Development of the hydrological regime of the Uzh River under backwater conditions to minimize the urban environment risks

The impact of high dams with large reservoirs with long-term regulation is covered in the scientific literature quite widely. The information and recommendations on the use of low-head dam for the creation of the low backwater are not enough. Large dams reduce the mean annual flow downstream, the magnitude and frequency of floods, lead to the accumulation of suspended solids, nutrients compounds (nitrogen, potassium, phosphorus), which, together with increasing temperature leads to the “blooming” phenomenon of the reservoir and the overgrowth, create barrier for sediments transferring and fish. The main reason of the deterioration of the water quality is the water retention time in the formed reservoir and anthropogenic pollution, although the literature rarely provides quantitative data of the water retention time for the evaluated reservoir. The environment risks assessment of the impact of the reservoir should include the calculation of the water retention time, the presence of pollutants in the river, which may indicate future negative effects. The hydrological regime of the river after the backwater creation should correspond to the natural flow with the floods passage by the entire width of the river bed, which will significantly improve flood management, sediments transport and washing the river bed. The significant risk is the shallowing of the river downstream, the amount of water discharged into the downstream should be at least 75% of the river flow. Releasing the flow to the downstream with an overflow or through fish passage structures allows to enrich the water with oxygen. In order to preserve biodiversity, it is necessary to create or leave islands and shallow water areas for the needs of waterfowl. To ensure an environmentally friendly regime operation of the low-head dam the crest of the spillway structure should be at the bottom level along the entire cross section. The low-head dam should be provided with automatic control system and communication with the hydrometric station for a quick response to the flow changes in the river.


Introduction
In recent decades, more and more attention has been paid to the impact of artificial hydraulic structures on the river environment and the minimization of risks.Considerable attention is paid to water replenishment of floodplain [1,2].The problem of disconnection between the floodplain and the river takes plays as a result of the flood control dykes' construction in the urbanized areas, and the water replenishment problem of the floodplain and oxbow lakes is possible to solve by the river flow management and artificial structures.The article [3] analyses the prospects of watering the old river bed and the oxbow lake in the urbanized area, which is protected from floods by earth dykes.It is not possible to restore the connection between the floodplain and 1254 (2023) 012082 IOP Publishing doi:10.1088/1755-1315/1254/1/012082 2 the river by destroying dykes on the urbanized areas.The old river bed is completely dry and the oxbow lake is silted and overgrowth because of the absence of the connection between the river and the floodplain in the summer.The analysis of the hydrological regime of the river and various replenishment scenarios showed that it is necessary to create backwater in order to water replenish the old river bed and increase the volume of the oxbow lake.
The large dam construction experience and the analysis of their operation indicates their significant impact on the hydrological regime of the river and the floodplain.High dams primarily affect the hydrological regime of rivers: decrease the mean annual flow downstream especially in the dry season, and decrease the magnitude and frequency of floods [4].
The large dams change the chemical parameters of water quality.Kamidis et al [5], Fantin-Cruz et al [6] indicate the significant decrease in suspended solids due to sedimentation in the reservoir, nitrogen concentration due to denitrification, decrease in phosphorus concentration in the water downstream and anaerobic conditions in the lower layers of the reservoir.Deep reservoirs are characterized by temperature stratification and the temperature increase in the summer due to long retention time.In the article [7] is indicated that large reservoirs in highly urbanized watersheds can improve the water quality downstream reducing the turbidity and concentrations of pollutants due to storage in reservoir.Shallow water reservoirs with the long water retention time accumulate nutrients (phosphorus, nitrogen), which, together with the warming of the water in the summer, leads to the water "blooming" which leads to decrease the oxygen in the water, and phytoplankton transporting into downstream [8].The water quality and temperature changing, nutrients and sediments storage are associated with long water retention time in large reservoirs, high dams also cause disconnect habitat along the river [9], decrease in flood flow, and as a result limit floodplain replenishment.
The researches of low-head dam are paid less attention.The study of five small reservoirs with storage capacity of 0.14-0.64hm 3 in the Nerbioi-Ibaizabal watershed showed that the lack of ecological flow in the summer negatively affects the density, richness and diversity of macroinvertebrates in the areas below the dam [10].Ignatius and Rasmussen [11] note the increase in temperature, the decrease in the concentration of nitrates and phosphates, and the dissolved oxygen increase in the water downstream when water overflow the crest of the dam.In the article [12] is noted that the impact of low-head dams depends on the composition of sediments, the length of the reservoir and the construction of the dam, which can retain sediments or allow them to be transported downstream.
At the same time, studies of the impact of dams built by beavers [13,14] also indicate the water speed decrease in the reservoir, fine sediment storage, the temperature increase, the dissolved oxygen decrease, the increase in the dissolved organic carbon and ammonium transport downstream, the decrease suspended solids and nitrates transport downstream.That changes lead to the complication of existence and as the result increase of biodiversity.
One of the important factors affecting the formation of the regime of dissolved and suspended substances, biogenic elements in water and bottom sediments, the conditions of life of aquatic organisms, their populations and groups is the water retention time, which shows the time that water or any dissolved substance remains in the reservoir.The water exchange is formed due to the water inflow from the catchment area and its outflow from the reservoir, contributing to the intensive passage of chemical and biological processes.According to Oksiyuk et al [15], the water retention time should not exceed 7-9 days for the optimal development of phytoplankton and reducing the probability of water "blooming".The optimal functioning of the hydrobionts is ensured if water retention time is rates to 4-6 days.The zoobenthic microorganisms change to the less valuable pelophilic species if the reservoir water retention time ranged from 5 to 11 days.In reservoirs with long water retention time more than 15-20 days, the accumulation of organic substances and biogenic substances can be observed, which leads to oxygen deficiency and "flowering" of water [6].

