Aim of Study
The aim of this study is to measure downstream changes in the hydraulic geometry and assess its influence on the changes in fluvial hydraulics in the lower reaches of the Morant River in St. Thomas, and further to show these influence depositional landforms.
Null Hypothesis
The changes in Hydraulic Geometry do not influence changes in fluvial hydraulics in the changes downstream of the Morant River in St. Thomas, and further, these have no effect on the development of the depositional landforms.
Location of Study Area
The study area is located in South-eastern Jamaica, as illustrated in Figure 1, along the Morant River in St. Thomas.
Methodology
The primary data for this study was collected on a field exercise to the Morant River in St. Thomas, Jamaica, on January 19, 2012. Four sites were selected to be studied in the lower region of the river long profile starting from the mouth at the delta, working 0.028km upstream towards the water source, Hillside.
A base map was carried into the field exercise which was used to identify each locality when each stop was made. It was also used to find the altitude of each locality, the distance between each locality, and to show the general area as well as the river itself, which was being studied.
At each locality, the depth and width of the channel were measured simultaneously using a meter rule and a measuring tape, respectively. The tape measure was stretched taut across the width of the channel from one edge to the next, and the meter rule was used to measure the depth of the channel at 1-meter intervals. This data was used to plot and calculate the cross-profile on a graph sheet using a scale of 1cm-1 unit for width and 1cm-0.5 units and then determine the wetted perimeter
The velocity at a point of the channel was also measured at each locality. This was done by stretching a tape measure for a specific distance and placing afloat to travel that distance. This distance was 15m for localities 2-4 and 11m was used for locality 1. This is because the length of the channel of flow was unable to be recorded because the channel was impounded by a channel mouth bar as it created a deep pool in the river channel. The time the float took to complete the journey was measured using a digital stopwatch.
The slope at the surface of the channel was measured using a clinometer which was placed tangential to the surface of the long profile. This clinometer was supported by a meter rule just below the surface of the river to obtain an accurate measurement.
Fluvial depositional landforms encountered at each site were studied, measured, and drawn. At locality 1 a sample of 30 sediments was selected and localities 2 and 3 a sample of 27 sediments all at random, so as to show that there is a greater number of sediment in this area as this is where everything is deposited, in comparison to other areas. These sediments were taken from the flood plain by taking two steps backward and taking up the sediment using the left hand, without looking down.
Sediment samples were collected at the first 3 localities, except at locality 4. After the sample sediments were selected the rock types and shapes were determined with the help of a geologist and then measured the rock’s dimensions using a centimetre ruler.
Using the cross-sectional area, which was obtained from the cross-profiles plotted on a graph, and the velocity at each site of study, which was calculated using the length and time of the section of the channel flow, the discharge was calculated at each locality. The hydraulic radius was also calculated using the cross-sectional area and the wetted perimeter, both of which were calculated by using the length, width, and depth of the channel, respectively.
Following the field exercise, books and maps related to the topic were consulted. The Spearman Rank correlation equation was used to show the relationship between the Fluvial Hydraulics and the Hydraulic Geometry variables were also used in calculations to help analyze the data collected. Further information needed was garnered from the subject teachers.
Presentation of Data
The Gradient of Morant River measured based on the water flow, spans from 2° for localities 1-3 and 1° for locality 4.
Channel width for each locality is seen to have increased steadily from 6.8m for locality 1 to a high of 22.5m for locality 4. Locality 3 however has narrowed to 9.8m from 15.3m, locality 2, giving a difference of 5.5m. In response to the channel width, there was a steady decrease seen in the average channel depth moving from 0.45m at locality 1, to 0.27m at locality 4. However, there was a slight increase of 0.6m at locality 3. Based on calculations, the cross-sectional area of each locality was found to steadily increase, with locality 3 changing with a difference of 1.11m².
The cross-sectional area of each locality increased continuously, with a drop at locality 3.Decreasing from 1.72ms-¹ to 0.33ms-¹ at localities 1-4 the velocity of each locality was calculated, with locality 3 increasing slightly. For the discharge of the Morant River, it decreases from 5.44m³s-¹ to 2.13 m³s-¹with a slight increase of 0.27m³-¹ between locality 1 and 2. The hydraulic radius of the areas studied was constant for the most part of 0.3m, with a high of 0.45m for locality 1.
According to data collected, sub-angular rock shapes were mostly found at Locality 1 which was 14 sediments. There was an even distribution of rounded and sub-rounded rocks of 6 sediments. At Locality 2, sub-angular rock shape was also seen in this area as the majority of 11 sediments. This was followed by angular-shaped rock with 2 sediments. There were no well-rounded sediments found at Locality 2. For Locality 3, the sub-rounded rock shape is the most dominant rock shape in this area which is equal to 11 sediments. Angular rock shape which is equal to 4 sediments was found the least.
At each locality, the rock type is different as each locality has a different rock type that is dominant. Locality 1 is an area where metamorphic rocks were found, locality 2 is an area where sedimentary rocks were mostly found and at locality 3, igneous rocks were found in abundance.
