An innovative rainwater system as an effective alternative for cubature retention facilities

Nowadays, rainwater retention is one of the most serious problems of water and sewage management. Proper management of rainwater is an extremely difficult task because of changing climatic conditions.

The purpose of rainwater management is to provide an effective way to manage excess rainwater based on principles of sustainable development and with the least possible interference with the environment [9, 15, 16].

Dynamic development of urban areas and progressive urbanization in recent years, have contributed to the reduction of green areas and have caused an increase of paved surfaces [12, 17, 40]. These phenomena disturb the balance between precipitation processes and runoff, soaking and transpiration of rainwater [3]. Due to the intensification of the degree of development in recent years, a negative impact of climate change is observed, which results in more frequent extreme rainfall [10, 16, 13, 38]. According to hydrological forecasts, the frequency of extreme precipitation will increase in the coming years [20, 31, 32]. These phenomena cause an increase of rainwater surface runoff, which negatively affects not only sewerage systems, but also water receivers [15, 33]. A lack of a sufficient hydraulic reserve in the existing sewage system makes increasingly local flooding and overflow rainwater from the sewerage system on the land surfaces [9, 4, 12, 13, 14]. All of these phenomena enforce to look for an effective water management method in order to reduce the risk of flooding in urban areas and prevent failure of the operation sewage system.

The existing sewerage systems, because of overloading, require an extension or building additional retention facilities. In the case of projected sewerage systems, the main problem is the cost of construction canals with significant geometries and cubature facilities for rainwater retention. Additionally, it is also necessary to have enough area of land for the construction of retention facilities, which in the case of urban areas is often impossible.

As it was shown in many works [8, 9, 23, 37] upper spaces in the canals are empty and are not fully used even during the maximum rains. The sewage retention canal [7] is a modern solution, which allows a practical use of this space and includes it into the usable retention capacity of the sewer system. In this solution vertical damming partitions are installed in manholes at certain distances.

Rainwater flows in sewage systems are most often rapid. A very large volume of rainwater are transported in a short time through the system of canals to the receiver. Such a situation causes numerous technical difficulties and a number of negative environmental consequences such as a rapid inflow of rainwater to the receiver, an increase in the speed of water flow in the river, floods, intensification of erosion phenomena, movement of river sediments, disturbances in functioning of aquatic ecosystems.

Limiting these negative phenomena is possible by regulating the runoff of rainwater from urbanized areas through the use of facilities for their temporary storage, e.g. retention reservoirs described in patents [41,42,43,44,45,46,47].

Traditional retention facilities usually occupy large areas, which in cities are valuable for residential, commercial and service development. In addition, they are expensive investments but necessary due to the regulation of the outflow of excess rainwater. All activities supporting this process and reducing the cost of its implementation are expected and valuable.

In the paper the role of retention in drainage systems is discussed. A hydraulic model of an innovative rainwater system equipped with a retention canals is presented, the model subcatchment is characterized, and the research methodology is described. The results are obtained based on simulations using hydrodynamic modelling. Additionally, the hydraulic functioning of traditional stormwater drainage system and innovative storm water drainage system equipped with retention canals were compared. On the basis of the analysis, a lot of advantages of the innovative sewage systems over the classic sewage system were shown.

In recent years, there has been a rapid development of the urbanization which, according to forecasts, will be growing rapidly [30, 34]. A replacement of natural permeable areas with paved surfaces brings an increase of surface runoff and more rainwater discharge through sewer system [17]. Additionally, an extreme weather phenomenon such as heavy rains have been observed more frequently recently [13, 32, 35]. These cause a number of negative effects, for example hydraulic overloading of the rainwater system and treatment plants, local flooding, overload and pollution of the rainwater receiver [15, 29]. As a result, an increasing part of the costs is spent on repairing the consequences of flood. Therefore, it is needed to improve methods to design sewage systems and search for new effective ways of retaining and controlling rainwater flow in sewage systems [28, 17, 19, 35]. At first, the rainwater flow in stormwater systems should be reduced and delayed using infiltration and retention devices at the place of rainfall generation [27, 10]. These solutions are not always able to use, so a careful analysis of their advisability should be conducted [9]. Retention tank (fig. 1) and an additional transit canal (fig. 2) have been the most well-known design solutions to reduce hydraulic overloading in the sewer system so far [28, 27]. The use of retention tanks has both economica and environmental advantages. The problem of hydraulic overload sewage system and the objects working with it is solved and the stormwater receivers are protected against an excessive volume flow and pollutants by using retention tanks. Additionally, they allow the use of smaller geometries of sewer pipes and prevent overflowing the sewer system during heavy rains [26].

