Geospatial technologies plays a vital role in the efficient and accurate assessment of building or infrastructure damage. In hilly regions rainfall triggered mass movement is one of the most widespread earth surface hazards. Mapping the spatial distribution of vulnerable built up areas can help the decision makers for proper planning and initiating preventive measures. In 2014, a rainfall triggered debris flow site, Kathbangla in Dehradun, killed at least 10 people and destroyed many houses in the area. To reduce such disastrous impact on human life and property, risk quantification forms a fundamental tool in risk management. Even though the large landslides are extensively studied and monitored, the smaller ones do not receive due attention due to their localized nature. Close Range Photogrammetry and Terrestrial Laser Scanning offer capability of very high resolution data acquisition, hence become a useful tool for studying small landslide which are difficult to monitor using satellite imageries. The study illustrates an application of 3D GIS for estimating damage in vulnerable built up areas on slopes susceptible to collapse. Since the study area is very small (2910 m2) and the variation in slope is also not very large, hence the conventional Earth Observation (EO) datasets with large to medium resolution are not suitable for capturing the elevation details of the area. Terrestrial Laser Scanner (TLS) was used to generate a dense surface model with a very high point density. The TLS data was segmented and filtered to extract the terrain points. The terrain points were imported to generate a surface model. This surface model was taken as prime data source for simulation of debris flow. Trivim, which uses the concept of digital terrestrial photogrammetry was used to generate a 3D building scenarios using a set of 2D images. Corresponding database was attached to individual building. The RAMMS (Rapid Mass Movements) numerical dynamic model developed by Swiss Federal Institute for Snow Avalanche Research was used to model the run-out of the debris flow. An open source software Trivim results in aggregating all the data pertaining to a block and reconstructing photo-textured 3D models with sub-meter accuracy in planimetry as well as elevation that can be visualized on a geo-portal. 3D GIS database is created by attaching user defined attributes with the individual segments of the model allowing users to perform spatial database query. Trivim generated 3D buildings and associated database was used in conjunction with the simulated debris flow for estimating the risk zones. The technique developed can be an effective method for identifying potential risk prone built up areas.
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