Ecological environmental flow estimation for rivers with complicated hydraulic conditions

Estuaries are semi-enclosed coastal transition zones, where freshwater inflow from rivers mixes with saline water from the sea due to tidal action. The environmental flow for estuaries is normally defined as the level of freshwater inflow required by the estuarine ecosystem to achieve satisfactory ecological objectives. However, quantifying this value is generally difficult, as the traditional methodology for studying the fluvial systems cannot be directly employed in estuaries (Xie et al. 2022; Gusti et al. 2023). Where rivers encounter estuaries, a transition zone called tidal river reach develops where riverine and tidal processes both affect flow and sediment transport processes. With the rapid growth of anthropogenic activities, frequent eutrophication, river bed sedimentation and ecosystem degradation occur (Walsh et al. 2013; Yang et al. 2014, 2015; Seijger et al. 2019), flows are not sufficient to sustain the deltas. As a result, environmental flow estimation in tidal rivers has become one of the major challenges for sustainable water resource management in estuaries and river basins. Previous studies have highlighted the significance and urgency of this challenge (Richter et al. 1997; Sun et al. 2009; Poff et al. 2010; Arthington et al. 2018).

The hydrodynamic process of the tidal river is a complex phenomenon that relies upon the upstream freshwater inflow and downstream tide. However, investigations into tidal rivers have been relatively poor due to difficulties in addressing complex responses of estuarine ecosystems to freshwater inflows and collecting required ecological data (Arthington et al. 2006; Alcázar et al. 2008). Research on environmental flows in tidal rivers began in the 1990s (Adams & Bate 1994; Matsumoto et al. 1994; Peirson et al. 2002; Liu et al. 2005). However, previous studies only focused on limited functions of estuarine ecosystems, leading to oversimplified descriptions of the characteristics of environmental flows. More recently, Sun et al. (2015) proposed an approach to environmental flow assessment that considers spatial pattern variations in potential habitats affected by river discharges and tidal currents in the Yellow River Estuary. Akter & Tanim (2018) established an environmental flow threshold in an ungauged semidiurnal tidal river in Bangladesh. Van Niekerk et al. (2019) presented an environmental flow methodology that was developed to accommodate shallow, highly dynamic micro-tidal estuaries found along the wave-dominated coast of South Africa. However, very few studies have been conducted to estimate the environmental flow in tidal rivers along the Yellow Sea coast of China.

Jiangsu Province, located in southeast China, with a coastline of over 1,000 km along the Yellow Sea, possesses the largest amount of coastal area in China. Over the past few decades, numerous projects such as beach reclamation, river dredging and the construction of sluices, dams, bridges and culverts have been implemented in the coastal regions of Jiangsu. However, these projects have resulted in water scarcity (Maren et al. 2023), severe riverbed siltation (Tao et al. 2012), saltwater intrusion and a pervasive downward trend in aquatic biodiversity and ecosystem condition (Xu et al. 2016; Cui et al. 2018; Cao et al. 2020). These modifications have significantly altered the hydrologic and hydrodynamic regimes of the tidal river reach and have threatened the health of the river ecosystem. Despite this, very few research studies on the environmental flow in this area have been conducted. Therefore, the development of approaches for calculating environmental flows in these estuaries is highly desired.

Yancheng City, located in the north of Yangtze Delta economic zone, has the Yellow Sea in its east and is situated downstream of the Huaihe River basin. Its coastline is 582 km long, with a beach area of 4,553 km². The tide level range falls within 2–3 m, making it a medium-tidal estuary. The plain landforms have long been influenced by the interlacing of the Huai River, the Yangtze River, the Yellow River and the Yellow Sea. The sediments carried by the Yellow River in the north and the Yangtze River in the south from alluvial deposits are under the influence of wind, waves and tides.

All the major river channels into the Yellow Sea in Yancheng City have been equipped with gates for tide and brine prevention, desalination and irrigation, and flood control and drainage. However, the original balance of sediment movement was disrupted, and the tidal current is unable to carry away all the sediment brought by the tidal current during low tide. This results in the sediment settling and gradually accumulating under the gate. To make matters worse, the amount of sediment under the gates is increasing every year, leading to a continuous decline in the river's drainage capacity.

The background analysis conducted revealed that all the tidal rivers above the sluices were characterized by low water quality, whereas the river reach located downstream of the sluices was plagued by severe sedimentation.

The single cohesive layer models were selected since the study river reach was dominated by the cohesive sediment with the particles ranging in size from 0.0076 to 0.0147 mm.

The calculation and storage time step of the hydrodynamic model, water quality model and sediment model were set to 1 and 60 min, respectively. The spatial step for each model was set to 200–500 m. The Shengjian Bridge was chosen as the control section for both the hydrodynamic and the water quality models (Figure 1(b)).

The flow discharge process was adopted as the upstream boundary of the model, while the tide level process was used as the downstream boundary. Due to limited observations, only the observed daily flow discharge and water level data for December 2006 were selected for model calibration. The results showed that the roughness coefficient fell within the range of 0.02–0.03. Three tide processes (spring tide, moderate tide and neap tide) were selected for model validation, and the determined coefficient was all higher than 0.80 (Table 3). This indicates that the established model performed well and can be used to simulate the tide movement in the Xinyang River.

