Updating of the 1:10,000 Scale Geohazard Map of Molave, Zamboanga del Sur

By Lawrence R. Zamoras, PhD.

 

The Geosciences Division of Mines and Geosciences Bureau-IX undertook a follow up geological hazard mapping and updating in the Municipality of Molave, Zamboanga del Sur on October 19 to 31, 2016. The program primarily aims to update geological hazards assessment and mapping at 1:10,000 scale completed by the agency in 2012. The team is composed of MGB-IX geologists; Lawrence Zamoras, PhD. (Team Leader), Paul Yecyec, Jerome Real and Administrative Assistant Rodolfo Eje.

The Municipality of Molave is susceptible to rain-induced flooding (e.g sheet flooding and flashfloods) particularly those areas adjacent to the two major rivers, Salug Dacu (Dipolo) River and Salug Diut River that transects the area on the west and east respectively and also serve as the boundary between adjacent municipalities. The northern section of the area is characterized by moderate to highly undulating terrain mainly covered by unconsolidated Quaternary deposit and volcaniclastic rock deposits coming from the past eruption of Mt. Malindang volcano. This area is susceptible to landslide prompted by torrential rains or during the monsoon season. Other forms of mass wasting have been noted along unprotected riverbanks, streams and creeks.

Molave Municipality is divided into two topographic regimes namely the Salug Valley in the south and the Malindang foothills in the north. The Salug Valley portion is part of the floodplain of Salug River which drains the southwestern flank of Mount Malindang. The Malindang foothills portion includes some of the ridges that stand between the drainage channels that surround the Mount Malindang in a radial pattern. In general, it is the low-lying flat floodplain of the Salug River that gets high flood susceptibility while the more elevated areas of Malindang foothills gets high susceptibility to landslide.

The barangays of Molave situated in the Salug Valley are Miligan, Rizal, Dipolo, Culo, Maloyloy-on (Pob), Madasigon (Pob), Makugihon (Pob), Sudlon and Blancia. During intense rainfall these barangays experience frequent flooding. In the Municipality of Molave, there are two main rivers that brings in flooding namely the Salug Dacu River and Salug Diut River. The Salug Diut River is closest to the urban area of Molave passing through barangays Culo, Maloloy-on, Sudlon and Blancia. Its frequent floods strongly affect not only these barangays but also Poblacion barangays of Madasigon and Makugihon. The Salug Dacu (Dipolo) River is a bigger river system that catches much of the southwestern slopes of Mount Malindang. Its floods mainly affect the western barangays of Miligan, Rizal and Dipolo.

In the Malindang foothills, the barangays are clustered into Upper and Lower portions. The barangays in the Upper portion are Bag-ong Argao, Upper Dimorok, Lower Dimorok, Santo Rosario, Simata, Silangit, Alicia, Ariosa and Gutlang, comprising the high elevation barangays of Molave. Their creeks and river channels converge downstream to be part of Salug Daku (Dipolo) River. In the Lower portion are barangays Dalaon, Bogo Capalaran, Lower Dimalinao, Mabuhay, Parasan, Dontulan, Gonosan. Their creeks and river channels converge downstream to become the Salug Diut River. All barangays in the Malindang foothills are characterized by relatively higher landslide susceptibilities. But as settlers cluster along gentler terrains, they are relatively safe.

The Municipality of Molave has a fairly high level of Disaster Preparedness including high public awareness, with emplaced Early warning systems and a number of Evacuation Centers. The LGU engaged in a dredging project along Salug Diut River in 2014 along the segment which often overflows and causes flooding in the Poblacion area. The dredged materials were dumped over the river embankment, in effect forming an artificial levee which is 5-meter high above normal water level and 2-meter higher than the original ground, in effect producing a river wall. This project has brought two benefits: (1) the enhanced capacity of river channel to contain flood water, and (2) the formed artificial levee that prevents flooding in Poblacion Molave when the flood volume exceeds the channel capacity.

This structural mitigation has lowered the Flood Hazard Susceptibility Ratings in the different puroks of barangays Culo, Maloloy-on, Makugihon and Madasigon. The High ratings in 2012 is now Moderate to High; the Moderate to High rating is now Moderate; and for the Moderate rating it is now Low to Moderate. This lowering of ratings by half level is also based on the positive feedback of the local residents regarding the construction of the “river wall” and its effect on flooding. However adverse effects have been experienced downstream in brgys Sudlon and Blancia. The containment of floodwaters along the river channel in Brgy Culo and Brgy Maloloy-on transfers the impact of flooding downstream, thus, generating more intense flooding than before.

Generally there is no change in the Landslide Susceptibility ratings in Molave. Previously identified landslide susceptible areas are mostly in the Malindang foothills that are sparsely populated. There have been no incidence of serious landslide in these mountainous areas in the past five years and neither any slope stabilization project has been implemented. Thus the previous Landslide Susceptibility ratings for the entire Molave are retained.


Photo 1. Dredged river channel of Salug Diut with extracted sediments dumped on top of the riverbank, in Purok Evergreen, Brgy Culo: [Left] artificial levee on the left riverbank formed out of the dredged sediments coming from the river channel, [Right] the 2-meter height of the levee from the original ground, at the back of which is the river channel of Salug Diut.

Photo 2. Informal built-up structures over the concrete dike along Salug Diut River in Brgy. Bogo Capalaran, Purok Tarsing.

 (This excerpt was taken from the MGB-IX Technical Report entitled “Updating of the 1:10,000 Scale Geohazard Map of Molave, Zamboanga del Sur”.)

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