Most of the natural rivers in Hong Kong are channelized by river training work to become engineered channels. The main river channelize method are straightening and deepening the natural rivers and cementing riverbed and riverbank. It is important and necessary to let stormwater discharge as soon as possible to decrease the flooding opportunity. Base on the riverbed a riverbank is straightening and deepening, the channel capacity increase, it is benefit to increase the flow rate of channel, reduce the opportunity of overflow during high flow capacity, lead casualties and properties loss. In addition, some of the channelization measures which has been adopt in Hong Kong’s rivers like concert grass grid, riprap and gabions are necessary to protect the riverbank, prevent soil erosion situation increase the carry burden of channel then river siltation. (Green Power, 2019)
One of the example is river in Tai Po, after a heavy rain for two days, this rainstorm has prompted Hong Kong Observatory to issue amber rainstorm warning signal. The flooding had influent to village like Tai Mei Tuk, Ting Kok and Shan Liu etc. due to the drainage capacity of river near Ting Kok was inadequate. (Elizabeth Cheung, 2017)
River engineering have the benefit on prevent flooding and protect human life and properties, but over engineered will be both impact to ecological and biological. Over engineered to widen the river will be loss of wetland habitat since channelization project will remove the wetland habitat beside the river. The engineered, channelization project involve to build concrete river embankment to steady the original one, this way will cause habitat destruction, loss in habitat complexity, habitat destruction is a great threat to wetland ,riverine ecosystem, (River, Wetland andWild Birds in Hong Kong, )like Bitterns, Crakes, Snipes, Rails etc. those species will lose their living resources and condition after their original habitat removed, than extinction in a large scale due to those habitat can’t maintain their life without suitable living resources. Sheung Yue River, which is the river that frequently experienced flooding in 1990’s, so Drainage Service Department decided to implement channelization work aims to reduce the frequently flooding situation by straighten the original alignment of the river to enlarge its hydraulic capacity. Although the hydraulic capacity of original river can be increase and flooding situation can be reduce afterward, but the habitats at both coasts of river has been destroy. (Richard LEUNG Wah-Ming, et al.,2017)
In addition, over engineered and channelization river to make the river become straight will cause the discharge volume and speed due to the fact that the bend that reduce the discharge speed disappear and shorten the time that river-water to downstream from upstream after channelization, it is ensure that the riffle and pool habitats at downstream will be disrupt by frequently erosion (Anon.2007). As same as wetland, the erosion to downstream riffle and pool habitats will influent to the life of amphibious fish to case extinction. The other influent to ecology by over engineered on channelization is water pollution because the suspended material case turbidity increase in river due to the fact of soil erosion, it is mansion that straighten and width the river will increase the discharge speed and volume of river, when flooding appear, the soil which loss the vegetation to grasp become loose and rush to surface water (river) because vegetation are removed to build embankment, soil erosion risk must be increase. On the other hand, over engineered on river channelization just concern on discharge to prevent flooding, but ignore the ecology distraction, over engineered of channelization is a behavior to destroy the nature purification of river. The microorganism can be biodegrading organic pollutants, it is helpful on decrease the turbidity, pH value of river water.
The linkage between biodiversity and habitat of river are consider on the balance of stormwater discharge control and river ecology system. This linkage is depends on the relevant abundance, composition etc. Once the habitat is destroyed, the river ecology system is also be destroyed, case losses on biodiversity. Biodiversity losses on one habitat can be cause decrease in ecosystem stability. This means channelization engine should consider numerous of measures to implement conservation under flooding treatment. One example is the river habitat of Ho Chung River. Ecological features have been adopted in the drainage improvement work. For example, flow deflector which form by big boulders, this habitat not only is a refuges of fish, but also provide a micro-habitats, this measure are considered the custom of fish, they will protect themselves by hide in hiding space due to high flow volume of stormwater. Thus, the micro-habitats can also provide suitable survive conditional to let different variety of fish to breed, to form a biodiversity situation. In addition, to control the flow capacity of river, weirs are installed in Ho Chung River, but those weirs are brother fish to migration to upstream. Fish ladder are also construct in Ho Chung River, the main usage of fish ladder is to help fish to go upstream, to overcome the border case by drop. The appearance of fish ladder has been follow the habitat of upstream, this measure enhance the grouping of ecological habitat and channelization engine (Richard LEUNG Wah-ming, et al., 2017),one side to conservator river wetland habitat and one side to implement channelization work.
Apart from the measures mention above, the naturalization measures implement by Drainage Services Department are attempts currently in some of the river in urban area. Kai Tak River is one of the example, Kai Tak River has been implement a series of naturalization measures to form a“green channel corridor” natural habitat by introduce landscape and ecological features, planting shrubs and plans along the river bank; introduce over 10 species include Lpomoea pes-caprae, Crinum asiaticum, Ficus pumila, Clerodendrum inerme, etc. to provide survival resource to butterflies build the fish shelters along the riverbank. After naturalization, the habitat can attract 7 kinds of fish species and 28 kinds of bird species back to the river include Gerres filamentosus ; Egretta garzetta etc. (Drainage Services Department, 2018)Overall, the ecological status of Kai Tak River after naturalization work can be restore in a large extent. Thus, there are still having the example that the ecology was damaged after naturalization project. The naturalization work of Tung Chung River is threat to the nature habitat of this river since 90 centuries. Tung Chung River is a river that having high ecological value with a lot of rare species or even endangerous species like Acrossocheilus beijiangensis, , Paramesotriton hongkongensis and Tachypleus tridentatus etc. and also rich in freshwater fish species, case Tung Chung River to be the “Ecologically Importance Stream”. Thus, when Tung Chung New Town developing, Tung Chung River Wong Lung Hang section was implemented naturalization work follow by the Tung Chung development. After naturalization, the nature landform and ecology were all disappeared. Government try to add the ecological conservation element to Tung Chung Rive, but the design of river habitat are different from the original habitat, in addition, Hong Kong are lack of construction procedure that proper care to the river ecology, flora and fauna that live in the original river are hard to survival. It can be said that the ecology of Tung Chung River has been destroyed when the Tung Chung New Town Extinction work is implement. (Green Power, 2016)
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