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Title: An investigation of how discharge influences the amount and size of the bed load of the Negro River in St. Thomas Jamaica.
Candidate Name: Shomari Fagan Country:
Jamaica
Centre:
Camperdown High
Centre Number:
100015
Year of Exam:
2012
It has been shown that the discharge along with the bed load of a
river varies as it travels down stream. The purpose if this Internal Assessment (I.A.) is to determine the changes in discharge and the bed load of the Negro River from its upper course to its lower course and to account for how the discharge influences the materials transported. This will be done by calculating the velocity along with the cross sectional area and collecting samples of the bed load within the river at different points along the river’s course.
On February the 16th 2012, the courses of the Negro River in St. Thomas Jamaica were visited, from its upper course in Cedar Valley to its middle course in New Monk Land and finally its lower courses in Danvers Pen and York. The river was visited during the late morning and early afternoon. At this time the river was in its low peak flow stage. It was at these points where several measurements were done in order to collect the primary data needed to complete this I.A. Such measurements included the cross sectional area of each course along with the velocity of the course. Samples of the bed load were also collected and analyzed. The velocity was calculated by choosing a specific distance in the river channel for each courses and allowing a ball to flow from on point of the distance chosen to the next and the time for this recorded. This was done several times in order to get an average time for the balls journey. The cross sectional area was calculated by measuring the width and average depth of the river channel. The width was measured by stretching on end of the tape measure from one bank of the river to the next and the distance obtained recorded. The depth was calculated by using the meter rule and the tape measure. The tape measure was stretched horizontally across the river channel and the rule place in the water and the depth at different intervals (10cm in Cedar Valley, 10cm in New Monkland, 0.5m in Danvers Pen) on the tape measure recorded. 20 random stones were taken from the bed load at each course of the river and the width and length of each measured. This was done to determine the various changes in the size of the bed load as it goes downstream.
The Negro river is located on the south coast of Jamaica in the parish of St. Thomas. The source of the river is located within the Blue Mountains. The river has latitude of 17.98 degrees and longitude of -76.65 degrees. The stream is 75km east of the approximate centre of Jamaica and 19km east of the country’s capital Kingston. The drainage basin size of the river is approximately 160km squared and has an average height of 225 meters above sea level. The river joins the larger West Arm of the Morant River in Sea forth which later empties into the sea at Morant Bay.
Characteristics of the River Valley at different points along its course
Cedar Valley
This area can be considered as the upper course of the river valley due to several factors. Such factors include: 1. The sizes of the bed load in the river. The materials present at Cedar Valley averaged sizes of those similar to boulders. Average sizes included the long axis of the material being 7.5cm and the short axis being 5.9cm. These figures represent the average sizes of the bed load material in the upper course of a river. Figure 2 shows the boulder size materials at cedar valley with the upper course of the river.
Boulder size materiasl
Figure 2 shows the boulder like materials present at cedar valley.
2. The width and depth of the river. The width of the river was found to be very narrow at Cedar Valley and the depth was found to be shallow in this area as well. These factors are characteristics of any typical river making this area the upper course of the river.
3. The flow of the river. At this point of the river turbulent flow was present. This type of flow is mainly present in the upper course of a river channel. This type of flow can be shown in figure 3 below.
Turbulent flow
Figure 3 shows turbulent flow of the river at Cedar Valley.
4. The gradient of the valley sides. The gradient of the valley sides in the area we found to be very steep. Steep valley sides are found in the upper course of a river. Figure 4 below show s the steep valley sides of the river.
Steep Valley sides
New Monkland
Newmonk land shared similar characteristics to that of Cedar Valley, this make the area a part of the rivers upper course. Similarly the area had: 1. The area had boulder size bed load materials present as well in the river channel averaging size n the long axis of 7cm and short axis of 6cm. figure 5 shows the boulder size materials present in the river channel at New Monkland.
Boulder size bed load
Figure 5 shows the bed load materials present in New Monkland.
2. The width and the depth of the river channel in this area were similar to that of Cedar valley. The width of the river channel was narrow and the depth shallow. 3. Turbulent flow was also present in this area as it was also present in cedar valley. Figure 6 below shows turbulent flow within the river channel.
Turbulent flow
Figure 6 shows turbulent flow in the New Monkland area of the river channel.
4. The gradient of the valley sides. At this location the valley sides were found to be relatively steep. Steep valley sides are found in the upper course of a river.
Danvers Pen This area can be considered the middle course of the river due to several factors. Such factors include: 1. The sizes of the bed load materials. The bed load had decreased in size and was now averaging sizes of stones and gravel. Theses sizes are normally found in the middle course of a typical river valley. Figure 7 below shows the stone and gravel size materials present in the river channel.
Gravel and stone size materials
Figure 7 shows the materials present at Danvers Pen.
2. The width and depth of the river valley. The width of the river valley had become widen becoming almost broad. The depth had changed as well, becoming less shallow to near deep. Figure 8 below shows the widen width of the river channel in Danvers Penn.
Widen river channel.
Figure 8 shows the widen river channel in Danvers Pen.
Cross sectional area The cross sectional area of a river is the product of the average depth and width of the river channel. The table below shows the changes in the depth and width of the river channel.
Location
Average depth(cm)
Average width(cm)
Cross sectional
Cedar valley
190
13.2
area(cm2) 203.2
Newmonk land
789
18.4
14,352
Danvers pen
840
20.2
16,994
York
1200
28.7
34,440
Velocity The velocity of a river can be considered to be the flow of water over a specific distance per unit average time. The table below shows the changes in velocity of the river along its course. Location
Velocity (cms-1)
Cedar Valley
53
New Monkland
71
Danvers Pen
120
York
143
DISCHARGE
The discharge of a river is the flow of water as it passes a particular point within the river channel along its course. This can be calculated by multiplying the cross sectional area of that point in the river with its average velocity. The table below shows the change in discharge of the river as it travels downstream.
Location
Discharge (cm3/s)
Cedar Valley
107.7
New Monkland
10, 189.9
Danvers Pen
20, 392.8
York
49, 249.2
BEDLOAD
The bed load of a river is the particles or materials that are transported along the bed of the river. The river below shows the change in the sizes of bed load materials within the river channel as it travels down stream.
From the data presentation is was noticeable that several changes were evident along the course of the river. Several factors can be held accountable for these changes in the cross sectional area, velocity, discharge, and bed load materials. These factors will be discussed below. The table below gives a general summary of the changes within the river channel.
Location
Cross sectional area (cm2)
Cedar Valley
203.2
New Monkland
14,352
Danvers Pen
16,994
York
34,440
Cross Sectional Area
Velocity (ms-1)
Discharge (cm3/s)
Bed load size
Several factors can be held accountable for the change in the cross sectional area of the river channel. The main factor has been erosion from the flowing water. From the data it was seen where the velocity had increase down stream. This increase in velocity led to an increase the rate of erosion. Erosion is what causes both the width and the depth of the river valley to increase by cutting away the sides of the banks of the river along with its bed causing an increase in the size of both thus increasing the cross sectional area of the river channel. Other factors can be held accountable for this increase. Such factors include the material that are transported also causes some amount of erosion. By this the materials bounce along the bed and bank of the river channel when been transported, overtime this causes these surface areas to ware away increasing the width and the depth.
