To accommodate economic development, more and more anthropogenic interventions, such as reclamation, harbor constructions, flood control measures, and channel dredging, have occurred in estuaries around the world. Because of the need for port development, channel deepening is often needed to maintain and enhance navigation. It is critical that sustainable port development preserves ecological integrity. The impact of channel deepening and its impact on the environmental integrity of an estuary are concurrent concerns.
Many studies have shown that channel deepening can affect tidal characteristics, resulting in as increase and a decrease in tidal range, estuarine stratification, and estuarine circulation [1
]. Channel deepening often causes a decrease in dissolved oxygen (DO) [7
]. A recent study of the Chesapeake Bay showed that a sea-level rise of 0.5 m increased bottom DO and reduced summer hypoxia volume by 12% [11
]. DO is an important measure of health in estuarine and coastal waters, which has attracted more research attention in recent years. Oxygen deficiency can degrade an ecosystem by destroying the bottom fauna habitats, therefore, altering the food web and aquatic ecosystem with undesirable consequences in estuaries and coasts [13
]. On the one hand, if the estuarine stratification increases, the vertical DO exchange is reduced; consequently, the bottom DO condition becomes worse [15
]. On the other hand, when the strength of estuarine gravitational circulation increases, there is less time for DO consumption as water parcels travel upriver [13
]. Gravitational circulation is one of the key mechanisms of an estuary that transports freshwater and pollutants out of the estuary near the surface and transport waters and pollutants from the outside to the estuary near the bottom [1
]. The classical estuary circulation theory shows that a depth change has a significant impact on the strength of the gravitational circulation [1
]. A change in gravitational circulation due to channel deepening changes estuarine residence time [16
], which can have a profound impact on estuarine hydrodynamics and the ecosystem [19
]. Although channel deepening improves navigation, it can cause unexpected consequences to the environment and ecosystem. Because the rate of global sea-level rise is increasing more rapidly than in the past [22
], this can put more pressure on the ecosystem, similar to channel deepening [4
]. Therefore, an investigation of the influences of channel deepening and sea-level rise on the ecosystem is highly warranted.
Human interventions, such as widening and deepening of estuarine and tidal river channels, can impact the distribution of suspended particulate matters, with important ecological ramifications including deteriorated light, primary production, and oxygen conditions. Previous research results have shown that tidal channel deepening could led to a shift of trapping locations of suspended particulate matter, which could lead to a significant oxygen deficit [8
]. After deepening of the navigation channel of the Elbe Estuary, it was found that enhanced oxygen deficiencies occurred and there was a permanent loss in the capacity of branches for reaeration of the open water [23
]. An additional study found that an increase in tidal range and non-tidal circulation resulted in salinity increase and attendant consequences to the estuary’s ecology after the channel deepening in Tampa Bay [24
It is commonly thought that an increase in channel depth results in increases in both stratification and gravitational circulation. However, recent studies have shown that a change in channel depth and sea-level rise can alter the tidal range, thus altering mixing processes [18
]. A change in tidal range is not uniform along an estuary and depends on both the length and depth of the estuary [4
]. The impact of nonlinear effect of tidal range on DO has not been fully studied.
Previous studies have indicated that channel deepening could affect both biochemical and dynamic conditions [8
], consequently, affecting DO condition. However, the impact of channel deepening and sea-level rise on hydrodynamics and DO has not been fully studied for estuaries with complex geometry, which is the focus of this study. The James River, a western tributary of the Chesapeake Bay, USA was used as a prototype estuary to explore this impact. The James River is a typical partially mixed estuary [2
]. To continue port development, channel deepening in the lower James River and Elizabeth River has been planned. One of the concerns is whether the increase in water depth could affect the DO and preserve ecological integrity because DO is one of the important indicators of the health of the aquatic ecosystem. Although the James River receives massive discharges of nutrients, DO conditions remain above the hypoxia level due to its strong gravitational circulation that transports a significant amount of high DO water from Chesapeake Bay into the James River [16
]. Here, we address the potential changes in stratification and gravitational circulation and their impacts on DO levels in the James River.
