Abstract

Direct losses of the episodic flood events, whether of life or property in north of Tehran (capital city of I.R.Iran) confirm that watershed management has to review on flood control projects, and they should consider of the episodic hyperconcentrated flows in the mountainous north
of Tehran. Identification of unconsolidated material sources was done through application of remote sensed data, aerial photos, and field observation. Available geologic, soil, topographic maps, and hydrometric, and climatic records were the other required data that by application of GIS
approach contribute for preparing risk-zoning map for mud and debris flows.

Occurrences of mudflows are reported in Kan watershed but no large mudflow deposits are found. Mass movement deposits are limited to a few rockflow deposits, and shallow soil flows. In adjacent catchments with the same lithology and relief, one large rockflow and mudflow deposits were found.

Scree slopes are the dominant factors for originating mudflows because they contain large quantities of debris. Most scree slopes occur within 1400 meters distance far from the incising river network. The slope steepness of the screes varies much, up to 70 degrees. The composition of the screes is highly variable, as expected with the large range of slope steepness. The NDVI values give an indication of the percentage of fine-grained materials of the screes. uspended loads of high discharges can be more than 10 mg/L, indicate episodic large supply of sand, silt, and clay.

By stereo-aerial photo interpretation, a risk-zoning map was made. The high-risk zone consists of screes, active soil flows, slope with thin screes, and debris (old mass wasting deposits). The moderate zone consists predominantly of dip slopes, and the low risk zone includes scarp slopes, slopes with steep dips, thin local screes, and valley fill. The
“Infinite Slope Analysis” method was used to make a prediction of future risks with various recurrence intervals of 24 hours rainfalls. Analysis indicates the potential amount of unconsolidated materials that can be entrained to form
mudflow.

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