A Study of the Changes in River Processes
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A Study of the Changes in River Processes
This is a study of the changes in river processes along the long profile of a river. To study this we will use a sample river. The river the study will be based on Loughton Brook, which is a river situated in Epping Forest in Essex and is also a tributary of the river Thames. A journey will be made to the river and measurements will be made at three different sites. The measurements that were taken will be studied so conclusions can be made about the changes to characteristics of a river with distance downstream. Aims and Hypotheses The overall aim of this study is to investigate the changes in river characteristics downstream along Loughton Brook. To investigate these changes in more detail a range of hypotheses will be tested. These hypotheses are: 1. The width of the river channel will increase with distance downstream. I expect to find this because in the long profile of a river channel width increases. This is because there is a greater volume of water at a faster velocity. This causes more hydraulic power erosion in the river channel downstream, which makes the channel wider. 2. The depth of the river channel will increase with distance downstream. I am expecting to find this because as the velocity of the river increase further downstream there will be more hydraulic power erosion which will cause the river to become deeper. 3. The wetted perimeter increases with distance downstream. This is because I expect the depth and width increase so the wetted perimeter must also increase. 4. The gradient will decrease with distance downstream. I expect to find this because in the long profile of a river the further downstream along the river the less high the river is above its base level. 5. The velocity increases with distance downstream. I expect to find this because as the river flows downstream there will be a larger volume of water. This is because as the river travels downstream more water will accumulate in the channel as the water enters the river from the drainage basin. 6. The bed load size will decrease with distance downstream. This is because I expect the velocity of the river to increase and so there will be more erosion of the rocks. This type of erosion is attrition and as the rocks collide because of this they will break up and become smaller. 7. The bed load shape (roundness and smoothness) should increase with distance downstream. This is because as the velocity of the river increases further down stream there will be more attrition as the rocks collide causing the rocks to become rounder. 8. The crosssectional area of the river channel increases with distance downstream. I expect to find this because I expect the depth and width to increase with distance downstream and these results will be use in the calculation to find the crosssectional area. Also this is because I expect the velocity to increase and so there will be more hydraulic power erosion, which will make the cannel larger. 9. There is a relationship between the depth and width of the river. I expect to find this because further down the river the depth and width both increase because there is an increased velocity because there is a greater volume of water. This increased velocity will cause more hydraulic power erosion which will cause the channel to widen and deepen. 10. Location Description The study that will be carried out will be based in Loughton Brook, which is in Epping Forest in Essex. The forest around Loughton Brook has been there for 5000 years. In 1878 an act was passed to stop development of the forest area and the site is now a SSSI (site of special scientific interest so there are still many trees large, old new and a lot of vegetation in the Loughton Brook drainage basin. The area was a good site for the investigation because it has a field centre, which has information on the river and also because it is local. The river also is a tributary of the river Thames. The ground in the area is mostly soil with leaves covering it. At the source of the river the soil is Pebble Gravel soil, a light brown/ orange coloured, sandy type of soil. This grainy consistency of the soil causes it to be porous and so easy for water to seep through. This can cause the soil to be damp and this is why the source of the river is a very saturated collection point of water with areas of very wet ground. The water seeps through the soil and into the river by infiltration and water can only be seen clearly at the source after rainfall because Loughton Brook is a flashy river so the water seeps quickly into the river channel and flows away downstream. During the months of April – September, when there is less rain, there is hardly any water in this part of the river unless it rains. The drainage basin of Loughton Brook is fairly small so the river features and processes are present but on a smaller scale and not always very obvious. Further along the upper course of Loughton Brook the soil is Clay Gate Beds. This type of soil is a grey/ brown colour and its consistency is like plasticine. It is also impermeable so water cannot pass through easily and flows towards the river instead of infiltrating downwards. This type of soil continues along the river until there is London Clay, which is impermeable like the Clay Gate Beds. Other features of the area of Epping Forest are that there are roads passing through the forest and the Epping New Road crosses Loughton Brook. There are also footpaths, nature trails and a Tea Hut for the public. The spring at the start of the Loughton Brook river. There is not much water because this is a flashy river so the water reaches the channel very fast. [IMAGE][IMAGE] Site Descriptions Site 1 At this site