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The reason for the overgrowth of water bodies with higher aquatic vegetation is the high biogenic substances concentration (nitrogen, potassium, phosphorus) and the low water depth.Nowak and Lawniczak-Malińska [16] showed that reeds can grow in the large amplitude of lake depth, but its optimal growth is observed in the range from 0 to 1.6 m, while the roots reach the maximum depth of 1.7 m.At the same time, less overgrown water bodies are characterized by the depth of more than 2.5 m with the low nitrogen and potassium concentrations [17].
The aquatic ecosystem sustainability is also affected by the amount of water discharged downstream.Many authors confirm the need to ensure the natural regime of the ecological flow: variability, magnitude, frequency, duration, timing and rate of change, which should be from 75% to 100% of the river flow [9,18,19].Waligórski and Janicka [8], Ignatius and Rasmussen [11] emphasize the importance of water overflow downstream, which create additional water aeration downstream.
The anthropogenically altered water environment requires the assessment of many factors that may negatively affect the environment in the urbanized areas.Urban river demande integrated water resources management even for environmental improvement projects.Relevance of the topic: floodplain water replenishment aimed to improve the urbanized environment requires intervention in the hydrological regime of the river bed, which requires a comprehensive assessment and development of hydrological regime means of minimizing the environment risks.
The aim of the work is to assess the main parameters that undergo changes during the backwater creation and to develop an environmentally friendly hydrological regime operation of the low-head dam to minimize the environment impacts.

Study area
The natural flow of the Uzh River in the central part of the city is characterized by significant irregularity and frequent floods.The most catastrophic floods occur in the cold period of the year, there are low water levels with periodic rain floods in the summer.The floodplain of the Uzh River is limited by flood protection dyke in the central part of the city.It is necessary to create the backwater near the area of Bozdos Park in the central part of the city to water replenishment of the floodplain lake and old river bed.
To replenish the old riverbed, it is planned to install the flap gates at the entrance to the old riverbed and at the end to create the required depth of up to 2.0 m.Water replenishment of the floodplain lake is carried out through the tubular spillway.To provide water exchange in the lake, it is connected to the old riverbed by tubular spillway.The backwater on the Uzh River is planned to be created by two flap gates to minimize main riverbed narrowing.The small reservoir is formed as the result of the backwater with the height of approximately 2.5 m (figure 1).The fish passage is provided for free fish movement.The backwater creation for water replenishment is necessary during the dry season.Since there is the spawning period from March 25 to May 15 which prohibit the activities that may affect fish spawning in the Transcarpathian region, it is actually possible to start to create backwater after the May 16th.

