Effects of Forest Fires and Slope Failures on Rainfall Threshold to Induce Bedload Discharge in Forested Watersheds, Republic of Korea
Abstract
Among natural disasters, forest fires and landslides are typical disasters that occur in mountain areas. Over the past 5 years in Korea, an average of 661 ha of forest has been damaged each year by forest fires. With respect to the annual average area damaged by forest soil sediment disasters such as landslides and debris flows, there is a gradually increasing trend with 231 ha, 349 ha, and 713 ha being damaged during the 80s, 90s, and 2000s, respectively. In Korea, when an area is damaged by a forest disaster, such as forest fire or landslide, erosion control works are implemented for afforestation of the damaged area and prevention of secondary damage due to sediment discharge in downstream areas. Among these, the most common type of erosion control is the construction of an erosion control dam. To maximize the efficiency of the design of erosion control dams and their effectiveness after construction, it is important to quantitatively estimate the amount of sediment discharged in the area damaged by forest disasters and to obtain information related to sediment discharge, including an understanding of the mechanism involved in sediment discharge. In the present study, we constructed the bedload monitoring system to analyze the rainfall characteristics that initiate bedload transport in areas damaged by forest fires or landslides and unaffected mountainous watersheds. We also aimed to identify the effects of forest fire and landslide damage on the rainfall threshold of bedload transport. For the comprehensive monitoring of bedload discharge, a bedload monitoring system was established by combining observation sensors and equipment. Sensors that can take direct or indirect measurements were built into the monitoring system for use in calibration and interpretation of indirect measurements. For the indirect method, a Japanese-developed hydrophone incorporating acoustic sensors was installed. For the direct method, a slot sampler with load cell sensors was used. Additionally, a rain gauge, water level meter, water velocity meter, turbidity meter, and CCTV were set up for hydrological observations. The discharge characteristics of bedload in forest areas damaged by the forest fire, slope failure, and deforestation were analyzed. The results showed that the area damaged by forest fire showed the lowest threshold rainfall conditions, which is associated with the discharge of bedload, and the highest bedload discharge. Such findings provided quantitative evidence for the importance of emergency restoration measures in forest fire-affected areas, as well as the need for erosion control to prevent secondary damage due to soil erosion. Compared with the control site, the forest damaged sites showed bedload transport events even in the low rainfall conditions of about 72 - 92 mm in the total rainfall amount and 2 - 10 mm/h in the maximum rainfall intensity. It was thought that forest fire and slope failure (felling) gave an impact on the rainfall threshold on the bedload transport event. Also, it can be thought that it is important that not only the prevention of direct damage caused by a forest fire and slope failure (felling) but also the prevention of secondary damage such as soil erosion. It is expected that a larger database will increase the reliability of this result, and will help better understanding of the soil erosion control works and the bedload transportation mechanism.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMNH33E0957S
- Keywords:
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- 1810 Debris flow and landslides;
- HYDROLOGY;
- 1826 Geomorphology: hillslope;
- HYDROLOGY;
- 4306 Multihazards;
- NATURAL HAZARDS;
- 7212 Earthquake ground motions and engineering seismology;
- SEISMOLOGY