Attenuation Flow Control
The purpose of attenuation flow control is to limit the peak discharge generated from a site during a storm. The difference between the inflow and outflow is the storage volume required.
Flow Control devices are typically:
Orifice: An orifice is a simple hole which throttles the flow. The outflow is dependent on the height of water also referred to as the driving head. Typically these are suitable on larger sites as it difficult for an orifice to achieve low flow without resulting in a small diameter which may result in blockage. They maybe suitable for sites which have upstream screening limiting the blockage risk.
Floating Orifice: The elegance of the floating orifice is that the head above the orifice is fixed and so the outflow is fixed and unrelated to the inflow. The added benefit is that since the orifice sits below the water surface is not subject to blockage from floating debris. Likewise a floating orifice can be much larger than a standard orifice and so blockage risk is reduced.
Vortex Flow control: This type of device generates a vortex in the outflow of the water. This vortex in turns slows the outflow. The benefit of this type of device is the inlet to the vortex can be quite large compared to a standard orifice . For example the outflow diameter on a vortex flow could be twice that of a standad orifice but still generate the same outflow. A larger opening reduces the blockage risk.
Submersible lift pump: In some cases restricted gravity discharge cannot be achieved to a sewer and so flow control can be achieved by using a variable speed submerible pump which can be calibrated on site. Alternatively a standard submersible pump can be used which feeds a chamber with flow control downstream. On site commissioning should be performed to ensure the downstream chamber does not surcharge (overflow).