Author(s): Julia Wing-Yin Chan; Anne F. Kerr; See-Hoi Wong; Shing-Cheong Lau
Linked Author(s): Edwin Lau Shing Cheong
Keywords: Channelization; Ecological value; Green channels; Ecological enhancement; Rapid stream evaluation
Abstract: A local study entitled “Eco-hydraulics Study on Green Channels, Stage 1” has been commissioned by the Drainage Services Department (DSD) of the Government of the Hong Kong Special Administrative Region (HKSAR) to review green channel design in DSD with respect to their specific ecological value. Ecological assessment is a lengthy and resource demanding process. As there are approximately 50 green channels in DSD with a total length of more than 100 km, it is necessary to identify a systematic prioritization system. The prioritization requires a scoring system in order to rank individual channels. There is currently no established method in DSD to evaluate ecological value of channels or streams by such a scoring system. References have been made to a scoring system developed for assessing the relative ecological importance of sites under the New Nature Conservation Policy of the Government of the HKSAR and established overseas techniques on rapid stream evaluation. A preliminary scoring system has been formulated based on desktop study of baseline information to compare 10 representative green channels. The ranking is further refined based on recent water quality monitoring and ecological field survey for the prioritization process. The water quality monitoring parameters included pH, dissolved oxygen and nutrients, which indicated health of the river channel in supporting aquatic life. The ecological survey covered the study area within 30 m of the riparian zone of the channel and focused on taxa associated with the river channel (wetland plants, waterbirds, herpetofauna, dragonflies, freshwater fishes and aquatic invertebrates). It was noted from the ranking that channels with a mix of natural substrates support a more diverse wildlife. Channels with “fair” water quality are often invaded by exotic species, such as fish and snails, tolerant of polluted conditions, resulting in ecological value being relatively lower than channels of similar design where water quality is better. It is also observed that freshwater fish and aquatic invertebrates were more or less affected by channel design. The more natural the channel bed, the more native fish species and aquatic invertebrates recorded. The study suggests that the rapid stream evaluation only provides a snapshot of the environmental conditions in green channels for prioritization of maintenance works or potential for further enhancement. The season and timing selected for ecological survey is important, as some species only appear in certain periods that may affect the overall scoring e. g. the peak season for dragonflies is April to July while herpetofauna are nocturnal and more active at night. A longer period of post-construction monitoring for water quality and ecological conditions is recommended to review effectiveness of the proposed ecological enhancement designs and developing robust management plans.
Year: 2013