Study methods
The seven cross-sections were analysed to establish the hydrological regime of the river in the area of the future reservoir, which located at the distance of 250 m, the first at the flap gates cross-section and the seventh at the top of the reservoir.The curves of the h = f (Q) were calculated for each cross-section on the base of the curve for Uzhhorod hydrologic station and the water levels were calculated in the natural riverbed for typical years of the stable summerautumn low flow of the 95%, 75% and 50% probability.The function of the average velocities and river flow in the cross-sections were determined.The simulation of the backwater creation and water level fluctuations in the formed reservoir including the water replenishment of the floodplain lake and old riverbed was carried out by the water balance method according to the equation: where ∆W -change in storage, m 3 ; W inf -inflow volume, m 3 ; P -precipitation, m; freservoir area, m 2 ; E 0 -evaporation from the water surface in the ice-free period, m; W outvolume of the ecological flow downstream, m 3 ; RI lake -volume of the flow into the floodplain lake, m 3 ; RI orb -volume of the flow into the old riverbed, m 3 ; SI -seepage outflow from river, m 3 .Meteorological data (temperature, wind speed, precipitation) for the specified years are taken from the data of the Uzhhorod climate station.The riverbed is the lowest points for the groundwater discharge in the area from the backwater beginning to the territory of the Bozdos Park.The water rising will cause temporary soil wetting losses in the aeration zone, but compared to permanent seepage into the Bozdos Park area, these losses can be neglected.Bozdos Park is located in the middle of the river loop, and the ground flow in the middle loop has the direction from upstream side of the dam to the downstream of the river loop.These seepage losses were calculated according to equation (4) of the article [3] as the seepage losses from the river with backwater to the river downstream.The methodology for determining precipitation, evaporation from the water surface as components of the water balance is described in work [3].
The length of the backwater in the Uzh River is calculated according to the equation: where L -length of the backwater, m; k -coefficient for mountain rivers is 1.2; H -hydraulic head, m; i -hydraulic gradient of the river.
The hydraulic retention time calculated as relation between reservoir volume and mean river inflow during the period of back water formation by equation [20]: where HR -hydraulic retention time, day; V -reservoir volume obtained from the curve of water levels and volumes, m 3 ; Q mean -mean river inflow during the period of backwater formation, m 3 /s; 86400 -coefficient to covert seconds into day.
To regulate the water flow in the river, the flap gates were chosen, which allow in horizontal position to pass sediments without creating barrier during the flood.The lifting of the flap gates to create backwater begins with flow increasing in the river after May 15th (136 calendar day).The ecological flow to the downstream is at least 75% of the flow entering the reservoir.At the same time, the floodplain lake and the old riverbed are being filled, which is possible due to the spring flood period.To maintain the water retention time at the range of 4-6 days in the old riverbed and the floodplain lake, the optimal water supply to the old riverbed is 0.5 m 3 /s, to the lake is 0.3 m 3 /s.The simulation carried out for two scenarios of the reservoir operation in the interaction of Uzh River -floodplain lake -old riverbed: maintenance of the constant maximum water level of the backwater (scenario 1); using the flow accumulated in the reservoir to increase the ecological flow up to 75% to the downstream (scenario 2).

