Beach Profile of Paea Lagoon Fish Pond: Tahiti, French Polynesia, Summer 2024
Authors: Abby Nestor, Kendall Bangert, Lily Silverstein
Introduction
The objective of this monitoring project was to establish a baseline beach profile and a comparison of the varying current speeds in and around the perimeter of the Paea Lagoon fish pond. The expected outcome was a higher beach profile on the inside of the south wall due to sediment accumulation resulting from the current that flows south to north. The bathymetry of the beach from the back wall of the property to the back wall of the fish pond was mapped using cross-shore transects along the inside and outside of the pond walls and in the center of the pond. The current speeds were measured in these same areas to evaluate the possible influence of the current and the sediment accumulation it causes on the bathymetry of the beach.
Methods
To create a profile of the beach's changing elevation, 5 cross-shore transects were used: 1 transect 1 meter from the outside of the south wall, 1 transect 1 meter from the inside of the south wall, 1 transect 1 meter from the outside of the north wall, 1 transect 1 meter from the inside of the north wall, and 1 transect down the center of the pond. Along the transects, any change in the slope of the beach was measured using the Emery Method, which uses two rods, each with a marker noting the eye level of the observer, a tape measure to measure the distance between the two rods, and a protractor apparatus to measure the change in angle of the slope. To begin, the 0 m point was defined as the apex of the angle at the base of the back wall under the fourth diving board from the stairs. For each transect, the distance between the 0 m point and the starting point was measured and recorded using the two marked rods and a tape measure. One rod was placed at the 0 m point and one rod was placed at the starting point of the transect, which was at the back wall of the house. The distance between the two walls was measured using a tape measure. The change in angle between the two points was also measured and recorded using an apparatus made of a protractor with a straw taped parallel to the flat side and a string tied through the hole of the protractor with a weight attached to the end. The string fell along the 90 degree mark when the protractor was held flat side up. The protractor was held flat side up at the marker point on the rod at the starting point and the observer looked through the straw, shifting the end of the straw on the protractor to point up or down until the marker on the rod at the 0 m point was in view. A second observer recorded the angle at which the string fell on the protractor in relation to the 90 degree line. The difference between this measured angle and 90 degrees is the change in angle between the 0 m point and the starting point. The rod at the starting point was kept in place, while the team member holding the rod at the 0 m point moved the rod to the point directly before the first change in the slope of the beach that occurs directly in front of the rod at the starting point. The process used to measure the distance between the 0 m point and the starting point was repeated between the starting point and the point directly before the first change in slope. The same technique used to measure the change in the slope of the beach between the 0 m point and the starting point was also used to measure the slope of the beach between the starting point and the point before the first change in slope. The rod at the starting point was then moved to the point directly before the next change in slope, and the techniques for measuring the distance between the rods and the slope of the beach between the two rods was used again. This process is repeated for each change in slope until the back wall of the fish pond is reached.
The function tan (θ) = H/D was used to solve for the change in height (H) between each change in slope, using the angle of the slope of the beach measured using the protractor (θ) and the distance between the two rods (D). The cumulative height was calculated for each of the 5 transects and graphed with the cumulative distance from the starting point.
To measure the current speed, 10 points were first chosen in and around the fish pond where a tape measure, buoy, and stopwatch were used to record how many seconds it took the buoy to travel a given distance. One point was 5.5 m from the shoreline, 1 m outside of the north wall, one point was 10.5 m from the shoreline, 1 m outside of the north wall, one point was 5.5 from the shoreline, 1 m inside of the north wall, and one point was 10.5 m from the shoreline, 1 m inside of the north wall. On the south wall, 4 points with these same distances from the shoreline and the wall as used for the north Wall were chosen as well. The last 2 points were 5.5 m from the shoreline, directly down the middle of the pond, and 10.5 m from the shoreline directly down the middle of the pond. At each point, two team members held each end of a tape measure and one of these two team members dropped a buoy at the end of the tape measurer at which they stood. A 1 m buffer was used to allow the buoy to reach a constant speed, and once the buoy traveled 1 m, a third team member started a stop watch and stopped the stop watch once the buoy traveled a given distance. For the 4 points outside of the pond, a distance of 5 m was used and for the 6 points inside of the pond, a distance of 3 m was used. This was repeated 5 times at each of the 10 points, and the average time it took to travel the given distance was calculated. The amount of meters traveled was divided by the average time to calculate the average current speed in m/s at each chosen point.
Results
Figure 1. Beach profiles of 1 meter to the right or left of the respective wall and the center of the fish pond.
https://docs.google.com/document/d/18pQvFWGDjtg89jIlTOwIRssvCENwRNFxEPDVx1oRX_w/edit?usp=sharing
The outside of the south wall had the lowest beach profile of all 5 sites. The shoreline was reached at around 9.27 meters with a total distance of 34.6 meters. As seen in Figure 1, this profile lies significantly below the other sites sampled and reaches its steepest slope at 22.31 meters. It briefly increases in height before reaching the back wall of the pond. This profile had an overall slope of -0.115. This was the steepest slope of all sites.
