Urban hydrology was created in order to improve methods of managing the runoff of precipitation in towns and protect them from flooding while also protecting public health and environment. The essence of a future solution consists in finding an acceptable compromise of an alternative solution for draining rainwater from a territory. The content of this work is a study focused on resolving the percolation of water from surface runoff and the confrontation between a field test, laboratory analysis, and numerical analysis. By confronting and subsequently proposing conditions for percolation, documents will be created for making urban drainage better and more efficient. The reason for the origin of the subject work follows from the insufficient information on infiltration systems in Slovak technical standards and, likewise, the lack of support for the percolation of water from surface runoff. This work points out the approaches, principles, and fundamentals of a proposal for percolation. The aim of the work is distribution of scientific knowledge in the field of research and solutions for the percolation of water from surface runoff, with emphasis placed on the retention capacity of the selected territory and the intensity of precipitation. A geological study (orientational, detailed or supplementary) must always be conducted with any decision on rainwater percolation in a certain locality. Its range is dependent on the difficulty and type of construction. The preliminary study of areal condition should be focused on detailed engineering-geological and hydrological information. After this work, it is concluded that the percolation of rainwater in urban areas with suitable hydrogeological condition is an effective rainwater management technology as well as protection to congestion of sewer systems.
The important drivers for urban drainage are hydrology and hydrogeology. Hydrological data are served as the foundations for investment activities in nearly all branches of the national economy; they are necessary especially for water management investments, such as the construction of dams, spillways and reservoirs, the alteration of water courses, and for flood-protection measures. Detailed hydrological data are required for improving agricultural lands, mainly in the areas of irrigation, drainage, the construction of ponds, overall watershed improvements, anti-erosion protection, the controlling of streams and the like. Transport constructions, mainly bridges and culverts for railways and roads, require very responsibly determined hydrogeological data on average and maximum values of flows in water courses (Vranayová et al. 2011; Zeleňáková & Hudáková 2014; Zeleňáková et al. 2015; Markovič & Zeleňáková 2017; Romanescu et al. 2018).
There are plenty of studies regarding urban drainage, mentioned in the following text. Impermeable surfaces have impact on hydrology by eliminating through filtration dramatically increasing the volume of surface runoff. Although street and sidewalks are generally considered to be impermeable, their hydrological behavior is based on the intensity and the duration of the precipitation (Butler & Davies 2000).
Relationships between urbanization and subsurface drainage processes are described in several in-depth studies (Price 2011; Hamel et al. 2013). They appear to be complex, based on the variation of natural drainage characteristics (geology, topography, vegetation, etc.) and the characteristics of urbanization such as spatial arrangement of impermeable surfaces, type of dewatering, etc. (Hamel et al. 2013; Mesároš & Mandičák 2018). Consequently, subsurface runoff in urbanized basins may increase or decrease. Impervious surfaces result in a decrease in tapping, but reduction in vegetation in other permeable surfaces can reduce evaporation and potentially increase tapping. So the loss of vegetation can reduce the intake, thus reducing the subsurface runoff (Price 2011; Hamel et al. 2013). Urban drainage, sewerage and infrastructure can have important interactions with underground drainage processes. Berther et al. (2004) proposed a detailed model of interactions between the soil surface in the cities and the drainage duct in order to simulate case variations in the effluent coefficient. Interactions between water in the soil and sewer networks were modeled by Goebel et al. (2004). Lowering the groundwater level (Lee et al. 2005) is a common, but not necessary, consequence of urban infrastructure such as sewer systems. Endreny & Collins (2009) investigating the impact of rainwater intake on groundwater, recommending to reduce the risk of contamination and increased groundwater levels. Karpf & Krebs (2011) developed a model to demonstrate groundwater filtration into sewer systems based on the sewer pipeline classification by year of production. In the research, they created a relationship between these classifications and the takeover behavior. The importance of incorporating geological data in hydraulic tomography surveys was proved by Zhao et al. (2016). The complexity of subsurface drainage processes means that predicting changes brought about by urbanization and urban infrastructure remains an important gap in our knowledge. The development of appropriate tools and a common set of indicators is therefore a very important area of future research, especially in view of the growing importance of the role of subsurface effluent processes in water environment (Hamel et al. 2013).
