Environmental flow (e-flow): from the Tennant method to holistic approaches

Environmental flow is one of the few E&S topics where a single value, a figure in cubic metres per second, condenses months of negotiation. Too low, it desiccates an ecosystem and triggers non-compliance. Too high, it cuts the generation potential and weakens the economic model. This article compares the main families of methods for determining it, from the empirical Tennant rule to holistic approaches, and details what lenders expect today from an environmental flow dossier.

What an environmental flow covers

The term has evolved. We spoke of reserved flow, then of minimum biological flow, today of environmental flow or e-flow. This shift is not cosmetic. It reflects a paradigm change.

A minimum flow amounts to a threshold: never drop below. An environmental flow describes a regime. The IFC Good Practice Handbook defines environmental flows as "the quantity, frequency, timing and quality of water and sediment flows necessary to sustain freshwater ecosystems and the human livelihoods that depend on them" (IFC, Good Practice Handbook on Environmental Flows for Hydropower Projects, 2018).

Note the four dimensions: quantity, frequency, timing, quality. A constant flow all year round, even generous in volume, does not reproduce the seasonal variability on which fish reproduction, sediment transport and the maintenance of riparian habitats depend. The spring flood has an ecological function. So does low flow.

This framing directly aligns with IFC PS6. In natural habitat, the standard requires aiming for no net loss of biodiversity. A flow regime that trivialises a watercourse and causes a migratory fish species to disappear comes into direct conflict with this requirement. This issue connects with the biodiversity issues that every project owner must anticipate in the studies phase. Environmental flow is therefore not an isolated hydraulic constraint. It is a link in the mitigation hierarchy.

Hydrological methods: Tennant and its limitations

The first family of methods looks only at the hydrological regime. The environmental flow is calculated as a percentage of the mean flow or of a characteristic flow.

The Tennant method, also known as the Montana method, is the archetype. It proposes percentages of the mean annual flow associated with qualitative habitat states: of the order of 10% for minimum survival, 30% for fair habitat, 60% or more for excellent habitat. The thresholds vary by season. Flow duration curve methods (for example retaining a flow exceeded for a large part of the year) follow the same logic.

The strength of these methods is their simplicity. They require only a flow record and can be calculated in a few hours. Their limitation is fundamental. They postulate a universal relationship between a percentage of flow and habitat quality, valid everywhere, for any river and any species. This relationship does not exist.

The same percentage of the mean flow does not produce the same water depth, the same velocity or the same temperature depending on the channel morphology. A value calibrated on temperate rivers in North America has no reason to suit a tropical watercourse with high sediment load. Lenders still accept these methods as a first approach, on projects with low stakes. They reject them as final justification as soon as a sensitive habitat or a species of concern is involved.

Hydraulic methods and habitat simulation

The second family links the flow to the actual geometry of the watercourse. One no longer reasons in abstract percentages, but in physical conditions measured in the field.

Simple hydraulic methods, such as the wetted perimeter method, seek the flow beyond which gaining water barely increases the surface in contact with the bed. This inflection point serves as a threshold. The method remains rudimentary, but it is based on field surveys.

Habitat simulation methods go further. They combine a hydraulic model of the reach and species-specific preference curves for depth, velocity and substrate. The PHABSIM-type approach produces a relationship between flow and the surface area of habitat actually favourable to a species, at a given life stage. One can then make an informed trade-off: such flow preserves spawning, such other sacrifices it.

These methods are more demanding. They require field campaigns, biological data and hydraulic expertise. In return they produce a defensible justification, species by species, that lenders' E&S teams know how to read and challenge.

Holistic approaches and the spirit of the IFC handbook

The third family changes perspective. Instead of starting from the flow to deduce an impact, it starts from the ecological and social functions to preserve, then reconstructs the flow regime that sustains them.

Building Block Methodology or DRIFT-type methods bring together a multidisciplinary panel: hydrologists, ecologists, sediment specialists, socio-economists. The panel describes the natural regime, identifies the components that matter (geomorphic floods, spawning flows, low flows), then constructs an environmental regime element by element. The result is not a single figure, but a monthly regime, sometimes with scenarios.

This is the logic carried by the IFC Good Practice Handbook. It does not impose a single method. It requires an approach proportionate to the risk, consideration of human uses downstream, and above all an Environmental Flow Management Plan with monitoring and adaptive management. The idea is simple: an environmental flow is not fixed on the day of commissioning. It is verified, measured and adjusted over the concession period.

For a developer, this requirement for adaptive management has a practical consequence. It is not enough to propose a value. One must propose an arrangement: measurement points, biological indicators, alert thresholds, revision procedure. This arrangement aligns with the logic of an environmental and social management system that lives on after construction.

Run-of-river: misconceptions and real risks

A persistent belief is that a run-of-river scheme, without a large reservoir, poses few flow problems. This is false as soon as a bypassed reach exists.

Between the intake and the release point, the bed receives only the environmental flow. Over several hundred metres, sometimes several kilometres, the ecosystem depends entirely on the value adopted. The length of the reach, the gradient, the presence of intermediate tributaries radically change the level of stakes. Two run-of-river projects of identical capacity can present ecological risks of a completely different order.

A second risk, often underestimated, relates to rapid flow variations linked to operation. When the plant modulates its production, the released flow varies abruptly. These hydropeaking events dry out then flood the banks at a rate that fauna cannot follow. Fry are stranded, invertebrates are dislodged. A credible dossier addresses these variations, not just the minimum flow.

Finally, sediment continuity and ecological continuity are played out in the same place. An intake that traps sediments deprives the downstream of materials and modifies the bed in the long term. An impassable structure cuts off migrations. Environmental flow is conceived with the fish pass and sediment transit, not separately.

What DFIs verify

Beyond the value adopted, lenders' E&S teams examine the approach.

Environmental flow is not a figure to defend, it is a demonstration to construct. Three reflexes avoid study revision. Choose the method according to the real stakes of the reach, never according to available time. Reason in regime terms, with seasonality, not as a single threshold. Propose a monitoring and revision arrangement, not just a commissioning value.

The right question is not "what percentage of the mean flow to adopt", but "what ecological functions and what uses must this reach retain, and what regime sustains them". A dossier that answers this question passes due diligence. A dossier that delivers a formula undergoes it.