Volcanic b Fig. A typical example of temporal variation in a cross-profile of gully, Sakurajima after Shimokawa and Jitousono h Field Survey for Debris Flow in Volcanic Area 53 The cross-section has unequally developed by undercutting sometimes and by lateral expanding sometimes because the resistance to erosion varies with the geological constituents of gully bed and side wall. Mean annual sediment production per 1 k r n 2 by the interrill, rill and gully erosion on the hillslopes of Sakurajima is not less than m 3, coming to over 93 mm in annual erosion rate Shimokawa and Jitousono c.
Judging from the previous works in Usu volcano Chinen and Mt. Helens Collins and Dunne t , it seems that a radical rise of the sediment Field by sheet-rill-gully erosion on the hillslopes thinly overlaid with pyroclastic airfall and surge deposits was nearly finished within a year to 2 years after the completion of volcanic eruptions because of washing out of fine pyroclastic materials by sheet erosion and of the recovery of infiltration rate.
Recent Developments on Debris Flows | Aronne Armanini | Springer
However, the high erosion rate have been kept over 20 years under a long term volcanic activity with frequent ash eruptions since in Sakurajima. Merapi volcano located in the central part of Java island, erupted with large scale pyroclastic flows in June The pyroclastic flow deposits of approximately 6.
A total amount of sediment yield by sheet-rill-gully erosion from the pyrocIastic flow deposits covered hiUslopes was evaluated and the rates of sediment yield were estimated based on the field survey and interpretation of aerial photographs. The sediment yield at the basins rapidly increased to a peak for a year from November to October and then 54 E.
Jitousono slowly declined with time keeping high level for approximately 4 years until October This is harmonious with that the debris flows occurred many times for a period of approximately 4 years after the pyroclastic flows. Comparing with the effect of airfall tephra, it is assumed that the pyroclastic flows in Merapi had the effect on the sediment yield over a longer period, probably because the deposits prone to erosion filled drainage channels and the recovery of infiltration rate was retarded.
Hydrological and Hydraulic Surveys on Debris Flow Video camera and ultrasonic waves water gauge are usually used for investigating the hydrological and hydraulic characteristics of debris flow. Dynamic picture photography by use of video camera is a most common method of field survey for the debris flow.
In this system, arrival of a debris flow is detected by cutting of the wire sensors set across the river and simultaneously video camera and video tape recorder start recording motion picture of the debris flow. Many debris flow motion pictures have been obtained in Yakedake Okuda et al. Helens Pierson These motion pictures are available for analyzing geometrical, hydrological and hydraulic characteristics of the debris flows. The ultrasonic waves water gauge is an uncontact type apparatus in which water stage is detected by a round time of ultrasonic waves sent from a transmitter and receiver to water surface.
This apparatus is effective for observing the debris flow with much sediment as well as flood and is possible to get the records of all the debris flows and floods including small scale one throughout the year. The data are available for analyzing runoff characteristics of debris flow, water balance and sediment yield in a basin. Jitousono installed at each site of the lowest reaches of Saido-gawa and Arimura-gawa, Sakurajima volcano just near which video camera for observing the debris flow had been set before. All the rivers including these rivers developed radially from the summits towards the surroundings, have running water 0nly at rainfall.
The apparatus is going on operation throughout the year.
Approximately 80 per cent of all the debris flows and floods were observed during the summer season from May to September with much monthly rainfall. The distinction between debris flow and flood depends on magnitude of peak discharge and shape of hydrograph. Using these records including rainfall data, the critical rainfall which is defined by two parameters, a rainfall just before the occurrence of debris and cumulative rainfall from its beginning, and the effect of volcanic activity on its temporal variation were examined.
The rainfalls just before the occurrence of debris flow was 4 to 5, 3 to 4 and 2 to 3 mm in minute intensity in correspondence to the cumulative rainfall of approximately 10, 20 and 30 mm, respectively. And the critical rainfall showed a little variation corresponding the rise and fall of volcanic activity within 13 years from to The occurrence condition of debris flow due to rainfall was investigated in Merapi volcano, also.
The critical rainfall was as low as Sakurajima for approximately 4 years after the pyroclastic flows and then increased with year Jitousono et al. Runoff characteristics of the debris flows and floods were analyzed by use of the water-stage observation records at the two volcanoes, Sakurajima and Merapi. Jitousono area at the observation station is 1. A nearly linear relation is obtainable on a logarithmic graph paper with three parallel alignments each other depending on catchment area and observation period.
According to the observation by the video camera at the Sakurajima, the debris flows are mostly muddy type including much volcanic ash. So, the most part of debris flows at the Merapi may be mudflow. M e a s u r e m e n t s of Sediment Concentration and Evaluation of Sediment Yield Sediment concentration of a debris flow is an important factor to evaluate sediment yield as well as to examine flow behavior.
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A hand-powdered equipment for dip sampling the slurries was suspended over the center of a cable crossing the channel by pulley at the same site as the ultrasonic waves water-gauge, the lowest reaches of Saido-gawa, Sakurajima. Sampling of the slurry of debris flow was done many times by use of this equipment corresponding to the water-stage hydrograph of a series of debris flow. The sediment concentration linearly increases with the discharge with a extent of scatter from the regression line.
The annual sediment yield by both of the debris flows of 18 in number and the floods of 17 observed at the Saido-gawa basin in was evaluated from both of the hydrographs and the sediment concentration at each stage of a hydrograph. The annual sediment yield from the basin of 1. In this calculation, the high sediment concentration for the larger scale debris flows was obtained by extrapolation. It is available only for evaluating the total sediment yield by debris flows triggered by a rain.
Sh]mokawa and T. A relationship between total runoff and sediment yield of debris flows induced by a rain Jitousono et al. C o n c l u s i o n s The hydrological and geomorphologieal characteristics of debris flow associated with volcanic eruptions were examined mainly on the basis of field observations and measurements at the three volcanoes, Sakurajima and Unzen and Merapi.
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This generated surface runoff on the tephra covered hillslopes even under a less rain with 1. Shlmokawa and T. Jitousono variations depending on the type and magnitude of volcanic activity. The sediment yield from the pyroclastic flow deposits covered hillslopes in Merapi kept high rates for approximately 4 years after the eruptions.
Usu and other volcanoes: Environ. H0kkaido, 9 1 C Collins, B. Helens: Geol.
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Erosion Control Engineering, 37 2 , in Japanese with English abstract. Helens: Washlngton 1 : Jour. Erosion Control Engineering, 37 2 , Janda, R. Helens, Washington 2 : Jour. Erosion Control Engineering, 37 3 , Jitousono, T. Forests, 15, in Japanese with English abstract. Jitonsono, T. Erosion Control Engineering, 42 3 , in Japanese. Erosion Control Engineering, 43 6 , in Japanese with English abstract. Erosion Control Engineering, in print.
Super Typhoon Bopha and the Mayo River Debris-Flow Disaster, Mindanao, Philippines, December 2012
Helens, Washington: In Abrahams, A. Erosion Control Engineering, 39 6 , in Japanese. Erosion Control Engineering, 40 1 , in Japanese. Shlmokawa, E. Union, 8, in Japanese with English abstract. Erosion Control Engineering, in print Suwa, H. Kyoto Univ. Swanson, F. Helens and other volcanoes: Proc.
Related Recent Developments on Debris Flows (Lecture Notes in Earth Sciences)
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