The construction of fully closed check dam (CD) is a conventional flood prevention mechanism implemented on rivers. Fully closed CDs trap large amounts of sediments in rivers to stabilize the river slopes and control erosion. However, fully closed CDs cannot selectively trap sediment and may easily overflow, causing them to losing their ability to mediate and hold sediments. Previous studies proposed the concept of "breathable CDs". The researcher introduced metal slit dam (SD) that could be assembled and disassembled quickly and conveniently. Once a CD reaches maximum capacity, operators must ensure that the water channels of the dam are free from blockage. Moreover, they must inspect the internal accumulation conditions of the dam periodically or immediately following heavy typhoon rains. When necessary, either the sediment buildup in the upriver blockage must be cleared, or the transverse structure of the dam must be removed to allow fine particles to be discharged along with a moderate amount of water. These actions can free up the sediment-storing capacity of the dam for the next heavy typhoon rains. In addition, operators should also inspect the damages inflicted on the dam, such as erosion, wear and tear, and deformation conditions. Damaged components should be disassembled and repaired if possible, or recycled and reused. The present study performed channel tests to simulate closed CDs, SDs, steel pipe dam (SPDs), and steel pipe plus slit dam (SPSDs) for 50-year and 100-year frequency floods. Results were then analyzed to determine the sediment trapping (ST) effects of various CDs, the effects of "adjustable CDs", and the changes of moderated riverbeds.
Lacus Creek is one of the important tributaries of the Laonong River upstream of the Gaoping River, which is an important river in southern Taiwan. After Typhoon Morakot in 2009, severe sediment disasters occurred, which affected the security of local revitalization, and the government has since invested in numerous projects to reduce the impact of sediment and flood disasters. In 2017, the Kaohsiung City government built a sabo type dam upstream of Fuxing Bridge in Lacus Creek; however, it was severely damaged by a heavy rain less than a year later. To investigate the cause of the damage, site investigations, concrete sampling tests, hydrology analysis, and reviews of the planning and design methods, construction process, etc. have been conducted, in order to understand the cause of the problem, avoid repeating mistakes, and clarify the responsibility of the administrative agency. The investigation and research found that many recent cases of torrent management have considered the transverse structures as isolated wildlife corridors, reduce the height of the transverse structures, create gentle slopes of the downstream surfaces, add fish passages, cut (openings) export slits, etc.; however, after setting up these environmentally friendly facilities, their effect and impact have not been explored in depth. This research survey found that, although the frequency of heavy rain events is not high in recent years, the current iterative impact of continuous rainfall may exceed the original design requirements. If the overflow mode with a single export opening is added, the scouring energy will be concentrated, and this additive effect is likely to cause the foundation of the sabo dam to lose bare space, and finally, the dam will be damaged. Therefore, it is recommended that when designing these torrent transverse structures, try to avoid using the method of a single narrow opening: it is better to increase the number of export openings (like a slit dam), expand the size of a single export opening, or adopt a fully enclosed dam design.
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