Figure 1

Figure 2

If an underground infrastructure is limited, the additional transit canal can be put in with the existing sewer system. The next possibility is to put the additional transit canal outside the urbanized area if there is dense underground infrastructure and surface development. However, location on the sewer system would interfere with investment and generate high investment costs [27].

The solutions mentioned have a limited scope of applications despite lots of advantages. There is no space for construction of such objects due to a rapid development of buildings and underground infrastructure. In addition, there are high investment costs. These are the basic disadvantages of current retention facilities. The lack of ability to use them and the growing problems of rainwater management make it necessary to look for modern solutions for rainwater retention [23, 25].

One of them is the sewage retention canal (Fig. 3). This solution can be applied to both designed and already existing sewer systems. The innovative retention sewage canals can replace a retention tank or reduce its required volume. That makes the investments costs lower. It is an effective solution compared to traditional ones. It does not require an additional area to build the special retention facilities [8, 9]. The main advantage of this solution is maximizing the retention capacity of the sewage systems. This in turn, allows hydraulic relief of the sewer systems, and gives an opportunity to connect new subcatchments to the existing sewage, and reduces the cost of constructing new sewage systems. The use of innovative retention canals equipped with damming partitions does not even require simple control systems as well as energy supply [28].

Figure 3

Such a sewerage system can be a successful alternative for Low Impact Development facilities and traditional retention reservoirs or cooperate with them in order to maximize the efficiency of the whole sewerage system [9, 23]. This solution minimalizes the risk of urban flooding, does not interfere with the natural environment, protects rainwater receivers and complies with the principles of sustainable development. The hydraulic model of the innovative rainwater network is discussed in sec. 3.

Retention sewage canal is a patented solution RP no. 217405 [7]. Its primary advantage is an ability to utilize the capacity of the sewer systems, including pipes and manholes, which had not previously been utilized in full. It enables retention of excess rainwater. In many cases, this solution makes sewer system to function without any additional retention facilities, especially retention tanks [4].

This solution consists in equipping the canalization with a system of retention canals with special damming baffles. The damming elements are installed in inspection manholes, perpendicular to the flowing wastewater (fig. 4). Damming partitions enable damming of rainwater throughout the sewage systems [28]. There is an opening flow at the bottom of each baffle and an overflow edge at the top, which is the leading discharge overflow [8, 24]. The damming baffles are mounted to the inside walls of the canals.

Figure 4

The principle of operation of the innovative rainwater sewage system is shown in Figure 5 allow for effective use of the drainage system capacity [23]. Mounted damming partitions into canals create rainwater retention chambers. It is recommended to start filling these chambers from the highest chamber which has the smallest opening. The next lower chambers have larger flow openings [28].

Figure 5

The rainwater inflow to the accumulation chamber located below depends on the rainwater outflow from the chamber located above and the surface runoff entering the sewage systems. The efficiency of the innovation sewer systems is determined by the critical values of the stormwater outflow from the damming baffle Q_oImax. The slope, diameters of retention canals and the geometry of the damming partition have a significant influence on the outflow Q_oImax. A designer should not only fit slopes and diameters of canals but also design correctly the dimensions of damming baffles, including their height and size and the shape of flow holes. This is a basic task to be performed by a designer. The establishment of critical rainfall is necessary to project damming partitions. A full utilization of the space in the canal ensures the lowest rainwater outflow Q_oImax from an outlet node. It is determined by the value of the rainwater flow reduction coefficient β_KR [28].

A model catchment consists of 80 sub-catchments, where a total drained catchment area equals F = 80 ha. The same hydrological parameters were assumed for each sub-catchment. Three concepts of rainwater sewer system were considered, varying in canal slopes.

Concept I - the canals bottom slope amounts i_k = 1 ‰.

Figure 6

Hydrodynamic modelling with the SWMM 5.1 program was used for the analysis. The surface runoff coefficient Ψ = 0.5, the slope of the drainage area i_ż = 10 ‰, the catchment roughness coefficient n_z = 0.015 s/m^1/3 and the canals roughness coefficient n_k = 0.010 s/m^1/3 were assumed.

