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Compare the observations and recordings made at the two sites with predictions made from the characteristics of a ‘theoretical’ river.

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Introduction

Introduction At the beginning of July 2002, we went to study two sites on the River Bovey in Devon. The initial site of enquiry was at the source of the river (Site 1 - grid reference SX 694829) where we collected data on the characteristics of the bedload, the velocity of the water and the cross-sectional area and bankfull level of the river. We then repeated this data collection at a second location (Site 2 - SX 779801) approximately 15km downstream near the mid-point of the river's course. The River Bovey is situated inside the Dartmoor National Park in the Southwest of England. Dartmoor is a large intrusive granite landform with many areas exposed through years of erosion and weathering. The moorland's high altitude results in extensive amounts of relief rainfall which, along with the park's impermeable rock-type, means the area has a high drainage density. The two sites in the catchment area studied have very similar rock-type and climate conditions but differ slightly in other relevant features. Site 1 is approximately 415m above sea level and is situated in an area of steep, upland moorland on peaty gley soils. The vegetation surrounding the source consists of a variety of shrubs, ferns and heathland, the culmination of these characteristics and the local climate and impermeable rock-type results in the ground being saturated for lengthy periods of the year. Site 2 on the other hand lies roughly 345m lower at 70m above sea level and is in a gentler-sloping landscape with mainly brown earths. The vegetation surrounding the immediate area of the second site is that of deciduous woodland. The two separate sites were chosen because the different characteristics they have resulting from their separate points along the river. This should give us a clear view on how a river changes from its youthful stage in it's upper course to the forms and processes it has later on it's journey towards the sea. ...read more.

Middle

This was done by taking a stratified sample of 50 pieces of bedload and grouping them into five classes from smallest through to largest (smallest being grains of coarse sand and largest being rocks and small boulders between 0.2m and 1m long). Measurements were then taken using callipers of the long axis and the short axis of each item of bedload. Another recording was made of the angularity of the selected bedload. This was done by making a subjective observation using the Power's Angularity Chart (see right) which uses a set of six pictures to aid judgement in identification. Analysis Site 1 Site 2 Cross-sectional Area 0.278m� 2.22m� Wetted Perimeter 2.12m 9.4m From the above data, presented in the channel cross-sections (Figures 4 and 5), it is possible to calculate the hydraulic radii at the two sites on the River Bovey. The hydraulic radius is the ratio between the area of the cross-section of the river channel and the length of its wetted perimeter: Site 1: Hydraulic radius = 0.278/2.12 = 0.131m Site 2: Hydraulic radius = 2.22/9.4 = 0.236m The river channel at Site 2 has a larger hydraulic radius, meaning that it has a smaller amount of water in its cross-section in contact with the wetted perimeter. This creates less friction which in turn reduces energy loss and therefore allows for greater velocity. This indicates that the river at Site 2 is more efficient than Site 1. The river at Site 1 has a smaller hydraulic radius meaning that a larger amount of water is in contact with the channel bed and bank sides. This results in greater friction, more energy loss and reduced velocity. Therefore Site 1 is less efficient than Site 2. The shape of the cross-section controls the area of maximum velocity in a river channel and the point of maximum velocity at each site varied. The cross-section of Site 1 (Figure 4) ...read more.

Conclusion

A more systematic sampling method for taking readings at Site 1 would have been useful in calculating a more reliable average velocity and thus a more accurate calculation of the rivers discharge at that site. The measurements of velocity at Site 2 were again sufficient to identify areas of increased velocity and also adequate to work out the average overall velocity, but they were not comprehensive enough to construct accurate isovels on the velocity diagrams (Figure 6). The most ambiguous results collected were those of the bedload, despite the use of a sampling technique with appropriate classes of strata selected. A stratified sampling method was employed to identify the proportions of bedload size, but it was based on our own personal observations that may have been inaccurate or semi-unintentionally biased towards establishing the hypothesis as correct. A stratified random sampling technique could have been used to make the data collection as fair as possible, which would have meant the avoidance of any bias or misleading results. However, it may have involved considerable time, energy and difficulty in selecting and visiting each area of the riverbed required. In order to have identified an even greater variation in velocity, discharge and bedload size between points on the river, I believe a site that was further downstream than Site 2 could have been studied. This would have established a more comprehensive understanding of how a river's form, velocity and load change during its course. Also, if other sites very close to the ones studied had been selected, it would have been quite likely that the results would have differed to a noticeable extent. The same is true if the study had been undertaken during a different, wetter period of the year. Overall, the fieldwork exercise was extensive and comprehensive enough for the desired level of accuracy required to produce a clear and adequate understanding of the theories and concepts involved. The River Bovey appears to behave how a typical river should - adopting channel form, shape and levels of discharge that best fulfil its two main functions: transporting water and sediment to the sea. ...read more.

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