2 rivers now share one mouth

JADE Valley Subdivision in Tigatto, Buhangin, was once the Davao River, and Talomo and Pangi Rivers constantly choke up and belches back floodwaters because these two rivers now only have one river mouth.

As Jade Valley is once again submerged by the waters of Davao River, we review two articles published in August 2011, two months after the flooding of Pangi River that killed 31 residents. ("Subdivision was once Davao River" published Wednesday, August 17, 2011, and "Choked rivers also caused Davao killer flood" published Friday, August 19, 2011).

These articles are based on a geologist's point of view on why Jade Valley will always be flooded up to the rooftops whenever Davao River overflows, and why Pangi and Talomo Rivers often choke up and flood Bangkal and Talomo areas.

The articles came out from the presentations made by the Mines and Geosciences Bureau office in Davao Region during the "Flooding the Watersheds Forum" organized by the Interface Development Interventions on August 10, 2011.

The facts remain the same, being all science-based, but nothing has been done hence.

Jade Valley was the river

Jade Valley Subdivision and neighboring Juliville Subdivision at Sitio Oynaguran in Tigatto, Buhangin, were built on oxbow lakes, which means, these subdivisions are where the Davao River used to be.

In that August 2011 forum, Mines and Geosciences Bureau (MGB)-Davao Senior Science Research specialist Beverly Mae Brebante pointed out two geological formations along the Davao River using the topographic map by the National Mapping and Resource Information Authority (Namria), which she identified as "oxbow lakes."

As explained, an oxbow lake is a stagnant lake that is formed alongside a winding river when the river changes path because of soil erosion, leaving an abandoned stream channel, cut off from the rest of the river.

"Meaning, previously eto yung daanan ng river, but because of the channel migration lumilipat ang river sa kanyang floodplain. In layman's term, yung playground ng river. When you say it's the playground of the river, bahala an ang river maglipat-lipat, it is the natural characteristic of the river," Brebante was quoted as saying in the August 2011 report.

"Eventually, pag naka-cut ang river because it will take the shorter route, may maiiwang lakes dito. Since this is the floodplain of the river, this river could return to its original position. Itong mga oxbow lake, yung portion na iyan, ito yung kanyang balik-balikan," she added.

Reviewing the National Mapping and Resource Information Authority (Namria) map presented by Brebante and super-imposing this on a Google map image of the Davao River, Sun.Star Davao found that the lower of the oxbow lakes Brebante pointed out is now Jade Valley.

Namria maps were drawn from wartime reconnaissance aerial photography by the US Armed Forces in the 1940s, when there was nothing in Tigatto but coconut plantations and the mighty Davao River.

Sitio Oyanguren in Tigatto is among the 404 flood-prone areas in Davao City. It has never been said, however, that Sitio Oyanguren is not just flood-prone, but was once the river channel.

"Sa atin once na wala na siyang water, either gagawing agri land or since maganda ang flat and near sa river, this will be developed, not knowing that it is an old channel," Brebante said in that forum.

But she warned, "Itong mga oxbow lake, yung portion na iyan, ito yung kanyang balik balikan (This oxbow lake, particularly this portion, will be the area constantly flooded by the river as it traces its old course)."

Jade Valley and Juliville Subdvisions first suffered roof-high floodwaters in January of 2002 after days of continuous rainfall.

Straightened and choked: Talomo and Pangi

With regard why Pangi and Talomo Rivers often causes flooding in Bangkal subdivisions, the reason is that both rivers are now sharing one rivermouth.

It's unusual, and by the shape of Talomo River today, which has an unusually straight stretch on its way out to the sea, geologists suspect that this particular stretch of the river has been straightened by human engineering some years ago. When the Pangi flooded in 2011, the city environment and antural resources office’s first suggested solution was to straighten eh river, not considering the fact that meanderings of a river allow more time for water level build-up, while straightening it is another recipe for disaster.

The same 1940s topographical map shows the Talomo River to have slight bends and curves as it goes out to the beach in Talomo, some distance from where the mouth of the Pangi River is, as opposed to the very straight length that can be seen on the Google map today.

Brebante said what happened to the two rivers is a not so common geological formation made worse by the shallowing of the beach from regular dumping of silt from both Pangi and Talomo Rivers, which can cause a backflow.

"In 1950s, the mouth of these two rivers are relatively separated, meaning may distance 'yung mouth headwater from these two rivers," Brebante said, comparing the 1950s Namria topographical map as against an image of the same portion of Davao City lifted from Google Earth.

"As we go forward, we can see the development of the mouth of rivers. At present, halos iisa na ang river mouth. Water draining upstream drain only through one outlet," she said.

She likened what could have happened in June 2011, which saw vast residential areas in Bangkal and Matina Pangi flooded, to a funnel.

"If you have one outlet na lang of all water from the headwaters of these streams, pag marami ang nahuhulog na tubig sa mouth and the mouth is not capable to carry the load, posibleng nag-backflow," she said, in theorizing what could have happened in Matina Pangi in June 2011.

It’s what happens when you pour water from two sources into a narrow funnel, or drink from two soda bottles at the same time. Most likely, you will choke.