Water Quality

Assessment Result

The potential water quality impacts, the scale, extent and severity of potential net (i.e. unmitigated) construction and operation impacts were evaluated, taking into account all potential cumulative effects including those of adjacent projects, with reference to the WPCO criteria.

Construction Phase

	River water will be diverted from the works area before the commencement of excavation and construction activities by constructing a cofferdam which will be made of hessian bags with clay. In addition, the construction of cofferdam does not involve any excavation works and hence, will not lead to significant disturbance of the river sediments. Impact to water quality due to the river diversion is thus not expected to occur.
	Modelling results show that SS concentrations associated with foundation pit drainage and demolition of cofferdams using open grab or closed grab are compliant with the assessment criterion. (Click here to see graphical presentation of result)
	Modelling results show that during construction, SS concentrations at the WSRs will be the same as the baseline concentrations. Disturbed sediments will thus not disperse to the WSRs from the Project Site. Therefore, the potential of release of total nitrogen, ammoniacal nitrogen, and total phosphorus from the disturbed sediments at the WSRs as a result of the construction of the Project is considered negligible.
	An adequate number of portable toilets will be provided at the Project Site to ensure that sewage from site staff is properly collected.
	Land based construction works will not cause adverse water quality impact with proper implementation of general good construction site practices.

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Operation Phase
Hydrodynamics
Modelling result shows that implementation of the Project will not significantly affect the hydrodynamic conditions downstream of the Project Site, and hence Wetland Conservation Area and Mai Po and Inner Deep Bay Ramsar Site near the Shenzhen River mouth. (Click here to see graphical presentation of result)

Erosion and Sediment Deposition of Riverbed
Modelling result shows a general trend of decreasing deposition volume and thickness with time after implementation of the Project at river section downstream of Ping Yuen River confluence. Maintenance dredging would be required downstream of the Project Site to remove the deposited sediment in order to maintain the overall flood prevention performance of the Shenzhen River. (Click here to see graphical presentation of result)

Water Quality
The modelling results show that no unacceptable water quality impacts are expected from operation of the Project. Unacceptable impacts to SS concentrations caused by maintenance dredging are not expected to occur. (Click here to see graphical presentation of result)