Bathymetric Mapping of a Dynamic Nearshore Environment (on a budget)

Mapping Playa Cocles On a College Student Budget

With some of some of the most beautiful environments in the world, it's understandable why Costa Rica's Eco-tourism industry is the fastest growing sector within its economy. Over 55% of tourist visiting the tropical country are coming specifically for its breath taking white sand beaches. Unfortunately, many foreigners wishing to soak up the sun are unaware of how dangerous some of the beaches in Costa Rica can be. Between 2000 and 2014, Costa Rica has seen an annual average of between 50 and 60 drownings. Researchers attribute many of these deaths to the powerful collapsing breakers and rip currents in the near-shore environment. Playa Cocles, a popular destination located on the east coast, just south of Limon, is one of these beaches. It is famous for its huge swells which attracts not only surfers, but swimmers and other water sport enthusiasts as well, many of them being tourist and unaware of the dangers these beaches pose. Playa Cocles is currently being studied by coastal geomorphologists Dr. Chris Houser and Dr. Sarah Trimble who are researching the sediment flux between land and sea and how rip currents are formed. Many of these small beach towns rely on tourism as a major source of revenue. To create a safer environment for ecotourists, Houser and Trimble are working with the local government to better understand how the beach environment changes over time and how these changes can lead to such dangerous rip currents. The goal is that this cooperation will lead to increased safety measures along the beach, as well as a more efficient education system that will inform tourist and locals how to enjoy the beach in a safer manner.

Figure 1 - One of the trained lifeguards appointed by the government of Puerto Viejo identifying newly formed rip currents.

As part of their research on Playa Cocles, Houser and Trimble required in-situ bathymetric measurements of the near-shore environment to create a baseline to compare with data derived from the WorldView-2 satellite. During the summer of 2015, I attempted to create a bathymetric model using a similar method that was used in Nearshore Bathymetry: Mapping on a College Student Budget. Following a similar procedure, with me I brought: Garmin-76 handheld GPS, 50/200 kHz Skimmer transducer, and the Lowrance Fish-finder.

Below is a report describing the project:

Unable to find a suitable watercraft to use for the survey, I strategically arranged the instruments inside a lightweight waterproof container. The transducer was sealed to the bottom on the inside of the container in a well of water (reduce inaccuracies caused by pockets of air) using water-resistant, malleable setting compound. The container was weighted so that is was neutrally buoyant, with the waterline at approximately half of the height of the container. A bright nylon swimming aid was fastened around the circumference of the container to ensure it would stay above the water level; it also assisted in tracking the movements through rip currents. The container was completely sealed using the setting compound mentioned above, with the Lowrance GPS secured* on the top of the container. The Garmin-76 handheld GPS acted as the base station for DGPS, which was utilized during the post-processing of data.

Figure 2 - The container was brought out from the crest of the beach to roughly knee-high water where it it entered the rip current and circulated. It was retrieved after half-hour intervals when it approached shore.

To create an accurate bathymetric profile, the surveyor must account for the variations in wave height due to wave action and changes in tide. Otherwise, measurements taken using sonar will only log values of ship's height above the floor at that time. To compensate for this, surveys measure the depth using sonar and while measuring the height above the WGS-84 reference ellipsoid. With those two measurements, surveyors can subtract the depth can be subtracted from the height, resulting in seafloors elevation relative to the WGS-84 model. Couple these elevation values with lat/lons and you have a bathymetric profile.

The data was processed in a similar way as the previous project; the data was outputted in a SD card in a format that is unknown by mapping software. The data was ran through the 'fishBelly.py.' scr ipt where superfluous information was removed and created a .csv file with coordinates in decimal-degrees. Afterwards, the data was converted into a shapefile which was used to create raster files displaying the elevation (digital elevation model) above the WGS-84 using the Inverse Distance Weighting interpolation method.

Figure 3 - This map shows the points at which depth was measured along the breaking zone of the beach.

Figure 4 - This map shows the IDW interpolation method of the depth measurements. The colors represent distance in feet from the WGS-84 reference ellipsoid, and were corrected using the Garmin-76 handheld GPS which acted as a DGPS base station.

Although the project did not run as smoothly as its predecessor, I saw it at it as an opportunity that tested my knowledge of geomatics in the field as well as my resourcefulness and grit. Factors like not having access to internet and little knowledge of the native language made for an interesting challenge that required much patience and ingenuity. The results were not as accurate as I initially planned; this was due to the design of the container which at the time I recognized had flaws. However, at that point it became a challenge to see how well I could conduct the experiment with what tools and resources I had available. Through trial and error, I gained experience on leading a project and sticking to the main objective regardless of the challenges that presented themselves.

Below is a list of things that occurred during this project that I thought were worth mentioning because they were funny or extremely frustrating at my expense, enjoy:

The only place to obtain a kayak in the entire town was shutdown, indefinitely.
The nearest hardware store which contained the tools needed was located 3 miles away with no way to drive there.
Tourist began taking pictures of me while I led the container out to rip currents. They all had assumptions on what I was doing including: I was afraid to leave my belongings on the beach, my wife died and her ashes were in the container, and I was inebriated after consuming illicit drugs.
The lifeguards insisted on assisting me place the container in the rip currents after I got pulled in multiple times.
The wave height was under exaggerated and were actually anywhere between 3-6ft, not 2-4ft.
During my nine days at the site, there was stormy and inclement weather for three of them.
Unable to find a battery with the correct specs, I used a small wet-battery which needed recharging at a mechanic shop a mile away after every 3-4 hours of use.
Dr. Houser's GPS became unattached while in circulation and fell into the water. Luckily it was waterproof and some small children found it deposited further down the beach.
A howler monkey showed interest in the container while on shore and appeared to be considering stealing it until a local ('Tico') ran and chased it away.