A rain overflow (RÜ) , also mixed water discharge , is a relief structure in the mixed system of urban drainage with overflow into a body of water . In addition to mixed systems, rain overflows can also be used in modified separation systems or in sewage treatment plants for hydraulic relief. Part of the inflow arriving at a rain overflow is passed on through the sewer system to the sewage treatment plant . This diverted drain is known as the throttle drain. The other part of the inflow is discharged into a body of water via a threshold. Rain overflows usually relieve a body of water between 10 and 40 times a year. In contrast to other relief structures such as B. Rain overflow basins (RÜB) and storage ducts (SK) there is no intermediate storage of the runoff in the event of a rain overflow .
A rain overflow reduces the runoff to the sewage treatment plant when there are high mixed water runoff peaks. In a mixed system, all types of runoff, i.e. dirt , foreign water and rainwater , are drained together. Different amounts of wastewater flow into the rain overflow. These fluctuations result from the different consumption of wastewater dischargers over the time of day, week and year , as well as the irregular accumulation of rainwater. The amount of extraneous water that occurs is usually constant. The amount of rainwater is usually greater than the amount of waste water, and there are also significantly greater fluctuations in the flow rate of rainwater. One speaks of a mixed water discharge peak if the discharge to the sewage treatment plant is greatly increased by rain runoff. Relief of a rain overflow into a body of water usually occurs when the rainwater runoff is approximately 30 times greater than the wastewater runoff.
In order to prevent rain runoff in the sewer system, it can be useful to build a rain overflow. However, this only makes sense if, for technical, water management and economic reasons, no measures for wastewater collection and rainwater treatment are possible.
Rain overflows may only be set up at points in the sewer system where the critical mixed water runoff (Q crit ) can be passed on completely as a throttle discharge and this passed on runoff can be treated in a subsequent storage structure , e.g. B. a rain overflow basin. The critical mixed water runoff is the sum of the daily mean values of the dry weather runoff and the critical rain runoff from the directly connected catchment area , as well as, if applicable, the throttle runoff of all relief structures immediately above.
For the construction of a rain overflow, the structure geometry, the altitude , as well as the type and location of fixtures , such as throttling devices , rule recorders , diving walls, etc. are relevant. For the hydraulic dimensioning of a rain overflow, the inlet design, the throttle discharge, the length of the structure and the shape and height of the relief threshold are of particular importance. In addition, information on the catchment area sizes and the receiving waters are required.
Dimensioning according to ATV-A 128
According to ATV-A 128, not less than twice the amount of the daily peak of the wastewater runoff plus the daily mean value of the external water runoff from the mixed areas in dry weather is to be assumed for the mixed water runoff. The maximum value of the wastewater runoff that occurs over the course of a day is called the daily peak of the wastewater runoff. Sewage treatment plants are usually dimensioned for this mixed water runoff. Since the runoff in the sewer system increases many times over when it rains, relief structures are required in the sewer system.
It is important to ensure that excessive contamination of a body of water is avoided by relieving a rain overflow. For this reason, rain overflows are to be designed for at least critical rainfall (r crit ) between 7.5 and 15 l / (s * ha). The critical amount of rain depends on the flow time (t f ) and can u. a. can be determined using the following formula:
r crit = 15 * 120 / (t f +120) in l / (s * ha) for t f <120 min
r crit = 7.5 l / (s * ha) for t f > 120 min
The flow time describes the time that a water particle needs to cover a certain distance. The longest flow time from the directly connected catchment area to the rain overflow without taking into account the flow time in pure transport collectors is used as the flow time .
Relief in bodies of water that temporarily carry little or no water, a critical rainfall of at least 15 l / (s * ha) is required. The critical rainwater runoff (Q rkrit ) is obtained by multiplying the critical runoff donation by the impermeable area of the catchment area (see under special requirements).
Minimum mixing ratio
A minimum mixing ratio (m Rü ) between rain and dry weather must be ensured so that a rain overflow into a body of water can relieve the pressure . The rainwater runoff is usually less polluted than the dry weather runoff. The minimum mixing ratio is intended to achieve sufficient mixing of the two types of wastewater in order to prevent excessive contamination of the water. The mixing ratio results from the following equation :
m Rü = (Q d -Q t24 ) / Q t24
with Q d - throttle discharge
and Q t24 - daily mean value of dry weather runoff
The minimum mixing ratio of 7 must not be fallen below, i.e. H. the throttle discharge must correspond to at least eight times the value of the daily mean value of the dry weather discharge.
m Rü ≥ (c t -180) / 60
Minimum throttle discharge
The throttle discharge of the rain overflow corresponds to the critical mixed water discharge that the rain overflow has to pass on in full.
The minimum throttle discharge is calculated as follows:
Q d = (m Rü +1) * Q t24
If the minimum throttle outflow is greater than the critical mixed water outflow, this is decisive.
The forwarded runoff of a rain overflow should be at least 50 l / s. The impermeable area is purely a calculation variable and indicates the proportion of a catchment area from which rain runoff reaches the sewer system. The impermeable area connected to a rain overflow should be at least 2 hectares in size. In dry weather , the flow velocity in the inlet and outlet areas of the rain overflow should not fall below 0.5 m / s, otherwise adequate flushing must be ensured.
Rain overflows on one or both sides can be implemented with a raised threshold and throttle or a floor opening. Rain overflows with openings in the bottom are mainly arranged in areas with a shooting drain. The rain overflow tapers evenly towards the throttle device. It is important to ensure that the bottom gradient of the rain overflow is greater than the gradient of the inlet channel. Otherwise a backwater can occur. It must be ensured that there are enough access openings so that the duct sections can be adequately ventilated, monitored and cleaned.
Important elements of a rain overflow are the overflow threshold and the throttle device . The overflow threshold is often made of concrete , reinforced concrete or clinker brickwork . To avoid water pollution, the overflow threshold can be designed with a floating material retention system, in the form of a baffle , a rake or sieve system. The throttle device regulates the outflow from the basin and can, for. B. be designed as a vortex throttle , controlled throttle valve, slide or a long throttle sections. The throttle section should only be used in exceptional cases, as it can no longer be changed afterwards. The largest possible bottom fall is to be provided, as this can prevent sludge deposits in the drain.
There are also rain overflows that are carried out without an overflow threshold. In the case of these rain overflows, the discharged runoff is discharged through a floor opening, whereas the discharge discharge flows away above the floor opening.
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in combined sewers, 1992, p. 2, paragraph 1,
- German Association for Water Management, Wastewater and Waste eV (DWA): DWA-M 103 - Flood protection for waste water systems, 2013, p. 10
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in combined sewers, 1992, p. 10 paragraph 4.3.1
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in mixed water sewers, 1992, p. 2 paragraph 2
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in combined sewers, 1992, p. 11, paragraph 4.3.1
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in combined sewers, 1992, p. 24, paragraph 6.2.7
- German Association for Water Management, Wastewater and Waste eV (DWA): DWA-M 180 - Framework of action for planning runoff control in sewer networks, 2005, p. 18
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in combined sewers, 1992, p. 21 ff. Paragraph 6.2 ff.
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in mixed water sewers, 1992, p. 49, paragraph 10.1.1
- Abwassertechnische Vereinigung (ATV) and German Association for Water Management and Cultivation (DVWK): ATV-DVWK-A 157 - Sewer structures, 2000, p. 21 ff. Paragraph 5.6
- Abwassertechnische Vereinigung (ATV): ATV-A 128 - Guidelines for the dimensioning and design of rain relief systems in combined sewers, 1992, p. 54, paragraph 10.2.4