The main focus of research and development of all types of hydrological models is abroad, particularly in works by Refsgaard & Storm (1996); Beven (2001), Blöschl & Grayson (2002), Smith et al. (2004, 2005) who have applied various hydrological models and have significantly contributed to the development of methods describing the precipitation-runoff process (Slys et al. 2012; Stec et al. 2017).
The purpose of this work is the proposal of conditions for percolation, which are derived from an analysis of precipitation, geological survey in the selected localities and from existing known equations for calculating the percolation of precipitation from surface runoff, for the needs of urban drainage.
MATERIAL AND METHODS
The basis of a solution for urban hydrology consists in finding a favourable compromise alternative solution for the drainage of rainwater from the land. The subject of the survey described in this study is resolution of the infiltration of water from surface runoff and a comparison of the results obtained from field measurements, laboratory analysis, and numerical calculations. A field test uses the real measuring of precipitation with parallel percolation. With laboratory analysis, a grading curve is determined and subsequently the filtration coefficient and numerical analysis deals with the modelling of percolation. Empirical relations and dimensional analysis belong under numerical analysis.
This section of the work deals with resolving the draining away of rainwater in specific locations on the basis of knowledge and measurement of qualitative and quantitative indicators of rainwater runoff. The territory where the field tests were conducted (Figure 1) are located in region of eastern Slovakia, Košice City.
Košice is the second-largest city in Slovakia and the largest city in eastern Slovakia. Its geographic coordinates are 48°43′12″N and 21°15′29″E. The city is situated at an elevation of 208 m a.s.l. From a geological point of view, the entire territory of the Košice Region falls into the zone of the Internal Western Carpathians. The subbase of Košice is made up lengthwise of water courses of quaternary rock (Holocene), of bottom land sediments and irrigation water and further of water courses of quaternary rock (Pleistocene), loess and loess clays. Lake and river sediments are also found here.
Košice – technical university campus
The first territory of interest is located in Košice, specifically in the grounds of the Technical University (TUKE). The study was carried out in the buildings shown in Figure 2.
These are building Park Commenius (PK6) – the computer center of the TUKE Faculty of Electronics and Informatics and the building of the Library Information Center (LIC) of TUKE. Percolation shafts were located near the building of the computer center of PK6. Rainwater from the roofs of the buildings is carried into the percolation shafts A and B and subsequently percolated into the subbase. In the grounds of TUKE, perhaps 300 m from building PK6 is the LIC building, on the roof of which is equipment for measuring precipitation – a rain gauge serving for monitoring the amount of precipitation throughout the entire year.
The shafts are located on the eastern side of the subject building PK6. The runoff from perhaps 1/3 of the roof is taken to percolation shaft A and approximately 2/3 of the area of the roof is drained into percolation shaft B. The shafts are made from concrete rings with an external diameter of 1,000 mm. Both shafts are secured with a cast-iron hood. The shafts are similar in size; the difference of more than 100 m2 is in the surface of the roofing construction from which the rainwater is taken. Hydrogeological surveys in the past were conducted in the grounds of the Technical University in Košice, from which it follows that anthropogenic sediments – made-ground – are present on this territory, under which are fluvial deposits of the River Hornad and under them sediments of the Neogenic age. The made-ground is, for the most part, from gravelly clays, and the construction waste is made up of a pebble component of natural gravel. Geological boreholes have confirmed the thickness of the made-ground and the result was a value of 0.5–0.6 m. Under the made-ground an associated layer of flood-loam clays with a thickness of 4.0–4.5 m was confirmed. This is clay with moderate to low plasticity. Beneath the flood-loam sediments are gravels (most often with a mixture of fine-grained soil) with a thickness of 5.0–7.0 m. In the subbase of quaternary sediments there are neogenic clayey gravels and clays with a thickness of 0.7–2.7 m (Medveď 1988).
Instrumental and software equipment
The first measurements of precipitation began in March 2011, at the building of the University Library in parallel with measurement of the amount of incoming rainwater and the height of the level of rainwater in percolation shaft A. In March 2012, similar measurements also began in percolation shaft B. The measuring instruments used and their placement in the percolation shafts are shown in Figure 3.
Instruments are identical in both shafts, but the YSI (Yellow Springs Instrument Company) multiparametric probe, levelogger and barologger are located only in shaft A.
