Example Task Question: The channel characteristics of a river change along its course.
Purpose Focus on velocity Sub- focus questions:
1. To what extent is there a regular /even change in velocity downstream? 2. Do changes in velocity downstream match the expectations of the
Bradshaw model? 3. What are the reasons for the change in velocity with progression
downstream? Location and background research The River Horner is located in the southwest of England, in Somerset. It is found on the northern edge of Exmoor (Figure 2) and has its source about 40km north-west of Taunton. In its lower course the river runs through Horner Wood, between Porlock and Allerford and through the village of Bossington. The river tends to flow in a northerly direction from source to mouth and is 9km long. This makes it ideal for study since a number of sites along the whole river can be accessed relatively easily. The main valleys between the hills are filled with alluvial deposits from the hills or sea. Figure 2 is a geology map that was sourced from the British Geological society (GIS kml Google Earth overlay http://www.bgs.ac.uk/data/services/home.html ). Figure 1 (opposite) Bradshaws model (source: the internet) Bradshaws model will provide the focus and context for this study. Figure 2 Geology map of south west England.
A risk assessment is also included as part of this purpose. Note how risks have been managed.
Figure 3a Figure 3b Figure 3a Inset regional map. Figure3b location of the River Horner (source: OS and http://wtp2.appspot.com/wheresthepath.htm Scale 1km = 1 square), Note, river is indicated in blue on the Figure 3b. Risk Assessment Hazard Risk Likelihood Control Traffic Hit by moving
vehicle Low Careful of
vehicles, cross at designated points; awareness
Rocky footpath and Uneven ground
Slipping and trips, scratches and cuts, twisting ankle
High Awareness and care, move slowly and watch out for each other
Ticks Limes disease and/or a bite
Medium Long sleeves and awareness
Planning and methodology A collaborative exercise was used to think about the influences on river velocity (thought to be very important in terms of characteristics) and then a mind-map was created online (source: www.bubbl.us). This helped me to prepare for the fieldwork. Figure 4 mind-map on measuring velocity
Figure 5- float method
It was decided that velocity would be measured using a float method. This was the most practical approach given the limitations of equipment. The float method is based on speed = distance / time (see Figure 5 opposite source:
http://labspace.open.ac.uk/file.php/1872/Mu120_3_028i.jpg) Choosing a float is also important here. After consultation with a number of textbooks, it was decide that dog-biscuits would be used. These float just below
the surface and are therefore not affected by the wind. We timed them over a distance of 5 metres. This was repeated 5 times at each site to get a more reliable average. Figure 6 below (photograph) shows accurate recording of times.
Figure 6 recording times Ten sites were selected down the River Horner. This was considered to be enough to link to the Bradshaws model so that significant differences could be seen. The sites are shown on Figure 7 Other variables were measured by other groups; this information may be used later as part of the understanding. Systematic sampling was used as it is straightforward to carry out, but in some places the systematic approach had to be abandoned due to access difficulties so the sites were not always evenly spaced. Figure 7 GIS Google Earth map showing the location of the River Horner, plus fieldwork sites, with an OS grid overlay (source: www.neraby.org).
Systemic sampling is used when there is an expected change between two locations (an 'environmental gradient'), then systematic is used to take regular samples at known distances. Therefore this is suitable for the River Horner
Methods of presenting data
A scatter graph is the most appropriate way of indicating the strength of relationship between site number and distance downstream. Figure 8 shows this, including a line of best-fit which has been added using Excel. Photos also show changes in the river channel characteristics from the upper to the lower sites.
Figure 8 Our groups data
Data from the other groups has also been used, but this forms part of the analysis. Figure 9 Flow diagram shows changes in velocity along the course of the River Horner. Length of arrow is proportional to average group velocity (total sample)
0 2 4 6 8 10
Using other groups data (pooled set) we were also able to plot a cross section based on one groups data for site 7 (see Figure 10). Figure 10 - a cross section based on one groups data for site 7
Secondary data from the National Rivers Flow Archive allowed us to find out more about the catchment of Exmoor. The reason is that discharge is proportional to discharge. This will help with the evaluation of the data.
Figure 11 (below) spread sheet data from NRFA plotted for 2011 to create a regime
Data analysis and conclusions
The data shows that the velocity of the stream starts off quite slowly at 0.16m/s at site 1. It then rapidly quickened up to 0.41 and 0.38 m/s at sites 2 and 3. It then dropped again in the middle sites (0.05 and 0.08 m/s) before gaining speed again at site 7 and 8 0.51 and 0.31 m/s. Overall this shows a very mixed pattern for velocity with considerable range (0.05-0.51 = 0.46 m/s). The average velocity is 0.24 m/s, but the median is slightly higher at 0.25 m/s. This is not really a very good measure of central tendency since the data seems to be set into two groups: Table 1 Correlations between the site and velocity Group 1 = low values (0.05-0.16) Group 2 = higher values (0.34-0.51) Site 1, Site 4, Site 5, Site 6 Site 2, site 3, Site 7, Site 8 I also used an Excel spreadsheet to look for any correlations between the site and velocity see screen-print for evidence (Table 1). My hypothesis was: There is a relationship between velocity and distance downstream
(Null = There is no relationship between velocity and distance downstream)
A very low number of 0.13 means that the results are not statistically significant. Therefore the null hypothesis should be accepted, i.e. there is no linkage between distance downstream and stream velocity. This is also verified by the scatter graph, Figure 8. A further part of the analysis involved looking at other groups data to see whether there were any more obvious patterns that were different to our own, individual groups data. Table 2 Group data with quartiles (Q1 + Q4) and median and standard deviation
The results are most reliable at sites 1 and 7 (lowest standard deviation), whereas sites 8 and 4 have greatest variation. I have also calculated the upper and lower quartiles Q1 (Lower) + Q4 (Upper), plus the median values for all the groups at each site.
Figure 12 Plot of quartiles, median and standard deviation for all groups data on the River Horner. This shows variations both downstream and between groups; perhaps median is the most reliable indicator of change.
Q1 Median Q4 SD Site 0.01 0.04 0.16 0.06 1 0.04 0.05 0.41 0.16 2 0.04 0.15 0.52 0.21 3 0.08 0.48 0.64 0.24 4 0.28 0.62 0.62 0.20 5 0.45 0.53 0.71 0.21 6 0.51 0.35 0.61 0.08 7 0.29 0.13 0.91 0.32 8
The graphs also back up the idea that velocity does not change evenly with distance downstream. This doesnt fit so well to the original model, at least for velocity. The model is idealised and simplified. Conclusions and evaluations Overall there is only limited linkage between the results obtained on the River Horner. It is certainly obvious that there are some unexpected readings. According to geographical theory, velocity may be linked to some other variables see my model below (Figure 13): Figure 13: Model showing likely influences on the River Horner velocity
1 2 3 4 5 6 7 8
Crosssectionalareaatpoint of measurement