Hydraulic control on sedimentation processes and bottom sediments chemistry of Sulejów Reservoir in Poland

Reservoirs are created by dams, which result in breaking the continuity of sediment transport and initiating lacustrine conditions that enhance sedimentation. Dam reservoirs that were constructed early in the nineteenth century were relatively small and used mainly for domestic water supply, irrigation, energy production, and canal operation. The main purposes of more recent reservoirs are 38% irrigation, 18% hydropower, 14% water supply, 14% flood mitigation, 8% recreation, and 8% other (navigation, fish breeding, etc.) (Lakes and reservoirs in the EEA area 1999). Once a reservoir has been constructed, different types of environmental problems may occur such as the accumulation of nutrients and organic matter, often leading to a decrease in water quality, eutrophication, changes in temperature, dissolved oxygen concentration, and sometimes seasonal stratification (Winton, Calamita & Wehrli 2019). Another problem can be the ecological deterioration of the river system, especially downstream of the reservoir; dams disrupt the continuity of the river and are barriers to migrating fish, and they change the hydrological cycle (Kondolf et al. 2006). In the floodplain wetlands, decreased flood pulses caused by dam operations reduce both inundation frequency and the areal extent of floodplain wetlands (Chen et al. 2021). Damming can impact the wetland function in alleviating floods, with a 7% decrease when compared with the natural conditions without a dam. Consequently, the supporting effect of wetlands on baseflow is weakened substantially by damming regulation (Wu et al. 2021). In addition, sedimentation of dams reduces storage capacity, and global water storage per capita is decreasing (Wisser et al. 2013). The effect of water damming depends on the size of the dam and the topography of the upstream reservoir or the length of the impounded river. This means that the above-mentioned effects can occur in all reservoirs but the extent depends on local circumstances (Lehner et al. 2011). Dams often have a profound negative effect on water chemistry and quality in rivers. Dams lead to the transformation of the river invertebrate fauna as a result of hydrodynamic and water quality changes in the impoundment (Gough, Garrido & Herk 2018). Nutrients in river damming show overall increases with considerable alteration of dominance for dissolved and solid particle species. Furthermore, dissolved inorganic nitrogen has increased eightfold over the last 50 years (Li, Xu & Ni 2021). The hydrological changes have made a significant contribution to the 44% reduction in riparian wetlands following the dam's construction. Also, hydrological alterations caused by dam regulation led to the area reduction of downstream riparian wetlands (Zheng et al. 2019). Storage of organic material and nutrients in the reservoir often leads to algal blooms in the summer, and to changes in water temperature in the reservoir and the river downstream (Zhang et al. 2023).

Although reservoirs represent an important water resource, very few monitoring networks devoted exclusively to European reservoirs exist (Romero et al. 2016). It is known, however, that monitoring of many major reservoirs is carried out, although data are generally held by numerous diverse organizations (in particular, reservoir owners), making efficient data collection problematic (Wang & Yang 2014).

Artificial reservoirs located on lowland rivers are susceptible to sedimentation and eutrophication processes, caused by high nutrient loads (nitrogen and phosphorus compounds) that are delivered from large areas of the catchment where they were released from diffuse and point sources. For lowland reservoirs, operated in such a way that the inflow is discharged and the water level (head) at the dam is kept stable, the influent rivers are the major determinants of the water quality in the reservoir. In other words, the longitudinal variation of water quality is determined by the influent flows and the water circulation within the reservoir (Ryu, Yu & Chung 2020). This is because the inflow river acts as the main source for substances, both solid (suspended material) and dissolved.

Generally, the highest amounts of biogenic components are deposited in sediments of the lacustrine zone with stagnation zones and prevailing fine-size material (Smal et al. 2013). Understanding flow through open channels or man-made structures, including artificial reservoirs, is fundamental for proper water management and understanding hydro-chemical and ecological processes. The hydrodynamics of the reservoir control water retention time, and nutrient and sediment transport. There are many examples of using CFD (computational fluid dynamics) methods for understanding the flow pattern and hydraulic properties of the artificial lowland reservoirs in Poland in the context of sedimentation processes (Bogucka & Magnuszewski 2006; Sabat-Tomala et al. 2018; Ziemińska-Stolarska, Polańczyk & Zbiciński 2015). The main purpose of this research is to use a 1D HEC-RAS hydrodynamic model to improve the knowledge of sedimentation conditions and of the chemistry of bottom sediments in the context of reservoir eutrophication and algae bloom.

