Beach Profile- Fish Pond Monitoring FP Spring 2024
By Elyse Hartmann, Madeleine Yang, and Marina Thompson
Survey dates: 5/19/24 - 5/21/24
Location: Paea, Tahiti, French Polynesia
Introduction
In this study we started a beach profile to monitor how the beach elevation changes over time. The objective given was to measure the degree of which the terrain slopes by looking at transects going through the Paeanfish pond and outside of it, by our client Thomas. Our monitoring reveals information on the relationship between current and sand deposition, as well as documenting erosion.
Methods
We started by measuring the dimensions of the fish pond. Each wall of the fish pond and the opening on the beach were measured to the outermost part.
In order to get a representative profile of the beach’s elevation, we split the area into four, parallel, 32 meter transects with 5 measurements taken within. Two transects went through the fish pond and two were on either side of the fish pond walls.
To gather the measurements, we used the south most noni tree in front of the pond as a fixed point to standardize the starting point for measurements. In the future if this tree is no longer there, start measurements 0.34m to the left of the southern mist metal pole in front of Thomas’ property. To calculate the slope of the beach, tape lines 120cm high on two sticks were used in conjunction with a protractor: we recorded the angle from the higher stick to the lower stick by looking at the other tape mark through a straw on a protractor. The protractor had a string with a weight attached to it, so one team member could read the angle indicated by the string, while one was looking through the straw. This was repeated to get the angle and hypotenuse 20 times around the fish pond in a systematic manner. Then the height from the original (horizontal point near noni tree) was calculated using cosine.
In order to measure the speed of the current we strategically choose five spots around and within the fish pond, making sure to intercept all the previous four transect lines. We took measurements on each side of the fish pond’s interior (north and south), two outside the fish pond’s walls (north and south), and one west of the fish pond outside the walls. At the spot, one team member held each side of the tape measure and the third team member recorded the time of when the buoy went past the designated marks. The buoy was dropped at 0m and floated downstream to pick up speed, once it reached 3m a stopwatch was started. When the buoy reached 7m (a distance of 4m recorded) the stopwatch was stopped and the time was recorded. The data was then averaged to find the speed in meters per second (m/s).
Safety precautions such as wearing shoes were taken when walking in the lagoon to avoid complications with venomous benthic animals.
Results
NOTE: Our depth measurements are not correct. We have determined that our measurements are proportional to each other and the beach slopes and current speeds are correct. (Therefore, figures 2-3 and 5 are all correct). However, the depth measurements do not reflect the real depths from point zero, so figure 1 is visually correct, but numerically incorrect. This is due to an unknown error in methods or calculation. We suggest future groups look further into this.
The measurements of the fish pond walls were 14.5m in the South wall, 17.1m for the East opening, 15.7m for the North wall, and 16.8m for the West wall.
Figure 1 shows cross sections of the beach along four different lines. It shows that the southmost line (site 1) has the lowest depth at the start and highest depth before the waterline. Whereas, the part of this beach cross section that is below the waterline has the second largest depth from point zero. If you look at site 2, the opposite is happening, the depth is the 3rd largest before the waterline and the shallowest after the waterline. Site 3 on the northern side inside the fishpond has the greatest depth throughout the whole transect line with the deepest part at 5.32 meters below the wall of the property.
These results correspond with calculations of the slopes showing that site 3 has the greatest negative slope overall of -0.22 and Site 2 has the lowest negative slope of -0.13. All of the slopes within each transect line had standard deviations between 0.08 and 0.15 showing a large amount of variation in slope within each site line. Though, transect 3 had the most slope variation.
In addition, the ocean currents at each location along the transect line sites differed. The current speed was highest deeper into the water west of westward fish pond wall where the current speed was 0.36 m/s. This is to be expected since the measurement was taken much deeper into the water. Location 1 was the next greatest along transect one (south most outside the fish pond) with a speed of 0.17 m/s. The lowest current speed was measured at site 2, where the average slope was the lowest as seen in figure 2. T-tests showed that there was a significant difference between the current speed between sites 1 and 2 with a P-value of 0.072.
Figures: https://docs.google.com/document/d/1s2taUzmCdVAy7Pd_E-1flnjZBLYdMdLJDsTSKIdLg3o/edit
Figure two represents the average slopes of our four transect lines. Site 3 had the greatest slope and figure two had the smallest. These were both the transects within the fish pond.
Figure 3 depicts the different current speeds at 5 locations, three outside of the fish pond and two within. Locations 1, 2, 3, and 4 each intercept the corresponding depth transect perpendicularly. Site 5 is parallel to the west wall, and had the highest current speed.
Figure 4 is useful in visualizing the different depths from the starting spot at different points in and around the fish pond. The depths ranged from .3 to 5.32 meters. The scale of colors goes from white/yellow (shallow change), to red/purple (deep change).
In order to establish long term visuals of how the fish pond changes shape and the walls shift, we took a photo standing from a perch on the bathroom wall. Future photos can be compared to this baseline- 5/21/24.
Discussion
Our results present a couple different patterns and findings that could be relevant to continue monitoring overtime.
The south side of the fish pond interior has a shallower depth and slope than the north side interior. Correlating this finding with lagoon current strength and direction can tell us about the possible future of deposition of sediment and changes of depth within the fish pond. The measurement can be used as a baseline measurement and repeated in the future to compare and quantify changes occurring within the fish pond.
Specifically, we found that the current is traveling northward, and we recorded a 0.6 meters per second slow down after the water crossed the south wall of the fish pond. Therefore, the higher elevation on site 2 can be correlated to sediment deposition due to slowed current speed after the fish pond wall.
Additionally, we want to note the relationship between the largest depths (at site 3), and the hole in the fish pond’s west wall, which overlaps with site 3. We observed that when a buoy was placed in a few meter’s radius of the opening, it would float towards that exit, slowly increasing in velocity, exemplifying this as an outflow spot for currents. This could help explain why the north interior side of the fish pond is deeper than the south, because of sediments being carried with currents leaving the pond at this exit (and other cracks and crevices along the north wall).
Numerically we recorded an average 0.3 meters per second increase in current speed on site three compared to site 2. But it’s relevant to note that we did not calculate current velocity specifically at the mouth of the west wall opening - which is where it would have been even faster, as observed visually by buoy movement. This solidifies the relationship between current direction, velocity, and deposition of sediment in and around the fish pond, especially when considering different physical features.
In the future we suggest groups do more measurements around the hole in the west wall. These could be current, depth, or sediment related to learn more about the erosion and deposition occurring in the fish pond and around the opening.
Errors
As this is the first beach profile there were a couple of human errors that can be addressed. One error would be that when measuring the dimensions of the fish pond we were at the water level. This may cause error as we may have not been aligned with the fish pond walls.
During the days that data was collected there were varying wind speeds between 3 and 5 on the Beaufort scale. We recorded the wind speeds at the time of each current trial in order to include a more accurate depiction of what was happening and why.
The wind made it difficult to fully straighten long transects and made it difficult to use the string method on our protractor when calculating angles of the topography’s slope. To reduce error, one person would look through the protractor while another person ensured the weight at the end of the string was indeed oriented downwards.
The large swell two weeks ago and other temporal weather could have affected the depth of the sediment and the current speeds.
In addition, there was an unknown error during data collection and processing, influencing the accuracy of our data.
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