Stormwater Attenuation: An Urban Flooding Mitigation Technique

Stormwater attenuation has become a very important part of construction in recent years due to increased urbanisation and harsh weather conditions that have been continually on the rise worldwide. In accordance with SANS 20252-2, stormwater attenuation refers to the collection and storage of excess runoff from a site so that it can be released slowly into the storm water system. This is in an effort to mitigate the risk of urban flooding in areas located further down from the rainfall site.

City Storm (Canva Image) 

When land is developed, it’s surface - which was once porous soil able to absorb much of the excess rain water- is then replaced with impermeable surfaces like concrete, brick and tar. Since the storm water is unable to seep into the ground, it collects and rushes down, wreaking havoc in lower lying areas and carrying pollutants to nearby rivers and oceans. 


Urban Flooding (Canva Image)

Attenuation tanks and soak pits are used to temporarily store excess runoff on a development site so that it can be released at a slower rate into the stormwater system. Soak pits (also referred to as leach pits) are wells that have permeable walls allowing water to be absorbed slowly into the ground. Attenuation tanks come in many forms including plastic tanks, large steel or plastic piping systems or pre-cast concrete culverts which are used to create spacious underground reservoirs. When space allows for it, features like ponds and wetlands can be created to simulate natural water management systems but sometimes development conditions don’t allow for this and so an underground storage system is built instead.


Civil Engineer (Canva Image)  

Understandably, there is a lot of preparation and work that goes into projects of this nature. Engineers & developers are required to submit calculations for approval. Such stormwater management plans include calculations like area of site, length of estimated drainage flow path, fall/slope of the site, rainfall intensities, runoff calculations etc. Once the plan is in place and the calculations submitted, developers can decide which stormwater attenuation solution is best suited for the space that is available.

For more on this topic you can check out the following blog on GRAF UK’s website: What is a storm water attenuation tank?

 Thanks for reading & happy plumbing :)