The TP concentration was chosen as the control indicator of the water quality model, with the goal of reaching Class IV of the surface water environmental quality. The TP concentration of 0.250 mg/L (inferior Class V water quality) was chosen as the initial condition of the water quality model to calculate the ecological environmental flow. The gauge sections of the Chengbei bridge and the Xinyang sluice were selected as the upstream and downstream boundaries for the model, respectively.

The downstream section of Xinyang sluice was selected to simulate the sludge height of the riverbed along the river. The immediate downstream of the Xinyanggang sluice was set as the upstream boundary by assuming its sediment concentration was zero, and the sediment gauging section nearest to the estuary was regarded as the downstream boundary of the model.

Because water quality differs significantly between wet and dry seasons, the EFw was calculated separately for each season. Correspondingly, July, which has the highest TP concentration during the wet season, and November, which has the highest TP concentration during the dry season, were selected as the representative months.

The EFb for the Xinyang River was calculated and presented in Table 4. It can be seen that the optimum range of flow during the wet season is 53.4–89 m³/s, while during the dry season, it is 20.1–33.5 m³/s.

The environmental flow above the Xinyang sluice can be obtained from the higher value between the EFw and the EFb. Therefore, for the wet season, the environmental flow above the Xinyang sluice is 220 m³/s, and for the dry season, it is 60 m³/s. For the environmental flow below the Xinyang sluice, it is mainly up to the EFs and thus it equals 40 m³/s instead.

Special geographical location determines the estuary region always being affected by intensified anthropogenic activities, which leads to frequent water pollution and persistent sedimentation. Therefore, ecological water requirements for water quality and sediment movement must be considered during the estimation of the environmental flow for tidal rivers. To address these challenges, the EFw is calculated to satisfy the water demand for dilution and self-purification of the water body. This can help maintain the water quality in the estuary region and ensure the health of aquatic ecosystems. Similarly, the EFs are calculated to meet the water demand for sediment transport. This helps maintain a certain channel volume and drainage capacity in the tidal reach, which is critical for navigation and flood control. By ensuring sediment movement, the EFs can also contribute to preventing the sedimentation of the riverbed and maintaining the stability of the ecosystem. Therefore, considering both EFw and EFs is crucial in estimating the environmental flow for tidal rivers, as it helps maintain the ecological health and functionality of the river system.

In the study, a framework for determining the environmental flow of medium tidal rivers was proposed. The framework integrates the instream water requirements of water quality, sediment and basic ecological flow, which satisfies the water demand for the sediment scouring to protect river channels and for the survival and reproduction of aquatic life in tidal river reaches. The framework considers temporal and spatial differences in the environmental flow requirements of tidal rivers, such as the wet season and dry season, above and below the sluice. It is easily understood and operational. In addition, the datasets required are not difficult to obtain. The relatively sound understandability and practicality make it have the potential to be widely applied in tidal river reaches dominated by cohesive sediment. To the best of our knowledge, this is the first attempt to estimate the environmental flow in the mid-tide rivers and also in the tidal rivers flowing into the Yellow Sea, China. What needs to be emphasized is that the selected sediment module should be matched with the sediment particle size. In addition, the objective of the water quality simulation should also be selected based on the actual water quality condition of the tidal rivers. For instance, the water quality conditions of the Xinyang River were assessed, and it turned out that only the TP indicator did not meet the required standards. Therefore, only the TP target concentration was set as the objective of the model simulation. Due to the lack of ecological monitoring data, such as fish spawning quantity and fish catch number, although it considered the ecological aspect by the basic ecological flow, the study did not consider the specific ecological factors. The four major carp are the common fish taxa in the Jiangsu coastal region. This demonstrates the significance and necessity of continuing to monitor ecological data when determining the environmental flow of tidal rivers. Instead, we divided the periods into wet and dry seasons to better consider the water amount demand differences during the reproductive period and non--reproductive period for living creatures in river systems. Salinity was also not considered since it lacks monitoring. Therefore, the next step is to build a site monitoring station to monitor consecutive salinity concentrations to improve the ecological environmental flow of the tidal river reaching the Yellow Sea estuary.

It is important to maintain ecological environmental flows in rivers to ensure the health and stability of the river systems, particularly in the face of increasing anthropogenic activities. The Xinyang tidal river has been assessed and an ecological environmental flow has been estimated for both the wet and dry seasons, as well as above and below the Xinyang sluice.

The ecological environmental flow required above the Xinyang sluice during the wet season is 220 m³/s, which means that at least this amount of water must flow through the river to maintain the health and stability of the ecosystem. Similarly, during the dry season, a minimum of 60 m³/s is required. Below the Xinyang sluice, the required ecological environmental flow is 40 m³/s.

The output can be used as a reference to ensure the rational utilization and development of coastal water resources in the area, taking into account the needs of the ecosystem. It can also help promote the health and stability of the river systems in the face of changing environmental conditions and increasing anthropogenic activities.

This work was supported by the National Key Research and Development Programs of China (2022YFC3203902) , the Water Science and Technology Project of Jiangsu Province (2022031), and the Research Project of Nanjing Polytechnic Institute (NJPI-2023-03, NJPI-RC-2024-10).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.