The Pilica River is the largest left-bank tributary at the middle reach of the Vistula River in Poland. It is 342 km long, with a catchment area of 9 258 km² (Fig. 1). The reservoir was built in the period 1969–1973 and its construction aimed to provide a stable drinking water supply for the city of Lodz. The earth dam, located in Smardzewice village, has a height of 15.4 m and a length of 1 210 m. Sulejów Reservoir has a length of 15.5 km, a maximum width of 2.1 km, and a surface area of 22 km².

Figure 1.

The two main rivers supplying Sulejów Reservoir are Pilica and its left tributary Luciąża. The long-term average discharge of Pilica at Sulejów gauge is 22.8 m³s⁻¹ and that of Luciąża at Kłudzice is 3.03 m³s⁻¹. Water retention in the Sulejów Reservoir is long, reaching an average discharge of more than 30 days. This feature enhances the sedimentation processes through stable hydrodynamic conditions in the reservoir.

Reservoir sedimentation is not very intensive; Pieron et al. (2021) calculated that after 50 years of reservoir exploitation, the annual loss was 46 000 m³ of volume.

The Pilica River catchment land use is predominantly agricultural (60% of its total area), and forests occupy 31%. The differential catchment of the Sulejów Reservoir is exposed to the highest in the whole Pilica River catchment anthropogenic pressure expressed by the highest population density, and the highest load of water released from the municipal water treatment plants (Magnuszewski et al. 2014).

The Sulejów Reservoir has been the subject of many studies, including analysis of the chemistry of its bottom sediments (Ziemińska-Stolarska et al. 2020), hydrodynamics (Ziemińska-Stolarska, Polańczyk & Zbiciński 2015), and ecohydrology (Zalewski, Wagner-Lotkowska & Tarczynska 2000).

The concentration of biogenic compounds and heavy metals in the bottom sediments of the Sulejów Reservoir (Central Poland) was measured by Ziemińska-Stolarska, Polańczyk & Zbiciński (2015). It was found that the distribution of examined compounds was largely influenced by the agricultural activity in the studied area, as well as the presence of ports and summer houses. Based on the measurements, it was observed that the highest amounts of biogenic components were deposited in sediments in deep parts of the reservoir, slow-flowing waters, stagnation zones, areas adjacent to arable land, and sites where fine-size fractions prevail in the deposited material. Biogenic compounds in sediments showed a pattern of a gradual increase along the reservoir from lower values in the backwater part. A similar relationship is visible for heavy metals. Referring to the eco-toxicological criteria, it can be stated that bottom sediments from the Sulejów Reservoir collected in 2018 are not toxicologically contaminated in terms of cadmium, lead, and chromium content (Ziemińska-Stolarska et al. 2020).

A research study conducted by Izydorczyk et al. (2008) to evaluate core variables of the ecological potential of the Sulejów Reservoir (temperature distribution, flow velocities, and concentrations of selected indicators – e.g. phosphates, nitrates, the abundance of phytoplankton) used a three-dimensional hydrodynamic model, GEMSS-HDM (generalized environmental modeling system for surface waters), coupled with a water quality model, WASP EUTRO (GEMSS-WQM). As a result, proposed scenarios for nutrient reduction – a 50% reduction of phosphate-phosphorus (PO₄-P) and nitrate-nitrogen (NO₃-N) from agricultural areas, and a 50% reduction of discharge from septic tanks – led to a considerable reduction of nutrient concentrations in the reservoir waters. The multifaceted usage of reservoirs is possible only if the ecological values of their geo-systems, especially the quality of water, are retained. This quality is derived from environmental conditions and from the way maintenance drainage areas are developed. The identification of relationships and interactions in the drainage basin–reservoir system helps indicate optimal forms of land use, triggering the protection of water resources (Ziemińska-Stolarska & Kempa 2021).

Another study was carried out to determine the role of lowland reservoirs in transporting micropollutants. It demonstrated that the dam section of the Sulejów Reservoir plays a role in the hydraulic transport and deposition of measured pollutants in the reservoir's sediments. The results obtained also revealed the reduction of nutrients and SPM (suspended particulate matter) concentrations. A 45% reduction of SPM, 28% of Total Phosphorus (TP), and 34% of Total Nitrogen (TN) were observed between the water inflow and outflow from the Sulejów Reservoir (Urbaniak, Kiedrzyńska & Zalewski 2012).

