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Storm water retention

Storm water retention

Draw of storm water retentionDraw of storm water retention

Not only global warming but also population growth lead to a change of the usual conditions in many areas, which require a reaction to the “new”, but also necessary and current building measures lead to changes.
One sector of these changes are building measures which are realized in gaps between buildings or on open spaces (e.g. through sealing) and therefore lead to increasing water quantities which have to be drained, which in cases of average and heavy rainfalls the existing sewerage network cannot absorb. Underground buffers are a widespread solution here, because they don’t overload the sewerage system due to a delay of derivation.
In most cases the network operator determines the conditions for the opening, he defines who, when and how much can be fed to the network, since an overload can lead to widespread damage, e.g. through overflowing.
For such changes the installation of underground restraint systems, especially of storm water retention from PE-HD, has proved, since discharge systems can be realized in a simple, fast and effective way and is therefore a benefit to the whole network , as well as a relief to sewage treatment plants.

In principle the storm water retention consist of a gully, the actual storage and a control chamber, which limits the inflow. Furthermore an overflow option has to be planned, which prevents an overload of the storm water retention and can drain the rain water into a dry well without any damage.
The ATV worksheet A 128 is decisive for measuring and designing rainwater drainage systems in the catchment area of water treatment plants. Storm water retentions (SK) in a rainwater / mixed water system are discharge facilities with overflow in waters. These special buildings are built in rainwater and combination sewers due to economic reasons. If the discharge systems aren’t measured and designed according to the technical policies, high dirt loads can get into the waters during rainfalls and put a heavy strain on them. The objective of mixed water treatments is the best possible reduction of total emissions from the sewer system and treatment plant. Sewer system and treatment plant are connected and have to be coordinated.
To design a storm water retention effectively and sustainably the decisive evaluation parameters have to be defined:

hna - long-time average height of rainfall
The long-time average height of rainfall is a location-dependent factor and has a considerable influence on the exonerative effect of the rainwater reservoirs. With increasing height of rainfall more mixes water gets into the waters. Details of the annual height of rainfall hna can be seen from the  German Metrological Service or similar.

Au - impermeable total area (fixed/unpaved)
The fixed or impermeable areas have to be carefully determined since they have a considerable influence on the size of the storage volume of the storm water retentions. This applies particularly during the forecast calculations for prospective construction or industrial areas. It has to be critically tested if the estimated areas are appropriate for prospective construction or  industrial areas. An area increase of more than 20% should be calculated in any case.

Qm - Combined sewage flow
The combined sewage flow Qm is the parameter who has the most influence on the volume determination of the storm water retention. It is often unnoticed that the parameter Qm has to be drawn from the sewer system calculation, not from the sewage treatment plant dimensioning. If a new sewage treatment plant is planned or an existing plant has to be renovated and extended, and therefore has to be measured with a Qm, which is to contain reserves for a longer period of time (15 – 25 years), this Qm has to be used in the dimensioning of the rainwater drainage system.

Qt24 - Middle dry weather flow
The actual water consumption figures are to be used. The sum of the dry weather flow from the sewer system usually has to correspond to the annual dry weather inflow of the sewage treatment plant. Within the planning the consumption figures have to correspond to the sewage treatment plant inflow.

 

Storm water retention - Lifted Pipes at construction site
Lifted Pipes at construction site
Storm water retention - Jointing pipes at construction site
Jointing pipes at construction site
Storm water retention - Retention pipes onsoil
Retention pipes onsoil

By calculating the crucial parameters the required buffer volume results, from which the individual assemblies can be measured and constructed. Under consideration of the local conditions the storm water retentioh from PE-HD can be manufactured custom-fit, whereby the maximum possible execution is chosen instead of the required execution due to reasons of sustainability. The costs of an immediate volume increase are in no commensurate to extensions works which have to be carried out 25 years later.
The most important formula is QZ ≤ QA + QÜ, to exclude a collapse of the restraint system.
The version of the storm water retention always conforms to the local requirements and installation conditions:

  • Type of restraint (above, middle or on the ground)
  • Type of restraint of dirt loads
  • Type of throttling
  • Type of overflow 

The thermoplastic material PE-HD has established itself as material for big volume storage sewer systems since multiple constructive and creative possibilities can be combined with optimum workability, long operating life and tightness of welded systems. The “dissolved” cross section which acts raw material saving, as well as the almost unlimited recyclability, even after decades of installation, complement the excellent features.

Author:
Steffen Lampe
Henze GmbH