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C-DRONE GUIDE · 26 AUGUST 2026

Green Tide: Drone Monitoring of Algae Strandings on the Brittany Coast, Method and Prices

Every summer, in the bays of Saint-Brieuc, Lieue-de-Grève or Douarnenez, the tide leaves tonnes of sea lettuce (Ulva) on the sand — the green algae that has proliferated for half a century along parts of the Brittany coast. The pile greening the beach is not just an eyesore : as it decomposes it releases hydrogen sulphide, a gas whose toxicity has already forced beach closures and caused documented incidents. For the municipality or intermunicipal body running the collection, knowing where the algae has washed up, over what extent, and how it changes from one pass to the next, before sending out a machine or a worker, reshapes how the season is run. Here is what a drone survey brings, what it never replaces — fixed H2S sensors first among them — and the prices seen in 2026.

Published on 26 August 2026, reviewed on 29 August 2026 — regulations in force as of August 2026.

Green tides in Brittany: a well-understood phenomenon, not a fatality

Ulva — filamentous or blade-shaped green algae, sometimes called sea lettuce — proliferates every year in a handful of shallow, poorly flushed Breton bays : Saint-Brieuc, Lieue-de-Grève at Saint-Michel-en-Grève, La Fresnaye, Douarnenez, Concarneau, among others. The mechanism has long been documented : a landmark study by Alain Ménesguen and Jean-Yves Piriou, published in 1995 in the journal Ophelia, established the link between nitrate inputs from Breton agricultural watersheds and the massive accumulation of Ulva along the coast, and showed that cutting those inputs is the only durable lever for reversing the phenomenon (see the study on Google Scholar). Three decades on, the government's green-tide action plan still structures public policy : new prefectoral orders are due, by 1 September 2026, to set the measures required of farms in priority watersheds still short of the reduction targets.

The scale of the phenomenon remains, however, very real and closely tracked : a study by Louis and co-authors, published in 2023 in Marine Pollution Bulletin, analysed eighteen years of stranding monitoring across nine sandy Breton sites and measured an average annual cumulated stranded-Ulva area of 2.4 hectares, with more than half of all strandings concentrated in Saint-Brieuc bay alone (see the study on Google Scholar). It is this variability from one bay to the next, and from one year to the next, that makes localised, repeated monitoring more useful than a single national figure.

The health risk: hydrogen sulphide, invisible and sometimes serious

A stranded alga that dries out is harmless ; one that rots under a crust of sand or mud, in anaerobic conditions, releases hydrogen sulphide (H2S) — the rotten-egg-smelling gas detectable by the nose at very low doses, whose smell paradoxically stops registering as concentration rises, making it an insidious hazard for anyone lingering over a thick deposit. France's High Council for Public Health set an alert threshold of 1 ppm : above it, access without personal protective equipment must be banned ; below it, as soon as any detection occurs, access remains discouraged. This threshold frames the instructions given to collection crews and the public on affected beaches.

To track this risk continuously, a network of fixed H2S sensors is deployed every year, from 15 May to 15 October, on the most exposed Breton bays, with results published regularly. It is this statutory monitoring, and this alone, that grounds a decision to restrict access to a beach — never an aerial image, however precise. The drone has no business measuring a gas here ; its place is upstream : spotting where a thick deposit has formed so that collection crews — and the sensors themselves, whose placement can be adjusted — are positioned in the right spot, without a worker having to walk into a risk area on foot just to find it.

What an overflight maps: extent, density, change from one pass to the next

A flight along the foreshore at low tide produces a georeferenced orthophoto of the affected stretch of beach, on which the algae deposit stands out clearly against bare sand. A multispectral sensor sharpens the reading : near-infrared and red bands bring out plant biomass even under a light sand cover, and tell a fresh deposit from an already-decomposing crust. This approach draws on a method already validated on marine macrophytes : a study by Ana Román, Antonio Tovar-Sánchez, Isabel Olivé and Gabriel Navarro, published in 2021 in Frontiers in Marine Science, showed that a drone-mounted multispectral camera effectively classifies intertidal macrophyte formations and maps their extent with a precision unmatched by satellite imagery on this kind of confined site (see the study on Google Scholar).

