Abstract
In recent years, the Taiwan government has established a number of flood control facilities such as dikes, pumping stations and drainage systems to effectively reduce downstream flooding. However, with continued development and urbanization of catchment areas, the original designs of most flood control facilities have become outdated. Hillside lands in the upper and middle reaches of river basins have undergone urban development through unsound engineering practices, paving the way for heavy downstream flooding. Therefore, proper river basin management should include both upstream and downstream sides. The main purpose of the paper is to simulate non-urban inundation areas with various degrees of development (0%, 10%, 20%, 40% and 60%), over two different return periods of 25 years and 200 years, for intensive rainfall events in the Shi-Chi District, Taiwan. Through hydrological analysis and numerical simulations of inundation, quantitative data on inundation potential have been established based on the land development conditions along the hillsides on the upper and middle reaches of the Keelung River Basin. The simulated results show that the increase in the extent of land development in the upper reaches causes an increase in the area and depth of inundation, resulting in an increased risk of flooding in downstream areas. If the land-use policy makers in the upper reaches of the river basin’s hillsides do not properly manage the land development, the risk of flooding in downstream areas will increase. In such an event, the policy makers should first review the situation to understand the problem with the consideration of this study. Thus, proper development and flood mitigation in hillsides can be established.
Highlights
In recent years, one of the significant effects of climate change has been an increase in precipitation.In Taiwan, the most general flood disasters are storm floods when high intensive rainfall occurred in the upstream
The main goal of this paper is to examine the impacts of hillside development on downstream flood disasters and flood control mechanisms, as well as to examine the impact of the level of hillside development on downstream flood disasters
Because this study focused on developments in only hillsides with a slope less than 30° in downstream areas and the study area had a valley type of topography, changes in the time of concentration would be too small to have an influence on the rainfall intensity and hydrological conditions considered in developing the design hyetograph [12,13]
Summary
In Taiwan, the most general flood disasters are storm floods when high intensive rainfall occurred in the upstream. Due to record-breaking increases in the intensity and duration of precipitation, many flood control facilities have lost the ability to prevent increases in the volume of runoff induced by development in river basin. In other words, when rainfalls last longer, the retention facilities can no longer effectively prevent flooding downstream. The phenomenon occurs mainly in western coastal areas where there are high hillside developments without efficient facilities, which could balance the runoff. Extreme weather conditions, such as typhoons or heavy precipitation, might strengthen the phenomenon
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