Author(s): Ali Najafi Nejad
Linked Author(s):
Keywords: Debris flow; Sedimentology; Sediment yield; Smallwatersheds; Mass movement; Massive erosion
Abstract: A debris flow event occurred in summer 1999 in Ziarat watershed, Golestan province, northeast of Iran. The debris flow originated shortly after an intense rainstorm in the upper part of watershed whose runoff mobilized the debris. The watershed under study has six subwatersheds with total area of 9755ha and average annual precipitation of 550 mm. Debris flow event originated only from one subwatershed (Tollbooth). 12 sediment samples were collected from depositional sites, one day after event. Sediments were taken from different locations from upper part to lower part of main stream of Ziarat watershed. Grain size distribution analysis (granolometry) has been performed samples. For each sample, particle size distribution curve has been plotted. Different indexes including d10, d50, d75, and d90 and ¡- has been determined. Field observations showed that shallow land slides in the valley sides of stream and stream bank erosion was the main source of debris flow. So for comparison of sediments of source and deposition area 7 locations in upper part of watershed (source area) were selected and 2 samples from right and left bank of the stream were collected. The samples collected from high level mark of water. Also grain size distribution analysis have been performed for each groupie of samples and Krumbine and symmetry indexes and trask coefficient were computed. The sedimentological indexes computed for deposition and source area were compared with each other. The results showed that the trask coefficient was 9.6-this shows that prevalent process, was debris flow no a flood. Also the results of granolometry showed that the two groups of samples were very similar. So granolometry can be used as a suitable method for identification of source of debris flow in small upper watersheds. Total average of sediment production of debris flow is estimated to be121500 m3, that is 10 times more than the average of annual sediment yield according to the records of hydrometric station.
Year: 2001