Evaluation of habitat quality and ecological flow in a regulated waterway

Sluice construction and river regulation projects are widely used in hydropower development, flood control, disaster alleviation, urban water supply, etc. (Shukla & Mishra 2019). The curving cutoff is a common river regulation measure (Slowik 2016). Manually cutting and straightening some curved rivers can effectively improve rivers’ navigation conditions and flood discharge capacity. However, while this type of hydraulic engineering provides significant economic benefits, it also alters the natural hydrological rhythm and hydrodynamic conditions of the rivers, resulting in the simplification of aquatic community structures and the destruction of the river ecosystem (Nakamura et al. 2014). This study focuses on considering the ecological function of rivers while meeting their essential functions and the needs for production and existence.

To improve the quality of biological habitat in canalized rivers, scholars have carried out research and practice for many years and accumulated a rich research foundation. The concept of ‘ecological engineering’ was introduced by the American ecologist Odum and Odum in 1962, combining ecological and engineering theories (Odum & Odum 2003). In the mid-1970s, Europe began to carry out ecological engineering practices for river regulation (Seifer 1983). At present, the physical habitat restoration method is used mainly to restore and maintain biodiversity through river geomorphology transformation. Chang et al. (2019) monitored the biological communities around spur dikes used for waterway regulation, indicating that the spur dike group could provide habitat and shelter for fish and benthos to a certain extent, increasing benthos diversity and fish richness. Lee et al. (2010) improved the habitat of endangered fish species by arranging boulders in straight canalized rivers, providing a reference for restoring urban river ecosystem diversity. Mozzaquattro et al. (2020) sampled and analyzed a fish community under different river forms and water retaining structures, and considered that a reasonable arrangement of obstacles in the river had a positive impact on the structure of the fish community.

Physical habitat reconfiguration measures facilitate the maintenance of biological habitat areas under certain river conditions, so it is particularly necessary to determine the optimum conditions for organisms within the reconstructed river. A highly developed river is seriously canalized, and its downstream often suffers from dehydration, causing serious damage to the rivers biological habitat. Maintaining a suitable flow in a river is a prerequisite for ensuring the survival of aquatic organisms. Furthermore, an important approach to resolving this problem is to scientifically determine the ecological flow of such rivers. Chen et al. (2016) proposed the concept of minimum ecological flow: the minimum flow in a river to maintain the river's basic ecological function and shape. Chen et al. (2007) defined appropriate ecological flow as being the water condition of the critical state of ecosystem decline. Currently, the calculation methods of ecological flow mainly include hydrological, hydraulic, habitat, and comprehensive methods (Abdi & Yasi 2015), among which the hydrological and hydraulic methods have no specific ecological objective, making it difficult to accurately reflect ecological requirements. Therefore, these two methods are often combined with water resource planning, which has macroscopic guiding significance for the ecological management of rivers. A comprehensive method for ecological flow considers some of the ecological indicators that have the greatest influence on the protection of the biological habitat. However, this method involves many fields and has a large span of disciplines, making it difficult to be widely adopted. The habitat method, which is extensively used in ecological flow assessment around the world (Liu et al. 2022), is based on the relationship between the quality of the aquatic habitat and the flow in the river and then determines the river environment suitable for the survival of the species to be protected.

