A GIS-BASED APPROACH TO MODELLING DEBRIS FLOW HAZARD: KAN WATERSHED-TEHRAN CITY, IRAN

Debris flows are among the most rapid and dangerous geomorphologic processes in mountainous regions in the world. Because of their high mobility, they can easily affect channel paths and their surrounding environments. Urbanization in Iran is centralized and is expanding towards the surrounding mountains. This urban expansion is expanding the risk related to debris flows. Detailed hazard assessments are, therefore, necessary for the run-out areas on debris-flow fans to protect people, their properties and urban infrastructures against the threat of future debris flow hazards and to better manage the emergency situations.

Debris flows are strongly related to high amounts of precipitation or rapid snow-melt. This is particularly true for clay-rich Shales and pyroclastic basins of Tehran city which are highly susceptible to incidences of flash floods and the initiation of debris flows.

This study is a contribution to geomorphic analysis of debris flow hazards in Kan Watershed in northern Tehran. The history of the risks in this watershed shows that giant floods in the years 1954, 1995, 2012 and a recent flood with a high volume of debris flows in 2015 resulted in widespread destruction and 10 deaths. Therefore, the geomorphological, climatic and morphometric properties of Kan Watershed were taken into account to delineate the potential source of debris flows in northern Tehran.

The threshold of each factor involved in this rapid process was determined by field observation and investigating the characteristic of past evidence. Then, using Boolean expressions, those cells containing defined conditions were extracted and the remaining cells were excluded from the final calculation. The sum of four obtained layers was an initiation map of debris flow hazards within Kan Watershed. For the runout modeling, the Flow-R Model was applied, which calculated the spreading of flow based on a multiple flow direction algorithm using the initiation map as the main input.

To validate the results, a binary overlay of the output with the event inventories of debris flows of Kan Watershed was produced. The result of this study can be used as an input for further hazard assessment and risk management studies.