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Ecological Restoration of Urban Flood Channel: Balancing Flood Control and Ecosystem Conservation

Author(s): Yao Wang; Onyx Wing Hong Wai

Linked Author(s): Yao Wang

Keywords: No Keywords

Abstract: Ranking as the world’s fourth most densely populated territory, Hong Kong experienced rapid urbanization and land development since the 1980s. Extensive development in the rural areas has turned the natural ground into hard paved surfaces. The seasonal alternation of the subtropical monsoon climate in Hong Kong usually bring about hot and humid summer with showers and thunderstorms and mild and cool winter. Approximately 70% and 88% of the annual precipitation occurred in wet season over the year of 2016 and 2017, respectively. The intensive precipitation in rainy, summer season which formerly was retained, now quickly becomes surface runoff in metropolitan areas, leading to flood hazards and threat to safety and property. For the purposes of flood control, the government has formulated a set of flood protection standards for the planning and design of the public storm water drainage systems and has made efforts for regulation of the rivers in the territory. Streams that pass through urban streams are normally regulated into uniform trapezoidal or rectangular shapes and lined with concrete in a “three-surface-bare” form to enhance the drainage capacity to divert rainstorm water. This guarantee in flood prevention is accompanying by the sacrifice of natural river functions and deterioration of natural stream environment. River realignment causes the riffles located on the river network to be cut off artificially, leading to disappearance of natural features (e. g. riffle -pool sequence) and riverine habitats. Accompanied by the widen of river bed, streams will stay dry or maintain minimal water levels during dry season. In those low-flow channels, the longitudinal continuity for the transport of material flow, energy flow and information flow from upstream to downstream has been impeded significantly. And they wil l also hinder the movements of aquatic organisms and obstruct the free passage of migratory fish along the flow direction. Tall and steep river banks due to cross-sectional regularization will hamper the lateral continuity that links the aquatic, riparian and associated terrestrial ecosystem, resulting in habitat loss and decrease of biodiversity. The hard engineering techniques applied on river banks and bed will lead to vertical discontinuity by bringing difficulties for the exchange of river flow, hyporheic area and groundwater. The river continuum concept (RCC) states that river system with higher physical variation will lead to higher species diversity and complexity while river system with stabilized physical structure will have lower biotic diversity. It is implied by the theory that the structural and functional characteristics of natural river, especially the integrity of the river ecosystem, will be sacrificed by channelization.


Year: 2018

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