Results and discussion
The environmental impact of the dam on the hydrological, biological, geomorphological parameters of the reservoir and water quality can be summarized using the diagram (figure 2).At the same time, the hydrotechnical structures for water replenishment of the floodplain is not the permanent backwater like the ordinary dam.The backwater is temporarily formed by the flap gate after the end of spawning (after the May, 15) and is maintained until the end of October (before autumn floods).The ordinary dam creates the physical barrier for the fish movement.The fish passage is necessary to move cross the dam.
The main impact on the river is the creation of the backwater for the certain water volume accumulation, which create due to the reduction of water discharge to the downstream.The impact of the low-head dam with small reservoir volume on flood flow and flood levels is insignificant, and the risk can be minimized due to the gates design.The evaporation from water surface increase, but this loss is appreciable for big water surface and shallow reservoir.The water level increase due to the backwater increases the groundwater table in the hyporheic zone, which also leads to river flow loss.But at the same time, the recharge of groundwater can have both the negative (increasing ground water table) and the positive is the water replenishment to maintain the biodiversity of the floodplain during the dry season.Water level fluctuation could cause groundwater changes and lowering the soils bearing capacity and as a result banks instability and foundations subsidence on the urban area.
The water speed decreases in the reservoir, which leads to the sedimentation of suspended solids, pollutants, nutrients and the deposition of bottom sediments.The creation of non-flowing conditions in the presence of significant nutrients concentrations can lead to the overgrowth of the reservoir and, to its "blooming" under the condition of rising temperatures.The rapid development of algae leads to the decrease of the oxygen concentration in water and as a result the fish death.The pollutants and nutrients accumulation are directly related to the water retention time in the reservoir, and as shown by Oksiyuk et al [15], the water retention time in the range of 4-6 days provides optimal conditions for the biodiversity.It is possible to ensure short water retention time only by minimizing losses from the reservoir and regular ecological flow to the downstream, which should replicate the natural hydrological regime of the river.Sediment transport occurs mostly during the floods, therefore, to ensure the transport of sediments, it is necessary to ensure the free passage of flood discharges across the full width of the riverbed.Since the water replenishment of the floodplain should be carried out during the dry season, there is no need to create backwater in the period from November to May.So, the main floods will pass through the full cross section of the river, which will ensure the sediments transportation and washing of the riverbed.On the example of the Uzh river, the highest floods occur in March in the year of 95% probability; in March and April in the mean year; in January, February and March for the year of 75% probability.During these periods, backwater is not created and nothing prevents the sediments movement.The flap gate is completely going down when the flow exceeds 103.8 m 3 /s, which allows sediment to pass through the gate sections and ensures periodic washing out of the reservoir.
Ensuring the intensity of water exchange, which is formed due to inflow into the reservoir, has the greatest impact on water quality and reservoir overgrowth.Preservation of the shallow water part on the islands formed by sediments will preserve the biodiversity of wetland birds of the Uzh River.The reservoir volume without bottom clearing and preservation of the sediments side as shallow water for coastal fauna is 319,680 m 3 , the water retention time of the reservoir on the Uzh River is 0.44 days for the year of 95% and 0.3 days for the mean year and the year of the 75%, which can be considered sufficiently intensive and favourable for the biodiversity.The water retention time is 6.3 days for floodplain lake, and one is 1.5 days in the old riverbed, which is also quite favourable intensity.
The simulation showed that ensuring the short water retention time in the floodplain lake and the old riverbed while maintaining the constant maximum water level in the Uzh river at the level of 110.2 m leads to the reduction of the ecological flow to the downstream (red line on the figure 3, figure 4 and figure 5) by 40% of the inflow for the very dry year of the 95% and the year of 75% probability, which is associated with long-term low flows in the summer period.If ecological flow is supported at the level of 75% of the inflow (black line on the figure 3, figure 4 and figure 5), it is possible to use the accumulated during the rain floods water in the reservoir, that will decrease the water level in the reservoir (blue line on the figure 3, figure 4 and figure 5) due to losses on evaporation, filtration and discharges into the floodplain lake and old riverbed.For some summer days with low water flows, it will be necessary to close the gates and stop the fresh water supply to the old riverbed (green line on the figure 3), which will not significantly affect the water quality in the old riverbed due to the short water retention time, but the water level will slightly decrease.that is discharged to the downstream (scenario 1); black -ecological flow to the downstream that is higher than 75% of the inflow (scenario 2); green -water supply to the old riverbed; orange -water supply to the floodplain lake; blue -the water level in the reservoir on the Uzh River (scenario 2).
The constant water level in the reservoir makes it easy to realize in practice the uniform water supply into the floodplain the dry season.The use of the second scenario with water level fluctuation in the reservoir makes it difficult to regulate the flow into the lake and the old riverbed, and will require complex automation and manoeuvring by the gates at the entrance to the old river and the lake, otherwise the water supply into the lake and old riverbed will be decrease due to head decrease.

Conclusions
The general requirements for creating the hydrologically friendly river regime in urbanized areas: • the water retention time in the range of 4-6 days to avoid future environmental risks; • the depth of the reservoir should be more the 2.0m with shallow areas; • the hydrological regime of the river should be corresponded to the natural with ecological flow at least 75% of the inflow of the river; • the crest of spillway structure should be designed on the bottom level along the entire cross section to provide sediments transporting and decrease flood risk;

Figure 2 .
Figure 2. Environmental risks of the reservoir creating on the river: blue -hydrological and hydraulic; red -water quality; green -biological; violet -geomorphological.

Figure 3 .
Figure 3. Simulation of the hydrological regime of the Uzh River under backwater conditions during very dry year of the 95% probability: red line -the percentage of the flow (ecological flow)that is discharged to the downstream (scenario 1); black -ecological flow to the downstream that is higher than 75% of the inflow (scenario 2); green -water supply to the old riverbed; orange -water supply to the floodplain lake; blue -the water level in the reservoir on the Uzh River (scenario 2).

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automatic control system and communication with the hydrometric station should be installed for a quick response to the flow changes in the river.

Figure 4 .
Figure 4. Simulation of the hydrological regime of the Uzh River under backwater conditions during the mean year of the 50% probability: notations are the same as on the figure 3.

Figure 5 .
Figure 5. Simulation of the hydrological regime of the Uzh River under backwater conditions during the year of the 75% probability: notations are the same as on the figure 3.