Latest, 22 October afternoon: Deduru Oya is discharging 16,500 cusecs and Rajanganaya more than 13,200 into the Kala Oya — a figure that has risen by two-thirds since the morning. Thirteen divisional secretariat divisions are under a named flood warning, Colombo roads are inundated, and the Met Department expects the rain to continue into Friday. Skip to the latest.
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 | Time | Gates open | Opening | Discharge |
|---|---|---|---|---|
| 14 October | — | 2 | 1 foot each | 1,400 cusecs |
| 21 October | 7.56 p.m. | 2 | 2 feet each | 4,200 cusecs |
| 21 October | 8.56 p.m. | 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 release doubled within the hour
The middle row is new to this report and it changes the shape of Wednesday evening.
The Daily Mirror filed the two-gate stage at 7.56 p.m., quoting the Director of Irrigation (Water Management): two gates raised two feet each, releasing 4,200 cusecs, with residents downstream urged to watch river levels. Hiru carried the same two-gate state five minutes later at 8.01 p.m., attributing it to the Irrigation Department and citing the rising reservoir level under continuing heavy rain.
Hiru’s six-gate filing is stamped 8.56 p.m. — fifty-five minutes after the two-gate one.
So the reservoir did not simply open six gates on Wednesday evening. It went from two gates to six, and from 4,200 to 8,400 cusecs, in under an hour. The discharge doubled within the hour, and the two newsrooms five minutes apart confirm the intermediate state independently rather than one copying the other.
A third reading breaks the arithmetic rule this report had derived
The Irrigation Department publishes gate openings and a discharge rate but never the relationship between them. An earlier version of this article derived one and said it held precisely. The two-gate reading added above shows it does not, and the correction is worth setting out.
Counting each gate by how far it is raised gives a total “gate-feet” of opening:
| Reading | Gate-feet | Discharge | Cusecs per gate-foot |
|---|---|---|---|
| 14 October | 2 | 1,400 | 700 |
| 21 October, 7.56 p.m. | 4 | 4,200 | 1,050 |
| 21 October, 8.56 p.m. | 12 | 8,400 | 700 |
The first and third readings both give exactly 700 cusecs per gate-foot, which is why two data points looked like a law. The middle reading gives 1,050, and it is the most tightly controlled of the three — same reservoir, same evening, fifty-five minutes apart, so the water level behind the gates was all but identical.
Between those two readings the opening tripled but the discharge only doubled. That is the opposite of what a constant rate would predict, and it cannot be explained by the reservoir level changing.
The honest conclusion is that the published discharge figures cannot be reconstructed from the gate positions, and readers should not estimate a release from gate counts. Either the figures are rounded to convenient numbers, or discharge is being quoted on a basis the department does not explain. The earlier claim that the announcements “reconcile to the cusec” was an artefact of having only two data points, both of which happened to land on the same round 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.
Update, 22 October: eight gates, flood risk named for nine divisions, and a third reservoir
By Thursday mid-morning the position had escalated sharply. Deduru Oya now has eight spill gates open, discharging 16,500 cubic feet per second, Hiru News reported at 9.56 a.m., attributing the figures to the Irrigation Department. Two more gates had been opened because of heavy rain across many parts of the island.
The discharge has roughly doubled overnight and is now almost twelve times the 14 October figure:
| Date | Time | Gates open | Discharge |
|---|---|---|---|
| 14 October | — | 2 | 1,400 cusecs |
| 21 October | 7.56 p.m. | 2 | 4,200 cusecs |
| 21 October | 8.56 p.m. | 6 | 8,400 cusecs |
| 22 October | 9.56 a.m. | 8 | 16,500 cusecs |
The Irrigation Department has now named the areas at risk
Wednesday’s filings carried a general warning to stay vigilant. Thursday’s names divisions. The department says there is a risk of flooding in the low-lying areas of the river valley in the Wariyapola, Nikaweratiya, Mahawa, Kobeigane, Bingiriya, Pallama, Chilaw, Arachchikattuwa and Rasnayakapura divisional secretariat divisions, and has told people living below the reservoir to stay alert.
That closes part of this article’s own “Not reported” list: a specific downstream risk area now exists where on Wednesday there was only a general advisory.
A third reservoir is spilling
Hiru’s filing adds that Rajangana reservoir also has eight spill gates open, releasing 7,884 cubic feet per second into the Kala Oya.
The island-wide floor recorded above therefore rises from two spilling reservoirs to at least three. Still no outlet has published an island-wide count.
