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In water management, experience asserts that rainwater should be infiltrated at the place where it accumulates. If this is not possible, then in many cases the temporary storage attenuation or retention of rainwater is required in attenuation volumes in order to protect the drainage systems from overloading and to limit their dimension. The runoff from paved surfaces are classified into the categories of non-hazardous, tolerable and intolerable according to their material concentration and thereby possibly associated potential hazards to groundwater in targeted rainwater infiltration.
Non-hazardous rainwater runoff can be infiltrated e. Tolerable rainwater runoff can be infiltrated through the unsaturated zone after suitable pretreatment or using cleaning processes sedimentation system, rainwater cisterns, overgrown soil, etc.
The underground composition is of crucial importance for rainwater infiltration. The permeability coefficient kf-value is a measure of water permeability of the soil.
The permeability coefficient should be between 10 -3 and 10 -6 in order to ensure the functionality of the infiltration system. In order to avoid over-dimensioning of the system, the kf-value should be determined as accurately as possible through investigation. There are professional geotechnical experts for this purpose. If the kf-value is unknown, then an approximation of the underground infiltration can be isolated based on the following short test.
The contamination of underground and surface water from rainwater from roofs and traffic areas can be considered qualitatively and quantitatively by using simple assessment procedures ATV DVWK-M Depending on the result, various measures for handling rainwater must be taken to ensure adequate water protection.
According to DIN , underground infiltrations systems trenches are equivalent to infiltration systems in active soil areas in terms of qualitative aspects, provided the inlet water comes from a rainwater harvesting system with non-metallic roof areas. Systems with a settling chamber in which the flow conditions allow specific substances heavier than water sink and specific lighter substances float are referred to as sedimentation systems.
Collection and filter chambers consist of a sedimentation area in which heavy particles settle and a filter that prevents light coarse contaminants from entering the downstream storage. Even light materials are retained in the chamber with an immersion pipe. Depending on the amount of dirt, they must be cleaned regularly. The total water discharged from roof is filtered and supplied to the tank.
Contaminants from flowing rainwater are retained and stored or degraded by physical, chemical and if necessary, biological processes with passage through soil layers or in trough-trench systems or unsealed surfaces such as grass pavers. Thus passage through overgrown topsoil is more effective than through a non-vegetated soil zone. The protective cover layers over groundwater must not be penetrated. Should contaminants penetrate into the trench despite pre-cleaning, it is very important that subsequent cleaning is possible.
In many trenches e. However fine contaminates pass through the slots in the flushing ducts and gradually clog the floors and walls of these trenches. Ultimately these can only be dug out completely if they have lost their infiltration capacity. With DRAINMAX Tunnel trenches for example, the critical walls and floors can be inspected with a camera through adequate connection chambers and are completely flushable.
Contaminants either are retained in the coarse filter of the sedimentation and filter chamber or settle in the sedimentation area. The coarse filter can be removed and emptied after the flushing process. The parallel rows of trenches are additionally protected by the long settling section in the seepage pipe and the additional settling possibility in the inspection and flushing chamber.
This guarantees a constant infiltration performance long-term. The combination of rainwater harvesting and rainwater retention in a cistern is particularly interesting for smaller systems for single-family homes since the costs for excavation and delivery are incurred only once and the cistern is not significantly more expensive. The low height offset between inlet and outlet in combination with large space flexibility and a very high storage volume are the advantages of this variant.
If no water is allowed to enter the surrounding soil from the system, it can be sealed with an EPDM foil on site. In a retention system the water is supplied into the drainage system with a throttled flow rate. The throttle discharge corresponds to the permitted outflow of the sealed area connected to the drainage system.
Most of the time this discharge corresponds to the natural flow before sealing the area. In the retention system the permissible throttle discharge is either supplied to a downstream drainage system by means of a lift pump or through a discharge throttle provided the height conditions allow.
Compared to the vortex or fixed throttle, continuous throttles make sure that the maximum permitted water quantity Q drains constantly, irrespective of the impounding depth H. The simplest form of a fixed or static throttle is a simple flow restrictor. The discharge value Q of the fixed throttle depends on the hydrostatic pressure resulting from the impounding depth H. A spiral stream of variable strength with a central rotating air core is formed by the tangential feed in the vortex throttle depending on the water level.
However this does not lead to a continuous throttle outflow. The vortex throttle has the advantage of requiring less space and lower risk of blockages due to the larger remaining cross-section compared to the other throttle types. These advantages are rarely relevant with decentralized rainwater retention.
