Re ECF Project: 2011-39

Project Title: A holistic approach to a sustainable eco-flood channel design for Yuen Long Nullah

Applicant: Prof. WAI Wing Hong, Onyx, The Hong Kong Polytechnic University

Total Approved Grant: $2,071,880

Duration: 1/12/ 2013 to 31/5/2017

Project Status/Remarks: Project completion to be reviewed by the ECF Research Projects Vetting Subcommittee

The focus of this project is to use a holistic approach which includes ecological assessment and physical and numerical modeling methods to develop a design to naturalize the flood control concrete channels in Yuen Long. A full size eco-flood channel elucidating the low flow channel of the Yuen Long Nullah and its hydrologic floodplain will be constructed in an open area. The low flow channel model will have all the prominent hydro-morphological features of a natural lotic habitat such as pools, riffles, convex and concave banks, etc. Gabion baskets will be proposed as deflectors to enhance the formation of pools and riffles. The goal of naturalization of flood control channel is to create a near-natural and self-sustainable lotic system to restore the river corridors in Yuen Long with rich biodiversity and in the same time to meet the flood control requirement.

Summary of the Findings/Outcomes:
In 2014-2016, ecological assessment surveys have been conducted in 20 sites in Yuen Long including natural streams and concrete channel sections. It was noticed that although in general the concrete channel sections suffered more from water pollution and riparian habitat degradation, some of the sites that were connected to high quality upstream flows seemed to support a variety of living organisms. Also, over the period of the study, these sites showed evidences that the biota could recover after disturbances such as flooding and the transition of the dry season, which were essential to make rehabilitation designs successful. Besides connectivity, other characteristics of the channel sites, such as the presence of the low-flow channel, base flow condition, bank vegetation, and the surrounding human interference also contributed to the rehabilitation potential of the channel section and were important criteria for the site selection and design of eco-channel. Installation of instream deflectors in the low-flow channel was proposed to increase channel physical heterogeneity and trigger aquatic habitat formation. Results from the laboratory and in-situ physical modelling experiments supported the hypothesis that deflectors were effective in guiding the water flow as well as the bed sediment to generate fast and slow flow regions that perform the functions of the pool-and-riffle sequence and resemble natural meso-habitats. Field experiment observation revealed that animals such as macroinvertebrates, fish and birds were immediately attracted to the gabion basket deflectors for resting and hiding. However, spontaneous plant colonization surrounding the deflectors would require more time and favorable conditions to established and be observed. 2D numerical modelling simulation was applied for deflector performance comparison. Among the tested scenarios, the following deflector design configuration within a straight low-flow channel section was relatively the most balanced and was recommended: 30% blockage ratio, 1m space between deflectors, staggered placement along both banks, and as a continuous sequence.