Abstract
Recovery of tide-receiving is considered to improve the water quality in the Lianjiang River, a severely polluted and tide-influenced river connected to the South China Sea. A tide-receiving scenario, i.e., keeping the tide gate open, is compared with the other scenario representing the non-tide-receiving condition, i.e., blocking the tide flow during the flood phase, by numerical simulations based on the EFDC (Environmental Fluid Dynamics Code) model. The impacts of tide receiving were evaluated by the variation in the concentration of ammonia and its exporting fluxes, mainly in the downstream part of the river. With more water mass coming into the river, in the tide-receiving scenario, the averaged concentration of ammonia reduced by 20–40%, with the most significant decrease of 0.64 g m−3. However, the exporting flux of ammonia has decreased in the tide-receiving scenario, as the consequence of the back–forth oscillation of tidal current. In the tide-receiving scenario, the time series of ammonia concentration approximately followed the tidal oscillation, with increased concentration during the ebb tide and reduction in the flood tide. In the non-tide-receiving scenario, the ammonia concentration decreases when the tide gate is open which results in further intrusion of seawater. This was followed by an increase in ammonia concentration again after the currents shift seaward and water mass with higher concentration from the upstream part is transported downstream. Given the identical ammonia input and river runoff, the ammonia concentration stays lower in the tide-receiving scenario, except for short periods after the tide gate opening and neap tides in the downstream part which lasts for around half a day. This study highlights the importance of hydrodynamic condition, specifically tidal oscillation, in the semi-diurnal and fortnight cycles, for the transportation of waterborne materials. Furthermore, the operation of the tide gate was additionally discussed based on potential varied practical conditions and evaluation criteria.
Highlights
The highly urbanized estuary and coastal areas, due to its high industrialization, with dense popularity and variability of near-coastal systems, are susceptible to multiple pressures induced by anthropological influence [1,2,3]
Tide gates are generally doors or flaps mounted on the downstream ends of rivers near the sea, which allow upstream waters to drain while preventing inflows from the sea due to tidal surges or flood events [5,6]
Comparing the ammonia concentration between two scenarios, we found that in most part of the time period, the water quality in the tide-receiving scenario is better than the non-tide-receiving scenario, which is in agreement with the averaged pattern for a spring–neap cycle (Figure 3c)
Summary
The highly urbanized estuary and coastal areas, due to its high industrialization, with dense popularity and variability of near-coastal systems, are susceptible to multiple pressures induced by anthropological influence [1,2,3]. Tide gate could disrupt the natural ecosystem by reducing the water connectivity between the river and sea, which potentially would result in undesirable physical, chemical, and biological side effects, such as low dissolved oxygen, high nutrients concentration and intensified algae bloom [7,8], blocked or delayed fish passage [9], invasion of upland plant, and so on. In some areas where local industries have experienced rapid development, the degraded water connectivity could further deteriorate undesirable water quality [10]. Taking account of the pros and cons of coastal infrastructure, how to utilize or restore the previously constructed tide gate to adapt to the new requirements of economic and environmentally friendly development, has drawn increasing attention and been under fierce debate [12]
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