Browsing by Author "Shih, Dong-Sin"

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Applying Biomineralization Technology to Study the Effects of Rainfall Induced Soil Erosion
(2019-12-01)
Shih, Dong-Sin
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Lai, Tzu-Yi
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Hsu, Zuo-Ming
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土木工程學系
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Department of Civil Engineering
The rainless days and drought seasons reveal a tendency to lengthen the wet and dry period in recent years in Taiwan. In the bare riverbeds in central Taiwan, such as the Dajia and Zhuoshui rivers, fugitive dust is the common problem during the winter's dry period with northeast monsoon. The study aims to use the biological method, Microbial-induced carbonate precipitation (MICP), to solidify the soil and implement a series of rainfall simulator experiments to reduce the Aeolian dust emission problems. Accordingly, the relationship between rainfall-induced soil erosion and its soil specimens are also discussed. The soil conducted the MICP for seven days as the curing age, and then be analyzed the degree of soil solidification under different conditions, rainfall intensity of 41 mm/h and 61 mm/h by the soil erosion experiment. The effect of soil solidification with various relative density of soils of 60%, 70%, and 80%, and hillside slopes of 5 degrees, 10 degrees, 20 degrees were tested. The result indicated that, the higher the relative density of soils, the better the effect of soil solidification would be. The relative density of soils from 60% to 80% all kept the effect of soil solidification as applying to MICP. Therefore, it was important to select the curing age with the matching relative density of soils. Moreover, the most appropriate condition for the effects of soil solidification by MICP was the slopes below 10 degrees and the curing age for seven days. The effect of soil solidification was still preserved in that with the high intensity rainfall (66 mm/h) due to the Aeolian dust emission commonly happening in the bare riverbeds with mild slopes.
Experimental and Numerical Investigations of Turbulent Open Channel Flow over a Rough Scour Hole Downstream of a Groundsill
(2020-05-01)
Chang, Cheng-Kai
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Lu, Jau-Yau
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Lu, Shi-Yan
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Wang, Zhong-Xiang
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Shih, Dong-Sin
This study discusses the mechanism for the occurrence of equilibrium and non-equilibrium scour holes. By using a particle image velocimetry (PIV) measurement system, it measures the turbulent flow fields in an open channel moving through the rough bed below a groundsill. Then, the Reynolds-stress model (RSM), embedded in FLUENT software, is applied to perform a numerical simulation. The experimental results show that at equilibrium, the location of the re-attachment point is significantly affected by the flow discharge. Further, the re-attachment point of the scour hole affects the size and range of the counterflow zone, which becomes the main region for deposits in the natural channel. In addition, the formation of erosion is mainly affected by turbulence intensity and Reynolds stress. However, in non-equilibrium scour holes, our results clearly show that the turbulence intensity and the Reynolds stress are significantly larger at the end of the scour holes near the bed due to the continual development of the scouring. The correlation between the numerical simulation and experimental results are also examined. Overall, it can be seen that the simulated mean velocity profiles are quite consistent with the measured data. However, in terms of turbulence intensities and Reynolds stress, the simulated results could be overestimated when compared with the measured data; they are overestimated with a sudden decrease near the liquid surface. Although, the simulations in the near bed area show some divergence and the trend in the scour hole is quite consistent. Therefore, numerical simulations can be performed in advance to act as an important reference when evaluating the safety of downstream structures.
Reducing of Bend Scour by Setting Spurs in a Curved Channel
(2020-05-01)
Shih, Dong-Sin
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Lai, Tzu-Yi
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土木工程學系
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Department of Civil Engineering
In Taiwan, the rivers not only are fast-flowing with high discharge, but they badly erode their beds during the typhoon seasons. In addition, erosion on the concave bank in a meandering channel is the primary cause of levee break. Therefore, the study conducted a down-scale experiment on erosion induced by oblique flow in a laboratory. It was similar to number 27-34 cross sections of the Fengshan river of Hsinchu County, Taiwan. The region was chosen because there are some special attacking angles of water flow and historical precedents of levee break. The study adopted the discharges of return periods of 10 and 20 years and measured the flow field by laser doppler velocimetry (LDV). Then the protective effects with different spur types were examined. The results indicate that increasing velocity induces side erosion when the flow impacts with the adjacent angle on the concave bank. However, the decreasing of velocity causes deposition of sediment on the concave bank. Furthermore, based on the vertical velocity profile of water flow, a higher flow rate is measured in the downstream on the concave bank. After the spurs are installed, the velocity at the spurs in the downstream is reduced, and the cross section with the larger velocity is moved to upstream. In addition, after setting the spurs, the reduction rates in volume of scour are 7.97% of a 10 year return period and 4.65% of a 20 year return period, respectively. That demonstrates the scour is effectively reduced as long as the spurs are set. Although the erosion mitigation rate and protection effect are decreased when the velocity is high, there is still a good protection effect at the bank. The setting of spurs has the following effects: First, the maximum scour depth generates in the front spur, while the maximum scour position keeps away from the bank. Then, the overall flow rate can be reduced to approximately 35%-40% comparing with the original flow field. Lastly, the spur on the slope of 1/30 degrees demonstrated the best function of stretching the distance from the embankment.
Using a PIV Measurement System to Study the Occurrence of Bursting in the Flow Over a Movable Scour Hole Downstream of a Groundsill
(2020-05-01)
Chang, Cheng-Kai
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Lu, Jau-Yau
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Lu, Shi-Yan
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Hsiao, Kuo-Ting
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Shih, Dong-Sin
Generally, hydraulic structures are installed along with rivers in Taiwan to prevent erosion. The groundsill is one of the most common structures to protect the underlying riverbed. However, the occurrence of bursting during the process of scouring can intensify the disturbance of sediment in the bed, sometimes even causing hydraulic structures to collapse. This paper aimed to study the mechanisms of bursting, the effects of bursting, and the scouring exceedance probability of sediment movement. To study this topic, a particle image velocimetry (PIV) was used to measure the hydraulic characteristics of a scour hole under different flow conditions. The results showed that, firstly, the bursting and the sediment entrainment rate increased with time at the beginning. Secondly, when bursting occurred at the beginning stage of scouring, the averaged velocity of main flow was reduced by about 30% and the thickness of the riverbed was deepened by about 20%. Moreover, when scouring time was 15 min, at the location of maximum scouring depth, all the experimental groups carried the proximity values of the scouring exceedance probability that stuck to a range from 35% to 53% at the bursting stage. Therefore, the scouring exceedance probability of the bursting of the maximum scouring depth can be further applied to designs and to protect the foundation of hydraulic structures.