The main habitat simulation models include the1-D PHABSIM, EVHA and HABITAT models, the 2-D RIVER2D model and the 3-D DELFT3D software, etc. A 1-D model cannot adequately simulate the hydrodynamic characteristics in habitat assessment, while DELFT3D is mostly used for the analysis of the internal flow field of water bodies, especially in the aspect of pollutant diffusion (Ma et al. 2020). RIVER2D closely combines hydraulic simulation with ecological simulation, which is mainly used for the simulation and evaluation of microhabitats (water depth, velocity, and bottom sediment, etc.). Park et al. (2020) used a RIVER2D model to calculate the ecological flow of endangered fish in the Gam River, and proposed the concept of a drought index based on it, which has certain guidance for the management of upstream dams. Zhou et al. (2019) studied the water requirement to meet the habitat connectivity of Schizothorax during the spawning period, providing a reference for resource regulation and biological habitat conservation in the Yanni wetland. Hou et al. (2020a) compared the ecological flows calculated by the Tennant and habitat methods, and concluded that the habitat method was better to ensure the habitat requirements of Schizopygopsis younghusbandi during spawning in the Yarlung Tsangpo River. However, the above studies on ecological flow mostly focus on evaluating ecological flows of fish at a particular stage in a natural river, and few have looked at the complete process from river regulation to river habitat restoration.

The Shaying River is the largest tributary of the Huaihe River, and it is not only an important channel for flood discharging, water supply, navigation, and irrigation but also a typical sluice and dam control river. This study takes the river regulation measures of curving cutoff and curved river sections downstream to meet the channel's demand for navigation and flood discharge. Additionally, this study investigates the impact of project implementation on aquatic organisms in the river, taking grass, silver, and bighead carp as the research object and combining them with the fish habitat requirements of the juvenile, adult, migration, and spawning stages. The curving cutoff channel is naturalized by arranging gravel mounds, and the change in biological habitat in the reconstructed river is compared with that of the original river. The response relationship between the flow discharge and the weighted usable area (WUA) of the target fish is analyzed, and the ecological flow of the study reach is determined using the habitat method. The results can provide a reference for the naturalization of river biological habitats.

The Shaying River originates from the Funiu Mountains in Henan Province. It flows through the central part of Henan Province from northwest to southeast and then enters Anhui Province in Jieshou City and finally into the Huai River at the Mohekou in Yingshang County. The Shaying River basin has a high level of development and use of water resources. The basin is a typical artificially transformed river, containing more than 1,800 large- and medium-sized dams and more than 1,400 sluices. The study reach is located in the lower reaches of the Shaying River, 14.2 km from the Mohekou (Figure 1). The length is about 7 km, with an average width of 200 m and a U-shaped cross-section. The depth of the valley is 8–12 m, the channel gradient is 1:3000–1:10000, the roughness coefficient of the river bed is 0.02–0.03. The study area is a warm temperate semi-humid continental monsoon climate with obvious seasonal alternation. According to the records of Yingshangzha Hydrological Station, the maximum annual precipitation is 1,722.5 mm, the minimum is 406.8 mm, and the average annual precipitation is 946.4 mm. The interannual and annual precipitation distribution is very uneven, 40–65% of the annual precipitation is concentrated in June to September, and continuous dry or continuous abundant conditions often occur. The temporal non-uniformity of precipitation distribution leads to large flood peak flow in the flood season, small flow in the dry season, and frequent flow interruption in Shaying River basin.

The aquatic organisms in the study reach are affected by the upstream sluices and dams. In the results of Shu & Wei (2015)'s investigation on the fish composition of the Shaying River, a total of 36 species of fish were collected, including 26 species of Cyprinidae, a decrease of 25% compared to the number of species collected by Li & Wang (1983) from 1973 to 1980. With an average body length of 116.1 mm and average body weight of 62.7 g, the individual fish are small, while the overall fish population density is low and fishery resources are scarce. The downstream reaches of Shaying River are mainly composed of grass carp (Ctenopharyngodon idella), silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) (of the four major Chinese carps), and piscivorous fishes such as culter alburnus, channa argus, and some small omnivorous fish, with grass, silver and bighead carp accounting for over 60% of the total number of fish.

