A method of mitigating flooding in existing coastal plain areas, the method comprising the following steps:
- (1) positioning at least one dyke on a seabed outside an existing coastline of an existing coastal plain area to form an enclosed area, or positioning at least one dyke on a seabed outside an existing coastline of an existing coastal plain area and connecting to the existing coastline to form a partially enclosed area;
- on the existing coastline adjacent the enclosed area, positioning at least one reservoir inlet connecting at least one river network or at least one rainwater pipe network in the existing coastal plain area to the enclosed area, and on the at least one dyke, positioning at least one reservoir outlet connecting the enclosed area to a local sea;
- wherein the enclosed area, together with the at least one dyke, the at least one reservoir inlet and the at least one reservoir outlet, comprises a “Reservoir on coastal beach For containing Rainwater against flooding caused by rainfall in a locality” (RFR), and a crest elevation of the at least one dyke is higher than a design highest water level in the RFR and a design highest tide level of the local sea;
- (2) within a discharging step, which is a period before the start of a forecast heavy rainfall or a forecast continuous rainfall, discharging water in the RFR into the local sea through the at least one reservoir outlet, resulting in lowering a water level in the RFR to a design pre-lowered water level in advance of the forecast heavy rainfall or the forecast continuous rainfall, wherein the design pre-lowered water level in the RFR is lower than a historical mean sea level of the local sea;
- during the discharging step, opening and controlling at least one sluice at the at least one reservoir inlet, thereby causing water in a “Water area For containing Rainwater against flooding caused by rainfall in the locality in the existing coastal plain area” (WFR) and water in the at least one river network or the at least one rainwater pipe network in the existing coastal plain area to flow into the RFR through the at least one reservoir inlet, such that a water level of the WFR is also lowered in advance of the forecast heavy rainfall or the forecast continuous rainfall;
- (3) excavating existing seabed in the RFR to the design pre-lowered water level in the RFR when the elevation of the existing seabed within the RFR is higher than the design pre-lowered water level in the RFR;
- wherein the at least one dyke is configured to withstand actions of water level and waves in the RFR, actions of tidal level and waves of the local sea, and actions from water level difference inside and outside the RFR, as well as actions of seepage caused by water level difference inside and outside the RFR;
- (4) preventing the water level in the WFR from lowering below an allowable minimum water level and preventing the water in the RFR from flowing back into the WFR with the at least one reservoir inlet sluice, wherein the at least one reservoir inlet sluice is the existing sluice of the at least one river network or at least one rainwater pipe network in the existing coastal plain area, or a new inlet sluice installed on the at least one reservoir inlet;
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- (5) equipping the at least one reservoir outlet with at least one drainage sluice and at least one drainage pumping station, and applying different drainage modes to the at least one reservoir outlet, or equipping the at least one reservoir outlet with the at least one drainage sluice only.