This study was conducted at the Mpenjati Estaury which is located in Port Shepstone and lies along the south coast of Kwa-Zulu Natal. An estuary is a partially enclosed coastal body of water which is either permanently or periodically open to the sea and within which there is measurable variation salinity due to the mixture of fresh water derived from land, however the fresh water inflow may not be perennial, the connection to the sea may be closed for part of the year and tidal influence may be negligible. The beach slope and the flow rate relatively depend on several factors but mainly the wave action and the sediment transportation, as these two factors controls the activities at the mouth of an estuary The focus of the research was to measure the topology of the beach using the Emery Board method and examine the relationship between the beach profile and sediment. Furthermore, the research was to analyze the statistical structure of the natural stream flow into the estuary using flow-duration analysis in relation to monthly dynamics. The changes in elevation were observed, recorded and analyzed. While simultaneously recording the sediment size found at each elevation point, which the results and discussion form part of this report. Introduction
Estuaries are dynamic systems where both hydrodynamics and geomorphology changes throughout the year. They are partially enclosed coastal body of water which is either permanently or periodically open to the sea and within which there is measurable variation of salinity due to the mixture of seawater with freshwater derived from land drainage (Day, 1981). Estuaries are productive and very interesting natural environments, in which fresh water from a river mixes with the salt water of the ocean and quantities of salt, heat, and sediment on a daily basis are constantly changing, (Brian R. Allanson, 1999). The Mpenjati estuary is dominated by tidal flow of sea water, rather than river flow of freshwater (Perissinotto et al., 2002).
Beach processes such as wave action, sediment transport processes, long shore currents, tides, wind, changing sea level, sand supply and fluvial flow are very important in shaping coastlines and estuaries. In particular, the influence on estuary mouth dynamics caused by wave action, observations reveal that waves play a significant role in sediment distribution. They achieve this by stripping substrates, eroding shores and suspending sediment for dispersal (Cooper, 2001). The waves that occur in the estuary mouths are brought about the ocean and others are internally propagated. Material that is eroded is rearranged offshore or transported into the estuary. Tidal or wave dominance in an estuary is determined by sediment morphology or facies (Cooper, 2001). Sediment sources in estuaries are the river watershed, continental shelf on the mouth of the estuary, biological activity and erosion within the estuary. Sediments are provided in estuaries by river in flow and landward transport of sediments by tidal currents (Cooper, 2001). Sediments can also be transported by waves, tides, circulation mixing or by the force of the river flooding (Cooper, 2001). In TOCE sediments may accumulate for some time before they move out again, driven by storm or flood conditions (Cooper, 2001).
These estuaries are separated from the sea by a sand bar and this sandbars are generally breached during the rainy season allowing the interaction of saline water with freshwater(Froneman,2003). In calm conditions, waves generally build beaches that have mobile sediment (sand, gravel and cobble) by transporting sediment onshore (Walter Hans Graf, 1984). During storms, strong currents under the breakers (“undertows”) draw water and sediment back from the beach; this causes beach erosion. Therefore, waves can substantially change the shape of beaches according to the seasons (Walter Hans Graf, 1984). ). Extreme storms can have greater effects, for example by completely destroying sand bars or other sediment deposits. All this implies that when there are strong waves estuary mouths open wider and when there are no waves the estuary mouths remain closed. River flow or fresh water in flow has a significant influence on estuaries (Levin and Boesch et al., 2001).
Especially on the environmental conditions of the estuaries, such as salinity levels and estuary health, biological, physical, geological and chemical aspects of TOCE (Levin and Boesch et al., 2001). When river flow mixes with sea water along the mouth dynamics of estuaries this contributes to diverse habitats and biological abundance (Levin and Boesch et al., 2001). Fresh water inflows also have an influence on the mouth of estuaries together with the tidal flushing (Levin and Boesch et al., 2001). In some cases when there is low river flow, the entrances or mouth of estuaries may close off completely (Levin and Boesch et al., 2001). So river flow plays an important role in the functioning of estuaries. Material and Methods
The time in which this component too place was during low tide (midday) and the mouth of the Mpenjati estuary was taken as the starting reference point for measuring the slope. Group 2A was subdivided into three main groups, each of which observed, measured or scribed an element of the beach slope. In the subgroups there were four subdivisions; the seaward surveyor, landward surveyor, the geotechnical engineer and the data recorder. The seaward surveyor held the seaward board and ensured that the rope of 2m is level between the two boards when fully extended and taut, while the landward surveyor held the landward board, sights over the seaward board to the horizon to shout out the measurement to the “data recorder”.
