Browsing by Author "Lin, Ying-Tien"

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A field study of nearshore environmental changes in response to newly-built coastal structures in Lake Michigan
(2014-03-01)
Lin, Ying-Tien
;
Wu, Chin H.
;
防災與水環境研究中心
;
Disaster Prevention and Water Environment Research Center
In this study, we monitored changes of cohesive nearshore environment including bluff and lake bottom/bed response to newly-built coastal structures with a thousand-meter-long revetment in Lake Michigan shoreline over a six-year study period. Sequential aerial photos showed that excessive slumping occurred only on the south bluffs and no bluff recession in the middle areas with coastal structures. Field measurements using our recently developed integrated geophysical techniques provided information on bathymetry, sand layer thickness, and lakebed downcutting over the nearshore reach of Concordia University in Lake Michigan. During the study period, the bathymetry profiles at the study site fluctuated dynamically, especially in the regions outside the shoreline structures, suggesting continuous and ongoing sediment erosion and deposition. The lakebed downcutting in front of the newly-built coastal structures is correlated with CWIH (cumulative wave impact height). Significant differences of lakebed downcutting in the north and south natural beach regions were revealed and may be associated with the nearshore sediment budget. The southwardly dominant longshore current maintains the equilibrium state of beach profiles in the north region, but the coastal structures prevent sediment supply from the well-protected bluffs in the middle region to the south region. The possible source of sediment supply in the south region is therefore from lakebed or bluff materials, supported by excessive bluff failures and lakebed downcutting. Overall the newly-built coastal structures seem to pose negative impacts on bluff stability at the south shore of the coastal structures. (C) 2014 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved.
Application of Acoustic Signals to Investigate Macrophyte Growth and Bottom Sediment Stability after Carp Removal in a Small Lake
(2013-01-01)
Lin, Ying-Tien
;
Wu, Chin
;
環境工程研究所
;
Institute of Environmental Engineering
This study combined acoustic-signal based devices including subbottom profiler (SBP) and side-scan sonar (SSS) and in-situ sediment data to investigate macrophyte growth and stability of bottom sediments before and after carp removal in Lake Wingra, Wisconsin, U.S.A. In 2007, i.e. before carp removal, carp deteriorated water quality, and macrophytes only grew to the water depth of 2 m. Meanwhile, SBP data showed a loose and fluffy sediment layer in unvegetated region, which was easily affected by wave or current motions. In 2009, i.e., after more than half (51%) of carp was removed, SBP data presented disappearance of the fluffy sediment layer, and both SBP and SSS detected that submerged vegetation grew in deeper water depth. In-situ sediment data performed that bulk sediment density and critical shear stress collected in 2009 became greater than those in 2007, which means bottom sediments turned into more consolidated and were harder to be resuspended. Secchi depth obtained during 2008 to 2010 greater than that in the previous ten years indicated that water became clearer. In short, the fluffy sediment layer may be the main source for suspended sediments to deteriorate water quality, and removal of carp is crucial for stabilizing bottom sediment and improving water clarity in this small lake. By using several acoustic features, lake managers can monitor environmental changes more effectively and efficiently, and provide appropriate management efforts in short time.
ASSESSING VULNERABILITY OF RIVERBANK REVETMENTS IN A REGIONAL DRAINAGE SYSTEM DURING STORM OR TYPHOON EVENTS
(2015-01-01)
Lin, Ying-Tien
;
Yang, Jinn-Chuang
;
Hung, Sheng-Hsiang
;
Chiang, Shih-Wei
;
Chang, Yin-Lung
In this study, a quasi three-dimensional hydrodynamic model and field measurements were combined to assess the vulnerable segments of the De-Shang Creek drainage system during the 610 flood and Typhoon Kong-Ray. An automatic monitoring system was installed to record channel water levels, and the displacement and pore water pressure variations inside the riverbank revetments. The results show that the variations of water levels and pore water pressure were almost at the same pace, which indicates the drainage system inside the revetments work well. The flow-induced lateral shear stress and safety factor (S.F.) of river embankments from numerical simulations are the two indexes used to assess the vulnerability of the riverbank revetments. By using the 610 flood and Typhoon Kong-Ray as test cases, the combined hydrodynamic and revetment stability model is able to indicate if riverbank revetments are in danger during the storm or typhoon events. In the observational site, the damages of the revetments in the 610 flood may be due to the exceedance of the flow-induced shear stress over the critical resistance shear stress of the revetments. Overall, this study show the combined hydrodynamic and revetment stability model is a useful non-engineering method on assessing stability of riverbank revetments during storm or typhoon events.
