More wild, wet weather?

How are protecting ourselves from potential increased rainfall in the future?

Each and every new development that is created must be prepared to face what could be its worst situation.

In earthquake prone cities buildings are designed to withstand extreme forces.

What happens in rainy cities? And what about if rainfall increases in the future?

On a typical development site a flow rate (known as a discharge consent) is given by a water company or other governing body that the development can discharge their rain water (also known as surface water) into a sewer or other location.

Typically measured in litres per second (l/s), these can be from 0.5 l/sec (sometimes even less but rare) up to 1,000’s of l/sec. The flow rate from the development site is reduced by flow controllers, pump stations or a combination of both.

Where the rain flowing into the drainage system is more than the discharge consent, storage attenuation tanks or swales hold the remainder until the incoming flow has reduced to below the discharge consent at which point the attenuation tanks or swales start to empty.

How will this cope if rainfall increases?

With some sites you may not have been given a discharge consent, which means another interesting issue comes up. More on that in a minute. The development site must calculate what is the worst storm in the last 30, 100 or 200 years, add an extra 30% or 40% and then design the drainage system to cope with this. In a typical example if a site has designed its storage system for a 1:100 years storm plus 40%, the development can hold the water from a rain storm that gives 40% more rain than the rainfall from the worst storm that happened in that area in the last 100 years.

What about where no discharge consent is agreed?