Owing to the characteristics of Taiwan’s watersheds includes erodible solid, uneven rainfall, steep slopes and high mountains, only 30 percent of terrestrial surface is not occupied by mountains. Traditional river engineering work has always emphasized safe and practical, but not necessarily ecologically or environmentally sound designs. In recent years, however, environmental awareness and the demands of outdoor recreation have risen dramatically. Large scaled river "beautification" projects have received considerable attentions and many have been implemented. On the other hand, still relatively few engineering projects are designed for maintaining ecological functions or habitat protection and restoration. The lack of close communication among biologists, ecologists, and civil and environmental engineers has been recognized as an important factor. Another significant factor is the scarcity of data on the quantitative relationship between hydraulic patterns, streamflow structure and habitat requirement for specific target species. In this paper, a brief description of the status of activities regarding "ecological engineering methods" in Taiwan is given. A case example of recent collaborative efforts including the hatchery center, streambank treatment, and backwater area made for saving the Formosan landlocked salmon population is described, with emphasis on the quantification of the relationship between refuges and habitat requirements of the endangered fish. A two-dimensional computational fluid dynamic model is used to help the design of backwater area. It shows that the hydraulic model can be considered as a suitable technique for habitat restoration.
A roughness and time dependent mixing length equation is developed from an extension of von Kármán’s similarity hypothesis, based on local equilibrium (turbulent energy production balanced by the dissipation), with an algebraic equation for the shape of the turbulent kinetic energy. Our vertical mixing length profile and the related mean velocities approach the experimental data well. We show that our equation follows Prandtl’s mixing length equation only near a smooth wall. The use of the proposed time-dependent mixing length equation in a turbulent oscillatory boundary layer shows, like the k-ε model, an increase in the mixing length and the eddy viscosity near flow reversal.