Beach watch: keeping a record of the beach and the wave

Program started 2026-09-26 · by Addie Conner

The beach at La Saladita moves, and in the last few years rock walls and piles of boulders have gone in along parts of it. Many of us, the people who own and run this site included, worry that armoring the beach will change the sand on the bottom and, with it, the wave. Beach watch is a way to write down what actually happens, carefully enough that the question can be answered with evidence rather than guessed at.

Send a photo or report

Why this needs people on the beach

Satellites can trace the waterline, but only on cloud-free passes, only at whatever tide the satellite happened to catch, and at a pixel size (10 m for Sentinel-2) that is coarser than most of the changes people here are worried about. They cannot see when a wall went in, how steep the beach is, whether the sand in front of a wall is lower than the sand a few lots down, or whether the point still peels the way it did. People can. A photo from the same spot every day, a beach profile twice a year, a dated note when a wall is built and a rating after each session are the ground truth that turns a worry into a question with an answer. The satellite side of the analysis is being built at /findings/shoreline/ (coming soon). Time of day matters more than it looks: the tide at dawn here follows a strict calendar, so a photo at 7 am in January and one at 7 am in June catch very different water levels (see the tide clock). That is why every record below asks for the time.

1. The photo post

The method comes from CoastSnap, a community beach-monitoring programme started in Australia by the Water Research Laboratory of UNSW Sydney and now running at beaches in many countries. The method paper, Harley, Kinsela, Sánchez-García and Vos (2019), “Shoreline change mapping using crowd-sourced smartphone images”, Coastal Engineering 150: 175–189, showed that ordinary phone photos taken by passers-by from a fixed cradle can map the shoreline well enough to track storm erosion and recovery.

What the post is

A sturdy post (galvanised pipe or hardwood set in concrete) with a phone cradle at about chest height: a small open bracket that holds any phone in the same position and at the same angle every time. The post does not hold a camera. The camera is whoever’s phone comes by; the cradle makes sure every photo is taken from exactly the same point, pointing the same way. A small sign on the post says what to do and carries the QR code for sending the photo.

Where to put it

We are not naming a spot yet; the right one is a conversation with neighbours. What the spot needs:

• Height. A few metres above the beach if possible (a dune, a bluff, the edge of a raised terrace or path). Looking down on the beach, not along it, makes the measurements much more accurate.
• The view. The point and the take-off zone, plus a long stretch of beach that includes both a walled stretch and an open one, all in one frame.
• Access and permission. Somewhere people already pass on foot, with the landowner’s clear agreement, and reachable without walking through anyone’s private space.
• Safe from the sea. Out of reach of storm run-up and the lagoon mouth, on ground that will not be moved or rebuilt for years.
• Fixed landmarks in view. At least four fixed, visible features spread across the frame, near and far (a building corner, a concrete post, a painted rock). These become the ground control points. Something that is itself changing, like the face of a new wall, does not count.
• Light. Not aimed straight into the morning or evening sun.

How the photos are used

Each photo is rectified: using the ground control points, whose real positions are surveyed once with GPS, and the known position of the cradle, the oblique photo is turned into a map view in which every pixel has a real coordinate. On that map two lines are traced: the waterline, where wet sand meets the swash, which is then corrected for the tide at the photo’s time stamp to give a shoreline position; and the breaker line at the point, where waves first break, which is a rough indicator of where the bottom is shaping the wave. A year of photos is enough to watch a swell take sand and the calm months bring it back, and to compare the beach in front of walls with the open beach over the same days.

2. Beach profiles, twice a year

A profile is a cross-section of the beach from a fixed point on land down to the water: how high the sand is, metre by metre. Repeated on the same lines season after season, profiles show whether sand is being lost, where, and whether it comes back. Two people can do one line in under an hour with two poles and a rope.

When

• Late dry season, around April, before the summer swells and the rains.
• After the rainy and hurricane season, around November.
• Extra, if you can: within a few days after a big swell or a storm.

A calm morning near low tide is easiest, but any tide works as long as you write down the time.

The lines

Three or four fixed lines, each running straight from the back of the beach to the water, perpendicular to the shore:

• Line A: in front of a wall or boulder pile.
• Line B: open beach, several lots away from any wall, as the comparison.
• Line C: at the point, where the wave starts.
• Line D (optional): just past the end of a wall. Sand loss is often worst at the ends of a wall, so this line catches what line A might miss.

Each line starts at a back-beach marker that will not move: a concrete post you set, a painted mark on a foundation, the trunk of a big palm. Record its GPS position once and photograph it. To keep the line pointing the same way each time, note a second landmark behind it or a compass bearing toward the sea. If a marker is ever lost, say so on the sheet rather than starting a new one quietly.

Method one: Emery poles (two people)

This is the classic method from Emery (1961), “A simple method of measuring beach profiles”, Limnology and Oceanography 6: 90–93. It uses the sea horizon as a free, perfectly level reference.

You need: two straight poles about 1.5–2 m long, each marked in centimetres from the top down (0 at the top); a rope or tape cut to exactly 2 m; a phone; the data sheet and a pencil.

1. Stand the landward pole on the marker. Stretch the rope 2 m toward the water along the line and stand the seaward pole at its end. Both poles upright.
2. The person at the landward pole puts their eye level with its top and lines that top up with the sea horizon. The person at the seaward pole slides a finger down until it sits on that sight line and reads the mark. That reading is how many centimetres lower the sand is at the seaward pole.
3. If the sand rises toward the sea (a berm), swap: sight from the top of the seaward pole, read the landward pole, and write the value as a rise (+).
4. Write down the distance from the marker and the change. Move both poles one rope length seaward, the landward pole going where the seaward pole stood, and repeat.
5. Stop when the seaward pole reaches the wet sand where the waves run up. Write that distance and the exact time, and take one photo looking back up the line toward the marker.