Fish are the climax community in an aquatic ecosystem, and they play a crucial role in the existence and abundance of other communities, particularly in a river ecosystem. Therefore, fish are a better indicator species for ecological flow (Yan et al. 2009). Grass, silver, and bighead carp were chosen as the target species among the four major Chinese carps owing to their high economic value and adaptability to flow conditions. Their spawning types include drifting eggs, with grass carp preferring to live in the lower layer of the water body, silver and bighead carp both inhabiting the upper layer of the water body, which are the main fish cultivated in reservoirs, rivers, and lakes. Moreover, silver carp are filter-feeding fish, which can effectively improve water quality.

The River2D hydrodynamic module, an average depth finite element model based on the two–dimensional Saint–Venant shallow water equation (Alabyan & Lebedeva 2018), was chosen to simulate the hydrodynamic environment in the study area. It assumes that the vertical direction of the water depth conforms to hydrostatic pressure, and the velocity is constant along the depth. Coriolis and wind forces are assumed negligible. The governing equations of the model are as follows:

• (1)
  Conservation of mass:

  

  (1)
• (2)
  Conservation of x-direction momentum:

  

  (2)
• (3)
  Conservation of y-direction momentum:

  

  (3)

Delauney triangulation is adopted in the study model, and the mesh density was set according to the accuracy of the calculation results and the complexity of the riverbed topography. The original river channel model has 56,828 nodes, with a total of 113,600 elements; the mesh size of the mainstream is 3–6 m, and that of the floodplain area is between 5 and 10 m. The curving cutoff river channel is regular, with 24,738 nodes, 84,283 elements, and 8–15 m mesh size in the model. Local refinement was conducted for the area where gravel mounds are arranged and at the junction of the original river channel, to simulate the shape of the gravel mounds, the minimum mesh size was 2 m.

The corresponding flow pattern distribution was obtained from the numerical simulation results of the hydrodynamic environment in the river under different discharges. According to the suitability curve of the target species to the river characteristic conditions, the suitability index of the grid area under each working condition was determined, and the WUA and PUA under each discharge condition were calculated to estimate the ecological flow in the river.

The growth stage of the target fish can be classified into juvenile and adult stages, and the habitat environment in these stages requires the enrichment of nutrients, most of which are slow-water environments. In the feeding stage of juvenile fish (July to August), the optimum velocity range is 0.12–0.2 m/s, and the optimum water depth range is 0.7–1.6 m (Bai et al. 2013). The optimum velocity range of adult fish in the feeding stage (July to November) is 0.1–0.25 m/s, and the optimum water depth range is 2–4 m. The four major Chinese carps are all spawning migration fishes. The optimum range to meet the migratory passage of the target fish is 0.4–0.6 m/s, and the optimum water depth range is 2–4 m (Dang et al. 2018).

The spawning stage is the most fragile and important in the fish life cycle. The hydrodynamic habitat conditions in this stage are of great significance for protecting fish species resources and habitat restoration. The four major Chinese carps are typical drifting egg fish, and the fish eggs need to drift in the water for some time before they can develop into juvenile fish with swimming ability. In this process, if the eggs sink to the bottom and break or are buried in the sand, hypoxia and other phenomena will cause the eggs to die. Hence, it is necessary to meet certain hydrodynamic conditions to keep the eggs suspended in water. With economic development and urbanization, examples of hydraulic engineering, such as artificial transformation and sluice dams, are common in natural rivers. The hydraulic environment in a river changes greatly, which may lead to flow velocity, pattern, and other conditions that cannot maintain the drifting and hatching of fish eggs, destroying the complete life cycle of fish, affecting the number of communities, and even endangering the survival of species. The optimum velocity range of the target fish spawning habitat is between 1.4 and 1.6 m/s, and the optimum water depth range is between 0.75 and 1.5 m (Zhang 2017; Dang et al. 2018).