At the first stage, three concepts of traditional rainwater sewer system were considered. For each of them the maximum value of rainwater outflow from the sewer at outlet node Qo_Tmax and the calculative time for rainwater sewage system dimensioning t_m were determined. At the next stage, each of sewers was equipped with damming partitions. Three different spacings between the damming baffles L_KR were assumed, such that nine variants of innovative sewer system equipped with the retention canals system were obtained. For that model of the sewer system, the maximum value of the rainwater outflow from outlet node in the innovative systems Qo_Imax and the calculative reliable rainfall time to dimension the innovative rainwater canals t_M were determined.

Precipitation models are used in the design of rainwater and combined sewer systems and facilities working with them. They allow determining the relationship between the intensity of the critical rainfall and the rainfall time and the probability of its occurring. The knowledge on critical rainfall is needed during hydrodynamic modelling [11, 19].

The data on the functioning of the innovative rainwater system come from hydrodynamic modelling using the Bogdanowicz and Stachy rainfall model. It was developed on the basis of rainfall measurements from 20 meteorological stations of Institute of Meteorology and Water Management in the years 1960 – 1990 in Poland [8, 22]. It is a probalistic model of maximum rainfall heights. It considers the time of rainfall and probability of occurring [9]. It is described in the publication [21] by the following formula (1): (1) h=1,42 t0.33+α(R,t)·(−ln p)0,584 h = {\it 1,42}\;{t^{{\it 0.33}}} + \alpha \left( {R,t} \right)\cdot {\left( { {\text -}\, ln\;p} \right)^{{\it 0,584}}} where: h - maximum rainfall height, mm; t - rainfall duration, min; p - probability of rainfall occurrence, p ∈ (0;1]; α - parameter depending on the region of Poland R and time t, -.

The parameter α depends on the region of Poland and the rainfall duration [22, 21]. The precipitation model can be used for the whole Poland except for mountainous regions. The rainfall model of Bogdanowicz and Stachy is recommended for rainfall frequency C = 2, 5, 10 years [10].

The simulation of the phenomena in the sewer system was performed by using hydrodynamic modelling with the help of Storm Water Management Model program (SWMM 5.1). The probability of rainfall occurrence was assumed p = 50 %. The phenomena were simulated using the dynamic wave model. It can truly reflect the functioning of the sewer during changing water flows in time, the occurrence of backwater and the retention of rainwater in the sewer [39]. The flow along the sewer system is gravitational. The hydrodynamic models obtained reflect different conditions of sewer system operation considering three different canals bottom slopes and three different spacing between damming baffles.

The analysis of the innovative rainwater system functioning in relation to the classical rainwater system was based on the results from Tables 1 and 2. Table 1 presents the data from simulation for traditional stormwater system with three variants of canals bottom slopes like i_k = 1 ‰, i_k = 2 ‰ and i_k = 3 ‰. Drained catchment is F = 80 ha. For those three concepts, the values of the maximum rainfall outflow at the outlet node from catchment Qo_Tmax and the calculative times for sewage system dimensioning t_m were determined. Table 2 shows the values of parameters after equipping the classic stormwater system with damming baffles. Each conception of innovative rainwater sewage system takes into account three various distances between damming partitions like L_KR1 = 200 m, L_KR2 = 300 m, L_KR3 = 400 m.

In that way 9 different variants of innovative rainwater systems with retention canals were analyzed. TThe rainwater flow reduction coefficient β_KR of the outflow from the innovative rainwater system was determined along with the parameters Q_oImax and t_M. The cross-sections of conduits and slopes of canals bottoms were identical in each concept.

The data presented in Tables 1 and 2 are the results of simulation from hydrodynamic modelling [1]. They present that the value of the maximum rainwater outflow from the outlet node in innovative system Qo_Imax is lower than Qo_Tmax in the classic stormwater sewer system in each case.

The results of the research presented in Tables 1 and 2 confirm that with an increase of the slope of the canal bottom i_k, the critical time for dimensioning the traditional sewerage system t_m decreases. It simultaneously causes that the rainwater outflow at the outlet node Qo_Tmax increases. The canal slope i_k affects directly the retention capacity of the innovative system. The rainwater retention effects of the system increase as the slope decreases. This is why the higher canals slope i_k, the higher flow velocity of rainwater in the sewage ν_TK., so that the rainwater is transported faster towards the outlet node and the flow time t_p in the sewer decreases.

