Editor’s Note: To make us more informed on disaster preparedness, we are yielding this space to the Aerial Assessment Report of the Mines and Geosciences Bureau made on July 10, 2008 in the headwaters of Libacao, Aklan. This report was submitted to Aklan Governor Carlito S. Marquez.
Based on the aerial geological assessment that MGB-6 (Mines Geosciences Bureau) has conducted last July 10, 2008, there were numerous landslides observed at the mountain ranges in the headwaters of Aklan River, Libacao, Aklan. The landslides were confined along the gullies (landforms created by running water eroding sharply into soil, typically on a hillside) and steep slopes of the mountain ranges and in some instances at river banks. The occurrence of landslides in these areas was due to the high precipitation (rainfall), which has widened the gullies and exposed the underlying lithology/rock types. This is attributed to “Typhoon Frank” which hit Panay Island on June 20-21, 2008. In addition, the slopes of the mountain ranges in the headwaters of Aklan River were observed to be very steep (75-80) degrees).
The mountain ranges at the headwaters of Aklan River, Libacao, are underlain by the Late Miocene Lagdo Formation. It is composed mainly of andesitic breccias and lavas with tuffaceous layers. These are often intercalated with coarse graywackes, mudstones and conglomerates with clasts of volcanic rocks, limestones, gabbros and serpentinites.
It was also observed that there was no accumulation of debris at the river channels of Aklan River that could impede or block the passage of water. It is deemed that the volume of materials that came down from the mountains and carried by the river waters was well accommodated in the meandering (aimless wondering) channel of Aklan River. Thus, at present there should be no apprehension of damming effect, which could result to a sudden onrush of floodwaters, as the river channel is wide enough to accommodate the slide materials. However, this situation cannot be totally discounted as it may occur in narrow or constricted areas, such as tributaries between hills and mountains. A periodic monitoring for this particular possibility or condition is advisable.
It is further noted that the municipality of Libacao is very near and is almost surrounded by the Aklan River on account of its meandering thus the town is highly susceptible to flashfloods and siltation. A bigger threat is the possibility of an event wherein the river waters due to its volume and water velocity will seek a shorter route towards its downstream destination and by doing so will overflow its banks and cut across the town. While this process is natural and normal for rivers, the town should be prepared and protected for this event.
It is also worth to note that the floodplains of Aklan River within the jurisdiction of Libacao have been occupied as evidenced by the presence of houses and cultivated fields. These areas are considered a risk for the occupants in the event of floodwaters overflowing its banks and spreading throughout the floodplains.
Based on the foregoing assessment and findings, the following are our recommendations:
A. Flooding
1. No structure/houses along previous and present river channel, banks/large creeks/canals/shorelines and floodplains since these areas are highly susceptible to flooding;
2. The water levels and turbidity (soil content) in lakes/rivers/creeks, etc. should be constantly monitored. A rain gauge or makeshift rain gauge is recommended for each barangay or one for several adjacent barangays. Communities, especially those near bodies of water, should prepare for evacuation anytime and the concerned authorities (LGU officials) should be informed if the condition worsens;
3. There must be periodic monitoring of the river system, from the headwaters going downstream. In case of large landslides occurring in river channels, cooperative efforts should be exerted to keep the channels free from debris and wastes. This will help maintain the flow capacity of the river; and
4. Coordinate with the Department of Public Works and Highways to plan for the appropriate structures to be constructed to protect the town against flooding and bank scouring/erosion.
B. Landslide
1. No structure/houses should be constructed at the slope and foot slope of the mountains since these are highly susceptible to landslides;
2. If these are active/recent landslides in the area, residents should monitor the progress of mass movement (e.g. landslides, tension cracks, damming effect. Communities, especially in slope and foot slope of the mountains should prepare for evacuation anytime and the concerned authorities (LGU officials) should be informed if the condition worsens;
3. Residents in the area with the leadership of barangay/municipal officials should develop early warning device/system in order to alert the people of the impending landslides. Moreover, there must be person(s) in-charge to operate/monitor this early warning device/system always;
4. Residents should also watch for signs of impending landslides such as:
= Rapid increase/decrease in creek/river water levels, possibly accompanied by increased turbidity (soil content)
= Saturated ground or seeps in areas that are not typically wet
= Sunken or displaced road surfaces.
5. The Barangay/Municipal Disaster Coordinating Council should always be vigilant especially during rainy seasons. There must be periodic monitoring of landslide and its debris at the headwaters of the Aklan River especially during rainy season; and
6. The effects and occurrences of landslides could also be mitigated by various engineering measures, such as, slope reduction (reduction of slope angle, additional support of foot of slopes to prevent slide or flow at the base, reduction of load/weight of slope), use of retention structures or other slope stabilization measures.
C. Other General Recommendations
1. Barangay/Municipal officials should identify evacuation sites wherein the residents could go and if needed, relocation sites should be identified and assessed; and
2. Most importantly, geohazards such as landslides and flooding could be mitigated by proper land use plan and observance of proper buffer zones.
This report was signed by LEO VAN V. JUGUAN, OIC-Regional Director./MP
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