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 Seaforth

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.

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.

Boulder size materiasl

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.

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.

Turbulent flow

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.

Figure 5 shows the bed load materials present in New Monkland.

Steep Valley sides

Boulder size bed load

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.

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

Turbulent flow

middle course of a typical river valley. Figure 7 below shows the stone and gravel

size materials present in the river channel.

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.

Gravel and stone size materials

Figure 8 shows the widen river channel in Danvers Pen.

3. The flow of the river at. At this point the flow of the river had less turbulence in its

flow where it almost became gentle.

4. The gradient of the river valley had decreased as well. The valley sides had

become more opened where it almost had a gentle appearance.

York

This area can be considered the lower course of the river due to several factors. These

factors include:

1. The size of the materials of the bed load. The size of the bed load materials had

decreased and averaged the smallest of the three locations. These sizes are the

general size of materials in the lower course of a typical river.

2. The width and depth of the river channel. The width of the river channel had

become broad and the depth had become deep.

3. The flow of the river. The flow the river at this point had become gentle and little

or no turbulent flow was present. The flow can be considered to be calm.

Widen river channel.

4. The gradient of the river sides. The valley sides had open up making the land

become flat. The valley sides of a typical river become flat in the lower course of

the river,

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

area(cm2)

Cedar valley 190 13.2 203.2

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)

Velocity (ms-1) Discharge

(cm3/s)

Bed load size

Cedar Valley 203.2

New Monkland 14,352

Danvers Pen 16,994

York 34,440

Cross Sectional Area

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.