Changes of the canal slopes i_k affect the value of maximum rainwater outflow both in the classic sewage Qo_Tmax and in the innovative sewage Qo_Imax. This relation is shown in Fig. 7, and takes into account different spacing damming baffles L_KR.

Figure 7

In the case of traditional sewerage system for slope of canals i_k = 1‰, the maximum rainwater outflow from the sewage is Qo_Tmax = 2887.7 dm³/s. However, for slope i_k = 2‰ the outflow is Qo_Tmax = 3692.8 dm³/s, for slope i_k = 3‰ outflow intensity increases almost 1.5 times to the value Qo_Tmax = 4175.9 dm³/s.

For example, considering an innovative system for the slope of bottom i_k = 1 ‰ and damming barriers spacing L_KR1 = 200 m, it is possible to obtain an almost threefold reduction of flow from the value Qo_Tmax = 2887.7 dm³/s to Qo_Imax = 981.6 dm³/s. For L_KR2 = 300 m, the flow is reduced by 2.7 times (the outflow rate is 1063.4 dm³/s). For L_KR3 = 400 m, the outflow is almost reduced by 2.5 times (Qo_Imax = 1159.6 dm³/s) compared to the traditional storm sewer.

The results of the simulation showed that the spacing of damming baffles measurably affected the rainwater flow reduction from the drainage catchment's outflow. The flow reduction effects increase according to the decreasing slopes of the canals i_k and to the decreasing damming baffle spacing L_KR.

Table 3 presents the multiplicity of rainwater outflow reduction in the innovative rainwater system depending on the spacing of damming partitions for distance L_KR1 = 200 m, L_KR2 = 300 m i L_KR3 = 400 m and bottom slopes i_k = 1 ‰, i_k = 2 ‰ i i_k = 3 ‰.

The above results show that after applying damming partitions in a traditional sewerage system for the slope i_k = 1 ‰, the flow from the outlet decreases by more than two times in almost all cases and by almost three times in some cases. In sewerage system for the slope i_k = 2 ‰, there is a smaller reduction of the flow, around from 2.2 to1.6 times less. In sewerage system for a slope i_k = 3 ‰, the flow reduction was less than 2 times in every case.

Hydrograms are often used in order to reflect the reversibility of rainwater flow in the canal [2]. A comparison of rainwater outflow variability from a traditional and innovative system with retention canal system at time t was based on the hydrogram shown in Figure 8.

Figure 8

In the case of classical stormwater systems, the hydrogram has an unfavorable pointed shape. The use of damming partitions causes that the peak rainwater outflow intensity at the outlet node is significantly reduced and the shape of the hydrogram flattens. For a smaller distance between damming baffles, the hydrogram flattens more. The studies [1] have confirmed that spacing of damming partitions L_KR impact on the value of the parameter Qo_Imax independent of the considered time t. The smaller damming spacing, the greater the reduction of outflow Qo_Imax from the innovative system.

Figure 9 shows the relationship between the critical time for rainwater sewage system dimensioning t_m and the critical time for innovative rainwater sewage system dimensioning t_M. Different damming baffles spacing L_KR was also considered for the innovative sewer system. Establishing the correct value of rain duration gives the basis for its dimensioning. However, in practice it turns out to be a very difficult task to solve because of complexity of studied phenomena [33].

Figure 9

As shown by the curves in Figure 9, the change of canals slope i_k directly affects the determined value of critical time for dimensioning both the traditional and the innovative system. In both systems, an increase of canal slope i_k results in a decrease of critical time for dimensioning sewage systems. As the spacing of the damming baffles L_KR shortens, the value of the critical time t_M increases. Many factors, especially parameters characterizing the drainage catchment impact on the value of critical ran duration. The biggest differences between the critical time for rainwater sewage system dimensioning t_m and the critical time for innovative rainwater sewage system dimensioning t_M occur at slopes i_k = 1‰.