Taken together, Deduru Oya and Rajangana alone are discharging 24,384 cusecs into two separate river systems in the North Western dry zone.
The Kala Oya now has its own named flood warning
Just under two hours later, the Irrigation Department extended the same treatment to the second river. It warns of a flood risk in the low-lying areas of the Kala Oya valley and adjacent low ground, and tells residents in the Rajanganaya, Nochchiyagama, Wanathawilluwa and Karuwalagaswewa divisional secretariat divisions to stay alert, Hiru News reported at 11.46 a.m.
Thirteen divisional secretariat divisions across two river basins are now under a named flood warning, where on Wednesday there was none.
Update, 22 October afternoon: the Kala Oya release has risen by two-thirds
Two further Daily Mirror filings and one from NewsFirst confirm the Deduru Oya position independently and move the Kala Oya figure sharply.
On Deduru Oya, the Daily Mirror at 11.12 a.m. and NewsFirst at 12.19 p.m. both give 16,500 cusecs and the same nine divisions, matching Hiru’s 9.56 a.m. figure. Three newsrooms now carry the same reading.
The Daily Mirror quotes the department directly: “Due to the significant rainfall experienced in the upper and middle reaches of the Deduru Oya Basin, the water level of the Deduru Oya is currently approaching flood level.”
Rajanganaya: 7,884 cusecs at 9.56 a.m., more than 13,200 by 11.42 a.m.
This is the substantive change. Hiru’s morning filing put Rajanganaya at 7,884 cusecs. Less than two hours later the Daily Mirror reported the reservoir releasing more than 13,200 cusecs into the Kala Oya — an increase of about 67% in under two hours. The department says the discharge may rise further depending on inflows.
The two readings are not in conflict; they are an escalation, and the later figure supersedes the earlier one.
| Reservoir | 9.56 a.m. | 11.12–11.42 a.m. |
|---|---|---|
| Deduru Oya | 16,500 cusecs | 16,500 cusecs |
| Rajanganaya (into Kala Oya) | 7,884 cusecs | 13,200+ cusecs |
| Combined | 24,384 cusecs | at least 29,700 cusecs |
The disaster-management gap in this article’s own record is now partly closed
This report noted earlier that no filing gave a Disaster Management Centre advisory. The Daily Mirror’s Deduru Oya filing states that the Irrigation Department has asked disaster management authorities to take the necessary precautionary measures. That is a request from one agency to another rather than an advisory issued to the public, so the gap narrows without closing.
Colombo is now flooding too
The warnings above concern two dry-zone river basins. By Thursday afternoon the rain had reached the capital. NewsFirst reported at 1.49 p.m. that several parts of Colombo were inundated, citing police, with severe congestion at Armour Street–Barber Junction in the Grandpass police division, around the Jethawana Temple, on a stretch of Sangharaja Mawatha, and in the Maligawatte cemetery area, where submerged roads are affecting access into and out of the city. Motorists have been told to use alternative routes.
This is the first road flooding any filing in this thread has reported, and it closes another gap recorded below.
The rain is forecast to continue
NewsFirst carries the Met Department’s position alongside the flood warning: heavy rainfall may continue today and tomorrow under the influence of the low-pressure area, with further heavy falls possible in the Western, Sabaragamuwa, Central, North Western, Northern and Southern provinces. Meril Mendis, Director of the department’s Forecasting Division, has urged the public to stay alert.
That is the same system this desk reported forming on Wednesday, and it means the discharges above are being driven by rain that has not yet stopped.
One spelling, two renderings
The Daily Mirror lists the third division as Maho; Hiru and NewsFirst both render it Mahawa. These are the same division — Mahawa is the Sinhala form and Maho the railway-town spelling in common English use. Readers matching the warning list against a map should treat them as one place.
Not reported
No filing gives a storage level or percentage capacity for any of the three reservoirs — the figures that would show whether these releases are near their peak or still building. None says how long the gates will stay open, how many spillway gates each reservoir has in total (so the Upper Kotmale warning still cannot be quantified), or what downstream water levels are actually doing.
No filing reports any evacuation or displacement, or any casualty, in the thirteen divisions under warning. The Colombo road flooding is reported by NewsFirst citing police, but no filing connects it to the river basins above or gives a city-wide list of affected roads. The Disaster Management Centre has still not issued a public advisory of its own for either basin — only the Irrigation Department’s request that it act.
Upper Kotmale’s operating agency remains unnamed, and there has been no further filing on that reservoir since Wednesday evening, so its current gate position is unknown. No outlet has yet published an island-wide count of spilling reservoirs for either 21 or 22 October.
