🇬🇧 Technical note — Pyrenean Desman and high-head hydroelectric intakes
Pyrenean Desman and high-head hydroelectric intakes
Critical analysis of existing prescriptions and proposal of a proportionate dual protection criterion
Open accessTechnical noteEnglish versionDenis Bouzon — Cabinet Eau-Energie, Pau — 10 June 2026 — version 1.0 — licence CC BY 4.0
Abstract. The Pyrenean Desman (Galemys pyrenaicus), a species protected under Habitats Directive 92/43/EEC, represents a major regulatory challenge for hydraulic projects in the Pyrenean zone. LIFE+ Desman Booklet 4 recommends a screen gap below 15 mm for intakes, citing a risk of the animal falling into the water chamber, without distinction of structure type or aspiration velocity.
This note demonstrates that this fall risk is physically non-existent for high-head plants fitted with a settling basin. It further demonstrates that the 15 mm gap, without control of aspiration velocity, causes rapid clogging of the screen — creating a fatal impingement risk for juveniles, whose swimming capacity is lowest.
The note proposes an inseparable dual criterion: aspiration velocity below 35 cm/s and protective bar spacing of at least 20 mm. A verifiable calculation method is provided for Water Act authorisation files.
Purpose of the English version: to make the technical argument accessible beyond French-speaking instruction services, while preserving the operational method: verifiable hydraulic calculation, proportionality, and site-specific assessment of intake risk.
The Pyrenean Desman (Galemys pyrenaicus) is a strictly protected semi-aquatic mammal whose conservation must be considered when hydraulic works are designed, modified or instructed in Pyrenean watercourses.
Existing guidance has contributed to raising awareness of the species. However, for high-head hydroelectric intakes, some prescriptions require a more precise hydraulic reading. In particular, the risk described as the Desman “falling” into the water chamber does not correspond to the physical configuration of submerged high-head intakes fitted with a forebay.
The note therefore focuses on a practical question: how should the real hydraulic risk at the screen surface be assessed in a way that is measurable, proportionate and compatible with field operation?
2. The problem with the 15 mm prescription
A fixed screen-gap prescription below 15 mm may appear protective in purely geometric terms. Yet it does not, by itself, assess the force exerted by the flow on the animal. The relevant hydraulic variable is the aspiration velocity through the effective wetted screen surface.
In Pyrenean torrents, coarse sands, gravels and small pebbles are naturally transported. A screen that is too fine may retain these materials, reduce the available passage area and increase the velocity through the remaining open zones. In normal operation, this turns a nominal protection device into a potential trap.
3. Actual risk: impingement by clogging
The note identifies the main risk not as a fall into the intake chamber, but as impingement against a clogged screen. Once the effective wetted surface decreases while the captured flow remains constant, the local aspiration velocity increases mechanically.
This is particularly critical for juvenile Desmanids, whose swimming capacity is lower than adults. The note retains a precautionary threshold of 35 cm/s, corresponding to half the current velocity documented for adults during hunting activity.
Key hydraulic formula: V = Qmax / Swetted
Where V is the aspiration velocity at the screen surface, Qmax is the maximum captured flow, and Swetted is the effective wetted screen area after taking bar spacing and bar width into account.
4. Proposed dual protection criterion
The proposed method is based on two inseparable criteria. Neither criterion is sufficient alone: velocity without physical spacing does not address accidental passage, and spacing without velocity control does not address impingement risk.
Hydraulic criterionScreen velocity below 35 cm/s, calculated on the effective wetted screen surface.
Mechanical criterionProtective bar spacing of at least 20 mm, adapted to the sediment context.
Main risk preventedImpingement against a clogged screen, especially for juvenile individuals.
MethodVerifiable calculation, field measurements and adjustment to the actual intake configuration.
5. Operational scope
This approach is intended as an operational tool for project owners, engineering consultancies, hydraulic structure owners, operators and instructing services. It does not replace site-specific assessment; it provides a reproducible calculation framework for discussing proportional prescriptions.
The note may be used in Water Act authorisation files where the relevant question is whether the intake screen creates a real attraction or impingement risk for the Pyrenean Desman, taking account of flow, screen geometry, clogging and sediment dynamics.
Relation to the original French publication:
This publication is the English version of the French technical note: Desman des Pyrénées et prises d’eau de centrale hydroélectrique, available at https://www.eau-energie.fr/papers/cahier1-fr.html.
Pyrenean Desman and high-head hydroelectric intakes