Table 4 summarizes the values of the critical rain duration times t_m and t_M considering different slopes of the sewer canals i_k and the spacing baffles L_KR. For instance, in a traditional sewerage system with a slope i_k = 1 ‰, the critical rainfall duration hits the t_m = 32 min. After equipping this sewage with damming partitions with L_KR1 = 200 m, the critical t_M. hit 88 min. It shows a difference of ΔT = 56 min and is the highest recorded ΔT difference among all the considered design variants. For L_KR2 = 300 m spacing, the value of ΔT = 52 min and for L_KR3 = 400 m spacing, the value of ΔT = 46 min. This results in an important conclusion related to the damming partitions spacing L_KR. An increase of the spacing between baffles in the retention canal system brings the t_M values closer to the critical time for dimensioning the canals of the traditional system t_m.

The smallest differences between the times t_M and t_m occur for the sewage with i_k = 3‰ and spacing of damming baffles L_KR3 = 400 m.

The dependence between the values of the determinate time for the dimensioning of the traditional rainwater system t_M and the determinate time for the dimensioning of the retention canals system t_M is described by the coefficient of the determinate times γ_TM [1] by the following formula (2): (2) γTM=tMtm {\gamma _{TM}} = {{{t_M}} \over {{t_m}}} where: t_M - duration of the maximal (critical) rainfall determined for innovative rainwater sewage system dimensioning, min; t_m - duration of the maximal (critical) rainfall determined for traditional rainwater sewage system dimensioning, min.

The relationship between the coefficient γ_TM and the sewer slope i_k and the spacing of damming baffles L_KR is shown in Figure 10. The results confirm the rule that the value of the coefficient γ_TM is always larger than 1. This proves that the critical time for the dimensioning of retention sewer systems t_M is always larger than the critical time for the dimensioning of traditional rainwater sewage system t_m.

Figure 10

The value of coefficient γ_TM decreases with increasing canal slopes i_k and increasing distance between damming partitions L_KR. The biggest values of the coefficient γ_TM were determined for the canals slopes i_k = 1 ‰ and they are γ_TM > 2.

The sewage flow reduction coefficient is another important parameter which characterizes the work of innovative rainwater system. This coefficient plays a key role to determine the usable capacity of the retention tank [2, 5, 6]. Its value depends on an inflow and outflow rate. The larger the volume of rainwater necessary for retention, the smaller the value of the β coefficient. The value of coefficient β is greater than zero and less than unity for the classical sewage working with a retention reservoir [2, 5, 18]. In order to determine the reduction of the rainwater flow in the innovative rainwater sewage equipped with the retention canal system, the rainwater flow reduction coefficient β_KR was found. The value of coefficient β_KR is ratio of the critical rainwater outflow intensity Qo_Imax to Qo_Tmax. It is determined by the following relation (3): (3) βKR=OoImaxQoTmax {\beta _{KR}} = {{O{o_{Imax}}} \over {Q{o_{Tmax }}}} where: Qo_Tmax - maximum value of rainwater outflow from the traditional rainwater sewer at outlet node, dm³/s; Qo_Imax - maximum value of rainwater outflow from the innovative rainwater sewer at outlet node, dm³/s.

It can be confirmed that the use of retention canals provides the expected flow reduction by determining the value of the rainwater flow reduction coefficient in the innovative sewer system β_KR. The smaller the value of coefficient β_KR, the greater the effects of rainwater retention in the sewage with retention canals system. When classic storm water sewage system is designed, the value of coefficient β is determined at the initiation stage. This value affects the required volume of retention tanks [2, 5]. However, in the case of the innovative rainwater system, the value of the rainwater flow reduction coefficient β_KR is not determined at the design stage, but it is the resulting value. It is calculated on the grounds of formulated procedure at the final stage of simulation calculations conducted as part of hydrodynamic modelling.

The effect of slope of canals bottom i_k and damming partitions spacing L_KR on the value of reduction coefficient β_KR is shown in Fig. 11. The value of this coefficient increases with a growth of the sewer slope i_k. This is because the velocity of rainwater flow through the sewage increases and rainwater is retained in the sewage system for shorter time. The retention capacity of the sewage system decreases. Another parameter that affects the value of the reduction coefficient β_KR is the damming partitions spacing L_KR. The larger the damming baffles spacing L_KR, the smaller the retention capacity of the sewage system and the value of the β_KR coefficient is larger. On the ground of Figure 11 it can be ascertained that the largest differences between the values of β_KR coefficient are obtained when the slope of the canals bottom i_k is changed. Considering the constant damming baffle spacing L_KR and taking into account the change of canals slope i_k, the largest differences β_KR coefficient were recorded at spacing L_KR = 400 m. For instance, by reducing the canal slope from i_k = 3‰ (for which the reduction coefficient β_KR = 0.75) to i_k= 1‰ (β_KR = 0.40), it was possible to obtain the value of the coefficient β_KR as low as 0.35. For changing the canal slope from the value i_k = 3 ‰ to i_k = 2 ‰ (β_KR = 0.64), the difference between the reduction coefficients was 0.11. On the other hand, decreasing the canals slope from i_k = 2 ‰ to i_k = 1 ‰, the reduction coefficient was lower by as much as 0.24 was achieved. These results indicate that even with significant spacing of damming baffles L_KR, the use of retention rainwater canals for sewage with low slope of canals bottom i_k is fully justified.

