Stormwater Retention

Stormwater Retention – Storm Drain Installation And Repairs

Storm water and how it is dealt with is a major concern for New Zealand because of its potential effects on our environment. We place great value on the ecological health of our urban streams and coastal waters, as well as on the economic, social and cultural value of these environments. If systems are not in place to deal with storm water the occurrence of flooding and contamination rises dramatically.

What is “Storm water”?


Storm water is any form of water that does not evaporate, seep into the ground or is absorbed by vegetation. If it isn’t absorbed, storm water becomes surface runoff, flowing off the land and into storm drains, streams, rivers or lakes. Surface runoff can happen on all types of land – Agricultural, residential, industrial and commercial.

City centres also have large sewerage systems. Many of these sewers are in their original state of being “combined sewers”. A combined sewer mixes human effluent with storm water.  Water that is carried in a combined sewer originally ended directly into open waters such as rivers and the sea. Now it is generally led straight to a waste water treatment plant, where it is treated before being released. Because there are still some combined sewers and these are designed to overflow during heavy rainfall, they can still pose a threat to the local environ and human health.

Why treat storm water?


Due to the volume and timing of the surface runoff water and the potential pollution that it may be carrying storm water can be very dangerous.

Out in a natural environment where human development has not encroached, water is naturally processed through the water cycle. The earth’s atmosphere continually absorbs water from the earth’s surface, purifies it and then precipitation returns it to be used for vegetation growth. Eventually the water is once again evaporated and the cycle begins again.

However human development interrupts the natural water cycle. Humans have altered the surface of the earth with the addition of hard surfaces, such as roads and buildings and so have interfered with water’s ability to be absorbed back into the soil.

Recognising the need to run the storm water away from buildings and homes, storm sewers and different grading and paving techniques which in theory allow for rapid drainage have been implemented. However, this also alters the established storm water flow path and affects the natural water absorption process. This can lead to soil erosion as well as the transportation of toxic debris and chemicals, including oil and grease, fertilizers, pesticides, animal faeces and other refuse and contamination.

Retention or Detention?

A retention/detention system is an underground pipe system that stores surface storm water runoff. Underground pipe systems provide the added benefit of being able to use the land above the system for recreation, parking, and other purposes.

The decision to select a retention system or a detention system is usually determined by the local environmental regulations. A retention system typically uses perforated pipe, so that the stored runoff can recharge groundwater, and a discharge outlet designed to limit the flow rate into the receiving sewers or channels. A detention system typically uses non perforated pipe and watertight joints so that the stored runoff exits only through the discharge outlet.


In both systems ponds are used as a holding place for the water. There are two different kinds of ponds that are used for flood control and storm water runoff treatment: wet ponds and dry ponds. Both systems function to settle suspended sediments and other solids typically present in storm water runoff.

Wet ponds can also be called retention ponds and they hold back water similar to water behind a dam. The retention pond has a permanent pool of water that fluctuates in response to precipitation and runoff from the contributing areas. Maintaining a pool discourages resuspension and keeps deposited sediments at the bottom of the holding area.

Detention ponds serve as important flood control features. They are usually dry except during or after rain. Their purpose is to slow down water flow and hold it for a short period of time such as 24 hours. Urban areas rely on these structures to reduce peak runoff rates associated with storms, decreasing flood damage.


Another way of managing storm water is by the use of Detention Tanks. A detention tank purifies polluted storm water and reduces the environmental impact on lakes, streams and rivers.

Detention tanks capture silt, and other contaminants, and when the storm water has been processed what is released back into the environment is purified with less health risks. Detaining storm water in an underground Detention Tank will allow for a slow release of the purified water at a rate more akin to how water would be distributed naturally

Effective Storm Water Management

Effective storm water management potentially contributes to community safety by reducing the risk of urban flooding and erosion. If water detention and other efficient techniques were more widely used, urban environments could in theory become self-sustaining in terms of water.

In zones where new development (infill housing for example) is being made, often there are additional demands put onto existing storm water systems and, these systems may not have the capacity to handle further loads. In these situations, a storm water retention tank is the best solution. Storm water retention tanks are designed to store rainwater from roofs, drives, paths and other areas. The water is then discharged to the storm water system from the tank through a small pipe at a rate the storm water system can cope with.

Over the past decade, a shift in storm water management towards systems that reduce and manage flows and then treat storm water contaminants, has been seen. Storm water management systems are have been called low impact design (LID), water sensitive urban design (WSUD), sustainable urban drainage systems (SUDS) and best management practices (BMPs).

Common examples of storm water treatment devices used in LID include:


  • detention ponds and wetlands
  • infiltration surfaces
  • permeable paving
  • swales (vegetated ditches)
  • media filters
  • bio-retention
  • green-roofs .

Many of these devices are visually appealing and can be incorporated into landscaping to provide a blue-green space in the otherwise concrete and asphalt urban environment. Stormwater has truly become a liquid asset.

If you think you could benefit from a storm water retention/detention system for your home, business, farm or new development give Euro Plumbing a call and we will discuss with you how we can help you and your land.

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