Hydrology – precipitation intensity
A table with the values of total precipitation [mm] and the values of rainfall intensities [mm.min−1] in the area of Košice – Barca was available from the Slovak Hydrometeorological Institute (SHMI) in Košice. This table was used with a comparison of the graphically processed data on precipitation totals and the graphically processed maximum 30-min and 60-min precipitation intensities.
Hydrogeology – filtration coefficient
The percolation of precipitation waters into the rocky environment to a great measure depends on the local conditions, primarily from the measure of its permeability. The basic hydraulic characteristic of this measure is the filtration coefficient. Correctly determining its values represents a key question in the assessment of the percolation of water into rock or soil. Many different methods exist for its determination.
The essence of a test is the separation of the grains using sets of sieves into several grain classes of declining sizes. The sizes of the openings and number of sieves are selected according to the type of samples and the required accuracy (STN EN 933-1). The testing process consists of washing and sieving under dry conditions. Since washing can change the physical properties of a light aggregate, in such a case it is necessary to use sieving only under dry conditions. The weight of grains captured in different sieves is related to the original weight of the material (STN EN 933-1).
In the scope of research for the purpose of work three soil samples were made available, specifically, two samples from model territory I (Košice). Samples from the territories of interest were supplied to the laboratory GEO Slovakia, Ltd, in Košice, in order to determine the filtration coefficient. With calculation of the filtration coefficient the size of the gravel grains with an overflow of 10% and with 60% is important. The filtration coefficient is given in m.s−1.
The highest monthly total of precipitation (NPMU) for year 2012 was in June (128 mm) and in 2013 was in May (97 mm). Since the study began being conducted in August 2011, the corresponding value from year 2012 was considered for the given year, and similarly with year 2014, where the value of 97 mm was considered. The number of days in the month with the highest average rainfall total (PDM) is determined from the highest average monthly totals for the given year, which was divided by the number of days in the given month (May – 31 days, etc.). This calculation determined the value used for determining the number of days with the highest average total. Obviously, this and the higher value were taken for determining the number of days.
Modelling of the emptying time
A mathematical-physical model was created in the scope of this work on the basis of dimensional analysis. The following are among the relevant values selected that influence the percolation period:
rainfall intensity i [m.s−1],
percolation surface S [m2],
percolation period/emptying time Tvs [s],
measured weight of the soil ρ [kg.m−3],
filtration coefficient kf [m.s−1].
The relation is universally valid for determining the percolation period, but for each of them (rainfall event), it is necessary to calculate the new factors A and B.
In this section, the results of the study in model location is presented. The results include graphic processing of total precipitation and rainfall intensity in Košice, since the duration of precipitation segments from the SHMI were only available for this location. The filtration coefficients – obtained from grading curves and calculations according to empirical relations is presented. In the following section, the created model is presented, the use of which enables calculation of the period of percolation in the infiltration facility.
Evaluation of hydrological parameters – precipitation
Precipitation events for individual days and the total sum of precipitation for the given day were recorded. Calculation of maximum intensity with a 30-min and 60-min interval was according to Equation (1).
From measured data containing the maximum 30-min and 60-min intensities from 2011 and from the table of the SHMI from 1968 a graphs were made (the example is in Figure 4), from which the mutual relation of the measured and table intensities of rainfall follow. There were only a few values which exceed the 30- and 60-min intensity with a periodicity of 5.0 from all evaluated rainfall events.
Evaluation of hydrogeological parameters – filtration coefficient
Determination of the filtration coefficient from hydrodynamic tests
In territory, it was possible to define from the archive a sufficient number of hydrogeological boreholes, on which hydrodynamic tests were conducted for determination of the filtration coefficient.
Determination of the filtration coefficient from laboratory tests
The output of the sieve and densimeter tests is a grading curve processed by software in the laboratory GEO Slovakia, Ltd. The grading curve subsequently serves for determining the soil fractions and for calculation of the filtration coefficient. In the case of use of grading curves determined from laboratory tests kf moves in the range of 7.81.10−5 to 6.4.10−3 m.s−1 in model locality. In comparison with other archived studies (Tometz 2012) differences in the values of kf determined by the presented methods are even more notable, namely from 4.17.10−7 up through 5.63.10−3 m.s−1.