there was a relatively steep river valley with a leafy covering on the ground of the river valley sides. The valley did not have a steep gradient because of the clay soil, which slips down the bank of the river. The trees on the river valley helped to hold the soil together. On the left bank there was a steep river cliff and on the right bank there was an over hang caused by the erosion hydraulic power erosion of the riverbank. The tree roots held this soil together as well and also reduced the erosion slightly because the river had to flow around the roots. The main forms of vegetation were trees. The river channel was relatively straight and the water was clear. There was not much water in the river at this site because the water level increases only after rainfall because the Loughton Brook is a flashy river so the water quickly flows away. Also in the channel there were small waterfalls and debris dams and these could affect the results. Site 2 At this site the river valley sides are less steep and there is a leafy covering here as well. The soil is Clay Gate Beds and the vegetation is trees and moss. The right river cliff is steep and on the left bank there is undercutting due to erosion. There is a large tree root that stops the river cliff overhang from falling into the river but the river cliff will eventually collapse. There is sinuosity of the river channel, caused by erosion of the softer rock and not the harder rock, but there are no actual meanders at this point on the river. The water in the river channel is clear and shallow and the small pebbles on the bottom of the river channel can be seen. There is a man made log dam in the river channel at about three metres upstream that may affect the result. Site 3 There is still a leafy covering on the banks of the river valley but the valley is now flat. There are meanders at this point on the river so on the right bank there is an undercut river cliff due to but on the left bank, which is the inside of the meander, there is a relatively flat bank covered with small pebbles caused by deposition. The meanders at some points are very large and one is nearly an oxbow lake. On the inside of this meander there is a tree which slows down the erosion because it holds the soil together. This slows the process of the meander becoming larger and eventually becoming an oxbow lake. There are riffles in the river where the readings of velocity are faster and there are pools where the readings are slower. The measurements our group took were from a pool so the results may be slower. The water in the river channel is brown, because the river is carrying material that it had eroded, so the bottom of the river channel could not be seen. [IMAGE][IMAGE] Methodology The primary data that was obtained was collected at Loughton Brook river. This particular river was chosen because it is has the features and processes of a river for us to study. The river is situated in Epping Forest, which is not only local to the school, but has a good field centre where extra secondary information can be collected. Also Loughton Brook flows into the river Roding which is a tributary of the river Thames. The three sites were chosen because they were at the upper, middle and lower course of the river. This was ideal because the results from each site could then be compared easily so the hypotheses about how a river changes downstream could be tested. Primary and Secondary Data The information collected for this investigation is found in two forms, which are Primary and Secondary data. Primary data is the data that was collected first hand at the study of the river whereas Secondary data is the information that is usually obtained by research. The data collected on the day of the trip was primary data and the information we collected at the field centre was secondary data. To collect the primary data, the following range of equipment was used: · 1 metre ruler · clinometer · cork · paper · pencils · stop watch · tape measure · 2 metre ranging poles PHOTO Methods To test the series of hypotheses a range of methods were used. These are the methods used to test each hypothesis: Hypothesis 1: The width of the river channel will increase with distance downstream. To investigate this we measured with the tape measure from one bank to the other across the top of the water from the edge of the water. This method was quite good but the river had wider parts and thinner parts within a few metres of each other. If the measurements were taken at a particularly wide or thin place then the results would be affected. To solve this, measurements could be taken in more than one place at each site and then the average of these results could be used. Hypothesis 2: The depth of the river channel will increase with distance downstream. For this we used a 1 metre ruler to measure from the river bed to the top of the water three times at regular intervals across the channel. Then we took the average of these results. We did this because across the river channel from one bank to the other the water is at different depths especially on a meander. By using the average three depths at ¼ of the way, half way and ¾ of the way across the river the results for each site will be easier to compare. Hypothesis 3: The wetted perimeter increases with distance downstream. We tested this by measuring along the bottom of the channel, from the edge of the water on one bank to the other, with a tape measure and then recorded the results. This method works well only if the is clear, but if the water is not clear and the bottom of the river cannot be seen then we could not measure the river accurately. Also there were stones and debris on the river bed which could have affected the results. Hypothesis 4: The gradient will increase with distance downstream. To measure