Putting life back into waterways

Comissioning an eco-hydraulics study in Hong Kong

To review the effectiveness of the implemented green features on ecological enhancement and to recommend effective solutions on river revitalisation works, the government commissioned the eco-hydraulics study on these watercourses.

Designing and enhancing green channels

Stage one was to build an inventory of river channels in Hong Kong territory, then evaluate their ecological conditions, ranking them on their eco-hydraulics performance. Eco-hydraulics focuses on the interactions between hydraulics and ecological systems, including how vegetation influences the flow and function of a watercourse.

We developed a rapid stream evaluation (RSE) method specifically for the green channels in Hong Kong and used it to assess 22 river sections. The RSE comprises a set of physical, chemical and biological criteria, including:

• water quality
• type of bed substrate
• quantity and quality of in-stream habitats
• ecological connectivity of the river channels
• level of biodiversity
• type and quantity of native aquatic flora and fauna found

Our stage one work also included the development of guidelines on designing green channels – ensuring the optimal balance is achieved between ecological enhancement and hydraulic resistance or water flow – as well as a digital ecological data management system for the DSD.

Stage two of the project applied our recommendations and proposals from stage one to develop detailed designs for site enhancement trials and field assessments at Lower Lam Tsuen River and Ma Wat River. It also involved the quick-fix ecological enhancement design at Hung Shui Kiu Nullah to provide a cost-effective method for enhancing the ecological and aesthetical values of the existing concrete nullah.

As part of the trial at Lower Lam Tsuen River, we replaced the concrete riverbed on a 65m long X 28m wide section with natural materials – boulders, cobbles, coarse gravels, sand and mud – and implemented a host of other measures. These included:

• selective planting to promote ammonia uptake from the water column
• creation of sinuous flow features (bends and curves) through planting vegetation and building natural structures
• planting submerged vegetation, providing aquatic habitats, such as pools and riffles, and removing invasive species to improve habitat for wetland dependent species, fish and aquatic fauna
• planting riparian vegetation along banks to improve the ecological connectivity of the river
• installing wooden poles with sustainable materials as resting grounds for birds

Improving water quality and biodiversity

The 10.8km Lam Tsuen River provides local villagers in its 21km2 catchment with drinking and irrigation water as well as drinking water for citizens of Hong Kong. The ecological enhancement works on the lower section improved water quality. The RSE ranking for physical and chemical factors improved from fair to average, with reductions in biological oxygen demand (BOD5) and ammonia-nitrogen, while the biological ranking went from average to good.

Habitat condition has also improved and there has been an increase in the number of species recorded in the river channel. More birds, dragonflies, herpetofauna, freshwater fish and aquatic invertebrate species were recorded during a post-enhancement survey conducted after six months.

Initial findings from a study examining the impacts of Lower Lam Tsuen River improvements indicate that instream planting with full grass could improve the thermal environment of the region, including reducing the surface temperature by as much as 6°C. As Hong Kong warms due to climate change, extension of greening and reducing the concrete surface of drainage channels could help to reduce the heat island effect in the region.

The success of the enhancement trial on the Lower Lam Tsuen River has led local councillors to ask for similar measures to be introduced in the upstream and downstream sections, while environmental groups would like to see them replicated across the Territory.