Two spillway gates at the Upper Kotmale Reservoir were opened on Wednesday after continuous rainfall in the reservoir’s catchment, and engineers warned that the remaining gates could open automatically if the rain keeps up.
Residents living along both banks of the Kotmale Oya downstream of the dam have been told to remain vigilant, the Daily Mirror reported at 6.30 p.m., attributing the information to the reservoir’s engineers. Hiru News carried the same announcement in Sinhala about two hours later, with the same two gates, the same cause and the same warning.
The automatic-opening warning is the part worth reading twice. Both filings state that if heavy rain continues in the catchment, the reservoir’s remaining spillway gates will open by themselves — meaning any further release will not be announced in advance the way Wednesday’s was.
A second reservoir is also spilling — and its discharge has risen sixfold in a week
Within half an hour of the Upper Kotmale report, Hiru filed on the Deduru Oya reservoir, where six spillway gates are now open: two raised three feet each, two raised two feet each and two raised one foot each.
The Irrigation Department says that configuration is releasing 8,400 cubic feet of water per second into the Deduru Oya, and has asked people living along and near the river below the reservoir to stay alert.
Set against this desk’s record of the same reservoir a week earlier, that is a sharp escalation:
| Date | Gates open | Opening | Discharge |
|---|---|---|---|
| 14 October | 2 | 1 foot each | 1,400 cusecs |
| 21 October | 6 | 3 ft ×2, 2 ft ×2, 1 ft ×2 | 8,400 cusecs |
Deduru Oya is discharging six times as much water as it was seven days ago.
The two figures check against each other exactly
The Irrigation Department publishes gate openings and a discharge rate but never the relationship between them. It can be derived, and it holds precisely.
Counting each gate by how far it is raised gives a total “gate-feet” of opening. On 14 October that was 2 gates × 1 foot = 2 gate-feet, for 1,400 cusecs — 700 cubic feet per second per foot of gate opening.
On Wednesday it is (2 × 3) + (2 × 2) + (2 × 1) = 12 gate-feet. At 700 cusecs each, that predicts 8,400 cusecs — exactly the figure the department published.
Two independent announcements a week apart therefore reconcile to the cusec. That is worth stating for two reasons: it confirms neither figure is a transcription error, and it means readers can estimate the next release from the gate positions alone, without waiting for a discharge number.
This answers a question the 14 October report left open
That earlier article recorded explicitly that Hiru’s reservoir report “does not say whether any other reservoir is spilling, so the island-wide picture available on 10 and 11 October has no 14 October equivalent.”
Wednesday supplies part of the answer: at least two reservoirs are spilling simultaneously, one in the central hills and one in the North Western dry zone. Neither filing offers an island-wide count, so this remains a floor rather than a total.
Two different systems, two different agencies
The two reservoirs are not comparable facilities and should not be read as one event.
Upper Kotmale sits in the central highlands and releases into the Kotmale Oya, part of the Mahaweli system. Wednesday’s filings attribute the announcement only to “the engineers in charge of the reservoir” and do not name the operating agency.
Deduru Oya is an irrigation reservoir in the North Western Province, and its announcement is explicitly attributed to the Irrigation Department. It is also a repeat performer in this desk’s coverage, having opened gates on 8 October and again on 14 October, when the Navy held 171 relief teams ready.
What links them is the weather. The Met Department said earlier the same day that a disturbance near the island may deepen into a low-pressure area within 24 hours, bringing heavy rain across six provinces.
Not reported
Neither filing gives a storage level or percentage capacity for either reservoir — the figures that would show whether these releases are rising or easing. Neither says how long the gates will stay open, how many spillway gates each reservoir has in total (so the Upper Kotmale warning cannot be quantified), or what downstream water levels are doing.
No filing reports any evacuation, road closure or displacement, and none gives a Disaster Management Centre advisory for the affected river basins. Upper Kotmale’s operating agency is not named in either report. No outlet has published an island-wide count of spilling reservoirs for 21 October.