The Cross-Sectional Area of all four Localities which were studied is illustrated in figure 4. Locality1 is located at the river delta which exists out along the coast of Portland, Duhaney Pen, with a maximum depth of 0.75m as it is very steep in nature. The graph also illustrates the channel shape of Locality 2, which has a shallower river depth in comparison to Locality one and it is wide in width, with a length of 15.3m. Locality 3 is similar to Locality 1, in the sense that both localities are similar in-depth, but locality 3 is wider in channel width with a difference of 3 meters. The last visited area along the river channel, Locality 4, is the shallowest section of the river channel studied. It has a maximum depth of 0.44m and is the largest section of the river channel studied with a width of 22.5m.
In the graph below the average velocity for each locality is displayed which decreases steadily from the mouth of the river upstream. The first area which was studied, Locality1, has the highest average velocity of 1.51ms-1. Locality 2 was studied secondly, and the velocity of the channel was calculated which was 1.09ms-1. The velocity for Locality 3 was also calculated which resulted to be 0.74ms-1. In the last locality located near Hillside, the velocity for the section of the river channel was calculated to be 0.33ms-1.
Fluvial Landforms
Floodplain
During times of heavy rain, resulting in flooding this area of 0.4cm which when calculated is 2.14km wide at Locality 1, grid square 4975. This width continues towards the source for quite some distance, until it narrows in the vicinity of Locality 3, according to figure 2. As it narrows beyond locality 3, this width becomes 0.1cm on the base map.
Central Bar
This depositional feature, the central bar, which is seen in grid square, 4779, measures 2.1cm long on the base map. This central bar has resulted in the slight braiding of the river course and eventually causing the development of a thin floodplain of 0.1cm the smallest width, to 0.2cm at its largest on the base map( ). Around this central bar are smaller bars of 0.3cm long and 0.2cm, respectively.
Terraces
At locality 2 as well as at locality 3, terraces have been formed. These terraces are seen formed at the edges of the floodplains along the Morant River, creating the shape and size of the river channel. According to figure 2, the channel shape of the Morant River. At both localities, there is a set of paired terraces that are at the same elevation, as seen in plate 3, for locality 2.
Analysis of Data
With the use of the Spearman Rank Correlation a number of relationships have been determined based on the grouping of two variables, a dependent and an independent. The formula for this calculation is which was used.
The competence of a river is dependent on a positive correlation between depth and velocity. When velocity is increased and depth is increased then this allows for larger particles to be transported by the river, which determines the river capacity. This positive correlation is proven by calculations using the data collected which resulted in a +0.8 correlation.
During the field exercise there was evidence of a high positive correlation as further upstream the sediments were seen to be quite large. This correlation also shows that the river has the capacity, at certain times of the year, to transport large amounts of sediments as it resulted in depositional features such as the central bar in figure 7, and also a channel mouth bar located in the area of the delta.
When the correlation is negative, it suggests that the fluvial hydraulic is dependent on the hydraulic geometry. Therefore the negative correlation, -0.8, between width and velocity occurs as a result of the width increasing and velocity decreasing.
This is because the volume of water in the river which was compact in a smaller width channel is now given space due to the widening of the channel and therefore it slows the velocity because it takes a longer time to cover this new area of land with the same volume of water.
Discharge and depth have a +1.4 correlation based on calculations using the Spearman Rank correlation formula. This positive correlation states that as depth increases, then the discharge of the river increases as well.
This positive correlation is due to 6 second-order streams joining Morant River and a third-order stream, Negro River, joining as well along with its own tributaries contributing to the discharge of that river, to a further extent, the discharge of the Morant River, Figure 2. The width of the river has also increased slightly at localities 2 and 3 where the river is seen to have joined accommodating the additional discharge.
The rock type is a factor which is dependent on the shape of the sediment as some rock type erodes at a faster rate than other types. This rate of erosion is dependent on the general composition of the rock. Sedimentary rock is the easiest eroded rock, due to its simple deposition of sediment particles with a little cementation of these particles.
This may have resulted in some of the rocks being easily rounded or sub-rounded. The angular rocks seem to have been metamorphic and igneous rocks as these rock types have been under immense pressure and or heat, causing their composition to be tightly packed and harder to erode than sedimentary rocks.
The construction of the floodplain of the Morant River is a result of sediments from the direction of the source being deposited downstream. Looking closely at figure 2, it can be seen that the width of the floodplain increases as you travel from the source towards the delta. This increase in width is a result of sediments being carried from upstream during times of flood, downstream to where everything is regurgitated.
This reason for this deposition and formation of a floodplain in the lower course is due to the fact that these areas are of the low elevation of less than 200m, the end of the green area of figure 2, which allows for the river to spread out-as it goes as low as 40m-in times of flood, and also because in the middle or upper course of the river, there are only narrow river valleys where no space is available to deposit the bed load which is picked up.
Conclusion
In conclusion, it has been identified that the Fluvial Hydraulic is dependent on the Hydraulic Geometry as when discharge or velocity increase the cross-sectional area changes from one of shallow profile to one of deep profile. The Fluvial Hydraulic also determine formation or erosion of Fluvial Landforms, whether it is the formation of a channel mouth bar or not. Both variables are therefore responsible for the features form.
Recommendations
- The lower course of the Morant River should be all together studied over a period of 3 months than as separate localities as done in this study
- This type of study should be preceded as a reconnaissance of the study, in case a follow-up study is to be done on the river, it can be done
- That further studies upstream should be conducted to see if data which was collected and calculated is similar in some cases, as only a part of the river was studied.
