According to hydrologists, Metro Cebu’s rapid flash flooding is caused by its narrow, steep terrain, older mountain rock formations, and urban concrete expansion replacing porous Carcar limestone.
Engineer Maria Nenita Jumao-as and Dr. Danilo Jaque stated that generic flood-control templates, narrowed river channels, and clogged drainage lines further restrict water flow and cause overflows.
To manage runoff, hydrologists and the DPWH are discussing large or mini flood-control dams, while Jumao-as urges enforcing Cebu City's rainwater catchment ordinance for households.
WHEN heavy rain falls over Metro Cebu, floodwaters can rise within minutes. Cars stall on submerged roads and commuters get stranded. Residents barely have time to move appliances before their places turn into rivers.
For many Cebuanos, the pattern has become familiar. But hydrologists said Metro Cebu’s rapid flooding is not caused by rainfall alone. They said the speed and intensity of flooding are also shaped by the island’s geography and geology. Urban development and decades of planning decisions also played a part in the problem.
Topographic funnel
Maria Nenita Jumao-as, an engineer and executive director of University of San Carlos Water Resource Center, described Cebu as being shaped like a “kamote” (sweet potato) — “bulging at the center and tapering to the north and south.”
Cebu’s narrow shape and steep terrain, she said, allow rainwater to flow quickly from mountain areas toward coastal communities.
“Flashy atong floods because of the narrow island and steep ang ibabaw... rain just goes out very fast but very intense also,” Jumao-as said.
(Our floods are flashy because Cebu is a narrow island with steep upland areas. Rainwater flows out very quickly, but the flooding is also very intense.)
Unlike wider islands with large river basins that can hold rainwater longer, Cebu is narrow, steep and mountainous. Because of the island’s topographic profile, rainwater behaves like water flowing down a tilted roof. Hydrologists classify these events as flash floods because water levels rise rapidly instead of building over several hours.
Cebu’s geology once slowed part of that runoff. Much of Metro Cebu’s coastal area sits on Carcar limestone, a porous rock formation that absorbs rainwater and helps recharge underground water sources, Jumao-as said.
Urban expansion
Urbanization replaced many of these areas with roofs and concrete as Metro Cebu expanded economically, Jumao-as said.
The situation changes in the uplands.
According to Jumao-as, older and harder rock formations in the mountains absorb far less water. As more slopes are cleared and covered with roads, houses and concrete structures, less rainwater is absorbed into the ground and more flows directly into rivers and drainage systems.
“We replace vegetation with cement,” she said, adding that this increases surface runoff during heavy rains.
The debate over upland development has long been tied to government policy. Decades ago, steep slopes were generally considered protected forest lands to help prevent erosion and manage runoff.
But a Marcos-era decree, Presidential Decree 1998, later allowed development in Cebu’s upland areas under government regulation.
Still, Jumao-as said development alone is not the main problem. The bigger issue, she said, is how flood-control systems are designed and implemented.
She said some drainage and flood-control projects rely on generic engineering templates instead of detailed studies of how water moves through specific communities. In one case, the estimated flood volume was “three times bigger” than what a drainage structure was designed to handle.
Some flood-control structures may even worsen flooding.
“Gi-constrict nila... I think that’s the biggest ‘no’ in current flood control strategies,” Jumao-as said, referring to narrowed river channels.
(They constricted the rivers... I think that’s the biggest ‘no’ in current flood-control strategies.)
Dr. Danilo Jaque, managing director of HydroNet Consultants Inc., said earlier drainage studies recommended wider sections of the Mananga River to handle major flooding events. He said portions of the river should be about 64 meters wide and five to six meters deep.
Today, some sections are only about 20 to 25 meters wide.
“That will really result in an overflow,” Jaque said.
He added that more than half of Metro Cebu’s drainage lines are now clogged, further limiting the system’s ability to carry floodwaters during heavy rains.
Instead of giving rivers more space during storms, some flood-control walls and structures create bottlenecks that force water to overflow into nearby communities.
Interventions
Among the river systems that worry hydrologists the most are the Mananga and Butuanon rivers, which carry huge volumes of runoff from the mountains into Metro Cebu.
For Jumao-as, small drainage projects alone will not solve flooding problems in these watersheds.
“We really need to think clearly... put the big dams... 80 to 100 meters high,” she said, pointing to Lusaran as a possible site because of its year-round water flow.
The Department of Public Works and Highways (DPWH) has proposed building dams that could serve both flood-control and water-supply purposes. But instead of one large reservoir, the agency is studying the possibility of constructing several mini dams because they may be cheaper and faster to build.
“The intention is flood control, but during the discussion with DPWH, they are also toying with the idea that it can be dual purpose — water supply and flood control,” Jaque said.
Still, he warned that relying too heavily on mini dams could weaken Cebu’s ability to store enough water to significantly reduce flooding downstream.
Earlier studies for the Mananga watershed envisioned a large dam capable of storing around 30 million cubic meters of water.
“That will really be a significant volume of water that can reduce flooding downstream,” he said.
Beyond large infrastructure projects, hydrologists are also pushing smaller community-based solutions.
Jumao-as urged stronger enforcement of Cebu City’s rainwater catchment ordinance, which encourages households and buildings to collect and store rainwater for non-drinking uses such as cleaning and flushing toilets.
“Imbis nga ang ground mo-catch sa ulan, you trap it,” she said.
(Instead of relying on the ground to absorb rainwater, you capture and store it.)
For hydrologists, Metro Cebu’s flooding crisis is no longer just a weather problem. It is also a consequence of how the island has been built, engineered and managed over the years.
Unless flood-control planning adapts to Cebu’s steep terrain, narrow river systems and rapid urban growth, communities downstream may continue to face sudden and destructive floods every time extreme rains hit the island.