Abstract The study was carried out o the 6th of August at the Mpenjati estuary and was done using different data collecting methods and analysis methods. The purpose of study was to determine the profile of the Mpenjati estuary and the flow of the estuary, these are shown by graphs that are found in the results section. Introduction An estuary is a partially enclosed body of water along the coast line where freshwater from rivers meet and mix with ocean.
Estuaries are transition areas from land to sea, and freshwater to salt water. Although estuaries are influenced by the tides, they are protected from the full force of ocean waves, wind, and storms by landforms such as barrier islands (water. epa. gov). Estuaries are divided into four different types depending on how they are formed.
The four types are the coastal plain estuaries that is formed by rising sea levels that fill a already existing river valley, tectonic estuaries that is formed by folding or faulting of the land, bar built estuaries are formed when a shallow lagoon or bay is protected or covered by a sand bar or barrier land and fjords that are formed in a u shaped valley formed by glacial action (Garrison, 1995, Essentials of Oceanography). The term “beach profile” refers to a cross-sectional trace of the beach perpendicular to the high-tide shoreline and extends from the backshore cliff or dune to the inner continental shelf or a location where waves and currents do not transport sediment to and from the beach”( Michael J. Chrzastowski Encyclopedia of Earth Sciences Series, 2005). The beach profile shape is constantly changing due to the affects of wave action and the storms that occur on the coast line, these waves or storms or a combination remove or deposit sands to the profile (Michael J.
Chrzastowski Encyclopedia of Earth Sciences Series, 2005), another cause to beach profile change is human activity, we remove or add sand to create a more flat beach for recreational needs. The study was done at the Mpenjati estuary on Saturday the 6th of August 2011 and was carried out at 10:00am – 10:30am, the tide at this time was changing to low tide with a wave height of around 1. 11m. The objective of the experiment done at the Mpenjati estuary was to determine the elevation of the beach profile or slope and to analyze the natural stream flow into the estuary using flow duration analysis in relation to mouth dynamics.
Materials & Methods On the field the equipment needed is two measuring poles/sticks with measurements in centimetres and a two meter length of rope. The first thing that is needed is for the time and the tide to be recorded. Thereafter mark a parallel line from the back beach to the swash zone to fellow, appoint two people from the group to hold the measuring poles/sticks steady, move them around and keep the rope up between the poles/sticks, appoint one of the remaining people has the surveyor to check if the rope is horizontal between the measuring poles/sticks and the final person is to be used has a recorded to take down the readings.
Starting from the back beach move the two measuring poles/sticks to determine the beach elevation, this is done by moving the rope in between the measuring poles/sticks with one stick (the one facing the land) having a fixed measurement (eg 160cm) and then adjust the rope so it is horizontal and the record the measurement on the seaward pole/stick, do this till you reach the swash zone.
The next thing to do on the field is to calculate the estuary flow this is done by using the measuring poles/sticks, a watch and an object that is visible and is able to float, first determine the estuary depth and length by using the measuring poles/sticks and then determine the flow by placing two measuring poles/sticks over the estuary (left on the ground) over a recorded distance and release the object at the first measuring pole/stick and record the time it takes to pass the second pole/stick and then divide the distance between the poles/sticks by the time taken to get the speed or flow.
In the lan the collected will be plotted onto excel and then used to make graphs, the data collected on beach slope elevation will be used to plot the beach profile, first thing to do is convert your data into distance and height (in meters), distance is simply the cumulative of the rope length along the beach profile while to calculate the height you add the change in y to the height (starting height 0) and the divided it 100, this is done by excel thereafter plot a scatter plot using distance on the axis and height on the y axis.
Thereafter using the data given create a flow duration curve the calculations will be done by excel and then plot a scatter plot with % exceeded on the x axis and flow rate on the y axis. Results TimeHeight (m) High Tide:07:50am1. 47 Low Tide:01:50am0. 45 High Tide:21:10pm1. 53 Low Tide:13:55pm0. 52 The table to the left is shown the time of the different tides and the height of the waves at the specific tide. The study was carried out during the highlighted tide/s. Gradient = ? y/? x = ((y2-y1))/((x2-x1)) = ((0. 32-0. 1))/((17. 3-4. )) = 11/625 = 0. 0176 The above graph was used to determine the slope of the beach profile using the co-ordinates collected. This was done so that we could determine the gradient of the slope and compare it to one that was already done by another researcher. The above graph was done to determine the flow (m/s3) of the Mpenjati estuary, the graphs show the normal flow of the Mpenjati estuary, the flow of the estuary with an increase of 0. 3m/s3 due to sewage works increase and a decrease by 0. 1m/s3 due to removal of water for irrigation.