the gradient we used a clinometer with the two ranging poles. The ranging poles were placed three metres apart and then the clinometer was lined up using the stripes on the ranging poles. This method was quite difficult and it was easy to make an error. This was because the clinometer was difficult to line up with the ranging poles an d also the gradients were so small that it was difficult to read the results from it. Another problem was that the result was based on the opinion of the person taking the measurement, which may not be very accurate. The results could be more accurate if they are taken more than once at each site. Hypothesis 5: The velocity increases with distance downstream. We investigated this by measuring how long took for a cork to float one metre downstream. We did this three times and used the average of these results to find the average velocity. This was a more difficult method to follow accurately especially in shallow water where the cork would often become stuck on pebbles or debris on the riverbed. The cork did not always float in a straight line or floated to the side, which affected the results. Also at different parts of the river channel, the velocity is not the same for example the velocity of inside of a meander is slower than the outside of the meander. The results would depend on where the cork floated. A more accurate way of measuring the velocity than using the cork would be using a flowmeter. Hypothesis 6 and 7: The bed load size will decrease with distance downstream but the shape (roundness and smoothness) should increase with distance downstream. To measure this we picked ten random pebbles from the river bed at each site. We then used the roundness and sphericity index, which we were given at the field centre, to see how round and smooth each pebble was and then measured the pebbles at their longest point to find if the shape changed downstream. The roundness index was not very accurate because the results were based on someone’s opinion and not on a proper measurement. Also only a very small sample of pebbles were taken and these may not necessarily be representative of the all the pebbles at the site. PHOTOS D,A,T,A,P,R,E,S,E,N,T,A,T,I,O,N Channel Cross Section Diagram – Site 1 Depth was measured at three points across the river channel, at ¼, half way and ¾ of the distance across the river. Therefore, to produce the cross section, the points plotted on the Average Channel Width axis needed to be calculated with this method: Average channel width (m) = 3.5 = 0.875m 4 4 =============================================== Hydraulic Radius This calculation shows the efficiency of the river channel and is measured in metres. The larger the Hydraulic Radius the faster the flow of the river therefore the river more is efficient. Hydraulic radius is calculated using this method: Hydraulic Radius = Crosssectional Area (m2) = 0.11 = 0.120m Wetted Perimeter (m) 0.91 Channel Cross Section Diagram – Site 2 Depth was measured at three points across the river channel, at ¼, half way and ¾ of the distance across the river. Therefore, to produce the cross section, the points plotted on the Average Channel Width axis needed to be calculated with this method: Average channel width (m) = 3.4 = 0.875m 4 4 ========================================================= Hydraulic Radius This calculation shows the efficiency of the river channel and is measured in metres. The larger the Hydraulic Radius the faster the flow of the river therefore the river more is efficient. Hydraulic radius is calculated using this method: Hydraulic Radius = Crosssectional Area (m2) = 0.12 = 0.128m Wetted Perimeter (m) 0.94 Channel Cross Section Diagram – Site 3 Depth was measured at three points across the river channel, at ¼, half way and ¾ of the distance across the river. Therefore, to produce the cross section, the points plotted on the Average Channel Width axis needed to be calculated with this method: Average channel width (m) = 3.6 = 0.875m 4 4 ==================================================================== Hydraulic Radius This calculation shows the efficiency of the river channel and is measured in metres. The larger the Hydraulic Radius the faster the flow of the river therefore the river more is efficient. Hydraulic radius is calculated using this method: Hydraulic Radius = Crosssectional Area (m2) = 0.19 = 0.186m Wetted Perimeter (m) 1.02 Channel Crosssection diagrams The crosssectional area diagrams are linked to the hypotheses the width of the river channel will increase and the depth of the river channel will increase with distance downstream. This type of graph was used because it is easy to see the shape and size of the channel so it is easier to compare the average channel size of each site. Hydraulic radius – the hydraulic radius increases at each site. It increases from 0.120m at Site 1 to 0.186m at Site 3 and this means the efficiency of the channel increases at each site. Depth and Width Bar Chart This comparative bar chart is connected to the hypotheses the width of the river channel will increase and the depth of the river channel will increase with distance downstream. This graph compares the average width and depth and was used because it shows clearly whether the depth and width are connected. As the depth increases the width also increases on this graph. The deepest channel is the river channel at Site 3, the shallowest is at Site 1. The widest channel is at Site 3 and the thinnest is at Site 1. Wetted Perimeter Comparative Bar Chart This graph is related to the hypothesis the wetted perimeter increases with distance downstream. This type of graph was chose to compare the wetted perimeter of the three sites. The wetted perimeter increases over the three sites and this is shown on the graph as Site 1 being the