When Ordinary Rain Is Enough
A logistics corridor that carries Surabaya’s goods is being defeated by rain it was built to handle.
BY MUHAMAD DANDY SAFITRA
The rain had stopped hours earlier, but the water on Jalan Tambak Osowilangun had not moved. Residents of Benowo in western Surabaya know the rhythm by heart: a downpour after midnight, a flooded road before dawn, container trucks crawling through the shallows, drivers turning back. From January to April 2026, the district office recorded floods 20–40 centimeters deep lasting an average of 550 minutes more than nine hours whenever heavy rain arrived.
Those figures are more than a weather report. The nearly five-kilometer corridor linking Tambak Langon and Tambak Osowilangun is the logistics route between Surabaya and Gresik. Every hour of flooding slows supply chains and hastens damage to the pavement.
On 26 April 2026, a day after 65 millimeters of rain, water still stood 15 centimeters deep and submerged the entire channel. Even moderate rain is enough to make the drainage here give up.
Half the Corridor Can Only Cope With “Ordinary” Rain
The question is simple: how much water is this corridor expected to carry? To find out, I examined 15 years of daily rainfall (2011–2025) from the two nearest BMKG stations and calculated the flows the channels must handle for storms expected once every two, five, and ten years. I then tested those figures against 98 channel stretches, each 100 meters long, using field measurements, and simulated the whole system with drainage software.
A storm expected once every five years sends 52.8 cubic meters per second through the system 52,847 liters every second. Of the 98 stretches, 49 cannot cope: 34 overflow and 15 sit at the brink. Half the corridor is safe only when the rain is lighter than its design standard.
Part of the problem lies in the silt. Across 18 measured stretches, 2,936 cubic meters of silt have accumulated, enough to fill more than a hundred trucks removing 11.08 cubic meters per second of capacity. In some stretches, 30 centimeters of silt leaves an 80-centimeter channel with almost half its capacity gone.
The simulation shows the consequence. In a five-year storm, 14 points overflow, releasing 134,508 cubic meters about 54 Olympic swimming pools. The worst point is upstream of the channel draining into Kali Lamong; the tightest bottleneck is a small channel on the south side carrying 3.49 times its capacity.
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The key finding is not any single number but the pattern. From a two-year to a ten-year storm, the water leaving the system barely changes, staying near 9.6 cubic meters per second. Only the overflow grows.
The problem is not that the rain is too extreme. It is that the channels have run out of room.
Dredging and Widening Are Not Enough
The obvious response is to dredge and enlarge the channels. Both help, but less than we might hope. Dredging the five worst stretches cuts overflow by 7.1 percent; enlarging five others cuts it by 15.6 percent; doing both cuts it by 21 percent, to 106,257 cubic meters. About 79 percent of the problem remains.
The water released downstream rises from 9.65 to 13.98 cubic meters per second. If the main canal, Kali Branjangan, and Kali Lamong cannot take it, the flood does not disappear it moves, perhaps to neighborhoods never part of the conversation.
The order of work needs to be reversed. First, the city should make dredging routine, with a fixed calendar and budget, starting with the five worst stretches two north of the road, three south. There is no reason to wait for the next flood.
Second, five stretches lacking capacity one north, four south need to be made 0.35–0.45 meters taller. Third, before upstream channels are enlarged, downstream capacity must be audited, and options such as retention ponds or pump stations near the river mouths should be studied.
Fourth, industrial estates and residents share responsibility: stop dumping waste into channels, stop covering them with buildings, and report silt before it becomes a crisis. Drainage belongs not to the public works office alone but to everyone who uses that road every day.
Flooding on Tambak Osowilangun is not a story about rain arriving without warning. It is the result of problems that can be counted, measured, and therefore fixed if we stop treating drainage as a seasonal emergency. As long as channels are cleaned only after water covers the road, ordinary rain will always be enough to sink the route that carries the city’s economy.
About the Author
Muhamad Dandy Safitra is a final-year student in the Civil Engineering Study Program, Faculty of Engineering, Universitas 17 Agustus 1945 Surabaya, and the author of the thesis “Evaluation of the Drainage System against Flooding on Jalan Tambak Osowilangun–Tambak Langon, Surabaya”.
