C-DRONE GUIDE · 5 AUGUST 2026
Mapping green tide algae strandings by drone on the coast: method, collection, prices
Every summer, the beaches of the Bay of Saint-Brieuc, the Bay of Douarnenez or Locquirec see the same scene return: mats of green algae stranded over several kilometres, which the municipality must collect before they rot and release hydrogen sulphide, a toxic gas. Estimating the area and volume to be handled by eye, from the seawall, almost always understates the real scale of a stranding — especially on a wide foreshore where the mats extend well beyond what the eye takes in at low tide. The drone flies over in minutes what a team would take an hour to walk, turning a rough guess into a dated, measurable map from one tide to the next. Here is what it brings to a municipal technical department or a watershed syndicate, and what it costs in 2026.
Published on 5 August 2026, reviewed on 29 August 2026 — regulations in force as of August 2026.
What the drone maps on the foreshore
A standard orthophoto flight at low tide is already enough to outline the stranded mats precisely: their total area, their distribution across the foreshore, and their advance or retreat from one tide to the next by simply comparing two dated maps. To go further, a ground pass on a few sample plots — thickness measured with a ruler, mat density — converts the mapped area into an estimated volume, the figure that actually sizes the collection job (number of truck loads, buckets needed).
A multispectral sensor adds a useful refinement: it distinguishes freshly stranded algae from mats in advanced decomposition, whose spectral signature shifts as they yellow and then blacken. Automated classification of this type of coastal cover, using multispectral drone imagery and classification algorithms, was demonstrated by the team of Román, Tovar-Sánchez, Olivé and Navarro in a 2021 study published in Frontiers in Marine Science, applied to monitoring marine macrophytes in the Bay of Cádiz natural park (see the study on Google Scholar). An age map of this kind sends collection crews straight to the most urgent zones — those where decomposition, and therefore gas release, is most advanced.
Sizing collection and documenting the health risk
Collecting green algae is a municipal or inter-municipal responsibility; the State funds 100% of the collection cost borne by local authorities and 50% of its treatment, under the third Green Algae Action Plan (PLAV 3, 2022-2027, backed by €130 million). A dated drone map serves that pipeline directly: it objectively justifies invoiced volumes and grant applications, and replaces a verbal assessment that is hard to verify after the fact.
The second benefit is a health one. Decomposing stranded algae release hydrogen sulphide (H2S), a gas linked to several serious incidents recorded on the Brittany coast — a risk documented by health authorities, particularly in pockets of algae trapped behind a dune system or rock armour, where the gas stagnates. The drone spots these older accumulation pockets before any human intervention and guides fencing off or access bans without exposing an officer to a ground survey in the most concentrated zone.
Tracked from one season to the next, the same method also feeds PLAV 3's local indicators: across Brittany, stranding areas fell by half between 2021 and 2025, a trend a municipality or syndicate can objectify on its own stretch of beach rather than relying solely on the regional tally.
What the drone does not do, and the coastal flight framework
The drone maps a surface phenomenon: it measures neither the nitrate levels in the rivers feeding the bloom, nor the agronomic quality of practices in the upstream watershed — the preventive arm of PLAV 3 (€110 million of the €130 million total, with prefectoral orders expected on 1 September 2026 for farms falling short of targets in priority watersheds). On that agricultural ground, it is precision-agriculture drone work that applies, but upstream on the fields, not on the foreshore.
The flight itself usually falls under the open category, sub-category A2 or A3 depending on how busy the beach is. Many Breton stranding sites — mudflats, dunes, listed bays — are also Natura 2000 zones or nature reserves, requiring a site access permit distinct from the flight authorisation, particularly during nesting season. As with any coastal mission, checking the Géoportail map beforehand, registering the operator with AlphaTango and holding professional liability insurance remain essential — see also our guide on flying a drone at the beach for the rules that apply to individual use rather than a municipal mission.
2026 prices
Orders of magnitude observed in France in 2026 (excl. VAT):
- One-off survey flight (one beach, orthophoto + estimated area and volume): €350 to €700.
- Flight with multispectral sensor (age map, spotting H2S risk pockets): €600 to €1,200.
- Seasonal monitoring (regular passes from May to September, several beaches in a grouped round): on quote depending on the coastline length and frequency, with a tapering per-site rate for a mixed syndicate or a community of municipalities covering several coastal towns.
This service draws on our drone surveying and photogrammetry offer: request a quote stating the beach frontage concerned, the desired monitoring frequency and whether a multispectral sensor is needed.