Application of tank model and quantitative assessment to predict rainfall induced displacement on landslide – Case study on Nawalapitiya landslide, Sri Lanka

Introducing of advance non-structural methodologies has not established widely to reduce the impact from landslide to communities by understanding the behavior of groundwater in a slope. To understand the behavior of groundwater, different types of models can be used to simulate groundwater levels. Among them, a simple tank model can represent a non-linear flow behavior and can get solutions very quickly. Also it can be used for the long-term analysis of the runoff and groundwater table fluctuations. The tank model is based on an analysis of the water balance, a calculation model that follows water flows in and out of the relevant hydrological system in a hillslope. As a modified and advanced version of this tank model, a multi-tank model was used in this study to analyze the groundwater table fluctuations in the slope. Past data from water level gauges installed in B-5, B-6 and B-7 were used to calibrate and simulate the tank model which was used in this study. Heavy rainfall-induced displacement (HRID) and maximum increase in water level (MIWL) were used as two variables to describe the change of displacement and the level of groundwater in the event of heavy rains. HRID and MIWL relationship and the simulated groundwater level from the tank model were used to estimate the calculated heavy rainfall-induced displacement (CHRID) and it is shown a good relationship with the actual rainfall-induced displacement. Therefore, this approach will be very useful for predicting groundwater level and calculating heavy rainfall-induced displacement particular landslide. An early warning system can be developed using this tank model by providing quantitative assessment which gives us more time to avoid risk through appropriate warning and evacuating rather than actions on real time when the active movements of the landslide occurs.