Seasonal Variation Of River And Tide Energy In Yangtze Estuary And The Influence On Regime Equilibrium

The Yangtze estuary is a widely recognized "golden channel" for navigation in the world. The delta area, supported by the estuary, is a highly developed economic region in China. Over the past decades, Yangtze estuary is suffering from the most severe human impact along the whole history due to fast economic development, especially in the form of water/sand supply and geomorphological change. Furthermore, greenhouse effect and El Nino phenomenon results in worldwide accelerated sea level rise and abnormal precipitation, increasing the possibility of floods. This will massively threaten the safety and sustainable development in and beyond this area. On account of these facts, it is imperative to investigate on the system performance throughout wet and dry seasons, so as to carry out an overall vulnerability assessment of the ongoing situation. Therefore, we propose using a quantitative approach to calculate several physical indexes about morphology, hydrology and energy under a consistent framework of river and tide based on GIS technology and hydro-dynamic model. This result is reached by applying the classic theory of optimal hydro-morphology, in attempt to fully understand the evolution stage of the current Yangtze estuary. The research focuses on the following 4 aspects:(1) Hydrodynamics modelingBased on field measured data and historical records, we did the calibration and validation for both numerical modeling and analytical modeling covering the whole Yangtze estuary and nearby sea. The terrain data were processed with GIS techniques of geometric correction, datum correction and mosaic to make a full bathymetry DEM. All these data, such as geomorphology, water level, liquid boundary conditions and bottom frictions, were assimilated with the method of Delaunay triangulation and optimal interpolation to build a format compatible database. The numerical modeling of TELEMAC6.2 was set up with field measured bathymetry and TPXO7.2 global tidal model to capture the real feature of river-tide interactions, while locally non-linear fluctuation is dominant, and thus the overall trend is vanished. On the other hand, the analytical modeling of CST was set up with simplified exponential convergence geometry and symmetric tidal wave to reflect the overall trend, but sometimes the results are over simplified. The combination of these two different types of models is significant to improve the scenario simulation and vulnerability assessment over wet and dry season, as they all emphasize tide, river and river-tide interactions.(2) Scenario simulationsTo ensure highly precise scenario simulations, we did a sensitivity analysis for some of the important controlling factors. The results show that bathymetry, bottom friction, river discharge and tidal wave will exert an important influence on the tide transmission and energy dissipation, while long-term astronomic waves (18.61 yr and 4.4 yr nodal cycle), sea level rise and Stokes’ drift only have a relative limited influence on the modeling results. As a result, the scenario simulations were run withfield measured bathymetry and sectional tested bottom frictions. The 5 groups of different combination of tide and river discharges are as follows:● Mean spring tide with averaged peak river discharge of 60,000 m3/s;● Mean spring tide with averaged wet season river discharge of 35,000 m3/s;● Mean spring tide with averaged dry season river discharge of 15,000 m3/s;● Mean spring tide with no runoff (a representative input of 1,000 m3/s);● No tidal forcing with riverine discharges of 15,000 m3/s,35,000 m3/s and 60,000 m3/s;The results indicate that river is dominant at the up boundary of peak runoff condition, when tide only shows influence at the outer estuary of 130km, and meanwhile tidal discharge (71,322m3/s) is only 54% of total discharge at the mouth. Scenario simulation also shows that river discharge is a constant along the whole distance at the lower boundary of no tidal forcing. The other three scenario simulations are within the above two boundaries, providing the foundation to investigate the system vulnerability over wet and dry season.(3) Quantative assessment indexesThe assessment is performed by calculating the indexes of estuarine morphology, tidal current limit, mass transportation at tidal current limit, tidal prism, and energy as well as its flux and dissipation under consistent framework of river and tide over wet and dry seasons. The results demonstrate that 1. the funnel-shaped estuary shows "transitional" feature, i.e. the shape becomes more prismatic in wet season whereas tend to be more exponentially in dry season.2. The location of tidal current reverse is approximately at 250km, which moves up and down over wet and dry season.3. Shallow shoals at tidal current limit move downward at wet season, move upward at dry season, but overall it moves downward.4. Yangtze estuary is a large scale estuary (length>400km), the tidal prism within tidal current limit is controlled by high-low slack volume, however above tidal current limit is controlled by high-low level volume; nonetheless, on the whole, the tidal prism is not changing over wet or dry seasons (the constant tidal prism indicates constant ebb tidal energy over wet and dry seasons).5. The spatial distribution of energy (Eh) from river and tide shows obvious variation over wet and dry season; on the contrary, the river potential energy is dominant in wet season.6. Energy flux from tide suggests exponential decrease upstream ward, while energy flux from river shows exponential decrease downstream ward, overall it indicates maximum at both sides and minimum in between.7. During wet season, tide is doing minimum work and the whole system achieves minimum energy dissipation rate, while during dry season estuary is doing work as little as possible in consistent with the imposed constraints, i.e. a uniform local energy dissipation.(4) The equilibrium of wet season and the dynamic equilibrium processesFinally we relate the results of evaluation to the classic theory of optimal hydro-morphology in estuary, and proved that Yangtze estuary as a whole is performing most efficiently in wet season, on the condition that tide is doing minimum work, and river achieves uniform energy dissipation, and all the energy dissipation of the system is attributed to riverine potential energy decreasing. The energy dissipation mechanism analysis also demonstrated that river induced asymmetry energy flux (RIA) balanced with tide induced asymmetry energy flux (TIA) in wet season when the river net energy flux (RNE) is dominant. Our results are inconsistent with the ones by Langbein (1963). The CST analytical model also demonstrated a minimum work during dry season. However, both of them are based on the same analytical theory and imposed with symmetric tidal waves. Langbein (1963) even neglect the influence from river. Our study is based on the mechanism of dynamic responses in alluvial estuary, which are that the tide suggests asymmetry; both flood-ebb discharge and energy flux indicate self-adjustment; estuary shape also shows "transitional form" from funnel to prism under seasonal variation of river influence. All these factors determine a self-adjusting tidal river to achieve minimum work under constraints, one in dynamic equilibrium. This is a complex feedback between river, tide and morphology, which has not been studied thoroughly. Above all, Yangtze estuary is a typical alluvial estuary in coastal plain; the results can be applied to other estuaries of the same type.