To address the problem, we investigated the changes in hydrodynamic fields, including tidal rage, salinity, and stratification. Additionally, we investigated the changes in transport timescales because hydrodynamic transport characteristics of an estuary can be quantified by transport properties, including residence time, flushing time, and vertical transport time [25
]. As timescales provide a common measure of hydrodynamics, hydrodynamics for different estuaries can be compared using timescales [25
]. For example, many studies have used water age to diagnose the vertical transport process [25
]. These timescales provide measures of characteristics of estuarine transport of pollutants, nutrient retention, DO aeration, and algal bloom [25
]. To quantify the changes in stratification and estuarine circulation for different channel deepening scenarios, the vertical and saltwater transport times of the James River were compared using a three-dimensional (3D) numerical model to diagnose variations of the aquatic environmental condition. Although the investigation was conducted for the James River, the approach and results are applicable to other partially mixed estuaries.
Previous studies have indicated that channel deepening can cause DO decrease due to an increase in stratification and decreases in vertical mixing result in reducing DO aeration [7
]. A recent study showed that a relatively small (15%) increase in depth can double the exchange flow in an estuary [5
]. An increase in exchange flow indicates an increase in gravitational circulation, which is favorable for DO, if DO is high at the outside of the estuary [17
]. Sea-level rise impacts on hydrodynamics and DO are similar to channel deepening in a uniform channel. However, it can be very differently due to the depth increase in the entire estuary resulting in volume increase, especially for a relatively shallow estuary. Sea-level rise can increase stratification and decrease vertical mixing, but it can also increase gravitational circulation and residence time [18
]. Recent studies of the Chesapeake Bay have shown that a sea-level rise of 0.5 m reduced summer hypoxia volume by 12% [11
] and increased bottom DO [12
]. This was mainly due to increased estuarine circulation that promoted oxygen-rich seawater intrusion in the lower layer [11
]. For the James River, sea-level rise results in a slight decrease in summer DO, resulting from competition between changes in estuarine circulation and vertical mixing. An increase in water volume, saltwater age (Figure 5
), and a decrease in vertical exchange time are the main causes of DO decrease. When water volume increases, residence time can increase [18
], which could increase retention time of organic matters. It appears changes in DO due to channel deepening and sea-level can be diverse depending on the interaction of geometry, hydrodynamic condition, and DO condition at the open boundary.
The competition between a decrease in vertical exchange due to an increase in stratification and a DO increase due to an increase in gravitational circulation needs to be analyzed for different estuaries. The results may be difficult to compare for different scenarios and for different estuaries. To understand the impact of hydrodynamics on DO due to channel deepening and sea-level rise in a general sense, the impact of these two dynamic factors on DO was investigated by examining the corresponding transport times. According to Equation (3), the bottom DO can be determined by the timescales of vertical exchange process, gravitational circulation, and biochemical oxygen consumption. Because the change in DO is expressed by the timescales, the impact of hydrodynamics on DO can be measured by a common measure and is applicable to different scenarios. The non-dimensional parameters,
at Station LE5-4 are selected and computed from each scenario in summer. The constant DO consumption rate of 0.32 per day was used [32
] for the summer period. The saturation DO is 7.5 mg O2
is about 23.4 days. The results are plotted in Figure 6
. It can be seen that, for the channel deepening simulation, the vertical transport time increases as channel depth increases, resulting in a decrease in
with channel depth increase). In addition, the timescale measuring gravitational circulation decreases as the channel depth increases, resulting in a decrease in
as well. As these two factors compensate each other, the change in DO follows the same normalized contour line, suggesting that the DO will not change much. The results agree with the 3D eutrophication model results (Table 3
). It can be seen that the competition of these two dynamic processes modulates DO. For the sea-level rise scenario, the vertical transport time does not change much, but
increases slightly. Therefore, the normalized DO deviates slightly from the baseline condition, indicating that DO will decrease slightly, but no significant change in DO is expected. For the James River, sea-level rise decreases summer DO slightly resulting from competition between an increase in estuarine circulation and a decrease in vertical mixing.
For this study we focused on the impact of hydrodynamics on DO due to channel deepening and sea-level rise. Because a change of hydrodynamics can also affect nutrients and phytoplankton distributions, and nutrient deposition to the bottom sediment, it could also affect the biochemical process indirectly. For example, an increase in residence time due to sea-level rise can increase the volume of the estuary and affect nutrient retention time. According to current model experiments, the impact of hydrodynamics on DO is not large, but it can affect the nutrient distribution, which may affect phytoplankton. More studies are needed regarding the influences on biochemical processes, especially temperature effect considered to be due to climate changes.