Influence Mechanism of Geomorphological Evolution in a Tidal Lagoon with Rising Sea Level

A tidal lagoon system has multiple environmental, societal, and economic implications. To investigate the mechanism of influence of the geomorphological evolution of a tidal lagoon, the effect of critical erosion shear stress, critical deposition shear stress, sediment settling velocity, and initial bed elevation were assessed by applying the MIKE hydro- and morpho-dynamic model to a typical tidal lagoon, Qilihai Lagoon. According to the simulation results, without sediment supply, an increase of critical erosion, deposition shear stress, or sediment settling velocity gives rise to tidal networks with a stable terrain. Such an equilibrium state can be defined as when the change of net erosion has little variation, which can be achieved due to counter actions between the erosion and deposition effect. Moreover, the influence of the initial bed elevation depends on the lowest tidal level. When the initial bed elevation is below the lowest tidal level, the tidal networks tend to be fully developed. A Spearman correlation analysis indicated that the geomorphological evolution is more sensitive to critical erosion or deposition shear stress than sediment settling velocity and initial bed elevation. Exponential sea level rise contributes to more intensive erosion than the linear or the parabolic sea level rise in the long-term evolution of a tidal lagoon.

Quantitative Relationships between the Tidal Current Limit, Tidal Level Limit and River Discharge in the Changjiang River Estuary

Estuaries are areas where runoff and tide interact. Tidal waves propagate upstream from river mouths and produce tidal currents and tidal level variations along rivers. Based on the hydrological frequency analysis of river discharge in the dry season and flood season at the Datong hydrological station over the past 70 years, a three-dimensional estuary numerical model was used to produce the quantitative relationships between the tidal current limit, tidal level limit and river discharge in the Changjiang River estuary. The positions of tidal current limit and tidal level limit depend not only on river discharge but also on river topography. When river discharge varies from a hydrological frequency of 95% to 5%, the relationship between the tidal current limit and river discharge is y=2×10−13x3+3 × 10−8x2− 0.0074x+359.35 in the flood season, with a variation range of 90 km, and y=−4×10−10x3−1 × 10−5x2−0.1937x − 1232.9 in the dry season, with a variation range of 200 km. The relationship between the tidal level limit and river discharge is y=6×10−8x2−0.0096x+775.94 in the flood season, with a variation range of 127 km, and y=0.3428x2−17.9x+777.55 in the dry season, with a variation range of 83 km, which is located far upstream of the Datong hydrological station.

Spatial Distributions of Nitrogen and Phosphorus in Surface Sediments in Intertidal Flats of the Yellow River Delta, China

The spatial distributions of nitrogen (N) and phosphorus (P) in surface sediments are of great significance in studying the ecological process of nutrient cycling in intertidal flats. However, little is known about N and P’s spatial distribution in intertidal flats of the Yellow River Delta (YRD). We analyzed the N and P contents in surface sediments and Suaeda glauca density at the low-tidal level to identify the spatial distributions of nutrients and their influencing factors in coastal tidal flat sediments. The results showed that the total nitrogen (TN) and total phosphorus (TP) concentrations in this study were both lower than the background values of China’s shallow sea sediments. The spatial distributions of N and P had significantly spatial heterogeneity, while those of the nutrients at different distances from the low-tidal level to the coastline showed no significant distance effects. The spatial distribution of S. glauca in coastal tidal flats had significant location characteristics and was closely related to the distribution of TN and pH. The TN in non-estuarine intertidal flats was less than that in estuaries; in contrast, the TP was higher in non-estuaries. There are some differences of N and P between estuary and non-estuary areas.

