How does an attenuation tank work?

In this video, Callum and Lisa discuss the function and operation of attenuation tanks, which are an important component of sustainable urban drainage systems (SuDS). Attenuation tanks are used to hold excess stormwater temporarily, allowing it to be discharged at a controlled rate into the sewer system.
Lisa begins by describing attenuation tanks as simple tanks used as holding areas for excess water. The water enters the tank through various inlets and channels, and the tank provides a space for the water to build up when flowing quickly, such as during a large storm event. This buildup is intended to prevent overwhelming the drains downstream.
The outlet of the tank usually contains a flow control device, which slows the water's outflow to a rate that the downstream pipes can control. This device can take different forms, including a smaller orifice plate for smaller tanks or a proprietary vortex flow control system for larger tanks. The tank's size and allowable outflow rate are designed based on factors such as the rainfall for the area, the amount of surface area that has been paved over, and the predicted volume of water. This volume is calculated using traditional rainfall patterns, the surface area that has been paved over, and an additional percentage to account for climate change.
Lisa notes that attenuation tanks can have multiple inlets and one outlet, or one inlet and one outlet. The former is called an online attenuation tank because the inlets and outlets are in a line, while the latter is called an offline attenuation tank because the water is not flowing through the tank but is instead filling up and draining back out of the same inlet. Lisa explains that the height of the pipe in relation to the tank can help identify whether the tank is an online or offline attenuation tank. For online tanks, the inlets can be located midway up the tank or even on the top surface, but the outlet must always be at the base of the tank to ensure that it empties completely.
Overall, the purpose of an attenuation tank is to prevent stormwater runoff from overwhelming the drainage system, which can lead to flooding and water pollution. Attenuation tanks are an important component of SuDS and are designed to be able to cope with any site as long as they are designed properly and have flow control devices that have been designed specifically for the tank.
So key considerations are things like... Also, you just mentioned the soil type, other things would be things like rainfall, local rainfall for the area, climate change considerations, the size of the [inaudible 00:00:23] the size of your development. Discharge rate, allowable discharge rate into the local storm sewer system, and the slower the discharge rate allowed... The less the storm drains can cope with, the larger your tank has to be to because it has to be released more, whether it's released more slowly. You have to consider if there's groundwater in the area. You have to consider also the later use of the crates, what will it be under? Will it be under a landscaped area? Will it be under a really heavily trafficked area? There's quite a lot of different considerations to take in for design of your tank, but there are the main ones.