After the curving cutoff of the original channel, the shape of the riverbed changed significantly. The analysis of the Fr value and shear velocity magnitude in the channel shows that the flow velocity in the channel increases while the viscous force between the flow layers decreases, which is beneficial for improving the flood discharge and sediment transport capacity of the river channel. Under 3,760 m³/s, the water depth of the original channel is mainly concentrated at 11–14 m, and the water depth of the curving cutoff channel is about 8.7 m. The water level of sections 1 + 650 and 8 + 830 dropped by 2.53 and 1.93 m, respectively. At the same time, the flow velocity of the original channel is mainly concentrated at 1.5–2.6 m/s, and the flow velocity is evenly distributed in the mainstream. The flow velocity in the curving cutoff channel has increased but the flow velocity distribution is similar to that of the original channel, and the velocity in the mainstream is 1.6–2.9 m/s.

Habitat classification is an important aspect of river ecological assessment. The target fish are screened for a suitable environment at each growth stage and classified into different habitat types according to flow patterns. Hydraulic gradients or Froude numbers are widely used to identify habitat types worldwide. Based on the Fr value, the habitats of the target fish are classified into three types (Jowett 1993): supercritical flow (0.18 ≦ Fr < 0.41), riffle (Fr ≧ 0.41), and deep pools (0 < Fr < 0.18). The suitable habitat types of the target fish at each growth stage are classified according to the above methods, as shown in Table 2.

Under 3,760 m³/s discharge, the deep pool habitats of the target fish in the original river are the most widely distributed in the entire life cycle. The area of all types of habitats decreases to varying degrees after the curving cutoff, with the adult fish deep pool habitat showing the greatest reduction, at only 10% of that of the original river. The riffle and supercritical flow habitats in the original and cutoff rivers are quite limited, and the hydraulic environment in the river channel is relatively simple. At this point, the original river is quite similar to the flow pattern of the artificial canalized river. According to relevant research, the habitat stability of aquatic organisms in this type of river environment is poor, which is also an important reason for the lack of fish resources in the lower reaches of the Shaying River.

Generally, there is little change in the water surface elevation in the river before and after the arrangement of the gravel mound groups. Local backwater will be formed on the concave bank of the channel without arrangement, and the flow is most obvious when it enters the curving cutoff channel from the original river channel. The maximum water level elevation difference of each measuring point in section 1–1 is 0.046 m. After arranging the gravel mound groups, the backwater area on the concave bank side increased, in which the range of water level elevation at each measuring point of section 1–1 increased to 0.085 m. The average water level of section 1–1 is 0.013 m higher than that of the river channel without arrangement, and the water level increase in other sections is less than that of section 1–1. The average water level elevation difference of each section is less than 0.02 m, and the water level range of each section's measuring point is less than 0.05 m. To sum up, the arrangement of the gravel mound groups has no obvious influence on the overflowing capacity of the curving cutoff channel, which can meet certain flood discharge requirements.

The habitat area of juvenile fish feeding in the original river is less affected by the change in flow discharge. The obvious turning point of the Q-WUA curve is 20 m³/s, and the flow discharge is determined to be the minimum ecological flow of the original river. When the flow discharge is 40 m³/s, the WUA of the original river reaches the maximum value of 192,961 m², the PUA is 6.44%, so 40 m³/s is determined to be the appropriate ecological flow of the original river. As the discharge gradually increases, the WUA gradually decreases and stabilizes between 21,000 and 26,000 m². When the discharge is greater than 1,640 m³/s, the floodplain area begins to submerge, the WUA increases again, reaching 230,779 m² under a flood discharge of 3,760 m³/s. Although the increased water area brings uncertainty to flood control safety and biological habitat, it cannot be used as a stable habitat for fish; thus, it is not considered an ecological flow.