The bottom relief of the Sulejów Reservoir is known from precise echo-sounding measurements performed in July 2016 using a Lowrance HDS-3 sonar, equipped with a GPS receiver. The total number of echo-sounder measurements was about 440 000 points (Jaskulski, Wawrzyniak & Zbiciński 2018). The depth of the backwater of the reservoir was obtained from ground surveying, in cross-sections, from the year 2008. Using the point data and inverse distance interpolation method, a DTM of the reservoir was calculated with a spatial resolution of 5 m.

The Sulejów Reservoir has an elongated shape (Fig. 2). The reservoir's bathymetry characteristics reveal an upper basin (km 150–152), separated by a narrowing channel (km 148–150). The main body of water (km 138–148) is contracted by an island (km 145–146).

Figure 2.

To understand the dynamics of the Sulejów Reservoir, a one dimensional (1D) computational river dynamics model, HEC-RAS ver. 6, was applied. The geometry of the reservoir, as a set of cross-sections, was obtained from a 5-m resolution DTM. Along the main axis of the reservoir, 73 cross-sections were designed with an average spacing of 200 m. The Manning's roughness coefficient n=0.025 was designed for all cross-sections. That value represents a naturally fairly uniform channel without vegetation. The lower boundary condition was set as the water normal head of the dam, at z=166.6 m a.s.l. The upper boundary condition was the long-term average discharge of the Pilica and Luciąża rivers.

For the model verification, levelling measurements of the water surface elevation at cross-section nos. 27–29 of the Sulejów Reservoir back-curve, performed on 30.08.2011, were used. On that day, the discharge of the Pilica River was 14.1 m³s⁻¹ and the water head in the dam was z=166.63 m a.s.l. The difference in water surface elevation in cross-section no. 29, measured by levelling and calculated by the HEC-RAS model, was only 1 cm. That was an acceptable difference and proof of good calibration of the HEC-RAS model.

The hydraulic parameters in the cross-sections were converted by linear interpolation between the HEC-RAS cross-sections into a map showing the distribution of v_m – average velocity in a cross-section.

Sediment samples from the reservoir were collected twice – in the autumn of 2016 and in the summer of 2017, from sites located in different parts of the Sulejów Reservoir. Depths, from which sediments were collected, ranged from 0.5 to 11 m below the water table. The samples were collected using an Ekmann-type bottom sediment sampler. The collected sediments were transferred to polyethylene containers and transported to the laboratory in refrigerators. The sediments for the tests were homogenized and dried, first at a temperature of approximately 20°C, and then to a constant mass at 105°C. The samples were sieved through a mesh size of 0.2 mm. Bottom sediments were mineralized in a mixture of HNO₃ and H₂O₂ using the Multiwave 3000 microwave system (Anton Paar).

Heavy metal concentrations in the tested samples were determined using the atomic absorption spectrometry technique.

Total phosphorus was determined spectrophotometrically after mineralization in the Ethos Easy microwave mineralizer. The content of total organic carbon in the tested samples of bottom sediments was determined using the Behr apparatus.

The linear interpolation between the HEC-RAS cross-sections was converted into a map showing the distribution of v_m – average velocity in a cross-section – at MMQ=22.8 m³s⁻¹ (Fig. 3).

Figure 3.

Having the results of precise bathymetry measurements and a map of average cross-section velocities, distribution 4 sedimentation conditions were proposed:

TL – thalweg and lacustrine zone, representing the old Pilica River channel and the lacustrine part of the reservoir,

The chemical properties of Sulejów Reservoir bottom sediment, grouped by sampling points with different sedimentation conditions, are shown in Table 1.

The bottom sediment chemistry of selected elements in Sulejów Reservoir was plotted on the bathymetry maps, together with coloured dots representing sedimentation conditions at the sampling points. The following maps were created showing the distribution of Total Organic Carbon – TOC (Fig. 4), Total Phosphorus – TP (Fig. 5), and Cadmium – Cd (Fig. 6).

Figure 4.

Figure 5.

