Modelling the effect of buildings on water related rapid mass movements

Water related rapid mass movement such as landslides, debris flows, and slush flows are expected to become more frequent as the climate continues to change. Hazard zoning in urban areas is necessary to save lives and to prevent damages to existing and future buildings. Previous events have shown that the runout of such events can be strongly influenced by the buildings and infrastructure in the runout path. Buildings can stop or reduce, but also redirect the movement of the flow. Therefore, one of the challenges for hazard zoning in urban areas is determining how to take existing buildings into consideration when assessing runout of rapid mass movements.

In this work, we have explored to which degree RAMMS::Debrisflow can be utilized to estimate the effect that existing buildings have on the runout of landslides, debris flows and slush flows. We aim at developing a general procedure that can be applied for hazard mapping at a large scale. We have studied nine previous rapid mass movement events where buildings have affected the runout. For each event, the runout has been back-calculated in RAMMS::Debrisflow 1) without taking buildings into consideration, 2) using increased friction for areas with buildings, and 3) using “obstacle/dam”-mode for areas with buildings.

Our study shows that modelling that takes buildings into account more accurately represent the runout of the different historic rapid mass movements than modelling without taking the effect of buildings into account. We therefore recommend using such an approach when assessing the hazard of rapid mass movements in urban areas.

We propose to classify the robustness of buildings to account for the varying effect that different types of buildings have on runout. This can be accomplished on a larger scale by using public datasets that include attributes for buildings, such as the Norwegian FKB-dataset. For example, a large, robust concrete building might fully stop runout and is best represented in RAMMS::Debrisflow as an obstacle. Wooden residential houses and other buildings with moderate robustness might retard, but not fully stop runout and are best represented using areas of higher friction. Small and fragile buildings such as sheds or small garages are expected to have negligible effect on runout, and we suggest to not take these into consideration when modelling runout.

Predictably, the effect that buildings have on runout is depending on the intensity of the flow and construction method of the building. There will therefore still be a need for expert judgement when assessing resistance of buildings to the mass flow and in interpretation of results on a detailed scale. Our proposed method can be viewed as a first step towards such an assessment. By utilizing the large building datasets (such as the FKB-dataset), the practitioner can make a quick and practical substitute for a tedious structural assessment of each building, thus increasing efficiency for the hazard engineer.