From the stated, it follows that laboratory determination of filtration coefficient values from grading curves, due to the very wide range of values, does not represent a suitable method for the purpose of percolation of precipitation into the ground. The filtration coefficients for the two model territories were set from laboratory analysis. The grading curve of the soils was determined at the laboratory GEO Slovakia, Ltd. For the sample from shaft A in Košice the type of soil was determined to be well-grained gravel and sample from shaft B to be gravel with a mixture of fine-grained soil. Thus, the evaluation of the hydrogeological study from 2010 was confirmed. The filtration coefficient is also determined from grading curves as follows. The filtration coefficient for shaft A is 1.58.10−3 [m.s−1], and for shaft B in Košice it is 1.89.10−4 [m.s−1]. This analysis was necessary for comparing the results of the filtration coefficients determined by numerical analysis.
Calculation of the filtration coefficient using empirical relations
The component of the numerical analysis for calculation of the filtration coefficient in Košice used two equations. The first equation is according to mentioned standard ČSN 75 9010 (2012) and the second according to the final report of Tometz (2012), which differs with its input values.
The values in Košice move approximately from 10−4 to 10−6, which represents course-grained sand through powdery sand up to clayey powder.
Verification of the model for determination of the emptying time
For modelling the percolation period it was necessary to know the parameters which influence percolation and to calculate the dimensionless arguments and the regression factors. The values of dimensionless arguments π1 and π2 were calculated from the measured data. The difference between the measured and the calculated values for the percolation period was determined as a relative error (uncertainty). The value of the locating constant and regression factor is: A = 1.0947, B = −0.374.
From the obtained results of the percolation period, the significance of the uncertainty can be characterized as low to moderate (until 50%).
RESULTS OF THE STUDY
The total precipitation for the monitored time period in Košice did not exceed the totals with a periodicity of 0.2, which, according to German standard DWA-A 138E (2005) or Czech standard ČSN 75 9010 (2012) is used for the design of percolation facilities. From evaluated precipitation events it was determined that the maximum and minimum precipitation intensities for Košice, which was compared with the table values for rainfall intensity according to SHMI Košice for the rain gauge station Košice – Barca. Only the intensity with a periodicity of 5 was exceeded. The values for rainfall intensity for the monitored three-year period did not exceed the intensity with a periodicity of 0.2; 0.5; 1; 2 in the Košice – Barca station. From the mentioned it follows that the use of periodicity of rain of 0.2 for dimensioning percolation facilities is equally appropriate in the conditions of Slovakia. The duration of precipitation divisions processed by the Slovak Hydrometeorological Institute for rain gauge stations in Slovakia is also further useful, despite the period of processing of the given tables (year 1968). The validity of the 50-year tables was confirmed and their usefulness is recommended even in the present.
One of the main goals of applying percolation facilities is efficiency – the slow and gradual emptying of containers which should not, according to ČSN 75 9010 (2012), exceed 72 hours, i.e. 3 days and according to DWA-A 138E (2005) 1 day. The mentioned was confirmed for the individual monitored precipitation events. In the work a model for calculation of the percolation period was proposed, which can be used with variations in the size of the surface of the percolation facility. This requirement is closely associated with parameters of the soil and hydrogeological conditions in the subbase. According to available provisions the bottom of percolation bodies should be situated minimally 1.0 m, in the case of percolation wells minimally 1.5 m, above the maximum level of the groundwater, and the permeability of the percolation layer should be in the range of kf = 5.10−3 to 5.10−6 m.s−1.
The bottom of a percolation well which is in the grounds of the TUKE is located in a layer of gravels with a portion of fine-grained soil, for which kf = 1.10−3 up to 1.10−4 m.s−1. These are neogenic sediments, which create favourable conditions for concentrating a larger amount of groundwater. This is the reason why in the spring months water stays at the bottom of the well and flows out very slowly. With great certainty this is associated with the shifting level of the groundwater, which tends to be higher in the spring months after the melting of the snow, as well as with the placement of the bottom of the well and with the engineering-geological profile, where clayey gravel is found (GC, class G5) under the layer of gravels with a mix of fine-grained soil (symbol G-F, class G3), the filtration capabilities of which are lower. The results of filtration coefficients determined by different methods are shown in Figure 6.