Method two: phone level (one person)

Use a straight board or pole exactly 2 m long and a phone with a level or clinometer app (most phones have one built in). Lay the board on the sand along the line, set the phone on it, and write down the angle in degrees, negative when it slopes down toward the sea. Move the board forward end to end and repeat to the water line. We turn angles into heights (2 m × the sine of the angle). It is less precise than the poles, but done the same way every time it gives a steady record.

What to write down

• Date, start and end time (to the minute), line (A, B, C or D), who measured, which method.
• Whether the marker was found intact.
• For each step: distance from the marker in metres, and height change in centimetres (− for down, + for up), or the angle if you used a phone.
• The distance to the water line and the time you reached it.
• What the surface is: sand, gravel, cobbles, rock, wrack, a scarp (a small cliff cut in the sand; note its height).

Send a clear photo of the finished sheet, or a spreadsheet, through the form as a “beach profile”.

3. Sending photos and reports

Everything goes through one short form at /beach-watch/submit/: a photo, the date and time (filled in from your phone, and editable), what kind of record it is (photo post, beach profile, wall report, sand brought in or storm damage), notes, and your name if you want credit. Your name is optional. Each submission goes into a review queue and is looked at by a person before anything is used or published. If the form ever fails, the page shows an email version you can send instead to [email protected].

4. Reporting walls and storm damage

This is not about blaming anyone. People build walls to protect houses they love, and most of us would think about it too with the sea at the door. The point is to learn what walls do to the beach and the wave here, with evidence, so the next decision is better informed.

For a wall, boulder pile or other hard structure

• Where: the stretch of beach and the nearest landmark (“just south of the river mouth”, “in front of the third lot past the palapa”). A description is enough; we do not publish owners’ names.
• What: stacked rock wall, loose boulders, concrete, sandbags, rock-filled baskets (gabions), or other.
• How long and how tall: pace it (one long step is about 1 m) and estimate the height above the sand.
• When it was built: the month and year, or when work started and finished. If you are not sure, a range (“between Easter and the first rains of 2025”) still helps.
• Photos: the wall, the beach in front of it, and both ends. Any old photos of the same spot from before the wall are especially valuable.

For sand brought in

Trucked-in sand is as important to record as a wall. It moves the waterline seaward, so without a date it can hide erosion that is really happening, or make a wall look harmless. Sand that is coarser or finer than the beach's own can change how steep the beach is, and sand dug from a river mouth or arroyo is sand the beach would otherwise have received.

• Where: the stretch of beach and the nearest landmark. No owner names needed.
• When: the date, or each date if it happens more than once.
• How much: the number of truckloads, and the truck size if you can see it.
• From where: river mouth, arroyo, a quarry inland, or unknown.
• The sand itself: a close-up photo next to a coin, and if you can, a small dry, labelled bag of it.

For storm or swell damage

• The date, and the swell or storm if you know it.
• What changed: sand lost or gained, a new scarp and its height, rocks exposed, a wall damaged or undercut, the lagoon mouth opening or moving.
• Photos from places you can find again, so the same view can be taken later.

Why the dates matter

A dated wall is a natural experiment. With the month it was built, the shoreline analysis can compare the beach in front of that wall before and after, against open stretches of beach over exactly the same months, which have the same swells, storms and tides. That before-and-after-with-a-comparison test separates the effect of the wall from a stormy year. Without a date, the wall is just part of the landscape and the test cannot be run. The same logic is behind the site’s other natural-experiment findings.

5. Surf ratings: the wave is the outcome

Sand and walls matter here because of what they might do to the wave. So the most direct measurement is the one surfers already make without thinking: how good was it? Log your sessions on the field log surf form: the date, the time of your session, a rating from 1 to 10, the size, the wind, how much of the wave you could ride, and a few words.

How the ratings are used: once reviewed, each surf report is copied, with the name reduced to initials, into the ground-truth ledger that the forecast skill scoreboard is rebuilt from, where it is scored against what the forecast called for that morning. As of 2026-09-26 that ledger holds 17 rows, 13 of them owner-rated days, which is too few to say much; more raters on more days are what make the audit honest. Over years, the same ratings do a second job: if the bottom changes, the same swell, wind and tide should start to rate differently, and a long record of ratings next to the forecast inputs is the only way to see that.

6. Data policy

• Open data. What beach watch publishes is released under Creative Commons Attribution 4.0 (CC BY 4.0): anyone can use it, with credit to lasaladita.com and the contributors. By sending a photo or report you agree to that license for it.
• What is published: rectified (map-view) images, the waterline and breaker positions traced from them, profile tables (line, date, time, distances, heights), a catalogue of walls (stretch of beach, type, length, month and year built) and of sand placements (stretch, dates, truckloads, source), summaries of surf ratings, and contributor credit as initials, or “anonymous”.
• What is not published: faces (cropped or blurred before any image is published), anything else that identifies a person, your email or phone number, and owners’ names. Raw, unrectified photos are kept for analysis and published only after that clean-up.
• What the form stores about you: only what you type, plus a one-way scrambled fingerprint of your connection used to stop spam. No account, no tracking.
• Take-down: write to [email protected] and we will remove your contribution or your name.

Cite as: “Beach watch: erosion and the wave at La Saladita”, La Saladita Field Guide, lasaladita.com/beach-watch/, 2026-09-26. Method credits: CoastSnap (coastsnap.com; Harley et al. 2019, Coastal Engineering 150: 175–189); Emery 1961, Limnology and Oceanography 6: 90–93.