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 second year, including the planning and design of Eco-Engineering for Fengshan 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 Fengshan river, the NanHe levee, ShiTouKeng levee, and MaYuan levee are selected to be the demonstrated reaches. The proper dredging is conducted on the first and last two reaches while a series of 6 spur dikes are planned on the last reach. Based on the hydraulic simulation, it indicates 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 Fengshan river basin whereas the micro-habitat conditions are investigated on the reaches of NanHe levee, ShiTouKeng levee and MaYuan levee. 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 are poor due to floods. 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. Based on the relationship between WUA and flow rate, the optimal flow rate Qopt in the reaches of NanHe levee, ShiTouKeng levee and MaYuan levee can be determined as Q_opt≧1.86 cms, Q_opt≧2.26 cms and Q_opt≧1.37 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 NanHe levee, ShiTouKeng levee, and MaYuan levee are selected to analyze the micro-habitat and macro-habitat, respectively. In the reach of NanHe levee, the analysis results show that the WUA for all of the three target species with proper dredging are better than the case without dredging, especially for Acrossocheilus paradoxus. However, in the reach of MaYuan levee, the WUA for two target species of Rhinogobius candidianus and Zacco pachycephalus significantly decreases due to dredging. In the reach of ShiTouKeng levee, the WUA for two target species of Rhinogobius candidianus and Zacco pachycephalus significantly increases when the spur dikes are planned. Besides, the range of Qopt is extended, which means the probability of the actual flow rate within Qopt is increasing, especially for the low flow rate in the winter time.

Based on the analysis results of field cases for two years, one can conclude that the construction of spur dikes and grade control structures might increase the physical habitat conditions on the specified reaches, while no final conclusion has yet been reached on dredging with respect to the physical habitat conditions. In addition, more detailed investigation and research should be employed to further clarify the applicability of conclusions mentioaned above.

99I017(99.5.24~99.12.15)