The outlet flow is constant with the continuous discharge throttle irrespective of the impounding depth H. The float adjusts the restrictor opening at the impounding depth by means of a lever arm. Coarse pre-cleaning of rainwater is necessary for the trouble-free operation of the throttle.
The larger the permitted throttle outflow in relation to the connected areas, the greater the difference. This difference leads to correspondingly lower total costs for the retention system. Also see Online Planner. The calculation of rainwater runoff is based on the knowledge that heavy rains last short durations and low rains persist for longer. The rainfall yield declines at the same statistical frequency with increasing rainfall duration.
The relationship between rainfall yield, duration and frequency is determined by the statistical analysis of precipitation registrations. For this a statistical rainfall with selected duration D and frequency n should be used as load case for calculation. Infiltration: If only formulas 1. Retention : Here, only formulas 1. For statistical determination of rainwater outflows, the assumed frequency of rain from the rain yield curve is crucial.
This value depends on the economic importance of the area and is related to the frequency with which the proposed system is congested. Drainage systems for the discharge of precipitation water from small properties, so long as the sewer network provider has given no other guideline, without a more effective run-off surface, are sufficient for DN connection to the sewer.
It is assumed that due to terrain condition and architectural building plans no backed-up water from the system will penetrate into the connected or neighbouring buildings and exceed any other official regulations. Here the yearly occurance of the dimensioning rainfall cannot be less once within two years. If an exceptional level of safety is required, the yearly occurance of the dimensioning rain is chosen as greater than 30 a. The harmless flooding can take place on the property, e.
The following flood verification for guidance is dependent on the local conditions and if necessary for parts of the drainage system e. If the ground conduits are dimensioned according to DWA-A, Table 4, then the dimensioning discharge, usually larger, instead can be set according to Equation 21 for the maximum discharge of the ground conduits at full charge Q voll :.
Should the rain catchment surfaces of the property be mostly roof areas and not harmless, flood-prone surfaces e. For the calculation of volume related dimensioning exercises, such as the dimensioning of precipitation water retention areas, the mean discharge coefficients C m according to Table 9 are to be used to determine the effective area. The dimensioning of rain retention areas must considered according to DWA-A for the run-off entering the drainage system in relation to both the sealed area A E,b as well as from the non-sealed area Table 9, No.
The determined surface types will be simply called A FaG in this norm, with the mean run-off coefficients C m multiplied and linked to the calculated value A u. The required storage volume V RRR will be determined according to the maximum difference between the rainfall amount and the discharge volume through the throttle in a specific time period. The impounding volume from the standard calculation according to the inlet limitation results from Cap. This resulting larger volume is relevant for the calculations for flood verification and the inlet limitation.
The relevant size of the retention space is calculated from Equation 18 as With this based on the flood verification, the required retention volume is increased by 7. At the latest when the flood volume cannot be represented on the surface then the underground storage volumes must be constructed larger.
The following method of calculation can be used for a rough estimation of required retention volume with specified rain duration. As the base of infiltration system can clog up with time, only the side areas remain effective in the long run. A large retention volume is an advantage for variable infiltration efficiency i. The following simplified dimensioning methods shall apply according to DIN:.
When using a classic pipe trench, the largest flow rate must be determined. The largest flow rate is then calculated as follows:. When planning and installing an infiltration or retention system, the current versions of the following regulations must be observed:. Protection of groundwater against pollution by specific dangerous substances. Not logged in English Log in. Navigation Main page Random page Help. Languages Deutsch English. Wiki tools Special pages. Page tools.
Userpage tools. Namespaces Page Discussion. More More. Page actions View View source History. Other languages:. Green roofs, fields and cultivated land; roof surfaces without the use of uncoated metals copper, zinc and lead , terrace surfaces in residential and similar commercial areas. Roofs with usual proportions of uncoated metals, cycle tracks and footpaths in residential areas, calm traffic areas; lawns and car parking without frequent vehicle changes; as well as lightly used vehicle areas up to DTV vehicles ; streets with DTV - 5, vehicles, e.
Lawns and streets in commercial and industrial areas with significant air pollution; for special zones see DWA. Enclosed sheet basin made from EPDM and geotextile. In accordance with the WHG, infiltration plants require a licence, since the States have been able to abolish the licence requirement, groundwater regulation.
Infiltration and Retention
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