Responses of Hydrodynamics and Saline Water Intrusion to Typhoon Fongwong in the North Branch of the Yangtze River Estuary

The typhoon impact on an estuarine environment is complex and systematic. A three-dimensional hydrodynamic and salinity transport model with a high-resolution, unstructured mesh and a spatially varying bottom roughness, is applied to investigate the effects of a historical typhoon, Fongwong, which affected Shanghai, on the hydrodynamics and saline water intrusion in the North Branch (NB) of the Yangtze River Estuary (YRE). The model is well validated through observation data of the tidal level, current velocity and direction, and salinity. The numerical results of this typhoon event show that: (1) the tidal level and its range increase toward the upstream part of the NB due to the combined effects of the funnel-shaped plane geometry of the NB and the typhoon; (2) the current velocity and the flow spilt ratio of the NB varies with the tides, with a maximum increase by 0.13 m/s and 26.61% during the flood tide and a maximum decrease by 0.12 m/s and 83.33% during the ebb tide, i.e., the typhoon enhances the flood current and weakens the ebb current; (3) the salinity value increases in the NB to a maximum of 1.40 psu and water is well-mixed in the vertical direction in the typhoon’s stable and falling period. The salinity distribution gradually recovered to the normal salt wedge pattern in 3 days following the typhoon. Although this study is based on a site-specific model, the findings will provide valuable insights into saline water intrusion under typhoon events, and thus assist in implementing more efficient estuarine management strategies for drinking water safety.

Small Scale Factors Modify Impacts of Temperature, Ice Scour and Waves and Drive Rocky Intertidal Community Structure in a Greenland Fjord

Understanding the influence of physical drivers and their scale-dependent interactions on ecosystem structure and function is becoming increasingly relevant as ecologists are challenged to quantify and predict the biological implications of anthropogenic activities and climate changes. Here, we aim to quantify the impact of multiple physical drivers (ice scour, wave exposure, and air temperature) and their interactions with small scale modifying factors (tidal level, substrate rugosity, and canopy forming macroalgae) on rocky intertidal community structure. We did this by quantifying intertidal biomass, cover and species richness at three tidal levels (high, mid, and low) at four sites in a sub-arctic Greenland fjord. We found a well-developed intertidal community, with a total of 16 macroalgae and 20 invertebrate species. At one locality, the total biomass was dominated by canopy forming algae exceeding 16 kg wet weight per m–2. Physical stress from ice scour, waves, and air exposure had negative effects on all three community metrics but important interactions and modifying processes were identified. The effect of tidal level differed between sites ranging from an absence of organisms at both high- and mid-intertidal level at the most ice- and wave exposed site to extensive cover across all three tidal levels at the wave and ice sheltered site. Canopy forming macroalgae and substrate rugosity both modified the impacts of physical stress. In the absence of ice scour, canopy forming algae formed extensive cover that modified extreme air temperatures, and the abundance of dominant invertebrate species were all positively related to the biomass of macroalgae. Rugosity provided refuge from ice scour, facilitating increased species richness and cover at exposed sites. Moreover, we detected no negative effects of fast ice, and ice scour impacts were primarily found where presence of glacial ice was combined with wave exposure. Our results provide an example of how large-scale physical factors pass through a filter of several modifying smaller scale processes before their impact on plot scale community structure is manifested.

Data pre-processing and artificial neural networks for tidal level prediction at the Pearl River Estuary

Traditionally, tidal level is predicted by harmonic analysis (HA). In this paper, three hybrid models that couple varied pre-processing methods, which are empirical mode decomposition (EMD), ensemble empirical mode decomposition (EEMD), and empirical wavelet transform (EWT), with the nonlinear autoregressive networks with exogenous inputs (NARX) were applied to forecast tidal level. The models were, namely, EMD-NARX, EEMD-NARX, and EWT-NARX. The sub-series obtained by using EMD or EEMD or EWT were then used as the input vectors to the NARX with the original data as targets. Notably, the EWT-NARX model was employed to predict the tidal level for the first time. Simulations were based on the measurements from four tidal stations at the Pearl River Estuary, China. The results showed that the EWT-NARX, EEMD-NARX, and EMD-NARX outperformed the HA model. Specifically, EWT-NARX was optimal among the four. Moreover, from the Hilbert energy spectra we can see the EWT solved the mode-mixing problem that EMD and EEMD suffered, thus enabling precise tidal level prediction. Simulations and experimental results confirmed that the EWT-NARX model can achieve prediction for the tidal level with high accuracy.