lowest at 0.913 and Site 3 the highest at 1.018. Gradient Comparative Bar Chart This graph is connected to the hypothesis the gradient will decrease with distance downstream. This type of graph was chosen because the gradient of the three sites can be compared. The gradient decreases over the three sites and this can be seen on the graph as Site 1 is the highest with an average gradient of 2.6o and Site 3 is the lowest with a gradient of 1.6o. Velocity Scatter Graph This graph is associated with the hypothesis velocity increases with distance downstream. This type of graph was used because it is clear if there is a correlation between the velocity and the distance downstream. There is a positive correlation between the velocity and the distance downstream. The lowest velocity is at Site 1 and is 0.036 m/s. The highest is at Site 3 and is 0.638 m/s although this result is also one of the two anomalies marked on the graph. Pebble Roundness and Sphericity Pictogram The pictogram related to the hypothesis pebble roundness and sphericity increases with distance downstream. This type of chart was used because it shows clearly what types of pebbles are at each site. The chart shows that the further downstream the site is, the more wellrounded the pebbles are for example there are three very angular pebbles in site 1 but none in site 2 and none in site 3. In site 1 there is only one wellrounded pebble and in site 2 there are three and in site 3 there are three. Pebble Size Cumulative Frequency Graphs This graph is associated with the hypothesis bed load size will decrease with distance downstream. These graphs show the cumulative frequency of the pebble long axis size at the different sites. This type of graph was chosen because from the graph the approximate median result can be calculated. These can then be compared over the three sites. The approximate median long axis varies over each site and from these results no relationship can be seen between pebble size and distance downstream. The approximate median for Site 1 is 2.35cm for Site 2 it is 3.6 and for Site 3 it is 2.4. The lack of connection between these results may be because there may be an anomaly where the curve is not smooth in the results for Site 1 as circled on the graph. This may have made the median higher than necessary. Crosssectional Area Target Graph This type of graph was chosen because it is easy to see the increase in crosssectional area over each site. This graph shows that over the three Sites the crosssectional area increases. Site 3 has the largest crosssectional area and is represented on the graph as 100%. The crosssectional area of Site 1 is 58% of the crosssectional area of Site 3 and the crosssectional area of Site 2 is 63% of the crosssectional area of Site 3. Hydraulic Radius, Velocity and Wetted Perimeter Radar Graph This graph compares the hydraulic radius, velocity and the wetted perimeter. This graph shows that as the velocity and wetted perimeter increase at each site the Hydraulic radius decreases. Velocity and Gradient scatter graph Crosssectional Area Target Graph This type of graph was chosen because it is easy to see the increase in crosssectional area over each site. This graph shows that over the three Sites the crosssectional area increases. Site 3 has the largest crosssectional area and is represented on the graph as 100%. The crosssectional area of Site 1 is 58% of the crosssectional area of Site 3 and the crosssectional area of Site 2 is 63% of the crosssectional area of Site 3. Hydraulic Radius, Velocity and Wetted Perimeter Radar Graph This graph compares the hydraulic radius, velocity and the wetted perimeter. This graph shows that as the velocity and wetted perimeter increase at each site the Hydraulic radius decreases. Velocity and Gradient scatter graph D,A,T,A,I,N,T,E,R,E,R,P,A,T,T,I,O,N Data Interpretation Graph 1 – Bar Chart to Show Average Depth and Average Width of the River Channel Overall the bar chart shows that the average depth and width of the river increases with distance downstream. This supports the first two hypotheses. This is obvious because the bar at Site 3 is higher than the bar at Site 1 for both average width and average depth on this graph. The graph supports the hypothesis that the width of the river increases with distance downstream. This is visible on the graph because the average width increased from 0.84m at Site 1 to 0.98m at Site 3. This increase of 0.14m is evidence for the hypothesis. Also from this graph it is evident that the average depth of the river increases with distance downstream which supports the second hypothesis. This is apparent because the average depth increases from 0.18m at Site 1 to 0.20m at Site 3. This increase, although it is small, at 0.02 m supports the hypothesis. Graph 2 – Scatter Graph to Show Average Depth and Average Width of the River Channel This scatter graph shows that there is correlation between average width and average depth. Both average width and average depth increase with distance downstream. This is apparent on the graph because the line of best fit shows a quite strong positive correlation. This is backed up by the spearman rank correlation test that was used which shows a relationship of __% and this is a good correlation. Graph 3 – Target Graph Showing Average CrossSectional Area of the Three Sites The target graph shows clearly that the average crosssectional area increases with distance downstream. This is clearly visible from the graph because the average crosssectional area at Site 1 is 0.11m2 and the average crosssectional area increases from this to 0.19m2 at Site 3. This is an increase of 0.08m2 over the three sites and this demonstrates the eighth hypothesis. Graphs 4,5 and 6 – Crosssection Diagrams Overall these three graphs show that the channel gets