SCaRP: Simulating Cascading Rainfall-triggered landslide hazards in the Philippines

The Philippines is on the eastern edge of the Maritime Continent, the archipelago of tropical islands that sits between the Indian and Pacific Oceans. High solar input warms the surrounding seas, which supply an abundance of moisture to the atmosphere, turning the whole region into an atmospheric "boiler box". The whole Maritime Continent receives very high rainfall totals throughout the year but due to its location on the eastern edge of the Maritime Continent, the Philippines are also in the firing line of tropical cyclones (also known as typhoons or hurricanes) that form in the Pacific Ocean. Strong tropical cyclones can reach up more than 150 mph and deliver more than 450 mm rainfall in just a a few hours. These extreme rainfall events, combined with the steep, mountainous terrain over the Philippines can produce catastrophic landsliding and related sedimentation in rivers with major societal and economic impacts. The Philippines accounts for half of fatal rainfall triggered landslides in SE Asia despite making up only 6% of the land area. In 2004, a series of typhoons hit Quezon and Aurora Provinces on the east coast of the island of Luzon in the northern Philippines causing landslides and floods that left 1,062 dead, 1,161 injured and 552 missing, displaced almost 7 million people and caused massive economic damage and prolonged human costs. Under climate change, the frequency and intensity of typhoons can be expected to increase with implications for landslide hazards and sedimentation problems in rivers.

The simulation of landslides resulting from typhoons is a complex problem as landslides are also dependent on weather conditions leading up to the extreme rainfall event that influence the stability of the layer of soil covering hillslopes. If soils are already wet leading up to an extreme rainfall event, landslides are more likely. It is therefore necessary to understand the meteorological patterns operating over the Philippines and how these influence patterns of landslides in typhoons. Another major factor that may limit landslide occurrence and size is the depth of the soil layer. A critical soil depth is needed for a landslide to occur and the size of the landslide will be limited by the depth of the soil. Once a landslide has occurred, it takes time for the soil layer to regenerate, with implications for future landslide hazard in the same area. Whilst landslide scars on hillslopes are exposed landslides may continue to deliver sediment to the downstream river system. Hence sedimentation problems in rivers downstream of landslides may persist for a period of years after the landslide event.

The SCaRP project combines the research strengths of the UK and the Philippines and brings together experts in geomorphology, meteorology and hydraulic engineering to effectively and efficiently address the need for better understanding of the impact of hydrometeorological hazards and support increased preparedness and resilience to future events. First, the characteristics of past extreme rainfall-triggered landslide events and their meteorological and geological controls over the Philippines will be determined, using a combination of in situ station data from the Philippines and global data sets from satellites to map landslides and determine rainfall patterns. Second, a number of events will be used to develop and test models for simulation of landslides and downstream sedimentation. Once the model has been tuned, it will be used to predict landslide events and river sedimentation in the future using climate projections for the region. In conjunction with our Filipino project partners, PAGASA (Philippine Atmospheric, Geophysical and Astronomical Services Administration), (MGB) Mines and Geosciences Bureau and Weather Philippines Foundation (WPF), we will develop a landslide early warning system for the Philippines using our modeling framework, ensuring a legacy from the SCaRP project.

Here we summarise who is likely to benefit from SCaRP research and how they will benefit. The Pathways to Impact attachment describes what will be done during SCaRP to maximise the likelihood of the SCaRP research reaching those beneficiaries.

Scientists, government agencies and the general public of the Philippines will benefit from enhanced understanding of the causes of landsliding in the Philippines, and from the installation of a landslide warning system. As a result, it will be possible to save lives through better landslide warnings. Furthermore, by improving understanding of the drivers of landsliding at a range of timescales from hourly to seasonal, we will improve the ability of government agencies to prepare for hazards and increase resilience to these hazards.

Project partners at PAGASA will benefit through increased understanding of the drivers of extreme rainfall and landsliding in the Philippines, from large-scale climate variability to typhoon dynamics. Through improved understanding of the precipitation thresholds and conditions that trigger landslides these scientists will be better placed to issue warnings of upcoming hazardous rainfall. PAGASA will also benefit from capacity building and knowledge transfer in landslide modeling, and the installation of a new landslide modelling framework linked to Project NOAH (Nationwide Operational Assessment of Hazards) for issuing seasonal landslide forecasts. This framework will enable improvements to the reliability of seasonal landslide forecasts as it considers changes in preconditioning factors such as soil thickness through time, a significant improvement on static maps of landslide susceptibility currently used. Staff at PAGASA will benefit from training and continued technical support in the installation, tuning, initialisation, forcing and interpretation of modelling framework. This will be installed on their systems and will be available for their continued use as a legacy of the SCaRP project.

Project partners at MGB will benefit from higher resolution and dynamic landslide susceptibility maps related to extreme rainfall as well as simulations of landslides under climate change. This will enable to better manage the Philippine's mineral resource and critical infrastructure. Project partners at NWRB will benefit from simulations of sediment transport linked to landsliding under climate change which can be used to help them 1) ensure the supply of safe and clean water for communities in the Philippines and 2) help maintain the dams for which the NWRB are responsible.

Climate change policymakers more widely will benefit from projections of changing landslide hazard in the Philippines under high and low emissions scenarios. By demonstrating the change in landslide hazards, we will provide useful information to guide policy decisions on the value of global emissions reductions for saving lives and protecting livelihoods in the Philippines.

Through communication with the wider public in the UK and the Philippines, both online and at key events, we will improve the understanding of scientific research methods and applications. By demonstrating the direct benefit of international collaborative research to vulnerable communities we will inspire young people in the UK and the Philippines to be interested in science, and by doing so, we will contribute to reducing the skill shortage amongst UK & Filipino scientists.