ESG RISK 11: Downstream Flows
Overview
ESG Risk 11 – Downstream Flows assesses how well a hydropower option anticipates and manages the future water regime both for generation and for sustaining downstream ecosystem and community needs. Early-stage evaluation reviews projected river inflows against seasonal and ecological flow requirements, including minimum base flows, flood pulse releases and sediment transport. It also flags the need to plan for flow maintenance during construction and reservoir filling. Because altered discharge patterns can disrupt aquatic habitats, fisheries, agriculture, navigation and cultural practices, this risk overlaps with sediment, biodiversity and social-impact criteria. An Extreme Risk score here warns of potential regulatory rejection, severe ecosystem decline, downstream stakeholder opposition, reputational damage or project cancellation.
Additional Guidance
Hydropower project type (run-of-river, storage, pumped storage), scale, operational objectives and inclusion of diversions are determinants of downstream hydrological changes. Powerhouses located downstream of a dam can have reaches with lower flows in between the dam and powerhouse, with consequent impacts on aquatic habitat and water users in the dewatered reach. Inter-basin transfer schemes involve a powerhouse on an adjacent river to the one with the dam, which poses environmental and social risks to more than one river.
Characteristics on downstream flows by project type range from:
Run-of-river hydropower, which typically has a water residence time of less than a day. These projects maintain pre-project seasonality in river flows, and often flood flows pass through the spillway. In cases, without mitigating measures, daily water level changes may be a source of downstream hydrological impact.
Storage hydropower, which typically has a water residence time of weeks to months and even years. With these projects, the downstream flows does not mimic seasonal flow regimes. In cases where discharges go into downstream river reaches, without mitigating measures, this can significantly alter aquatic ecosystems downstream and can result in high impacts and losses to aquatic biodiversity, potentially accompanied by a range of social impacts.
Pumped storage hydropower, which has at least two reservoirs, involving pumping water to the upper reservoir and generating electricity when the water flows from the upper to the lower reservoir. After first filling, there is limited ongoing impact on a river system due to the recycling of water. Important considerations for pumped storage hydropower downstream flow commitments include during first filling, during ongoing operations if inflows are directed into the storage(s) to address evaporation and/or seepage losses, and during times of flood and spill. Because of the two reservoirs, more than one river will be affected.
Operational regimes and their implications for downstream flows could potentially be:
Baseload operation, likely to be used to meet some or all of a region’s continuous energy demand. These projects typically produce energy at a constant rate. Downstream flows can suffer from the loss of natural variability, and in cases the flow releases are continuously larger than pre-project flows.
Peaking operations, also known as hydropeaking, involve discontinuous releases of water through the turbines to meet peak energy demands, which causes downstream water flow fluctuations at short time-scales. Increasing penetration of intermittent renewables may require more peaking or more pumped storage hydropower in future, including changes to existing projects.
Load following operations, which typically involve day-to-day variations in the flow made by the hydropower plant operators, generally to better match electricity generation with demand. These can vary to differing degrees from natural pre-project flow regimes.
At the early stage, scenarios for commitments to downstream flow releases may entail:
Ensuring a continuous minimum flow to sustain critical habitats and species;
Implementing variable flow releases that mimic natural seasonal variations to support ecosystems; and/or
Introducing periodic high-flow events to simulate natural flood conditions, aiding in sediment transport and habitat maintenance.
Any or all of these measures would ideally be accompanied by adaptive management; i.e., continuously monitoring and adjusting flow regimes based on ecological feedback and changing environmental conditions.
Sources of information that can help inform ratings for ESG Risk 11 Downstream Flows include:
Basic characteristics of the project, namely the location, type and scale of hydropower project and key feature location(s) in the river basin.
The planned operating regime, which along with project type and scale are the key determinants of the degree of change to the natural or pre-project flow regime and resulting downstream impact.
Hydrology expertise engaged early in project studies to analyse how far downstream the project will affect, and to what degree.
For pumped storage hydropower involving two reservoirs, information on the (at least) two separate downstream rivers which may have different ecosystem and community impacts.
Opportunities for early stage actions that could reduce the risk for a project option might include:
Alternative locations and designs so that the project has little or no adverse or unmitigatable impact on downstream flows.
Downstream flow release mitigation measures included in project design and costings.
A regulation pond to dampen short-term irregular water level changes.
Offset (i.e. compensation measure) options such as protection of an unaffected downstream river reach.