In reconstructing the river channel, the juvenile feeding habitat is made significantly larger than the original river. This is because compared to the large-scale deep water and slow-water environment of the original river, the velocity in the river increases, and the water depth decreases after the curving cutoff, which can better meet the feeding demand of juvenile fish. The flow discharge of 40 m³/s is the obvious turning point of the Q-WUA curve, indicating that this flow is the minimum ecological flow of the reconstructed river. When the flow discharge is 60 m³/s, the WUA reaches the maximum value of 523,207 m², the PUA is 27.05%, and indicates that 60 m³/s is the appropriate ecological flow for the reconstructed river. With the increase in discharge, the velocity and water depth further increases, and the WUA decreases rapidly. After 240 m³/s, the WUA became smaller than the original river, with the smallest being at 840 m³/s, which is 7,000 m². In the process of a subsequent increase in discharge, WUA increases slightly. The WUA increases to 69,835 m² under a flood discharge of 3,760 m³/s, indicating that the juvenile fish habitat in the reconstructed river is greatly affected by the flow discharge. Therefore, the flow discharge in the reconstructed river should not be greater than 240 m³/s at the juvenile stage of the target fish.

The flow pattern of the original river is mainly in a slow or still water state, and the target fish prefer a slow-water environment during adult fish feeding, so the habitat of adult fish in the original river is widely distributed. The WUA of adult fish increases rapidly in the flow discharge range of 5–40 m³/s, where the obvious turning point of the Q-WUA curve is 30 m³/s, which is determined to be the minimum ecological flow of the original river, and WUA = 480,985 m² is the peak value of the curve at 40 m³/s, and the PUA is 16.05%, which is determined to be the appropriate ecological flow of the original river.

Because the river is widened by 100 m when the curve channel is cut and straightened, and the water depth in the reconstructed channel is smaller under low discharge, the adult fish habitat obviously decreases, and the flow velocity gradually increases when the flow increases. Therefore, the adult fish habitat does not obviously increase in the high-flow environment. The turning point of the Q-WUA curve of the reconstructed river is also 30 m³/s, which is determined to be the minimum ecological flow of the reconstructed river. The WUA = 183,040 m² is the peak value of the curve at 60 m³/s, and it is determined to be the appropriate ecological flow for the reconstructed river. Within the flow discharge range of 240–1,040 m³/s, the shrinking trend of WUA in the reconstructed river is similar to the original river. In the flow discharge range of 1,040–1,900 m³/s, the WUA in the original river will be stable at 60,000 m², but the WUA in the reconstructed river still decreases. Combined with the curve change trend of the juvenile fish stage, it shows that the biological habitat adjustment ability of the reconstructed river is weaker than that of the original river.

The target fish migrate in groups before and after the spawning period. Figure 10(c) shows that the changing trend of the Q-WUA curve in the original river and the reconstructed river is basically the same, and 140 m³/s is the obvious turning point of the curve, which was determined as the minimum ecological flow of the river. The maximum point 240 m³/s of WUA is determined to be the appropriate ecological flow; at this time, the WUA for the original river is 583,300 m², and that of the reconstructed river is 725,621 m², indicating that the reconstructed river will not destroy the target fish's migratory passage.

Within the flow range shown in Figure 10(d), the rapid growth point and peak flow discharge corresponding to the Q-WUA curve of the original and reconstructed rivers are the same, being 30 m³/s and 140 m³/s, respectively. The peak value of WUA in the reconstructed river is 87,237 m², which is 581% higher than that in the original river, and the quality of the target fish spawning habitat in the river is greatly improved. The WUA is the minimum when the flow discharge is 840 m³/s, and it increases slightly after this flow discharge. Under a flood discharge of 3,760 m³/s, the WUA of the original river reaches the maximum, which is 81,629 m², but the WUA in the reconstructed river is only 30,084 m² at this time, and the WUA is a theoretical value considering only the factors of water depth and velocity. In practice, the flood environment has high sediment content, poor water quality and unstable flow environment, all of which are unsuitable for fish survival. To sum up, the minimum and appropriate ecological flows of the study river are 30 and 140 m³/s, respectively.

