Velocity will increase
downstream because the
increase in cross sectional area
resulting from added water
(tributaries and run off) will
increase the efficience as
friction is reduced due to the
increase in hydraulic radius.
Pebble roundness
will increase
downstream -
because of increased
attrition and abrasion
Pebble size will decrease
downstream - because attrition
and abrasion breaks up
sediment and bedload as it is
transported downstream
Bradshaw's Model
Predicts how river behaviour
changes downstream
showing that some variables
increase and others
decrease
"It was useful because it allowed me to base my hypothesis
on these concepts and gave me ideas that I could test, and to
see to what extent these ideas are evident in a real river, and
if there are any other factors that are important, like geology
and land use."
2. Location
"Cowside Beck flows through a hilly
limestone area in the Yorkshire
Dales National Park. It flows into
the River Wharfe about 5km south
of Kettlewell."
5 km long, has a very
distinct upper, middle
and lower course.
Relitively
unchanged
by human
activity
Risk assessment - river is
safe - never fast flowing or
more than wellington
heigh deep with stable
banks
Safe and easy
access points
3. Risk Assessment
Drowning in deep water - river is
not very deep, has flat river terraces
to either side (easy to step out to the
side), worked in groups so nobody
was alone if they fell
Action - Member of staff
supervised each group with a
throw rope, survival blanket and
spare dry clothes to prevent
hypothermia
Forecast hot - took
suncream and fluids to
prevent dehydration and
sunburn
"We also recognised that
risks were ongoing and
were prepared to modify or
cancel the data collection
at any time. EG a bank had
become unstable at a
sample site since the
pre-visit so we moved the
site downstream by 10 metres"
4. Data Collection
One method - Pebble roundness (increase downstream)
1. Systematically picked 11 pebbles along the bed width divided by 11 to give correct
intervals. Did this at 3 sample sites.
2. Laid out at the side of the river in order from left to right
3. Compared each pebble to the Powers Scale, gave score between 1 and 6 (1
very angular.) The same person did this to help with consistence and recorded the
results on a frequency table.
We then photographed the pebbles to give further visual evidence
to compare back in college. Then safely returned the pebbles back
to the river.
4. Strengths/weaknesses
Worked well - quick and easy,
systematic sample allowed
representation of the full channel width
Choosing 11 pebbles from
50cm intervals gave us a big
enough sample size for
statistical significance
Using the Power's index gave us a quick visual technique
HOWEVER it was subjective and relies on a value judgement
which led to some inconsistances within the data.
"To reduce the limitation nest time, I would
standardise the Power's scores more by
getting everyone in the group to agree on
a score and also using a couple of
pebbl;es per site to be checked by all
groups to act as a moderator.
5. Data presentation
Proportional divided circles
1. Chose radius of 3cm for my largest mean pebble size -
workes well visually on the map. Calculated a constant by
dividing the 3cm by the square root of largest pebble size.
Gave me 0.9. 0.9 was then cused to calculate the other radii,
by multiplying it by the root of each mean pebble size.
2. Drew circles
using compass
next to 3 sample
sites on base map
of Cowside Beck.
3. Created pie chart inside each circle. To
calculate angle, divide frequency by the total
number of pebbles (66) then multiplied by
360. Then constructed angles using a
protractor working clockwise. Repeated the
same technique for the other circles for
consistency.
4. Then shaded in the 6 segments in different colours and creased a key.
7. Advantages/disadvantages
Easy to construct, clear and visual, easy to compare
between 3 sites due to the map. The diameters showed the
decrease in mean size from 11.1 to 9.1cm
Best way to show roundess. 6 Categories is perfect as it is easy to calculate the
angle. Having 3 pie charts on the map made this easy to spot anomolies such
as site 2 where angular material had washed in from scree slopes.
Anomalous data distorted the
averages. A graph showing the
range of sizes at each site
might have been better. Pie
charts did not show individual
values but can be resolved by
writing the percentages in each
segment.
8. Data Analysis
Spearman's Rank
1. Drew table and ranked 18 velocities in the first
column from lowest to highest. Wrote in the
corresponding average pebble roundness and ranked
from highest to lowest.
2. Next colum - calculated and recorded differences
in the rank. Squared the differences (808.5) and used
this to work out the formula. (0.166)
3. Checked for statistical significance by using a critical values table. My result
was less than 95% certain of being a valid correlation therefore accepted my null
hypothesis of no relationship between velocity and pebble roundness.
"One possible problem with Spearman's is
that I could have made a mistake in my
calculation. However a major disadvantage
is that the result swallows up anomalous
data in the middle course where scree falls
increase angularity, explaining this weak
relationship. So I think it is important to use
a scatter graph as well to analyse the data
successfully."
Advantages - ideal due to the relationship being linear.
18 pairs of data is also enough for significance. Quick
and easy, gives numerical value and is therefore
objective, allowing you to see the strength of the
relationship. More reliable than using a line of best fit on a
scatter graph as this is subjective and can be
misinterpreted.
9. Results and coclusions
1.Partly followed the Bradshaw's
model. Velocity did increase
downstream slightly from 0.09 to
0.32 m/s.
However site 2 was faster than
expected due to human
intervention - straightened to
speed up flow.
2. Mean pebble size decreased but only from 11.1 to 9.1cm
"My results would be useful to other
people/geographers as it shows that
Bradshaw's ideas are partly evident in
real life, but not always because of local
factors like human intervention and
geology."
3. Pebbles got rounder but located divided
circles showed some anomalous data in the
middle course near Darnbrook farm - 17/66 had
scores of 1 to 2 (very angular) - due to scree
slopes.
"Overall I was able to achieve my aim in testing Bradshaw's model,
however to further validate my conclusions I would also like to extend my
investigation to sampling another stream in the locality and collect larger
sample sizes to improve significance."