Discussion From the slope that was determined from the data collected at the Mpenjati estuary we determined that the gradient of the slope was 11/625 or 0. 0176 and from the slope of the beach profile from Bascom 1959 we see that there is a much greater gradient or steeper gradient than that of the Mpenjati beach profile, this gentler profile at the Mpenjati estuary is due to either having wave or storm activity that is mainly destructive meaning the waves action will mostly remove sand from the beach decreasing the gradient of the slope.
From the mouth state data in the appendix we that the mouth was open for 63% of the time period between 02/1993 – 06/2000, the mouth was open for 62% of the time between 07/2000 – 03/2005 and the mouth was open for 62% of the time between 04/2005 – 09/2006, from this we can say that the estuary is a open mouth system because the mouth is open for most of the time. A cumulative frequency curve that shows the percentage of time that specified discharges are equalled or exceeded, in the case of the Mpenjati estuary it will be the percentage exceeded.
We see that the mouth of the Mpenjati estuary will open at 0. 6% exceeded at a rate of 0. 5m/s3. If sewage works increased by 0. 3m/s3, it will affect the duration that the mouth is open by increasing it by 0. 3m/s3 meaning it will open at a earlier stage and it will remain open for a longer time. If water is removed at 0. 1m/s3 it will decrease the duration the mouth is open and will affect the time that it remains open. If either the mouth receives more or less water it affect the estuary mouth and change the components of the land and water in and around the estuary.
Conclusion The result of the slope of the Mpenjati estuary compare to that of Bascom show that there is a much gentle gradient in the Mpenjati profile and we see that a increase or decrease in flow will affect the mouth state and will either benefit or damage the estuary in many ways. References Garrison, 1995, Essentials of Oceanography: Wadsworth Publishing Company. Michael J. Chrzastowski Encyclopedia of Earth Sciences Series, 2005, Encyclopedia of Coastal Science, 2, Pages 145-147 Appendix
Table 1: data collected at the Mpenjati Estuary for beach profile analysis Beach Slope Calculations xy? x? yDistanceHeight 0 2000 2 2420. 04 4 2240. 06 6 2360. 09 8 2380. 12 10 24100. 16 12 24120. 2 14 26140. 26 16 24160. 3 18 24180. 34 20 24200. 38 22 24220. 42 26 2-3260. 39 28 2-4280. 35 30 2-6300. 29 32 2-10320. 19 34 2-14340. 05 36 2-636-0. 01 38 2-938-0. 1 40 2-1240-0. 22 42 2-1142-0. 33 44 2-1444-0. 47 46 2-1246-0. 59 48 2-1148-0. 7 50 2-1050-0. 8 52 2-1052-0. 9 54 2-1054-1 56 2-1056-1. 1 58 2-858-1. 18 60 2 60 Table 2: flow rate(m/s3) of the Mpenjati mouth
PercentileT40FMPENJATI0. 3-0. 1 % exceededFlow/MARFlow (m^3/s)TQ (days)Flow (m^3/s)Flow (m^3/s) 100. 00%0. 00%13. 9009. 5910. 369. 8919. 491 99. 90%0. 10%12. 5358. 6490. 408. 9498. 549 99. 00%1. 00%7. 2444. 9990. 695. 2994. 899 95. 00%5. 00%3. 9532. 7281. 273. 0282. 628 90. 00%10. 00%2. 7141. 8731. 852. 1731. 773 85. 00%15. 00%2. 0991. 4482. 401. 7481. 348 80. 00%20. 00%1. 5361. 0603. 281. 3600. 960 70. 00%30. 00%0. 8730. 6025. 770. 9020. 502 60. 00%40. 00%0. 4480. 30911. 240. 6090. 209 50. 00%50. 00%0. 3470. 23914. 520. 5390. 139 40. 00%60. 00%0. 860. 19717. 590. 4970. 097 30. 00%70. 00%0. 2340. 16221. 480. 4620. 062 20. 00%80. 00%0. 1760. 12228. 530. 4220. 022 10. 00%90. 00%0. 1240. 08540. 710. 385-0. 015 5. 00%95. 00%0. 1000. 06950. 320. 369-0. 031 1. 00%99. 00%0. 0690. 04872. 460. 348-0. 052 0. 10%99. 90%0. 0560. 03890. 500. 338-0. 062 0. 01%99. 99%0. 0490. 034102. 050. 334-0. 066 ASHLEN CHETTY 210518625 BIOLOGY 231: PHYSICAL COMPONENT TITLE: An integrated study of the Mpenjati estuary beach system dealing with the physical components of the beach profile and mouth of the estuary