Assessment of geophysical hazards in Urban Lacustrine Systems: under the program Ten Years after Millenium Ecosystem Assessment of Laguna de Bay: towards a sustainable future

Landslide and flood hazards constantly pose damaging impacts to upland and lakefront communities in southern Laguna de Bay [Philippines]. Major slope failures have occurred along the flanks of Mt. Makiling due to heavy rains brought about by typhoons such as Milenyo (2006) and Paolo (2017). Prolonged and intermittent rains would also cause persistent flooding in the low-lying urban areas and coastal communities bordering the lake. In this study, the authors estimate the impacts of landslide, flood, and liquefaction hazards in the municipalities of Calamba, Los Baños, and Bay through field surveys and semi-quantitative analysis. The municipalities of Los Baños, Calamba and Bay in Laguna are underlain by lava flow units (e.g. basalt, andesite, dacite) and capped by pyroclastic deposits (e.g., tuff) of the Macolod Volcanic Complex. Rock samples were collected and stratigraphic sections were described to further characterize the volcanic processes along the southwestern portion of Laguna de Bay. Geological and ground penetrating radar (GPR) data when integrated to characterize the varied materials (e.g., solis, sediments, rocks) that underlie certain areas of Los Baños, Laguna. In addition to recognizing the variable geophysical signatures of rock units at depth, deformed or fractured zones were also inferred from the GPR radargrams. Landslide hazard assessments were also carried out along the foot slopes where communities are exposed to potentially unstable hillslopes. We used analytic hierarchy process (AHP) and GIS techniques to evaluate the relationship of geological, geomorpholohical, and anthropogenic factor to hazard susceptibility. Our assessment reveals that slope failures in the area are strongly influenced by slope gradient, rock type, and degree of weathering. Floods generally affect lakefront and low-lying communities which have become more vulnerable due to urban development. Following the guidelines of the National Economic and Development Authority (NEDA) in disaster risk assessment, the risk of fatality and property damage to different hazards were estimated by multiplying the probability of occurrence of the hazard and its consequences (e.g., cost, extent of affected area). Building damage due to a Mw7.2 earthquake along the West Valley Fault was also assessed for highly urbanized barangay [villages] in Los Baños, where residential, commercial, industrial, and academic structures thrive. Assuming a felt intensity of PEIS 8, almost 40 percent of buildings in the barangay will have a high probability of complete damage – these are structures built from concrete hollow blocks (CHB-type) and those built from concrete hollow blocks with wood or light material (MWS-type). Information derived from the geological and geophysical surveys are important in assessing the effects of ground shaking related to the potential rupture of nearby active faults. Outputs from this project will be useful in mitigating the impacts of geophysical hazards that pose risks to lake-watershed communities, particularly those underlain by deformed and poorly consolidated materials or those that are situated along the low-lying coastal zones of Laguna de Bay. The assessment hazards and risks is also useful for local governments in prioritizing areas for further evaluation and in strengthening their disaster risk reduction and management plans.