In conclusion, it is necessary to state that the foundation of an optimally designed percolation system is certainly a quality geological and hydrogeological study. Determining not only the filtration coefficient or percolation coefficient of the first layer in contact with the built-in facility was shown to be important, but also the flow properties of the other soils. The importance of exact determination of the mentioned properties is in the fact that, to a crucial measure, the periods of emptying of the percolation facility enter into the calculation; thus, they contribute to the optimal design of the facility itself. In the case of use of inexact data, the overflowing of wells threatens at times of extreme precipitation events. The fact that Slovakia is lacking valid legislation which would exactly define the input requirements for the design of percolation facilities appears to be a problem. New legislation should thus resolve not only the determination of conditions for the percolation of rainwater but should also set the range and method of carrying out a geological and hydrogeological study.
Authorities in many countries (Germany, Switzerland, United Kingdom, The Netherlands, USA, Canada or Australia) have already been dealing with this issue for a certain amount of time. At present, this problem is now also addressed by new European guidelines. These mandate the handling of rainwater by percolation, directly where it falls if possible. If not, then releasing rainwater into sewerage or a water course is possible only through a retention container. Specifically, it is possible to speak about EC guideline 271/91/EEC, which arranges the design of percolation systems with a connection to the Report on the Water Shortages and Drought in the European Union from 2007.
On the territory of Slovakia percolation facilities are also used which may, with the help of filters and traps for oil and petroleum, also carry away water from car parks and pavements and other sites. Obviously, certain processes and provisions are used for suitable dimensioning of percolation facilities. Percolation facilities should be located so that no damage is caused to either the drained buildings or the neighbouring buildings and other facilities. Designers with the design of percolation facilities observe a minimal distance from buildings and from the level of groundwater in line either with ČSN 75 9010 (2012) or DWA-A 138E (2005). In German standard DWA-A 138E (2005) or Czech standard ČSN 75 9010 (2012), which was created on the basis of the mentioned German standard, are used as basic documents. The fact that no applicable standard for the design of the percolation facilities in Slovakia has been written causes a problem, however. A certain unification of the calculations and proposed processes for the territory of Slovakia would therefore be necessary, since both standards contain certain specifics related to the given countries. Some designers use the Czech standard, others the German one, and there are in them in places adequate differences in processes and thus also in the results of the solutions to the given problem.
During the construction of any building, an overview of all aspects of functionality and operating capability of the building must be taken into consideration. One of these problems is also resolution of drainage of precipitation water from roofs, whether classic or flat roofs, as well as car parks or pavements belonging to buildings. For a long time, a very suitable and simple solution until recently was the carrying of this water into the sewerage system; however, with constant development of towns and the construction of new buildings, existing sewerage pipes and mains become overloaded, and their continuous adaptation to actual needs is practically impossible. The present state of sewerage systems in this direction is not suitable, and so in the case of storm rainfall, when more rain falls in a few minutes than for an entire month, there is a threat of local flooding. In recent years, advancements have developed in science as well as in practices of urban hydrology from the viewpoint of basic understanding and approaches to management. New technologies have been developed for recording, analysing and predicting precipitation in urban areas with the aim of pointing out the issues of small spatial and time benchmarks for the reaction to precipitation and runoff in towns as well as the percolation of water from surface runoff, with an emphasis on the retention capacity of a selected territory and precipitation intensity.
At present, great emphasis is placed on carrying away rainwater from town and village lands through percolation facilities, which represent a sustainable method of handling precipitation water from surface runoff. Therefore, this study is focused on the percolation of rainwater from surface runoff. The research focused on determining the filtration coefficient, for which existing percolation facilities were used, so-called percolation shafts in model locality – TUKE Campus, on the basis of confrontation of theoretical and practical analyses and a synthesis of acquired knowledge in the conditions of eastern Slovakia. The use of percolation facilities is demonstrated in the work; this represents an effective method of taking away precipitation water from surface runoff for a specific project design.
This work was supported by a project of the Ministry of Education of the Slovak Republic 1/0217/19 Research of Hybrid Blue and Green Infrastructure as Active Elements of a ‘Sponge City and by project Slovak Research and Development Agency SK-PT-18-0008.