Cases Study Of Tidal Network Hydrodynamics By Numerical Methods

Tidal channels are important and distinctive features of coastal environments as they represents a basic circulatory system through which water, sediment, organic matters, nutrients and pollutants are transported into and out of intertidal wetlands. Here we choose two study sites of different background environments. One is the Jiuduansha Shoal within the Changjiang estuary, another is the tidal flat within the Luoyuan Bay in the Fujian Province. The former site experiences much complicated hydrodynamic condition and the channel networks are well-developed, while the later is characterized by small-scale channel networks.Based on the simplified Poisson hydrodynamic model, we compile a procedure achieving the watershed delineation for every sub-network and drainage scaling properties extraction in the tidal basin of Upper Jiuduansha, Changjiang estuary. To summarize the calculated and statistical results, we define two different evolution patterns. The "concentration type" is characterized by a single, strong-developed network with larger drainage area, developing under stronger hydrodynamic conditions, while the "scattering type" is represented by several, small-scale networks and smaller drainage area. Both in the two patterns, tidal channel stretch linearly with the increasing drainage area, which is consistent with the nearly same degree of channelization within the study area. In the case of Luoyuan Bay, a depth-integrated hydrodynamic model is constructed to study the relative impact of water level fluctuation, channel depth, macro-landscape and vegetation on the spatial flow pattern and amount of concentrated flow in a tidal marsh landscape during the one ebb period. The simulation results confirm the assertion that more homogeneous flow develops as the tidal platform gets deeper submerged or the channel depths decrease, because of the decreasing of frictional difference between platform and tidal channels. The amount of concentrated flow is loosely correlated to the channel area in our study site. Even if the potential catchment for one channel expands in size, the converging discharge may not necessarily increase as a response. As an integral system, change of macro-landscape directly induces relevant response of channels. The interaction among channels and the effect of the macro-landscape brings about inquiry about the reasonability of applying the catchment delineation theory to the tidal network system. Moreover, the growth of plant exerts additional resistance on the platform and decelerates the flow velocity as a consequence. In our simulations, the flow field changes slightly while variations of concentrated discharge take place in part of the channels. Our results suggest that the cluster of tidal channels within an embayment is distinctive from the river channel systems in terrestrial environment. The process of flow partitioning on tidal platform needs to be considered comprehensively, as the lack of topographical divide within tidal basin makes the hydrodynamics more complex and variable than the terrestrial counterpart.Furthermore, it has to be noted that the simplified model in the first case is solved by the Finite Difference Method while the depth-integrated model in the second case is solved by using the Characteristic-Based Split(CBS) method. The complex distribution of channel networks on the Upper Jiuduansha Shoal prevents the application of the CBS method. Thus we cannot replace the drainage delineation by the amount of concentrated flow in the first case. The correlation between the planar channel area and amount of concentrated flow in the second case may imply the immaturity of these channel networks. Thus the hypothesis of maturity in the first case may still hold and simplified model is sufficient for drainage delineation in the first case.