Modelling of hyperconcentrated flow in steep-sloped channels
(2018-01-01)
Chen, Chien-Hua
;
Lin, Ying-Tien
;
Chung, Hau-Rong
;
Hsieh, Te-Yung
;
Yang, Jinn-Chuang
An unsteady two-dimensional depth-averaged numerical model for hyperconcentrated flows in steep-sloped alluvial channels is developed and applied to the Zhuoshui River in Central Taiwan. In this model, the suspended load and bed load are both included to reproduce actual sediment transport characteristics of steep-sloped channels in the chosen study river reaches. A rheological relationship for Bingham fluid and the formula for the sediment carrying capacity, established through laboratory experiments, are used to describe the hyperconcentrated flow behaviour in the model. At peak discharge during Typhoon Morakot, bed shear stress from the hyperconcentrated flow model can be 150% larger than that from the clear water model. According to the simulation for Tung-Pu-Rey Creek, an upstream tributary of Zhuoshui River, the temporal evolution of bed changes obtained from the model can be used to provide plausible explanations for the reasons behind levee failure during Typhoon Toraji.
Response of bottom sediment stability after carp removal in a small lake
(2013)
Lin, Ying-Tien
;
Wu, Chin H.
;
防災與水環境研究中心
;
Disaster Prevention and Water Environment Research Center
This study combined several features of acoustic and electromagnetic (EM) waves-based devices including sub-bottom profiler (SBP), side-scan sonar (SSS) and ground penetrating radar (GPR), and in situ sediment data to study responses of bottom sediments after carp removal in Lake Wingra. In 2007, i.e., before carp removal, macrophytes only grew to a water depth of 2 m. Meanwhile, GPR data showed no visible sublayer in vegetated regions, while SBP data revealed a loose and fluffy sediment layer in unvegetated regions, easily affected by wave or current motions. The field data showed that suspended sediment concentrations (SSC) were much greater in unvegetated regions than those in vegetated regions during a one-day wind event, and the resuspended sediments could remain suspended in the water column for two days. In 2009, i.e., after more than half (51%) of carp were removed, the fluffy sediment layer recognized by SBP became thinner or even disappeared, and both SBP and SSS results showed that submerged macrophytes started growing in deeper water. In situ sediment data presented that bulk sediment density and critical shear stress became greater, and bottom sediments consolidated and were harder to be resuspended. Secchi depth collected between 2008 and 2010 was greater than that in the previous 10 years, indicated clearer water state. In short, the fluffy sediment layer because of carp activities may be the main source for suspended sediments to deteriorate water quality. Removal of carp is crucial for stabilizing bottom sediment and improving water clarity in this small lake.
The role of rooted emergent vegetation on periodically thermal-driven flow over a sloping bottom
(2014-12-01)
Lin, Ying-Tien
;
Wu, Chin H.
;
環境工程研究所
;
Institute of Environmental Engineering
Thermal-driven flow is generated due to topographic or vegetation-shading effects. Asymptotic solutions by assuming a small bottom slope are derived to discuss effects of rooted emergent vegetation and interactions between emergent vegetation and sloping topography on thermal-driven flow during diurnal heating and cooling cycles. The results show that the zero-order horizontal velocity is significantly reduced by vegetative drag, and the time lag between the change of horizontal velocity and the reversal of pressure gradient is also shortened. The solutions reveal that the viscous effect is dominant in very shallow water, and the drag force becomes important as the water depth increases. The inertial term is only important at the very beginning stage of flow initiation. Different vegetation distributions can significantly change the temperature fields that then affect patterns of thermal-driven circulation and exchange flowrates. For the case of tall vegetation growth in shallow water, and when the deep water side is open, the effects of vegetation shading may interfere with the topographic effects and dramatically alter the flow patterns. The blockage of solar radiation due to vegetation shading can determine the patterns and magnitude of thermal-driven flow. By means of the derived asymptotic horizontal velocity, exchange flow rates can be estimated, which are in good agreement with previous studies. The limitation and valid ranges of asymptotic solutions are finally discussed.