Technical Studies

Recently, the concept of river Eco-Engineering has become more and more popular in Taiwan. However, most river Eco-Engineering projects were emphasized on the materials of river protection techniques and flood prevention without the extensive discussions for the influences on the physical habitat. Thus, in this work, the Touchien and Fengshan rivers are selected to be the study reaches and the short-term and long-term effects of river Eco-Engineering on the physical habitat will be investigated through the concepts of macro-habitat (flow pattern diversity) and micro-habitat (weighted usable area, WUA).

The study will be fulfilled in two years. This report presents the results of the first year, including the planning and design of Eco-Engineering for Touchien river, current physical habitat analysis (without planned Eco-Engineering), and the investigation for the influences of Eco-Engineering on the physical habitat. For the planning and design of Eco-Engineering on Touchien river, the JuanChiao levee, ChuLin bridge, and Railway bridge are selected to be the demonstrated reaches. The groin dikes are planned on the former two reaches while the grade control structures are planned on the last reach. Based on the long-term hydraulic simulations, it shows that the erosion potential along the river bed and toe of bank can be significantly reduced due to the planned Eco-Engineering.

Based on the hydraulic simulations without considering the planned Eco-Engineering, the current physical habitat conditions, including macro-habitat and micro-habitat, are analyzed. The macro-habitat analysis is performed through the whole Touchien river basin whereas the micro-habitat conditions are investigated on the reaches of JuanChiao levee, ChuLin bridge, and Railway bridge. The flow pattern diversity (Shannon’s diversity index) is adopted to quantify the macro-habitat conditions and the analysis results show that the flow pattern diversities in the Shangping and Youluo rivers (the upstream tributary of Touchien river basin) are better than the main stream Touchien river. The concept of WUA, original developed by PHABSIM, is adopted to investigate the micro-habitat conditions. Based on the biological data, the Rhinogobius candidianus, Acrossocheilus paradoxus, and Zacco pachycephalus are selected to be the target species. The analysis results show that the Rhinogobius candidianus and Zacco pachycephalus have better WUA than Acrossocheilus paradoxus in the selected three reaches. Furthermore, based on the relationship between WUA and flow rate, the optimal flow rate Q_opt in the reaches of JuanChiao levee, ChuLin bridge, and Railway bridge can be determined as Q_opt³ 4.85 cms, 4.85 cms £ Q_opt£ 21.8 cms, and 2.5 cms £ Q_opt£ 27.5 cms, respectively.

To investigate the effects of Eco-Engineering on the physical habitat, the long-term migration of river bed with or without planned Eco-Engineering is firstly predicted through the numerical simulation, then, the reaches of JuanChiao levee and Railway bridge are selected to analyze the micro-habitat and macro-habitat, respectively. In the reach of JuanChiao levee, the analysis results show that after the long-term migration of river bed, the WUA for all of the three target species and the flow pattern diversity in the case with groin dikes are better than the case without groin dikes, especially when the flow rate is extremely low. Besides, the pool percentage has significant increase due to the planning of groin dikes, thus the range of Q_opt is extended, which means the probability that the actual flow rate belongs Q_opt is increasing. In the reach of Railway bridge, the macro-habitat analysis results show that compared with the case of no grade control structures, the flow pattern diversity has dramatic increase when the grade control structures are planned, the percentages of riffles and pools increase 32% and 24%, respectively. Based on the above analysis, one can conclude that the construction of groin dikes and grade control structures might increase the physical habitat conditions on the specified reaches.

98I017(98.5.22~98.12.15)