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].
An example of a retention tank (1 - inlet canal, 2 - outlet canal, 3 - flow chamber, 4 – accumulation chamber) [28].
An example of an additional transit canal (1 – sewer canal, 2 - sewer manhole, 3 - additional transit canal) [28].
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].
An example of a rainwater system equipped with a retention sewage canals (1 - canal, 2 - piling partitions, 3 - manhole) [28].
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.
Scheme of the damming partition installed in a sewage manhole (a) cross section; (b) longitudinal section; 1 - sewer manhole, 2 - overflow edge, 3 - piling partition, 4 - flow opening, 5 – canal,
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].
Scheme of the retention sewage canal with damming partitions that create stormwater canal retention spaces (the light blue - average distribution of the liquid stream mirror in the conduits of a traditional rainwater systems; the blue - liquid stream distribution and retention capacity of the rainwater sewage system after equipping it with damming partition),
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
A model catchment consists of 80 sub-catchments, where a total drained catchment area equals Concept I - the canals bottom slope amounts Concept II - the canals bottom slope amounts Concept III - the canals bottom slope amounts
It was assumed that the sewage system examined ha a linear sewer system in each design concept. It consisted of 80 pipes of equal length (Fig. 6).
The scheme of the model catchment, total drainage area
Hydrodynamic modelling with the SWMM 5.1 program was used for the analysis. The surface runoff coefficient
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
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):
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
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
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
A set of basic hydraulic parameters of the traditional rainwater system.
- | - | |||
1. | Conception I | 1 | 2887.7 | 32 |
2. | Conception II | 2 | 3692.8 | 26 |
3. | Conception III | 3 | 4175.8 | 25 |
A Set of the values of the basic hydraulic parameters of the innovative rainwater sewage with retention canals system.
- | - | - | β |
||||
1. | Conception I | Variant 1 with |
1 | 981.6 | 88 | 200 | 0.34 |
2. | Variant 2 with |
1 | 1063.4 | 84 | 300 | 0.37 | |
3. | Variant 3 with |
1 | 1159.6 | 78 | 400 | 0.40 | |
4. | Conception II | Variant 1 with |
2 | 1775.1 | 56 | 200 | 0.48 |
5. | Variant 2 with |
2 | 2120.8 | 46 | 300 | 0.57 | |
6. | Variant 3 with |
2 | 2362.8 | 42 | 400 | 0.64 | |
7. | Conception III | Variant 1 with |
3 | 2445.3 | 40 | 200 | 0.59 |
8. | Variant 2 with |
3 | 2899.5 | 34 | 300 | 0.69 | |
9. | Variant 3 with |
3 | 3118.8 | 30 | 400 | 0.75 |
In that way 9 different variants of innovative rainwater systems with retention canals were analyzed. TThe rainwater flow reduction coefficient
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
The results of the research presented in Tables 1 and 2 confirm that with an increase of the slope of the canal bottom
Changes of the canal slopes
Maximum rainwater outflow at the outlet from the drainage catchment for traditional storm water system
In the case of traditional sewerage system for slope of canals
For example, considering an innovative system for the slope of bottom
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
Table 3 presents the multiplicity of rainwater outflow reduction in the innovative rainwater system depending on the spacing of damming partitions for distance
A comparison of rainwater outflow from traditional and innovative sewer system taking into account different variants of their working.
2.9 | 2.1 | 1.7 | |
2.7 | 1.7 | 1.4 | |
2.5 | 1.6 | 1.3 |
The above results show that after applying damming partitions in a traditional sewerage system for the slope
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
Hydrograms of rainwater outflow from the traditional and innovative sewer system at rainfall duration
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
Figure 9 shows the relationship between the critical time for rainwater sewage system dimensioning
Calculative time for rainwater sewage system dimensioning
As shown by the curves in Figure 9, the change of canals slope
Table 4 summarizes the values of the critical rain duration times
Comparison of calculative time
1 | 32 | 88 | 56 | |
1 | 32 | 84 | 52 | |
1 | 32 | 78 | 46 | |
2 | 26 | 56 | 30 | |
2 | 26 | 46 | 20 | |
2 | 26 | 42 | 16 | |
3 | 25 | 40 | 15 | |
3 | 25 | 34 | 9 | |
3 | 25 | 30 | 5 |
The smallest differences between the times
The dependence between the values of the determinate time for the dimensioning of the traditional rainwater system
The relationship between the coefficient
Values of the coefficient of γ
The value of coefficient
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
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
The effect of slope of canals bottom
Rainwater flow reduction coefficient β
In the case of decreasing the distance between the damming partitions
The study [2] demonstrates that there is a close relationship between the rainwater flow reduction coefficient
The relationship between the rainwater flow reduction coefficient β
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
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 Equipping the innovative system with damming partitions enables effective use of the sewage system capacity. This, in turn, reduces the rainwater outflow The slope of the canal bottom An increase of the rainwater outflow flow from the sewage outlet Equipping the sewerage system with a system of retention canals allows beneficial flattening of rainwater outflow hydrogram. It has affect on the reduction of the required capacity of retention reservoirs cooperating with the sewerage system. The critical rainfall time for the retention canals dimensioning operating in the innovative system The value of the coefficient of the critical times γ
On the basis of the analysis carried out, it was concluded that the key parameter in the innovative rainwater system is the slope of the canal bottom