Response of Water Environment to Land Reclamation in Jiaozhou Bay, China Over the Last 150 Years

Jiaozhou Bay (JZB), located at Qingdao City, north China, is a semi-enclosed shallow bay that has undergone large-scale land reclamation and is suffering from a deteriorated water environment. Long-term evolution of JZB with respect of coastline, tidal prism, tidal dynamics, water-exchange capacity, and pollutant transport from 1863 to 2020 was investigated in this paper, using remote sensing images, historical charts, and a numerical model. The JZB was predominated by natural evolution from 1863 to 1935, during which the coastline barely changed. Thereafter, human intervention became intense and more and more natural tidal flats were replaced by salt ponds, aquaculture area, and reclamation very quickly. As a result, tidal prism, area of tidal flats, and area of JZB decreased sharply by 0.290 km3, 182 km2, and 223 km2, respectively, from 1935 to 2020, corresponding to annual decreasing rates being of 123 times, 10 times, 12 times, respectively, as that of before 1935. A numerical model showed that the residual current in JZB tended to be weaker due to the change of coastline and bathymetry, which is not favoring the water-exchange and pollutant transport, especially in the northeast of JZB. The basin residence time increased from 15.5 days in 1935 to 17.6 days in 2020, because of weaker residual tidal current and smaller tidal prism. Local residence time increased significantly near the area with large land reclamation, especially in the northeast and west of JZB. Distribution of dissolved inorganic nitrogen (DIN), in each year, which is the dominant pollutant in JZB, indicated higher DIN concentration and weaker transport along with reclamation. The research on JZB evolution over the last 150 years can provide useful suggestions for the decision-makers of the local government to improve the marine ecosystem. The systematic method to investigate long-term water environment evolution of JZB can be used to study other semi-closed bays.

Estuarine morphodynamics and development modified by floodplain formation

Rivers and estuaries are flanked by floodplains built by mud and vegetation. Floodplains affect channel dynamics and the overall system's pattern through apparent cohesion in the channel banks and through filling of accommodation space and hydraulic resistance. For rivers, effects of mud, vegetation and the combination are thought to stabilise the banks and narrow the channel. However, the thinness of mudflats and salt marsh in estuaries compared to channel depth raises questions about the effects of floodplain as constraints on estuary dimensions. To test these effects, we created three estuaries in a tidal flume: one with mud, one with recruitment events of two live vegetation species and a control with neither. Both mud and vegetation reduced channel migration and bank erosion and stabilised channels and bars. Effects of vegetation include local flow velocity reduction and concentration of flow into the channels, while flow velocities remained higher over mudflats. On the other hand, the lower reach of the muddy estuary showed more reduced channel migration than the vegetated estuary. The main system-wide effect of mudflats and salt marsh is to reduce the tidal prism over time from upstream to downstream. The landward reach of the estuary narrows and fills progressively, particularly for the muddy estuary, which effectively shortens the tidally influenced reach and also reduces the tidal energy in the seaward reach and mouth area.

On the Tidal Prism: The Roles of Basin Extension, Bottom Friction and Inlet Cross-Section

The prism of the Lignano tidal inlet was approximately constant over the last forty years, although the section width has halved. This has led to questions concerning the factors that most influence the tidal prism, and on the applicability of the well-known A–P relationship. A conceptual scheme of the sea–channel–lagoon system has been used to perform a sensitivity analysis of different parameters that characterize both the basin and the inlet cross-section. A 2D hydrodynamic model has been applied to evaluate the prism and compare it to the one derived by a static method, which is the basis of the analytical derivation of the A–P linkage. Three regimes have been found in the prism variability as a function of the basin extension: a linear static regime between prism and basin area; an asymptotic regime in which the prism depends only on the basin bottom friction; and an intermediate one. In addition, the roles of the inlet and channel sizes on the prism value have been investigated. The results, compared to the empirical relationships between the prism and the inlet cross-section, show that a variation in the cross-sectional area does not always corresponds to a change in tidal prism.

Breach the dikes! How to design saltmarsh restoration schemes for mitigating coastal flooding.