Figure 6.

The maps in figures 4–6 and Table 1 show that the highest concentration of analyzed chemical parameters occurs in the zone of the former Pilica River channel and in the deepest lacustrine part of the Sulejów Reservoir. Distribution of average velocity in the cross sections, obtained from the HEC-RAS model, shows that the water is almost stagnant in the reservoir between km 138–145. This creates favourable conditions for phytoplankton growth and sedimentation of fine particles, including organic matter.

Higher concentrations of TP and Cd are proportional to the high TOC content in the bottom sediments. This can be explained by the fact that sediments are the major reservoir of P and part of its internal load in water ecosystems (Dubnyak & Timchenko 2000; Förstner 2004; Wagner & Zalewski 2000). The sorptive capacity of sediments for P is related to the content of organic matter, Fe-Al-oxides/hydroxides, clay, and CaCO₃ (Gierszewski, Szmanda & Luc 2006; Trojanowska & Jezierski 2011).

A high concentration of TP in the bottom sediments of the lacustrine part of the reservoir creates conditions for a phytoplankton bloom at the end of every summer.

Sampling points, representing the littoral zone, are located in the shallow coastal area of the reservoir, which has higher water dynamics due to wind wave action. The analyzed chemical parameters of the bottom sediments show the lowest concentrations in the whole reservoir.

Accelerated flow areas are found at the narrowing of the reservoir where the water has a higher velocity and conditions for sedimentation are less favourable. Accelerated flow zones can be coincident with thalweg locations. In such a case, the old river channel, having favourable conditions for sedimentation, is exposed to a higher velocity of flow due to changes in the whole cross-section area and the narrowing of the reservoir.

Sediment traps are places of sheltered water, created by the configuration of the coastline. One point in the upper part of the reservoir, around km 151, was also included in this class because it is located in a back-curve with a steep decrease in the velocity gradient. The analyzed chemical parameters of the bottom sediments in sediment trap locations show high concentrations. In the case of coastal bays on a western bank at around km 140.5, the sediment traps can be explained by human activity related to tourism and large marina operations.

The proposed method to classify sedimentation conditions based on precise bathymetry and hydraulic 1D modelling helps to interpret the spatial distribution of the chemical analysis results. One of the common methods of visualizing chemical analysis results is the application of spatial interpolation (Ziemińska-Stolarska et al. 2020). In such an approach, we assume the homogeneity and continuity of the sedimentation conditions in the whole reservoir. Analysis of the Sulejów Reservoir bathymetry and hydraulics shows considerable variability in sedimentation conditions. There is a velocity gradient in the longitudinal profile from the backwater to the dam; additionally, different local conditions in the littoral zone and sheltered waters form sediment traps.

The results of the chemical analysis of bottom sediments from the Sulejów Reservoir show large variability and a range of concentrations. The method of classifying sedimentation conditions based on precise bathymetry and hydraulic modelling of the reservoir helps to interpret the chemical analysis results.

The Sulejów Reservoir acts, most of the time, as a sediment trap that accumulates sediments and pollutants. High concentrations of DOC and TP in the lacustrine part of the reservoir promote phytoplankton and cyanobacteria bloom, observed every year on the reservoir in the summer (Zalewski, Wagner-Lotkowska & Tarczynska 2000).

The method of bottom sediment chemistry visualization by dot maps or spatial linear interpolation used in many studies does not take into account the spatial variability of the sedimentation conditions. The application of the simple interpolation method, used by Ziemińska-Stolarska et al. (2020), for the graphical representation of the chemical properties of the Sulejów Reservoir bottom deposits shows the general situation of the chemicals but does not differentiate sedimentation conditions. The proposed method to delimit sedimentation conditions can be used for other artificial reservoirs and natural lakes. The delineation of lacustrine, transitional, and riverine zones is the first step in any programme to re-cultivate or dredge reservoirs. Our method gives a more detailed picture of sedimentation conditions in different zones of reservoirs showing the influence of water velocity, reservoir depth, and the location of the old river channel.

The application of the 1D HEC-RAS model, based on precise bathymetry data, can provide valuable information on sedimentation conditions, which are controlled by the velocity gradient in the reservoir. More advanced methods of flow simulation in the reservoir require use of 2D hydrodynamic models, which are more demanding in computational power and calculation time.