ESG RISK 3: Water Quality, Erosion, and Sedimentation
Overview
ESG Risk 3 - Water Quality, Erosion, and Sedimentation assesses risks related to reservoir and downstream water conditions, influenced by factors like reservoir depth, water residence time, and external pollution. These issues are closely tied to erosion, sediment transport, and impacts on biodiversity and local livelihoods. Effective risk management requires early investigations and careful scoping of potential causes and feasible mitigation measures. Ignoring these factors can lead to severe consequences, including regulatory non-approval, ecosystem decline, public health hazards (e.g., hydrogen sulfide emissions), infrastructure damage, and high costs for reactive solutions. Addressing these risks proactively is essential to safeguard project viability and environmental sustainability.
Additional Guidance
Factors that influence the type and degree of future water quality risks for a hydropower project include:
Reservoir depth. Deep reservoirs can stratify, meaning the upper zone of the reservoir differs in temperature and oxygen levels to the deeper zone, which can cause issues with anoxic and cold releases downstream if there is a low intake. Depending on location and climatic conditions, very shallow reservoirs may experience wind-induced mixing of bottom sediments, challenges with vegetation regrowth when the reservoir is drawn down, and also temperature and evaporation effects on water quality.
Reservoir water residence time. Inundation of existing vegetation can lead to later water quality issues as the vegetation decays and uses up oxygen. Tropical and subtropical climates typically have very rapid and dense vegetation growth, which can decay when submerged and create water quality issues. In general, short water residence times have fewer water quality issues emerging.
Large changes in water levels. Large changes in water levels will expose banks that have been affected by submergence of varying durations. Exposed dewatered banks can give rise to erosion, landslips, and wave risks which in turn can cause turbidity. These impacts may pose challenges if there are other uses of the reservoir and infrastructure around the reservoir, as well as being a potential source of visual impact. In tropical and subtropical climates, vegetation can rapidly regrow in exposed banks, and then decay seasonally and cause water quality problems upon re-submergence.
The frequency of changes in water levels. Pumped storage hydropower, and projects operating in peaking mode, can have rapidly fluctuating water levels in the reservoir and/or downstream. Frequent changes in water levels, as well as rapid drawdowns, can cause bank erosion. This in turn can contribute to sediment accumulation in the reservoir, turbidity affecting water quality, and loss of riparian habitats. Rapidly fluctuating water levels can also pose safety concerns if there is public accessibility and other uses of the reservoir and/or downstream river.
External pollution sources. Upstream polluters can cause ongoing water quality issues for a project. The reservoir may inundate contaminated sites that are not decontaminated prior to inundation, and so have pollutants sources within the reservoir. Surrounding developments may not ensure appropriate water treatment measures. Changes in downstream flow patterns from hydropower discharges can in cases cause less dilution or intermittent pulses of pollutants emitted into the river by external sources. Downstream uses may be sensitive to poor water quality.
Additional reservoir or downstream uses. Some reservoir uses can give rise to negative impacts, for example: bank erosion due to boat wakes; public safety risks; nutrient enrichment; and risk of algal blooms with aquaculture. Additional uses downstream may either discharge poor water quality into the river or rely on certain river water quality conditions for abstractions, both of which can be negatively affected by the flow regulation patterns of the power station.
Sources of information that can help inform ratings for the water quality aspects of ESG Risk 3 - Water Quality, Erosion and Sedimentation include:
National, state or local databases for government water quality monitoring;
GIS analysis, local history assessment, and local visitation to determine past and present water quality and contamination sources and issues;
Calculation of the likely reservoir water level changes (amount, rate);
Calculation of the likelihood of stratification = (length / depth) x (mean flow / volume);
Calculation of estimated water retention time = reservoir volume / mean river flow.
Actions that could reduce the water quality risks for a project option might include:
Design features that ensure adverse water quality can be managed, such as multi-level offtakes, air injection and oxygenation facilities, and sufficient environmental flow releases;
Alternative locations and designs so the project can avoid any ongoing external sources of pollution;
Treatment of contamination focal areas;
Agreement with and improvements to sources of polluting activities.
An often assumed mitigation for the risk of reservoir stratification is pre-impoundment clearing of reservoir vegetation; however, the net impact needs to be carefully weighed up against the anticipated benefits. Environmental costs include: increased soil erosion risks due to removal of stabilising vegetation; rapid regrowth before impoundment; and disposal problems with the vegetative debris. Excellent mapping of both vegetation and access is required, as well as proactive planning for future uses.
Sources of information that can help inform ratings for the erosion and sedimentation aspects of ESG Risk 3- Water Quality, Erosion and Sedimentation include:
HSA How-to Guide for Hydropower Erosion and Sedimentation;
Catchment surveys to assess potential for erodibility. Indicators include steep slopes, erodible soils, sparse or no vegetation cover, active deforestation, historical landslip evidence, evidence of more recent landslips following intense rainfall events, and/or poor land use practices;
Any information available on sediment yields in the catchment, instream sediment transport rates, and/or fluvial geomorphology studies, including from universities, government studies and databases, and research programmes;
Geomorphic information highlighting landslips, erodibility and sediment movement;
Historical erosion and/or sedimentation effects on any existing instream structures;
Local knowledge.
Actions that could reduce the erosion and sedimentation risks of a project option might include:
Liaison with authorities to limit or prevent land use changes in the catchment that could cause high degrees of erosion;
Planning for sediment through-flow or sediment flushing design features for the project (see the How-to Guide for Hydropower Erosion and Sedimentation);
Ensuring adequate area in the project reservoir for “inactive storage”;
Planning for sediment collection structures in the catchment, such as sediment check dams;
Advocating for a protected catchment.
A key opportunity in the area of water quality, erosion and sedimentation for hydropower projects is to anticipate the risks early. Early consideration is highly advantageous, as mitigation measures included at the design stage can be very cost-effective. Characterising the problems, paying for damages, and retrofitting solutions can be highly expensive and damage the reputation of the developer/operator.
Further opportunities lie in addressing legacy impacts, i.e. from activities prior to the hydropower development; addressing these may provide benefits for hydropower operations, and/or the benefits may be for the broader local communities. Additionally, the opportunity to improve the aquatic and terrestrial physical habitats can be highly beneficial for biodiversity (see ESG Risk 6) and contribute to a nature-positive development philosophy.