Then finally, the geotechnical engineer moved with a seaward surveyor to collect sand samples which was identified using the hand lens to determine the grain size comparison to the sand gage chart. The data recorder keep organized notes of each measurements including horizontal distance (x), measurement of change in elevation (y), cumulative change in elevation of all measurements, and sand size at each location. Starting at the baseline, cross-shore data points of elevation were recorded. From this data collection, gradients were calculated using the gradient equation. The data collected was change in height of the poles as we moved down the beach profile as for y-axis co-ordinates and x-axis values determined the change in distance position which was 2m throughout. Estimations on the mean grain size of sediment was taken from the samples observed at each at each sampling interval using a Grain Size Chart. The x and y co-ordinates were used to plot the beach profile graph which showed the steepness of the slope as you move seawards. We calculated the gradient by finding the change in y over the change in x (Δy/Δx). All the data and data calculations were imported, calculated, graphed and analyzed through Microsoft excel. Results
Figure 1: Beach slope of the Mpenjati Estuary showing the relationship between the accumulative height(m) and distance(D).
Figure 1 shows the change of the slope from the berm of the beach towards the sea at Mpenjati. The graph also shows that as the distance increases, the height decreases. As you move from low tide mark to high tide mark the slope is decreasing (downslope). Figure 2. The observations made there indicate that the more energetic and turbid an environment, the larger the median grain size found in that environment. Figure 3. Indicates that as the flow (m^3/s) rate increases, the percentage exceeded will increase. The Mpenjati flow duration curve shows the prediction of what will happen if you decrease or increase the inflows of an estuary, in this case by sewage and irrigation systems. (Refer to appendix) Discussion and Conclusion
From the results obtained in the study, it shows that 60% of the estuary mouth is usually open at a rate of 0.2m^3/s. Flow duration curve is defined as the graphical representation of a ranking of all flows in a given period from lowest to highest as a percentage of time a certain flow value is equaled or exceeded. The curves may be derived for flows in any time interval such as daily flow, monthly flow or annual flows (Jordan, 2010). If the curve represents a continuous flow in the river system then it can be used to predict the occurrence of the next flow. Calculations has shown that if you increase the sewage works inflow by 0.3m3/s the mouth of an estuary will remain open throughout the inflow because the new flow rate will be 0.5m^3/s. Furthermore decreasing the abstraction for irrigation by the flow of 0.1m^3/s will cause the decrease in the water entering the estuary (inflow) leading to the estuary entirely closing, the closed estuarine system are primarily characterized by the low level of salinity (Perissinotto et al., 2010). The flow duration curve shows that the flow rate decreases with the decrease in the exceeding of the estuary by the three inflows. However the sewage work has the higher flow rates when compared to the natural flow which is the second highest and lastly the abstraction for irrigation curve.
This reveals that the sewage works play a major role in the life of an estuary in terms of opening and closing of the mouth system. As the estuary experiences the river inflow, wave action and floods due to high tides, particles are transported into and out of the estuary at different levels, and the time it takes for one particle to travel through the estuary out the mouth is called residence time [TQ=volume/flow]. Residence time is important for when a toxic substance enters the estuary, residence time will be calculated to predict how long it will take for that toxin to exit the estuary. When the flow rate is 0.31m^3/s the residence time will be 967741.96 seconds which is equal to 11 days. And finally the swash zone and the back beach have a steady slope while the berm has a steep slope. Median grain size of sediment has shown that the back beach and swash zone carry a small size of sediments as compared to the berm. This is due to the fact that in the back beach there is a mixture of mud and sand due to the river inflow and in the swash zone the course sediments are washed into the berm by the strong tides. Beaches with coarser grains tend to be steeper than those with finer grains. (Bascom, 1959)
Walter Hans Graf W.H., 1984. Hydraulics of sediment transport Water Resources Publication. 513 pages. Allonson B.R., 1999. Estuaries of South Africa. Cambridge University Press. 340 pages Froneman HS(2002a,2002b).Marine Pollution. Berlin: Springer. ISBN 0387109404. Schwartz M.L. 2005. Encyclopedia of coastal Sciences. Published by springer 16 June 2005. Bascom, WN (1959). The relationship between sand size and beach-face slope, Am. Geophy. Union Trans. 32 (6): 866-874 Perissinotto, R., Stretch, D.D., Whitefield, A.K., Adams, J.B., Forbes, A.T., Demetriades, N.T., 2010.Ecosystem functioning of temporally open or closed estuarine system.Nova science. New York Cooper J.A. G., 2001. Geomorphological variability among microtidal estuaries from wave dominated South African coast. Vol 40, 1-2. Kibirige I and Perissinotto R. (2003). The zooplanktonic community of the Mpenjatiestuary, a South African temporarily open/closed system. Vol 58, 4. Levin A.L, Boesch D.F and Covich B.A. (2001) The function of marine critical transition zone and the importance of sediment biodiversity. Vol 4, 5.
Figure 2: A graph representing the beach face slope vs median grain size
.Figure 3: A Mpenjati flow duration curve also showing the changes of sewage work and obstructions of irrigation Tide Chart| | | |
Saturday 2013-08-03 | | |
Sunrise 06:41 Sunset 17:24| | |
Moonrise 03:51 Moonset 14:36| | |
| Time| Height (m)| | |
High Tide:| 05H00| 2.00 | | |
Low Tide:| 11H00| 0.17 | | |
High Tide:| 17H25| 2.02| | |
Low Tide:| 23H20| 0.22 | | |