The profile of the inside of the south wall was a total of 37.89 meters in length. The bathymetry showed a gradual decline in height with a significant increase in slope steepness at 32.46 meters. The profile has an overall slope of -0.0908.
The middle of the pond had a fairly high profile that was 35.5 m in length. The slope fluctuated between increasing and decreasing steepness until 17.73 m where it significantly increased in steepness. The profile had an overall slope of -0.0903, which is similar to that of the inside of the south wall.
The inside of the north wall had the highest profile overall and was 38.67 m in length. The slope decreased before flattening out and increasing slightly at 3.65 m. The slope then decreased gradually before a slight rise in elevation approximately 5.89 meters from the wall where there is a patch of rocky substrate. The overall slope was -0.0911, similar to that of the south wall and middle of the pond.
The profile of the outside of the north wall had a length of 35.37 m. The slope of the beach decreased gradually before flattening out at ~21 m and then fluctuated in steepness for the rest of the profile, with a final increase 2.11 away from the back wall where a rocky portion lies.
The overall slope was -0.0996.
Figure 2. The current speed is measured along the outside and inside of the walls and middle of the fish pond within two distances from the shoreline.
https://docs.google.com/document/d/1A05Mk31mAPxufpbUG6dC-EWkxcY2xQxDEcnoFefgBZs/edit?usp=sharing
Figure 3. The standard deviation of each measured current speed with a gradient depicting highest to lowest deviation value.
https://docs.google.com/document/d/1AlyQYfcdaSQjspsW31pykuVYXOHchfmQ0UlB-1LQC0g/edit?usp=sharing
As clear in Figure 2, current speed was relatively consistent with a range of 0.118-0.267 m/s. The slowest speed occurred 5.5 m from the shoreline on the inside of the south wall. The fastest speed occurred 10.5 m from the shoreline on the inside of the south wall. Figure 3 displays the standard deviations for each current speed. The lowest standard deviation was 0.025 and occurred 5.5 m from the shoreline on the outside of the south wall. The highest standard deviation was 0.125 and occurred 5.5 m from the shoreline on the inside of the south wall.
Discussion
Due to constantly changing currents, a correlation cannot be determined from this limited data set. The beach profiles and current speeds serve as a baseline to build upon in future surveys. The overall slopes of the profiles were relatively consistent between the five sites. The outside of the south wall had the steepest overall slope of -0.115 indicating a lack of sediment accumulation. This outcome was expected due to the current flowing south to north which pushes sediment over the wall to the inside, displacing sediment from the outside. The current speed complimenting this data shows small scale changes due to a heterogenous wall varying in height. The slopes of the profiles of the other sites were all relatively consistent.
The fastest current speed was 10.5 meters away from shoreline on the inside of the south wall and was 0.45 m/s faster than the outside. This was unexpected because the wall was thought to slow the current speed and therefore the speed should be higher on the outside of the south wall. These results could be explained by the changing wind speed which caused a large standard deviation of 0.125 occuring 5.5 meters away from the shoreline on that same side of the wall. Also, the tide was higher than usual so the rocks were deeper in the water which may have interfered with the usual current dynamics. As the current continues to flow northward, it would typically pick up speed due to scouring which makes the current stronger as it approaches the inside of the north wall. It loses speed when it passes through the wall resulting in a higher speed on the inside of the north wall compared to the outside of the north wall. This is shown in our results with 0.055 m/s decrease in speed.
During data collection, wind speeds varied between 2 to 4 on the Beaufort scale. This wind may have led to potential errors as in order to obtain an accurate angle measurement, the string needed to hang straight down and it was often blown by the wind. Furthermore, these wind variations caused fluctuations in speed at which the buoy traveled throughout each trial, leading to large deviations between measured current speeds at given location. Lastly, current speeds were measured at different distances from the inside and outside of the fish pond to reduce overlapping currents.
Proposal for Future Studies
In future studies, the accuracy would be improved with the survey being performed over several days, in differing conditions, and over different seasons to account for temporal changes in this fish pond’s bathymetry. Although potential errors have been accounted for in our data, it can be assumed that improvements can be made in order to effectively compare changes in these beach profiles. Substrate type in different areas of the pond may also be compared to its current speed.
Metadata
Beach Profile Data:
31 July, 2024
1 PM- 6 PM
Beaufort 4
Cloud Cover 90%
Current Speed Data:
1 August, 2024
11 AM-2 PM
Beaufort 2-4
Cloud Cover 65%
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