<p>Managed realignment (MR), a form of of nature-based coastal adaptation to reduce flood and erosion risk, involves the abandonment of existing sea defences and their relocation further inland. MR aims to (re)create intertidal habitats, such as saltmarshes, between the old and new lines of defence; as well as flood water storage. The newly created habitats dissipate wave energy and thus provide new natural coastal protection. However, the assessment of the success of MR is difficult, as restoration targets are often vague and data on project performance are scarce. The few studies that do exist show a lack of understanding about the effects of MR scheme design on high water level (HWL) attenuation and thus its coastal protection function.</p><p>Here we present the results of a 2-D hydrodynamic model, calibrated and validated against field measurements of equinoctial tides between August and October 2017, taken within, and seaward of, the Freiston Shore MR site, The Wash, eastern England. Using this model, we performed sensitivity analyses to explore whether or not, and how, the Freiston Shore MR scheme design affects HWL attenuation. For this purpose we changed the configuration of the old defence line and the breaches created within it for renewed tidal exchange and manipulated the digital elevation model of within-site topography. Specifically, we applied six scheme design scenarios (two scenarios with three breaches and varying MR areas, three single breach scenarios of different breach width and one bank removal scenario) and assessed High Water Level (HWL) attenuation rates for each scenario.</p><p>Our results show that scheme design, particularly storage area and number and size of breaches, of the Freiston Shore MR site had a significant effect on the site´s HWL attenuation capacity. When the tidal prism is varied by changing the number and size of breaches and the storage area kept constant, modelled HWL attenuation rates increased with decreasing tidal prism. However, largest HWL attenuation rates (> 10 cm km<sup>-1</sup>) were only obtained if the MR area was of sufficient size, therefore, it is only the larger sites which are exhibiting effective coastal protection. Consequently, the maximum modelled HWL attenuation rate occurred (up to 73 cm km<sup>-1</sup>) for the scenario with the largest area (142 ha).</p><p>The Mean High Water Depth (MHWD) from each of these scenarios explained most of the variation in HWL attenuation between the scenarios (R² = 0.996). This strong correlation may help to inform the construction of more efficient MR schemes with respect to coastal protection in the future.</p>

Small Scales Dynamics Inferred from Tidal Measurements to Mitigate Daily Floodings in the City of Douala: A Case Study of the Besseke's Flood Drain

The recently constructed Besseke’s flood drain is always filled with water due to individual or combined effect of the tide, urbanization drainage, underground plumes and precipitations runoffs. This study focused on the analysis of small scales dynamics inferred from short term tidal measurements to mitigate the daily flooding in the Besseke’s flood drain. The methodology used is based on field measurements observation. The sampling of water level was conducted during two (02) different tidal regimes in May 2019. The volume of brackish water moving in and out of the Besseke’s flood drain was calculated using the formula of O’Brien. The results showed that Spring conditions had greater amplitudes than Neap tide conditions. During Spring tides, the tidal prism that passed in the midsection of the Besseke’s flood drain (S4) was 3.5 × 101 m3. This means that only a negligible amount of the incoming brackish water reaches the Besseke’s flood drain, amplifies and causes the daily flooding. The unexpected stronger amplitudes and dynamics observed in S4 could be due to its sub estuary nature. Furthermore, the percentage composition of water in this section, showed that the fraction of brackish water changes from 85.7% during Spring tide to 77.8% in the Neap tide conditions. The overall spatial evolution revealed that, the trend in tidal prism (during Spring conditions) was (S0) > (S2) > (S1) > (S3) > (S4) with corresponding values of 2.1 × 104, 1.3 ×104, 1.0 × 104, 2.5 × 102 and 3.5 × 101 m3 respectively. Finally, Tidal prism and Cross-sectional area showed a perfect correlation (r2 = 0.96). The best fitted Cross-sectional area-Tidal prism relationship was obtained in S3 (Market) during Spring tide condition.

Managing Tidal Inlets

Sandy inlets are in a dynamic equilibrium between wave-driven littoral drift acting to close them, and tidal flows keeping them open. Their beds are in a continual state of suspension and deposition, so their bathymetry and even location are always in flux. Even so, a nearly linear relationship between an inlet’s cross-sectional flow area and the inshore tidal prism is maintained - except when major wind and/or runoff events act to close or widen an inlet. Inlet location can be stabilized by jetties, but dredging may still be necessary to maintain a navigable channel. Armoring with rock large enough to resist erosion can protect an inlet bed or river mouth from excessive storm flow erosion. Armoring can also be used as a stratagem to close inlets.

Managing Tidal Inlets

Sandy inlets are in a dynamic equilibrium between wave-driven littoral drift acting to close them, and tidal flows keeping them open. Their beds are in a continual state of suspension and deposition, so their bathymetry and even location are always in flux. Even so, a nearly linear relationship between an inlet’s cross-sectional flow area and the inshore tidal prism is maintained - except when major wind and/or runoff events act to close or widen an inlet. Inlet location can be stabilized by jetties, but dredging may still be necessary to maintain a navigable channel. Armoring with rock large enough to resist erosion can protect an inlet bed or river mouth from excessive storm flow erosion. Armoring can also be used as a stratagem to close inlets.