deeper and wider over the three sites. At Site 1 the deepest part of the river was 0.23m and at Site 3 it is 0.25m deep which is an increase of 0.02m. Also at Site 1 the river was 3.5m wide but at Site 3 it was 3.6 m wide so the was an increase in width of 0.1m over the three Sites. This is only a slight increase but this may be because the size of the Loughton Brook River is small so I would not expect to see a large change. Also the shape of the three diagrams shows that the left side of the river is deeper than the right side. This may be due to erosion of the left side and deposition on the right side, which suggests that the flow of the river is faster on the left hand side than the right. Graph 4 is the river crosssection diagram for Site 1. It shows that the river cannel is 3.5m wide and at its deepest point it is 0.23m deep. The shape of the riverbed can be seen using this graph. It shows that the left hand side measurement of the river is deeper at 0.23m than the right hand side measurement at 0.085m. This difference of 0.145m may be due to the erosion and deposition on the riverbed. Graph 5, the diagram for Site 2, shows that the river channel is 3.4m wide which is 0.1m narrower than at Site 1 and also the river is 0.22m deep at the deepest point which is 0.01m shallower than the measurement for Site 1. The left side is deeper than the right side like it is in Site 1. The left side measurement is 0.22m deep and the right side measurement is 0.09m deep. Again the difference of 0.13m may be due to erosion and deposition. Graph6 is the crosssection at Site 2. The diagram shows that the channel is wider than the channel at the other two sites as it is 3.6m wide and is also deeper at 0.25m deep at the deepest point. This is wider than Site 1 by 0.1m and deeper by 0.02m. The left side measurement is 0.25m, which is deeper than the right side measurement that is 0.11m deep. This is a difference of 0.14m. The difference may be caused by erosion and deposition. Graph 7 – Graph Comparing Average Wetted Perimeter Over the Three Sites ================================================================= The comparative bar chart shows that the average wetted perimeter increased from 0.91m at Site 1 to 1.00m at Site 3. This increase of 0.09m is evidence for the hypothesis that the wetted perimeter increases with distance downstream. Graph 8 – Graph Comparing Average Gradient Over the Three Sites The comparative bar chart shows clearly that the average gradient decreases from 2.6o at Site 1 to 1.0o at Site 3. This decrease of 1o supports the hypothesis that the gradient will decrease with distance downstream. Graph 9 – Scatter Graph Showing Correlation Between Average Surface Velocity and Distance Downstream This graph shows a correlation between the surface velocity and the distance downstream. This means that the further downstream the river is the faster the velocity of the river. This is evident because the line of best fit shows a strong positive correlation. This is evidence for the hypothesis the velocity increases with distance downstream. There are two anomalies on the graph one for group 10 a Site 3 because the velocity was measured at 0.64m/s, which is 0.5m/s higher than the highest of the rest of the measurements for Site 3. The other anomaly is for group 7 also at Site 3. This was measured at 0.05m/s, which is lower than the measurements for the rest of group 7’s velocity results. This is anomalous because the other groups have an increase over the three sites but group 7 have a decrease. These anomalous results at Site 3 may have be caused by measurements being taken in riffles or pools.  Graph 10 – Radar Graph to Show a Comparison Between Wetted Perimeter, Hydraulic Radius and Velocity This graph shows that the Hydraulic radius decreases from  Graph 11 – Pictogram to Show Pebble Roundness and Sphericity ============================================================ The Pictogram shows clearly that the pebbles collected at Site 1 are more angular than at Site 3. At Site 1 there are mostly angular pebbles and very few rounder ones but at Site 3 there are mostly rounded pebbles but no angular pebbles. The chart shows that at Site 1 there are three very angular pebbles, two sub rounded pebbles and only one wellrounded pebble. The pebbles collected at Site 2 are rounder than at Site 1 because there are no very angular pebbles, three sub rounded and three wellrounded pebbles. At Site 3 there are no angular pebbles, three sub rounded pebbles and three wellrounded pebbles. This is evidence for the hypothesis that the bed load shape (roundness and smoothness) should increase with distance downstream. Graphs 12, 13 and 14 – Cumulative Frequency Curve of Pebble Long Axis From these cumulative frequency curves the approximate median length of the pebbles can be found. This can be used to compare the data for the pebble length more easily. Graph 12 shows the cumulative frequency curve for the long axis of the pebbles at Site 1. At Site 1 the approximate median long axis length is 2.35cm. Graph 13 is the cumulative frequency curve for Site 2 and it shows that the estimated median long axis length is 3.6cm. This is longer than the median for Site 1 by 1.25cm. The last cumulative frequency curve is for Site 3 and this from this graph the approximate median length is 2.4cm. This is longer than Site 1 by 0.5cm but shorter than the pebbles at Site 2 by 1.2cm. Overall the pebbles at Site 1 are shorter than the pebbles at Site 3 but only by 0.5cm. The pebbles At Site 2 are the longest at 3.6cm although there may be an anomaly. This may be the reason that the results that were obtained do not entirely follow the hypothesis that the bed load size will decrease with distance downstream. How to Cite this Page
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