Environmental flow assessment for Dajia river under considering human activities and ecosystems

The environmental flow reservation is a solution to reduce the impact of the hydraulic constructions on natural river environment. How to determine environmental flow under concerning human well-being and ecosystems sustainability is uncertain in Taiwan; hence the foreign empirical environmental flow formulas were applied often. However, the foreign empirical environmental flow formulas could not be applied to every river in Taiwan because the regional properties, such as hydrological, ecological, economic, etc., are different. The study proposes an environmental flow assessment framework with management objectives considered by river properties analysis and resident expectation. The area, hydrological, hydraulic and habitat methodologies were used to illustrate the alternatives of environmental flow. Moreover, water allocation model and ecological evaluation model were used to analysis the impact of environment flow alternatives on water supply strategies and ecosystems reservation. The Dajia River is selected in this study to prove the feasibility of the proposed environmental flow assessment. The results show that the base flow, estimated according to the 10% of MAF, proposed to discharge everyday could be 4.62~9.36 cms for wet season and 1.98~4.01 cms for drought season. The flush flow, evaluated by V > 0.8 m/s, proposed to discharge once a month for drought season could be 12.14~16.29 cms.