Hsiao Wen Wang

Sustainable sediment management in reservoirs and regulated rivers: Experiences from five continents

By trapping sediment in reservoirs, dams interrupt the continuity of sediment transport through rivers, resulting in loss of reservoir storage and reduced usable life, and depriving downstream reaches of sediments essential for channel form and aquatic habitats. With the acceleration of new dam construction globally, these impacts are increasingly widespread. There are proven techniques to pass sediment through or around reservoirs, to preserve reservoir capacity and to minimize downstream impacts, but they are not applied in many situations where they would be effective. This paper summarizes collective experience from five continents in managing reservoir sediments and mitigating downstream sediment starvation. Where geometry is favorable it is often possible to bypass sediment around the reservoir, which avoids reservoir sedimentation and supplies sediment to downstream reaches with rates and timing similar to pre-dam conditions. Sluicing (or drawdown routing) permits sediment to be transported through the reservoir rapidly to avoid sedimentation during high flows; it requires relatively large capacity outlets. Drawdown flushing involves scouring and re-suspending sediment deposited in the reservoir and transporting it downstream through low-level gates in the dam; it works best in narrow reservoirs with steep longitudinal gradients and with flow velocities maintained above the threshold to transport sediment. Turbidity currents can often be vented through the dam, with the advantage that the reservoir need not be drawn down to pass sediment. In planning dams, we recommend that these sediment management approaches be utilized where possible to sustain reservoir capacity and minimize environmental impacts of dams.

Assessment of climate change impacts on flooding vulnerability for lowland management in southwestern Taiwan

Taiwan suffers from losses of economic property and human lives caused by flooding almost every year. Flooding is an inevitable, reoccurring, and the most damaging disaster in Taiwan since Taiwan is located in the most active tropic cyclone formation region of the Western Pacific. Flooding problem is further worse in land subsidence areas along southwestern coast of Taiwan due to groundwater overdraft. Increasing number of people is threatened with floods owing to climate change since it would induce sea level rise and intensify extreme rainfall. Assessments of flooding vulnerability depend not only on flooding severity, possible damage of assets exposed to floods should also be simultaneously considered. This paper aims at exploring how climate change might impact the flooding vulnerability of lowland areas in Taiwan. A flooding vulnerability evaluation scheme is proposed in this study which incorporates flooding severity (the maximum inundation depth determined by a two-dimensional model) and potential economic losses for various land uses. Effects of climate change on flooding vulnerability focus on alterations of rainfall depth for various recurrence intervals. The flood-prone Yunlin coastal area, located in southwestern Taiwan, is chosen to illustrate the proposed methodology. The results reveal that reducing flooding vulnerability can be achieved by either reducing flooding severity (implementation of flood-mitigation measures) or decreasing assets exposed to floods (suspension of land uses for flood-detention purpose). Performance of currently implemented flood-mitigation measures is insufficient to reduce flooding vulnerability when facing with climate change. However, the scenario suggested in this study to sustain room for floods efficiently reduces flooding vulnerability in both without- and with climate change situations. The suggestions provided in this study could support decision processes and help easing flooding problems of lowland management in Taiwan under climate change.

Three-dimensional saltating processes of multiple sediment particles

Abstract The purpose of this study was to investigate the interacting mechanism between the saltating particles near a channel bed. A three-dimensional real-time flow visualization technique was developed to measure the interparticle collision behaviors during the saltating process. Based on the experimental data, the distribution of the collision points was found to be symmetric. This confirms the assumption that the projections of the collision points onto the reasonable plane are uniformly distributed. A three-dimensional saltating model was also developed. This model produced satisfactory results. The model is able to simulate the continuous saltating trajectories of several particles. The simulated dimensionless saltating height, longitudinal and vertical saltation velocity components were found to increase as the dimensionless particle diameter and the dimensionless flow transport capacity parameter increase, while the simulated lateral saltation velocity component varies inversely with the dimensionless flow transport capacity parameter. A regression equation for the bed load transport rate was also obtained.

Community responses to dam removal in a subtropical mountainous stream

Dam removal has the potential to efficiently solve the problems caused by fragmented stream habitats but may simultaneously cause negative impacts on biotic communities. To conserve the critically endangered Formosan landlocked salmon (Oncorhynchus masou formosanus), a 15-m-tall check dam was partially removed from the Chichiawan Stream at the end of May 2011, before the flood season. Using this dam removal experience, we aimed to cast dam removal as an action comparable to a natural flood event. We applied a before-after-control-impact (BACI) design and quantified the environmental factors and major biotic communities at four sampling sites in the stream bimonthly before (2010) and after (2012 and 2013) the dam removal and monthly in the year of the dam removal (2011). After the dam removal, a faster current velocity and more turbid water were observed at the downstream sites, and the area’s deposition consisted of small-grained sediments. Despite this, our results show that the dam removal was performed during a suitable period. There was no obvious influence on tadpoles as they metamorphosed into adult frogs and left the stream before the dam removal. Fish exhibited a greater resistance to the alteration in flow resulting from the dam removal. An increase in fish abundance at the upstream sites after the dam removal suggests that the corridors created by the dam removal allowed access to more habitats for the fish. In particular, the periphyton biomass and aquatic insect densities decreased markedly at the downstream sites after the dam removal, but they recovered within a year, demonstrating the resilience of these taxa. Coleoptera, Plecoptera and Trichoptera were more resistant than the periphyton, Diptera and Ephemeroptera after the dam removal and an extreme flood event. In conclusion, the responses of stream communities to dam removal were similar to the responses to an extreme flood event. To mitigate the impacts caused by dam removal, our results suggest that stream communities may respond to dam removal as a natural flow alteration if the timing of the dam removal occurs just before the flood season.

Experiments on channel evolution caused by check-dam failure

A 10 m long, 0.2 m wide flume was employed to simulate the channel bed evolution of check-dam failure. The experiment longitudinal profiles, the gradient of channel bed, head-cutting propagation distance and deposition length were compared with the theoretical solution derived from a sediment transport diffusion equation. In contrast with the theoretical solution, two different gradients were obtained upstream and downstream of the check-dam. The theoretical solution provides a good description of the changes upstream of the check-dam. The ratio of clear water depth to sediment moving layer thickness in the experiment was analyzed and showed that high concentration sediment laden flow was taken in the incipient of check-dam failure, which may be the reason why the experiment result was slightly different from the theoretical solution in the downstream of check-dam.

Experiments on channel evolution due to dam removal in Taiwan

Due to safety concerns and habitat restoration for landlocked salmon, a 13-m high check dam on Chijiawan Creek was removed in late May 2011 in Taiwan. We conducted experiments to understand channel evolution of different scenarios. We further compared our experimental results of riverbed elevation changes with the analytical solutions derived from the diffusion equation and field dynamics as well after the creek experienced the first flood event. The results indicated that magnitude of discharges and notch size are dominant factors in resulting channel evolution. While the largest differences between grain size distribution are associated with discharge, the largest differences in net change in upstream volume are associated with notch size. While the theoretical equation could help understand the channel change after dam removal, it only explained the evolution closer to the dam. The physical experiments, on the other hand, provided insights especially with regard to comparing alternative proposed management actions. The discrepancies between predicted and actual outcome highlight more needed inputs for future dam-removal assessments.