Burbage Brook.
Why do we predict discharge will increase downstream?
- Tributaries will add extra discharge to the main channel.
- Drop of over 60 metres within the first kilometre.
- Steep slopes and minimal vegetation will cause less infiltration - esp. after prolonged/sudden rainfall.
Theory and why we chose it.
Bradshaw model -
- Helps to describe the changes in channel characteristics that you would expect to see as the river travels from its source in the upper course to its mouth in the lower course.
- Depicted as a series of triangles - if the triangle gets wider towards the downstream section of the model then, on average, that variable increases with distance downstream e.g. discharge, mean depth, mean width, average velocity andload quantity.
- Likewise, if the variable gets smaller towards the downstream section of the model then, on average, that variable decreases with distance downstream e.g. load particle size, channel bed roughness and slope angle (gradient)
Why did we choose this theory?
- Prior knowledge of river processes enabled us to make an educated hypothesis
- The location was sade, accessible and available.
- The equipment needed to test the theory didn't have to be sophisticated/expensive.
- The Bradshaw model allowed a comparison to a known and consistent trend, therefore it was a suitable investigation for varying levels of understanding.
Location - why we chose it.
Burbage Brook, Derbyshire.
- 8km north-west of Sheffield
- 3 miles east of Hathersage.
Why did we choose here?
- Accessibility - within 2hrs drive of school, access via the A6187 and Ringinglow Rd, car parking and Longshaw estate footpath.
- Safety - upper course was shallow, narrow and low and discharge was close to the source.
- Typical upper course - v-shaped valley sides suitable for testing Bradshaw model effectively.
- Safe terrain - owned by the National Trust, public land, not too steep, legal permission.
Features of location.
- Source 427m above sea level
- Tributary of the River Derwent
- Higger Tor - impermeable outcrop, 385m above sea level.
- Between Site 1 and 2, the river drops from 396m to 335m in under a kilometre.
- Moorland and peat bogs - acidic soil, low nutrients as rain washes them away, heather and moorland grasses
- Only trees - 83 acre Scots pine plantation.
- Beneath the surface is impermeable gritstone - moorland is very wet, vegetation doesn't rot easily which forms layer of peat over surface.
- Upper vallley - millstone grit at the top and shale beneath on the valley floor
- Millstone grit - resistant rock with distinctive bedding and jointing planes.
- Shale - soft rock, numerous thin beds, easily broken by weathering and eroded by the river.
Before: Undertook a pilot study to identify and potential hazards and make amendments to our methods. Studied OS map to familiarise ourself with the area under investigation.
During: data collected in February - high levels of rain and low temperatures.
- Hypothermia - caused by extreme exposure to cold weather. We made sure we wore appropriate warm clothing such as jumpers and raincoats and took spares with us.
- Falling on uneven ground/loose rocks - To prevent this we all wore sturdy footwear such as walking boots and made sure we followed the footpath at all times.
- Drowning - falling in the river and hitting head on a rock. To eliminate this risk, we walked in groups at all times, behaved in a sensible manner and ensured the water level never exceeded knee-depth in the deepest section.
Continuous monitoring throughout: e.g. weather conditions, water levels, human risks.
Risks involved in data collection: used group data for security and repeated each data collection method 3 times at each intervan to get a mean and eliminate any anomalous results.
Data collection method - velocity.
- Systematic sampling: At 500 metre intervals downstream, the channel was split into 3 equal 1 metre sections using a tape measure, 1 at each of the 2 banks and one in the centre.
- The float was then placed on a 1 metre ***** and timed, using a stopwatch, to test the length of time taken to travel the distance.
- This was repeated at each interval to gain an average at each site.
Relates to theory/aim because...
Discharge = velocity x cross sectional area.
Positives of data collection method.
- Easy to carry out
- Easily repeated for accuracy and to gain a mean average.
- Produces data that can be easily manipulated e.g. can be presented on a graph/examined in a statistical test.
- Gave us accurate results for one of the variables needed to calculate discharge in order to make a comparison with the Bradshaw Model.
Negatives of data collection method.
- Subject to human error e.g. timing and measuring - reaction times of individuals may differ, reducing the effectiveness of the data.
- Subject to natural influences e.g. the wind resistance against the float may have prevented it from travelling at maximum speed.
- The method only measured the surface velocity - channel velocity would have been more suitable.
- Inaccurate/sophisticated equipment was used - a hydroprop would have given us more reliable data.
How did we eliminate bias?
- Repeated the collection method at each 500m interval in order to gain an average at each site.
- Add a time buffer on each measurement to account for human error.
- Use a hydroprop in future - more accurate and reliable results.
- The same points were measured at each site (3 equal sections: 2 banks and the centre of the channel).
- The float was held by 1 person and the other independently recorded the time as soon as the float was released. The roles of each person were consistent throughout.
- Removed any debris in the channel before starting.
- Stood in the river channel before releasing the float - prevent abnormal currents.