Repeated on an identical flight plan — ideally at every significant low tide during peak season — this overflight produces a comparable series : the municipality or managing body can see whether the deposit is spreading, drifting to another part of the bay, or receding after a collection round. It is a direct aid to prioritisation : over several kilometres of beach, sending collection machinery first to the most heavily loaded zones, rather than working the beach in order, cuts the number of days a deposit sits decomposing in the sun. This periodic-monitoring logic echoes what we detail for other coastal issues in our guides to coastal erosion monitoring and, for inland water bodies, cyanobacteria and algae monitoring on a lake — a biologically distinct phenomenon, but one that calls for the same repeated-mapping logic.

Public maritime domain, Natura 2000 and a busy beach: the flight framework

A beach falls under the public maritime domain, whose management is most often delegated to the municipality : authorisation, or at minimum prior notice to the managing department, precedes any mission, just as it would for mechanised collection. Many of the bays affected by mass strandings are, moreover, designated Natura 2000 sites or part of a nature reserve because of their intertidal habitats : a systematic check of the zones on Géoportail before any quote helps anticipate any constraint the reserve's manager may impose, particularly during the coastal bird nesting season.

The flight itself follows the general rules detailed in our guide to drones and coastal beach regulations : open category, sub-category A2 or A3 depending on how close bathers and walkers are, with a morning slot or a low tide outside peak attendance simplifying the organisation. A busy beach in peak season inevitably puts people in the camera's field : a wide framing on the stretch of sand, with no zoom on beachgoers, is enough in the vast majority of cases to stay within the framework we detail for every professional operator in our guide to GDPR and professional drones. Up-to-date professional liability insurance remains, as for any mission, a non-negotiable prerequisite for the contractor.

Mission method and prices

The mission is organised with the technical departments of the municipality, the intermunicipal body or the joint authority in charge of collection : timed to low tide, with priority stretches identified in advance, and a take-off point clear of any supervised swimming area. The flight runs a photogrammetric pass for the orthophoto — supplemented by a multispectral pass where the service includes it — and concludes with a report locating the stranding zones, their estimated extent and, for a seasonal watch, how they have changed since the previous pass. The deliverable is designed to sit alongside statutory H2S monitoring rather than replace it : the municipality keeps, forwards or cross-references this data with the sensor network's results and, where relevant, with CEVA.

2026 prices (excl. VAT) : €350 to €700 for a one-off survey flight over a beach sector (orthophoto, outlined stranding zones) ; €600 to €1,200 for a flight with a multispectral sensor (biomass index map, thickness estimate) ; €250 to €500 per visit for regular seasonal monitoring on the same flight plan, from May to October ; coverage of several bays across an intermunicipal area : on quotation depending on the stretch of coast and frequency. A travel charge applies beyond a 30 to 50 km radius. This service falls under our drone surveying and photogrammetry offer ; request a quote stating the stretch of beach concerned, the monitoring frequency you want and how it should tie in with the existing H2S sensor network.

Frequently asked questions

Does the drone replace the fixed hydrogen-sulphide sensors?

No, never. The drone produces an image : the visible extent and density of an algae deposit on the foreshore. It measures no gas concentration in the air at all. Only the fixed sensors of the monitoring network, deployed every year on the most affected bays, quantify hydrogen sulphide and trigger access restrictions once the French High Council for Public Health's alert threshold is crossed.

Can a drone fly over a busy beach in summer?

Yes, under conditions. The flight most often falls under the open category, sub-category A2 or A3 depending on how close bystanders are, with authorisation from the manager of the public maritime domain — usually the municipality — and a prior check of the zones on Géoportail, particularly where the bay is a Natura 2000 site. Scheduling the pass early in the morning, outside peak attendance, simplifies the organisation and limits shots of identifiable people.

Does the drone survey give the tonnage of algae to collect?

It gives an order of magnitude useful for planning, not a contractual figure : the area covered and a thickness estimated by visual comparison give an approximate volume, converted to tonnage using an average density figure known from CEVA's work. The actual weighing of skips at collection time remains the only figure that counts for the campaign's tally and any resulting declarations.

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