4 minute read
Dahan River
Banqiao District, New Taipei City, Taiwan
Preserving a local ecosystem through a river island approach. The high bank of the Dahan River in metropolitan New Taipei City was the site of a landfill for garbage until 2009, when it was transformed into three artificial wetlands to treat urban sewage prior to discharge into the river. But in recent years, the artificial wetlands have become less effective at purifying that sewage because of dwindling water supplies and various other factors such as land reclamation and the invasion of nonnative species. Furthermore, the high right riverbank has caused the river channel to shift toward the left, affecting riverbank safety. Given the ecological disruptions caused by previous dredging projects, this initiative collaborated with ecological groups and nonprofit organizations through workshops, field surveys, and consultations. This project applied the principles of ecological hydraulics and undertook dredging activities that not only preserved bird habitats but also used local resources to create microhabitats and natural tidal wetlands to minimize its ecological impacts. The experience this project in northern Taiwan provides makes it a valuable reference for other river improvement projects. For its work, the project was given the High Distinction Award at the 4th Engineering Administrative Transparency Awards by the Water Resources Agency, Ministry of Economic Affairs (MOEA), Taiwan.
Article Cover: Aerial view of the riverine islands—the coexistence of artificial and tidal wetlands. Situated in an urban, populationdense area of New Taipei City, the river channel in this project requires measures to mitigate potential overflow during typhoons or heavy rainfall. (Photo by Water Resources Agency, Ministry of Economic Affairs [MOEA], Taiwan)
Producing Efficiencies
Originally constructed as artificial wetlands, the high riverbanks were transformed into natural tidal wetlands upon completion. Dredging and channel widening to preserve the ecologically rich area led to the development of riverine islands planted with cleared vegetation and tree trunks to protect the riverbank for 100 meters, creating a 2-hectare microhabitat. Natural tidal forces were used to create a 4.5-hectare tidal wetland whose gradual slope enhanced connectivity between the main river channel and the high riverbanks, facilitating the movement of living organisms. Two sloping surfaces transformed into vertical cliffs 10 meters long and 2 meters high attract nesting kingfishers (Alcedo atthis) and sand martins (Riparia riparia).
Using Natural Processes
The project’s goal was to enhance local habitats and natural processes. Decaying logs added at the head of an island created to conserve pristine forested areas are exposed at high tide, providing resting and roosting places for birds while stabilizing the shoreline. The natural tidal wetland provides a bare beach for waterfowl and benthos at low tide; the beach floods at high tide, creating a shallow area for fish and other organisms. At low tide, the tidal ditch carries mud and sand to the river’s deep main channel, reducing siltation and maintaining habitat for aquatic organisms and waterfowl; the ditch floods at high tide and provides deeper and wider water for fish.
Broadening Benefits
This project aimed to create a more resilient ecosystem that provides flood prevention, ecological preservation, landscape enhancement, and opportunities for recreation and environmental education. The project cleared 12,000 cubic meters of debris and excavated 200,000 cubic meters of soil from the Dahan River’s high riverbank, reducing the risk of flooding and enhancing the river’s flood-carrying capacity. The project area’s artificial and natural tidal wetlands boast high levels of biodiversity and primary productivity. The forested areas provide resting and roosting places for arboreal birds.
Promoting Collaboration
The project was jointly reviewed and designed by the public and private sectors. During implementation, government agencies, consulting firms, academic institutions, schools, and nongovernmental and nonprofit organizations were invited to participate. Multiple field surveys, ecological assessments, and workshops were organized, and stakeholder feedback was fully considered before proceeding to the planning phase. Progress during construction was shared on a web platform to encourage public participation and address any concerns. Further, local teachers and students were invited to visit for environmental education and community outreach.