Positives of data presentation - scattergraph
- Clear, visual representation of the relationship between 2 variables - uses bivariate data which aids interpretation.
- Best method to use for continuous data - allows for line of best fit which enabled me to predict variables in further sites
- Anomalies are easily identifiable.
- Easy to extract data
- Correlations between the variables can be examined in greater depth by using a statistical test such as Spearman's rank.
- Easy to interpret/plot.
- Able to compare individual points.
Negatives of data presentation - scattergraph
- Not statistically verefied on its own - you may see a correlation but will need a statistical test to confirm.
- Doesn't show cause and effect.
- Anomalies can sometimes skew your results,
- 'Overplotting' can be an issue with similar results - difficult to interpret the actual value.
- I could not plot discontinuous variables such as bed load roundness, therefore I couldn’t analyse all the variables against the Bradshaw model.
Alternative: comparative bar chart - located to give precise conclusions for aim.
Data analysis - Spearman's rank
Spearman's rank correlation coefficient -
- Used to examine the strength of the relationship between two variables e.g. distance downstream and discharge.
- Uses continuous data with a relationship.
- The test was suitable for the number of values I had collected for each variable as opposed to e.g. chi squared where the total number observed had to exceed 20.
- I could examine the strength to which two different variables were related - links closely with Bradshaw.
- It allowed me to test the significance in order to prove that the relationship was not by chance.
- Mann Whitney U requires the variables to be independent of each other therefore spearman’s rank was the appropriate choice as it allowed me to compare variables were one was dependant on the other.
My spearman's rank correlation coefficient result allowed me to conclude that, in line with the Bradshaw model, discharge did increase with distance downstream at Burbage Brook, Derbyshire.
This enabled me to accept my alternate hypothesis and reject the null, proving the investigation a success.
This is backed up by my results, which were presented on a scattergraph, whereby at site 1 discharge was recorded at 0.0152m/s and by site 10, this figure had increased to 0.506m/s.
It is possible that discharge increased due to surface run-off from the steep-sided valley slopes, lack of vegetation and sheep-trampling the ground. The river is also a tributary of the river Derwent.
There was, however, an anomalous result recorded at site 3, with an unexpected decreased in dicharge to 0.194m/s. We expected that the figure for this site would be of greater value.
Anomalous result caused as the overlying drift changed from boulder clay to aluvium (minor aquifer), causing the water to leach out of the channel, reducing velocity and discharge.
My investigation greatly improved my understanding of the fluvial changes of a river, especially that effecting discharge, as my alternate hypotheses proved correct - discharge increased with distance downstream, concordant with the Bradshaw model that underpinned out investigation.
Helped to develop our geographical understanding of the external factors e.g. changes in overlying drift, which affect my variables under investigation. I now understand better the processes affecting discharge, thus have better understanding of my geographical theory.
It also helped me to understand that rivers don't always conform to the 'textbook' model as they can be heavily influenced by other factors such as geoglogy/contours of the land. (Use figures from data collection to back up e.g. any anomalous results).
I also became familiar with using statistical test and understand their importance in giving objective proof/disproof. I was able to understand how downstream changes were linked as the data I collected supported the predictions it made.
- Only 2km of the upper course were investigated which is not parallel with the Bradshaw model as it is not representative of the whole river. In future, we could extend out investigation by testing more sites along the whole long-profile to ensure results are more accurate and reliable.
- Some methods of data collection were inaccurate and unreliable as they were too highly effected by human error and natural influence. We should have used a hydroprop (scientific tool) to measure velocity.
- We only did 1 investigation and did not repeat/take previous data/data from different seasons - may just have been an anomaly not something that frequently occurs.
- Our use of systematic sampling may have caused us to miss key sites between the 500m intervals. To avoid this, we should have used GIS to accurately measure the distance between the sites, or used the stratified sampling method which would have allowed us to choose sites in the upper, middle and lower course, ensuring a fair representation.
- Safely reached the aim of our investigation - accepted alternate hypothesis.
- While results were unexpected, could explain why using secondary data e.g. OS maps/environment agency geoglogy maps/river history data.
- Helped to develop our geog understanding of river processes & external factors e.g. change in overlying drift, which affect them: now understand better the processes affecting discharge, thus have better understanding of geog theory.
- Location successful – safe, easy to investigate & access - allowed theory to be tested in one day - didn't limit our time/resources.
- Results may be useful for: Farmers to protect livestock; EA for flood management; engineers such as bridge builders.
- Human influences - humans can either increase the discharge (e.g. at a sewage outfall) or decrease the discharge (e.g. abstraction of drinking water).
- Land use - discharge is higher in unvegetated, urbanised and deforested basins because there is greater surface run off.
- Rock type and structure - the surface runoff componant of discharge is lower in drainage basins of permeable from than impermeable rock.
- Gain more data - from 'The National River Flow Archive' to compare to the data collected.