Figure 11

In the case of decreasing the distance between the damming partitions L_KR and keeping the constant slope of canals i_k, the desired decrease of the value of flow reduction coefficient β_KR can be obtained. By reducing the distance between the damming baffles from L_KR3 = 400 m to L_KR1 = 200 m for the sewage system with a slope ik = 1‰, β_KR that was lesser by 0.06 was obtained. For the sewage system with slope i_k = 2 ‰, this difference is 0.16 and in the case of the sewage system with slope i_k = 3‰, the value of the reduction factor β_KR less by 0.16 was obtained. The key issue is the choice of an optimal solution [6]. Therefore, it is necessary to consider that the use of a smaller damming partitions spacing is justified and it provides the expected effect of reducing the rainwater flow. The studies have confirmed that each solution should be individually analyzed, both economically and ecologically. For example, for the variant with the slope of sewage canals i_k = 1 ‰ and the damming partitions were localized at L_KR3 = 400 m, the flow reduction coefficient was β_KR = 0.40. When reducing the spacing by 100 m, the flow reduction was determined to the value β_KR = 0.37. However, on reducing the spacing by 200 m, the value of β_KR coefficient was 0.34. In this case, increase in the density of the damming partition spacing causes a slight flow reduction. Considering the variant for the slope ik = 2 ‰ and L_KR3 = 400 m whose β_KR = 0.64, decreasing the spacing by 100 m causes a decline of the value of this coefficient by 0.07. In the case of decreasing the spacing by 200 m, the coefficient βKR is already smaller by 0.16. In this situation, changing the spacing of the baffles affects the value of the outflow reduction coefficient more than in the previous variants. As the results for some variants show, the effect of rainwater flow reduction using close partitions spacing is the same as for larger spacing or slightly more beneficial. Therefore, it is recommended to consider the economic costs which come from the implementation and subsequent exploitation of the innovative system.

The study [2] demonstrates that there is a close relationship between the rainwater flow reduction coefficient β and the critical time for multi-chamber tanks dimensioning T_MW in the traditional rainwater sewage system. The work [1] showed that there was a close relationship between the rainwater flow reduction coefficient β_KR in the innovative rainwater system and the critical time for innovative rainwater sewage system dimensioning t_M. This phenome is shown by the curve in Figure 12.

Figure 12

This relationship was formulated based on the pairs of results for all variants presented earlier in Table 2, including the calculative time for the innovative rainwater sewage system dimensioning t_M and the corresponding rainfall flow reduction coefficient β_KR. Based on this, trend lines were created and the equation describing this relationship was determined. To conclude one can say that there is a close relationship between the time t_M and the reduction coefficient β_KR. The results are well fitted to the curve as evidenced by the high value of the coefficient of determination R² = 0.9836. The studies [1] have shown that for specific design conditions, it is possible to establish an unambiguous curve of the relationship between the critical time t_M and the reduction factor β_KR.

The paper presents possibilities of rainwater outflow control in an innovative rainwater system using the canal retention. The hydraulic functioning of the traditional rainwater sewage and the innovative rainwater sewage after equipping it with a retention canal system were compared. A total of 9 different functioning variants of the innovative rainwater system were analyzed. Each variant of the innovative system with damming baffles showed more favorable hydraulic conditions than in the case of an identical traditional system.

On the basis of simulation studies and an analysis carried out on the model urban catchment, a number of important conclusions of cognitive and application significance can be formulated.

The value of the maximum rainwater outflow at the outlet node from the innovative rainwater system Qo_Imax is always lower than the value of the maximum rainwater outflow from the identical traditional sewer system Qo_Tmax.