How to size a stormwater attenuation tank

Before a stormwater attenuation tank can be installed there are a number of things to take into consideration. In this article we discuss the main factors that go into deciding how to size a stormwater attenuation tank.

Where you are located will dictate the amount of rainfall you experience during the year. For example, on average Manchester sees about 151 days of rain each year, producing 875mm, while much less rain falls in London, with an average of 109 days and 557mm of rain falling. This means the higher the rainfall and likelihood of storms developing, the bigger the tank is needed. 

When talking about a return period, this refers to the average time between severe storm events. This information is based on historical information and cannot be predictive. So if the storm return period is calculated at being 1 in 30 years, this only means it has happened once in the past 30 years and not that a storm can happen once every 30 years. 

Predicted changes to the weather also have to be taken into account, especially with the current data surrounding climate change. Defra (Department for Environment, Food and Rural Affairs) offer guidance on peak river flow allowances by river basin district, predicting changes between 2020 and 2115. These are based on a based on a 1961 to 1990 baseline. 

The catchment area of rainfall will be another key factor in determining the size of the attenuation tank required. This means calculating the surrounding area and the amount of rain that will run-off the catchment area so it can be processed by the tank. 

Not all stormwater attenuation tanks will sit in water below ground. But the groundwater table level has to be taken into account for all tanks as this will rise during periods of rain and the tank will need to be able to manage the increased water intake to process it accordingly. 

The invert level refers to the floor of the drainage pipe sending water out from the tank. The invert level should be lower than, or flush with, the bottom of the lowest unit to enable the tank to drain efficiently. 

Not all attenuation tanks will be connected to local sewers, with some using drainage fields, which means a drainage layout drawing will have to be composed. This details how the water is processed out of the tank out into the local environment safely. 

If a local sewer network is not be used but rather a soakaway, a soil infiltration rate into the soil has to be calculated. This is defined through metres per second, defining how many metres of water saturate the soil within that time. 

For attenuation systems that are connected to the sewer a similar rate has to be calculated. However, the difference is the water being allowed to flow from the tank is defined by litres per second.