Environmental flow and cumulative depletion in a run-of-river hydropower reach
A 25 MW run-of-river hydropower project is proposed on a gravel-bed Himalayan river with a catchment of 1,200 km2. The lean-season February flow at the barrage site was 18 m3/s before an upstream project began operating; that project now reduces the flow at the site to 14 m3/s in February. The developer proposes to divert 70% of the available flow, leaving 4.2 m3/s in a 9 km dewatered reach between the barrage and the powerhouse tailrace. The rapid EIA used a three-month post-monsoon survey, applied the Tennant method, and concluded that 30% of flow is satisfactory for aquatic life. Local records show that snow trout spawn in riffles during March-April, and four villages use the reach for bathing and livestock. The environmental clearance condition requires a minimum flow of 20% of lean-season flow but does not specify the baseline year.
1. Calculate the proposed residual flow as a percentage of the natural and current flows. Evaluate the EIA's claim that 30% is satisfactory and the 20% clearance condition.
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Residual flow = 4.2 m3/s.
Relative to the current depleted flow: 4.2/14 = 30%, which is the figure the EIA quotes.
Relative to the pre-upstream natural lean flow: 4.2/18 = 23.3%. The cumulative depletion is 1 - 0.233 = 76.7% of the natural February flow.
The 20% clearance condition equals 0.20 x 18 = 3.6 m3/s if the natural flow is the baseline, or 0.20 x 14 = 2.8 m3/s if the depleted flow is used. The proposal passes the numerical condition either way, but that is not evidence of adequacy.
The EIA's 30% is misleading because it is calculated after the upstream project has already taken its share. A 30% residual of an already-depleted flow is only 23% of the natural flow, and the Tennant 30% class is a broad 'satisfactory' band developed for average flows in temperate streams, not a February low-flow standard for a gravel-bed river with spring-spawning fish. The correct baseline for cumulative impact is the natural or pre-development flow, and the condition should state that baseline explicitly.
2. Critique the use of a single Tennant minimum and propose a seasonally structured environmental flow for this reach. Include the March-April spawning period.
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A single annual minimum fails because it treats the river as a constant-flow pipe. The Tennant method gives 10% as poor, 30% as satisfactory and 60-100% as excellent, generally for average flow, and it says nothing about riffle depth, velocity, wetted width, temperature or spawning cues. A low February flow and a March-April spawning flow have different ecological functions. The three-month post-monsoon survey also misses the lean season and the spawning season, so the baseline is not representative.
A building-block flow would include:
- Base low flow: at least 30% of natural lean flow, i.e. 0.30 x 18 = 5.4 m3/s, to maintain wetted riffles and dilution. The proposed 4.2 m3/s is below this.
- Spawning pulse: 40-50% of natural flow, i.e. 7.2-9.0 m3/s, for 4-6 weeks in March-April, with stable or gently rising flow to attract spawners and protect redds. A 4.2 m3/s flat flow does not provide this cue.
- Channel-maintenance or flushing flow: at least one short high-flow event during the monsoon at 60% or more of natural flow to mobilise fines and maintain gravel permeability.
- Water-quality flow: enough dilution to keep downstream temperature and oxygen within limits, especially at the tailrace.
Because this is run-of-river, the only way to provide these flows is to reduce the diversion or enlarge the bypass; storage is not available. The developer should be required to release the base flow plus the spawning pulse, and the 4.2 m3/s proposal should be rejected as inadequate.
3. Design a compliance and monitoring package that addresses cumulative impacts and the villages' use of the reach.
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The package should combine flow, habitat, fish and social monitoring, with enforceable triggers.
Flow compliance. Install telemetered gauges at the barrage, in the dewatered reach and at the tailrace. Compute daily natural flow from a catchment water-balance or a nearby reference gauge, not from the already-depleted flow. Publish the residual flow and bypass-gate position in real time. Set an automatic alarm and a shutdown or bypass-opening trigger if flow falls below the seasonal requirement for more than a defined period.
Ecological monitoring. Map riffle habitat and wetted width at several flows; count snow trout redds and juveniles before, during and after the March-April pulse; log temperature and dissolved oxygen in the dewatered reach. Use the results to adjust the spawning pulse in an adaptive-management framework.
Cumulative impact. Require a basin-level flow accounting that includes the upstream project and any other diversions, with a cumulative depletion cap and a share of the residual flow allocated to each project. A project-by-project 20% rule cannot protect the reach if each project applies it to the flow left by the previous one.
Village use. Maintain safe access to pools and bathing areas, monitor water quality at village use points, and include a grievance mechanism. The monitoring results should be independently audited and linked to the clearance conditions, not reported only in an annual self-declaration.