To meet the navigation conditions of the curved reach of the lower reaches of the Shaying River, the curving river was cut off, gravel mound groups were arranged in the cut-off channel, and the influence of the waterway regulation project on the living environment of fish has been discussed. Prior studies have noted the importance of flow to habitat (Nan et al. 2022); the annual precipitation distribution in the Shaying River basin is extremely uneven, the flood flow is much higher than the annual average flow, water reduction and dehydration occur frequently in the middle and lower reaches of the river in the dry season, which seriously destroys the habitats of aquatic organisms and blocks the migratory passage of fish resulting in a decline in the population of aquatic organisms (Ge et al. 2019). In this study, the ecological flow of the reconstructed river and the original river were estimated by the biological habitat method, the minimum and appropriate ecological flows of the target fish in the juvenile, adult, migration, and spawning stages were obtained respectively, and the results were summarized in Table 4. Li et al. (2012) used the 7Q10 method to estimate the minimum ecological flow of the Shaying River as 30.86 m³/s, which is in the ‘middle’ ecological water demand grade of the Tennant method and is close to the minimum ecological flow for adult fish and the spawning stage obtained in this study. Hou et al. (2020b) pointed out that even if the minimum ecological flow is met, biodiversity and ecological integrity are reduced. The appropriate ecological flow obtained by the habitat method takes into account the needs of organisms, so it can better ensure the health of a river ecosystem.

In the study, only two hydraulic factors, flow velocity and water depth, are considered, while the factors affecting fish survival are various. Studies show that temperature and the life behavior of fish are inextricably linked (King et al. 2016), and chemical factors such as ammonia nitrogen concentration and dissolved oxygen also have certain effects on fish habitat quality (Boets et al. 2021). In future studies, combining other habitat influencing factors should be considered to have a more scientific evaluation of habitat.

Based on the fish habitat changes in the river channel before and after waterway regulation, the minimum and appropriate ecological flows of grass, silver, and bighead carp in the study reach were estimated. However, the study area river length is about 19% of the length from Yingshang Sluice to Mohekou, and it is not certain that the ecological flows obtained in this study are optimal in other sections of the river. In the next phase of the study, the research area should be expanded, combining the life history of fish with the hydrological conditions of the basin, to estimate the ecological flow values for all life stages of fish in the entire area, and make reasonable recommendations for upstream reservoir dispatching.

• (1)

  The river channel can meet navigation requirements after the curving cutoff, the flow velocity increases, and the viscous force between the flow layers decreases. The water levels of the inlet and outlet decrease by 0.64 and 2.23 m, respectively, under a 20-year return period flood discharge, and the flood discharge capacity of the channel is improved. However, the flow pattern in the curving cutoff channel is single, and the habitat type of the target fish is mainly deep pools.

• (2)

  The minimum and appropriate ecological flows of the original and reconstructed rivers were estimated, and it was established that the minimum ecological flow for the grass, silver, and bighead carp spawning stage (March – June) was 30 m³/s and that the appropriate ecological flow was 140 m³/s. The minimum and appropriate ecological flows for the migration stage were 140 and 240 m³/s, respectively. In the growth stage (July – November), during which there are juvenile and adult fish stages, the appropriate ecological flow of the original and reconstruction rivers were 40 and 60 m³/s, respectively. From July to August, the ecological flow regulation should be dominated by the needs of the juvenile fish. Furthermore, the minimum ecological flow of the original and reconstructed rivers were 20 and 40 m³/s, respectively. From September to November, adult fish dominate, and the minimum ecological flow of the river was 30 m³/s.

• (3)

  The natural reconstruction of the curving cutoff river is conducted using an arrangement of gravel mound groups. Under the condition of appropriate flow, the peak area of juvenile, migratory, and spawning habitats of the target fish in the reconstructed river increased by 168, 24, and 581%, respectively, compared to the original river channel. Meanwhile, the habitat area of adult fish which prefer a slow-water environment decreased by 62%, and the habitat environment was more balanced.

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

The authors declare there is no conflict of interest.