Public car parks and flood risk in Bernis

Framework: Self-initiated pilot study based on public data. It was attached to a bid submitted in April 2026 to the EPTB Vistre Vistrenque, a public river basin authority.
Date: April 2026
Municipality: Bernis (Gard, southern France), 3,425 inhabitants
Data: RGE ALTI 1 m terrain model and BD ORTHO 20 cm aerial imagery (IGN), Vistre flood risk prevention plan (DDTM 30), Admin Express (IGN), OpenStreetMap, Mapillary
Tools: QGIS 3.44, GDAL, Python (pyproj)

Context

Bernis is a town of 3,425 inhabitants in the heart of the Vistre basin, in the Gard département. Its historic centre was built at the foot of the hills, on the low land of the alluvial plain, and is covered by the Vistre flood risk prevention plan (plan de prévention du risque inondation, PPRi), approved on 4 April 2014.

When a flood warning is issued, the town needs to know which car parks to clear first and which ones can take the vehicles moved to safety. This feeds directly into its municipal emergency plan (plan communal de sauvegarde, PCS).

I carried out this study on my own initiative, to test on a real town a method designed for the 42 municipalities covered by the basin authority.

The question

Which public car parks in Bernis are floodable, which need monitoring, and which can serve as a refuge during a flood?

Method

Gather open data. Municipal boundary (IGN Admin Express), RGE ALTI digital terrain model at 1 m resolution, regulatory zoning of the Vistre PPRi (DDTM 30) and car parks from OpenStreetMap, all in Lambert-93 (EPSG 2154), heights in IGN69.
Inventory the public car parks. OpenStreetMap listed 4. Photo-interpretation of the 20 cm BD ORTHO imagery at 1:2,500 added 9: ring boulevards, town hall, school, squares. Private access, underground car parks and on-street parking were left out; doubtful cases were checked on street-level photos (Mapillary).
Cross with the PPRi and the terrain. Each car park is joined to the PPRi zoning. Where it touches several zones, the most severe one is kept, as a precaution. Its mean elevation is computed from the 1 m terrain model.
Classify and map. Three classes: refuge (outside any PPRi hazard zone), to monitor (moderate hazard), floodable (high hazard). The result fits on an A3 map with an inset on the town centre, delivered with the GIS layer.

Results

Map of the 13 public car parks of Bernis, coloured by class (floodable, to monitor, refuge), over the Vistre PPRi zoning and the terrain, with an inset on the town centre — Summary map of the study (A3 format, reduced; labels in French). Sources: IGN (Admin Express, RGE ALTI), DDTM 30 (Vistre PPRi), OpenStreetMap contributors (ODbL). Open the full-size map

13public car parks analysed: 361 spaces over 12,948 m²

Floodable, high hazard4P07, P10, P11, P13 · 78 spaces
To monitor, moderate hazard8P02 to P06, P08, P09, P12 · 271 spaces
Refuge1P01 · 12 spaces

97% of the spaces lie in a PPRi hazard zone, 22% in a high-hazard zone. Only 12 spaces out of 361 (3%) are in a refuge zone: a single car park, Chemin des Capitelles, on the northern hills at 29.4 m (NGF datum). The four floodable car parks are all on the plain, between 20.0 and 20.8 m.

What the town can take from it

Too little refuge space. 12 refuge spaces for a total of 361, roughly one in thirty: not enough to move the town's vehicles to safety.
A distant refuge. The only refuge car park is 1.1 km from the most exposed one, on Boulevard Alexandre Ducros.
Boulevards first. The tree-lined ring boulevards (Charles Mourier and Alexandre Ducros) hold 60 spaces between them: the priority in a crisis.

An action sheet for the most exposed car park

The Boulevard Alexandre Ducros car park (P10) combines every weakness: high-hazard PPRi zone, a mean elevation of 20.02 m, the lowest in town, and 30 spaces over 1,343 m². Its action sheet sets out what to do at each alert level:

orange alert: inform residents and shopkeepers on the boulevard;
red alert: clear the 30 vehicles and close the access points;
at all times: a “high flood hazard” sign at the entrance to the boulevard and an evacuation plan displayed at the town hall.

It also gives the route to the refuge (1,083 m) and points out its limit: 12 spaces cannot take 30 vehicles, hence the recommendation of an additional refuge, to be found with neighbouring towns.

Deliverables

PDFA3 map with legend, sources and town-centre inset
GPKGGIS layer of the 13 car parks: PPRi zone, mean, minimum and maximum elevation, class and source of each feature
QGZReproducible QGIS project: layers, styles and layout
PDFAction sheet for the most exposed car park, A4 format
PDFMethod statement: every step, with the pitfalls met in Bernis

Files available on request (in French).

Limits

The study does not replace hydraulic modelling: it gives neither water depth, nor flow speed, nor flood duration. It ranks exposure from the regulatory zoning and the terrain, which is enough to pre-select refuge areas for a municipal emergency plan. The inventory was done remotely (aerial imagery, street-level photos); a site visit would confirm the doubtful cases.

Apply the method to your area

It relies on open data only and works for any French municipality covered by a flood risk prevention plan, with the same deliverables. In Bernis the full workflow took about 6 hours of production; for a mid-sized municipality, allow 4 to 6 hours once the département-wide data are prepared.

Day rate: €400, VAT not applicable (art. 293 B of the French General Tax Code). Quote based on the number of municipalities and the deliverables chosen.

Request the study for your area →

Reply within 24 business hours, quote within 3 business days.

← All case studies