Land Reclamation In Nearly 60 Years And Its Impacts On Water Age Of The Representative Region In Xiamen Bay

Located in the southeast coast of the Fujian province, Xiamen Bay is semi-closed with flexural coastline and complex terrain. Economic development which calls for land resources made Xiamen have serious problem of land shortage. Reclamation, as the most efficient way to resolve the problem, had already been applied in 1950 s. The negative impact occurred at the same time, such as the water environment quality was worse and tidal capacity was decreased sharply while hydrodynamic condition changed. These environment problems could be the hidden trouble for the harmonic development of this place.Environmental Fluid Dynamics Code(EFDC) is fully integrated Hydrodynamic, sediment transport, toxic contaminant transport, and water quality-eutrophication simulation capabilities. The model can be used for 1, 2, or 3-Dimensional physical and chemical simulation of rivers, lakes, reservoirs, estuaries, wetlands, and coastal regions. To study the Jiulong Estuary and Xiamen Bay with multi-islands, complex topography and flow patterns, the model was for the first time applied to the simulation on tidal and salinity field in this area. Some parameters were calibrated and verified using observational data. The model data-comparisons are water elevation, flow velocity, and salinity and the simulation results agree with measured data well. Xiamen Bay has a regular semi-diurnal tide and the average rising and falling time is approximately 6 hours. Currents almost flow rectilinearly along channels and the maximum surface current speed can reach 2m/s. About 74km2 tidal plats emerge at low tide in and around Xiamen Bay. Stratification and different kinds of salinity fronts in estuary are most strongly formed one hour after low water. The tidal current and salinity field were re-acted successfully in the study area and the model can be applied in estuary and coastal regions.Water age is defined as the time elapsed since the departure of a water parcel from an area, where its age is prescribed to be 0, to its arrival at a water body of interest. As a time variable, water age is an evaluating indicator of the process of material exchange and transportation. In this study, based on an established three-dimensional hydrodynamic model of the Jiulong Estuary and the Xiamen Bay, the concept of water age is applied as to calculate and reveal the spatial and temporal variations of ages of the study waters during tidal process, on the basis of which the hydrodynamic field is investigated. The result shows that horizontally, water age value of the Xiamen Bay appear to be larger in the northwest and smaller in the southeast, and water age gradually increase from open sea to the top of the bay, which range from 10~30 days in the area of Kinmen to 80~90 days in the area of the top of the Tongan Bay. In general, isograms of ages are vertical to the incident direction of tidal wave, reciprocating during tidal circles in small scale. Controlled by fresh water infusion from upstream and tidal currents from the open sea, the horizontal distribution of water age value of the Jiulong Estuary appears to be smaller on both ends of the estuary and larger in the middle, the range of which is from 10~25 days. In and around the estuary, a water mass of larger age value exists which reciprocates during tidal circles. Vertically, stratifications of water age in this region are mainly induced by estuarine salt-wedge front, shear front and local terrain structure, which are then demonstrated in the paper. Meanwhile some particular flow behaviors in this area is first reported and analyzed by using the concept of water age, which helps obtain a clear understanding of the complex hydrodynamic process in estuary and bay.Based on the 3-D hydrodynamics-water age coupled model of Jiulong Estuary-Xiamen Bay, a series of numerical experiments were conducted according to the variations of the Xiamen coastline during past 60 years. The hydrodynamics and water age in year 1955, 1956, 1970, 1984, 2003, and 2012 were simulated and the distribution of water age in each case was analyzed in order to reveal and quantify the impact of land reclamation on the hydrodynamics of Xiamen Tongan Bay in nearly 60 years. The results showed that the hydrodynamics of Tongan Bay was heavily affected by the land reclamation around Xiamen Bay in recent 60 years. Before year 1955 when the coastline was natural, the average water age of Tongan Bay was approximately 57.19 days, however, the land reclamation of Gao-Ji Levee, Bingzhou, and Dongkeng Bay resulted in dramatic increase of water age in Tongan Bay. From 1984 to 2012, the tidal dynamics of Tongan Bay was weak and the average water age remained large at about 120 days.