Geologists from the University of the Philippines National Institute of Geological Sciences (Nigs) have recommended that further real estate developments should be shifted upland away from the threat of floods similar to those which devastated the city last month.

This and other policy recommendations were presented by the four person team which included Prof. Fernando Siringan (now with the U.P. Marine Science Institute), Prof. Alfredo Mahar Lagmay, Prof. Emeritus Kelvin Rodolfo (also with the University of Ilinois) and Riovie Ramos, associate researcher from the U.P. MSI

Aerial photo by Prof. Fernando Siringan shows the alluvial plain built by accretion of sediments on which the central business district of Cagayan de Oro now lies.

The team conducted an aerial survey and rapid assessment of the city earlier last week and compared their findings with previous research conducted by Siringan under the auspices of the defunct Cagayan de Oro River Development Authority (Corda) headed by now Mayor Constantino G. Jaraula when he was still a city councilor.

“The central and business district of Cagayan de Oro is built on a delta plain, also called a flood plain,” said Siringan in his presentation. “It is built by the deposition of sediments and large portions of the delta plain have elevations which are within two meters or lower of sea level.”

On top of these, Siringan said key infrastructure like the recently completed rotunda of the Kagay-an bridge are located in former wetlands or recently abandoned river channels, making them naturally susceptible to floods.

Constricted and shallow portions of the Cagayan River Channel (the dark spots along the river channel near the Maharlika and Ysalina/Carmen bridges) slow down the flow of flood waters exacerbating floods and sedimentation.

“Floodwaters reached a high water mark of 5.5 meters above the river level at this point,” Lagmay noted in his presentation “Aerial Photography, Digital Photogrammetry and Flood Simulations.”

Because of its location, the city is influenced by tides, storm surges and tsunamis, making floods a part of its natural cycle. With the city’s rapid urbanization, floods have become a constant threat.

However, while the team recommends moving further development to the upland barangays which are relatively free of the flood threat, Lagmay said this should not compromise the city’s remaining forest cover since reforestation will play a key role in mitigating further calamities like last month’s floods.

“Reforestation is a must,” Siringan stressed. “If it will not take place upstream, the areas downstream will suffer.”

Longshore sediment drift affects the river mouth's discharge flow, worsening flooding and sedimentation.

The city’s official website lists sixty three of the city’s 80 barangays as urban and 17 rural. It further lists that geomorphologically, Cagayan de Oro has three broad landforms: lowlands; level uplands; and hills/mountains.

The lowlands consists of sandbars, tidal flats found mainly between the mouths of Cagayan River and Iponan River, a narrow strip of level area described as a coastal alluvial plain from Puerto in the southeast to the center of the city, a broad

alluvial plain in a limited patch around a small hill south of the city and the main “ River Flood Plain formed by the Iponan and Cagayan Rivers that coalesced north of the Cagayan-Iligan Highway which is most susceptible to flooding.

Its uplands consists of three distinct plateaus, the most extensive of which occupies the southeastern region; another situated west of Cagayan River along the road to Lumbia and Talakag, Bukidnon and a third is in the south-central region just north of Barangay Indahag.

Close up of landslide slumps or slips in the Iponan Watershed show the threshold for landslide occurrence in both forested and denuded areas were exceeded. (photo by Fernando Siringan, MSI-UP Diliman)

Siringan noted that floods in Cagayan de Oro have become more frequent and higher, in large part due to the 2,000 hectares of forest cover from its watershed which were converted to agriculture and other uses and never replaced. As a result, riverside areas suffer from poor water retention and contribute to the high sediment loads of rivers, constricting waterways and shallow river mouths.

Lagmay also urged the city stakeholders to conduct further research and zone flood prone areas. The team noted that rainfall data is not readily available and records are relatively recent. They urged the city to coordinate with Namria (National Mapping & Resource Information Authority) to have tide gauge stations installed to monitor the mean sea level and determine if it is getting higher or lower through time.

“Records are needed,” Siringan stressed. “We need to start recording.”

Lagmay said low-cost technology which recently became available in the market can empower city planners to do flood simulations using aerial photography with relatively inexpensive commercial digital still cameras and digital photogrammetry software in conjuction with field data.

- 30 -