Sri Lanka’s recent floods, triggered by intense rainfall in the central hills and amplified by spillovers from major reservoirs, have once again drawn public scrutiny to water management and power sector decision-making.
As families in low-lying regions were evacuated and dramatic images of overflowing dams circulated online, a familiar debate resurfaced: why were reservoir levels not pre-emptively lowered when severe rain was forecast?
Hydropower accounts for a significant portion of Sri Lanka’s renewable energy portfolio, and reservoirs double as both critical water storage systems and electricity generators.
The assumption among many members of the public was that, with ample hydropower generation recorded in the preceding days, authorities should have had enough flexibility to regulate water levels ahead of the storm.
But a closer examination of official power statistics, transmission constraints, thermal plant behaviour, and inter-agency responsibilities reveals a more intricate set of conditions that limited such actions.
Power generation data
Electricity generation data available for 4 December, a representative day after the floods, illustrates the structural dynamics that constrained reservoir operations.
According to Ceylon Electricity Board (CEB) statistics, Sri Lanka generated 43.84 GWh of net energy, of which 35.86 GWh, or 81.8%, came from renewable sources. Fossil fuels provided only 7.98 GWh, accounting for 18.2% of the total.
Within the renewable mix, hydropower played a dominant role, delivering 22.67 GWh, while solar contributed 8.98 GWh. Wind from CEB and private producers together provided 0.65 GWh, and mini-hydro and biomass collectively added a further 3.57 GWh.
During peak demand, which reached 2,188.8 MW, renewables supplied 67.6% of the load, leaving fossil fuels to support the remaining 708.8 MW. These numbers demonstrate that the grid was heavily reliant on renewable energy during the period in question, suggesting at first glance that hydro plants were given substantial operational space.
The data implies that high hydropower output should have been accompanied by reservoir drawdown, which in turn might have mitigated the severity of the subsequent flooding.
Realities of the national grid
However, senior technical officials argue that the relationship between hydropower production and reservoir levels is not linear, and that public perception often overlooks the operational realities of the national grid.
According to a senior CEB official, who spoke to The Sunday Morning on conditions of anonymity, hydroelectricity, despite its prominence in the energy mix, cannot shoulder daytime demand on its own. Even on days with optimal rainfall and reservoir inflows, Sri Lanka must operate large thermal power plants – particularly coal – to maintain grid stability, he claimed.
It is learnt that coal-fired generation units, such as the ones in Norochcholai, cannot be turned on or off at short notice. They operate best at steady loads, and unplanned shutdowns can take days to recover from.
This means that even when abundant hydro resources are available, operators must keep coal plants running at least at minimum stable generation levels to maintain frequency regulation, reactive power support, and inertia — core components of a stable electrical grid. While hydro offers flexibility, it cannot fully substitute the stabilising characteristics of thermal generation.
The national grid’s east-west transfer capability, particularly the transmission corridors that carry electricity from Mahaweli hydropower stations to the load centres around Colombo and the Western Province, remains a longstanding bottleneck.
According to the CEB official, on days with low national demand – such as weekends or rainy periods when solar output drops – the Mahaweli-Biyagama transmission lines often operate at near-maximum capacity. In such conditions, increasing hydropower output risks overloading the lines.
The technical official explained that if the grid attempted to push 80% or more of electricity from hydropower during low-demand periods, especially on weekends, the transmission system would become vulnerable: a single line fault caused by lightning or wind could cascade into a nationwide blackout. This is not a theoretical risk but a statistically well-documented phenomenon inherent in constrained power systems.
According to the expert, hydropower, despite being abundant, cannot be dispatched rapidly beyond what the transmission infrastructure can safely carry. Thus, reservoir drawdown is limited not only by generation needs but also by the physical limits of power flow.
These grid stability constraints intersect with another structural limitation: the slow recovery time of thermal plants, especially Norochcholai.
Experts from the Solar Industries Association have pointed out that sudden shutdowns of coal plants can require multiple days for restoration due to complex thermal cycling procedures, boiler stress limitations, and grid synchronisation requirements.
When floods damaged sections of the grid and forced curtailment of solar output, CEB operators were compelled to prioritise thermal generation at its lowest sustainable level, not because hydropower was insufficient, but because losing a major thermal plant during a storm could have resulted in a prolonged nationwide blackout.
Therefore, thermal units had to remain online even when hydropower resources were high, inadvertently limiting the opportunity to lower reservoir levels through increased generation.
A multi-purpose water storage system
Another factor influencing reservoir levels is the multi-purpose nature of Sri Lanka’s water storage systems. Unlike some countries where hydropower reservoirs are managed primarily for electricity generation, Sri Lanka’s major reservoirs serve irrigation, drinking water supply, industrial needs, flood mitigation, and hydropower simultaneously.
The Mahaweli Authority, not the CEB, controls water release decisions for many of the country’s largest reservoirs. These decisions are influenced by seasonal crop cycles, irrigation schedules, municipal water demand, and long-term hydrological forecasts.
Historically, the period preceding the southwest monsoon – particularly the weeks before the typical heavy rains of late May – corresponds with naturally low reservoir levels.
The senior CEB official stressed that in nearly two decades of experience, he had not encountered instances where water was deliberately released in anticipation of heavy rainfall in a manner that threatened water security. Instead, reservoirs gradually approach lower levels through natural dry-season consumption.
A perfect storm of constraints
Hydropower planning and management of reservoir capacity during floods introduces additional complexity.
Reservoir safety guidelines restrict the rate and timing of pre-emptive water releases to prevent downstream inundation. Large releases before the certainty of heavy rainfall may conflict with irrigation needs or cause controlled flooding in areas unprepared for sudden inflows.
Operators must balance the statistical likelihood of rainfall against the socioeconomic repercussions of premature releases, especially in a country with strong dependence on stored water for agriculture, according to the official.
Hydrological forecasts, despite improvements, still contain uncertainty regarding rainfall intensity, catchment distribution, and timing. If forecast rainfall fails to materialise after proactive water release, reservoirs may be left at vulnerable levels, reducing hydropower availability and water security for months. This risk often outweighs the potential benefits of early drainage during non-monsoonal periods.
Together, these factors formed a perfect storm of constraints leading up to the recent floods. The national grid was heavily renewable-dependent in early December – soon after the cyclone – but limited in its ability to absorb additional hydropower generation due to transmission bottlenecks and the operational requirements of thermal plants.
Reservoir management fell under agencies tasked with balancing multiple competing priorities, from irrigation to drinking water provision, and their decisions were influenced by forecast uncertainty and long-established protocols rather than short-term energy considerations.
The hydropower system simply could not be used aggressively enough to create the reservoir space needed to contain the unprecedented rainfall that ultimately arrived.
As learnt by The Sunday Morning, the flood crisis illustrates several deeper systemic issues. The foremost is the persistent weakness in Sri Lanka’s high-voltage transmission network, particularly the limited capacity to evacuate hydropower from the Mahaweli region. This bottleneck has been identified for decades but remains only partially addressed.