Attenuation tank: stormwater attenuation

Q: What is an attenuation tank?

An attenuation tank (also called a stormwater tank) is a buried storage volume that holds stormwater back and releases it in a controlled way to the sewer network via a flow regulator, so that the network is not overloaded during heavy rain.

Q: What is a flow regulator?

A flow regulator (vortex flow control) is a device on the outlet that limits how much water is released per second. It ensures that the tank only discharges what the local authority permits, while the rest is held back temporarily.

Q: How is an attenuation tank sized?

The tank is sized according to the permitted discharge (l/s), the design rainfall with a climate factor and the contributing area. The difference between incoming and outgoing water gives the required storage volume. Norrloop sizes the tank for the individual project.

Q: Attenuation tank or soakaway?

A soakaway infiltrates the stormwater into the ground. An attenuation tank holds the water back and releases it in a controlled way to the sewer network. An attenuation tank is chosen where the ground infiltrates poorly, or where attenuation is required before discharge.

Short answer: An attenuation tank (also called a stormwater tank) is a buried storage volume that holds stormwater back and releases it in a controlled way to the sewer network via a flow regulator. This avoids overloading the network during heavy rain. The tank is sized according to the permitted discharge in litres per second.

01What is an attenuation tank?

An attenuation tank is a buried storage volume that receives stormwater from roofs and impermeable surfaces, stores it temporarily and releases it slowly again. The purpose is to even out the peaks: instead of all the rain hitting the sewer network at once, the water is held back and released at a rate the network can handle. The terms attenuation tank and stormwater tank are used for the same principle.

02Why is attenuation required?

Densification and heavier rainfall mean that in many places the public network does not have the capacity to take all the stormwater at the same time. The local authority therefore often sets a limit on how much a building or property may discharge to the network at most, given in litres per second. Attenuation is the attenuation step of sustainable stormwater management: delay and attenuate the larger volumes of rainfall before they are released onward.

03What is a flow regulator?

A flow regulator (also called vortex flow control) sits on the outlet of the tank and limits how many litres per second are released onward. It ensures that the tank never discharges more than the local authority permits, no matter how full the storage volume is. The rest of the water is held back until the rain has eased, and the tank empties gradually.

04How is an attenuation tank sized?

The storage volume is calculated from three quantities:

Permitted discharge

l/s from the local authority

How much may be released onward to the network.

Design rainfall

return period + climate factor

How heavy a rainfall the system must withstand.

Contributing area

m2 impermeable surfaces

Roofs and asphalt that lead water to the tank.

The difference between the water that comes in and what may be released out determines how large a storage volume you need. A climate factor is added to allow for heavier future rainfall. Norrloop sizes the tank to the area, the rainfall and the local authority's discharge limit.

05Attenuation tank or soakaway?

The two solutions respond to different conditions:

Solution	Principle	When
Attenuation tank	Holds the water back, releases it in a controlled way to the network via a flow regulator	The ground infiltrates poorly, or attenuation is required before discharge
Soakaway	Infiltrates the water into the ground	The ground can take the water

They are often combined: a soakaway takes the small volumes of rainfall, while an attenuation tank attenuates the larger ones when infiltration is not enough.

06Where is the tank placed?

With sufficient cover for a frost-free and, where needed, trafficable solution
With access to the flow regulator and chambers for inspection and maintenance
With a sand trap upstream, so particles do not accumulate in the storage volume
Placed and set to level so that gravity flow to the network is possible where required

07Why an attenuation tank in PE?

Attenuation tanks in polyethylene are corrosion-free, watertight and easy to handle, and can be assembled in modules so that the storage volume is built exactly as large as the discharge limit requires. PE withstands the moist soil environment over a long period without sacrificial anodes or risk of rust.

PE material

corrosion-free

No sacrificial anodes, no rust. Watertight and long service life.

Modular

tailored volume

The storage volume is built to exactly what the limit requires.

08Frequently asked questions

What is an attenuation tank?

A buried storage volume that holds stormwater back and releases it in a controlled way to the sewer network via a flow regulator, so that the network is not overloaded during heavy rain.

What is a flow regulator?

A device on the outlet (vortex flow control) that limits how many litres per second the tank releases onward, so that the discharge stays within the local authority's limit.

How is the tank sized?

According to the local authority's permitted discharge (l/s), the design rainfall with a climate factor and the contributing area. The difference between in and out gives the required storage volume.

Attenuation tank or soakaway?

The soakaway infiltrates the water into the ground; the attenuation tank holds it back and releases it in a controlled way to the network. Choose attenuation where the ground infiltrates poorly or attenuation is required.

Need to attenuate stormwater before discharge?

Norrloop sizes and supplies attenuation tanks, flow regulators and sand traps according to the local authority's discharge limit.