Calculating the Size of Attenuation Storage Tanks

Attenuation storage tanks play a crucial role in managing stormwater runoff, particularly in urban areas where impermeable surfaces can lead to flooding. The size of these tanks is determined by several factors, including the discharge rate agreed with the local authority or water authority. 

This article will explain how the size of attenuation storage tanks is calculated and what happens to the water when a flow control device is installed in the last chamber before the local authority manhole or watercourse.

Factors Influencing the Size of Attenuation Storage Tanks

1. Rainfall Intensity and Duration: Understanding local rainfall patterns, including the intensity and duration of downpours, is critical for determining the size of your attenuation tank. Different regions experience varying levels of rainfall, so accurate data is necessary to ensure your tank can handle the expected water volume. Rainfall intensity refers to the rate at which rain falls, typically measured in millimetres per hour (mm/h), while duration refers to the length of time the rainfall event lasts. Both factors are essential in calculating the peak flow rates and total volume of water that the tank must accommodate

2. Impermeable Surface Area: The size of impermeable surfaces, such as roofs, driveways, and roads, contributes to runoff. The larger the area, the more stormwater will be generated, requiring a larger tank to manage the excess water effectively

3. Discharge Rate: The discharge rate is the rate at which water can be released from the tank into the drainage system. This rate is regulated by local authorities to prevent overwhelming the drainage infrastructure. The tank must be sized to store excess water until it can be safely discharged at the allowable rate

4. Soil Infiltration Rates: The ability of the soil to absorb water affects how much water needs to be stored in the tank. Areas with low soil infiltration require larger tanks to prevent flooding

5. Climate Change Allowances: Climate change is expected to increase the frequency and intensity of rainfall events. To account for this, current guidelines often recommend a 40% increase in rainfall intensity when designing attenuation storage tanks

6. Local Regulations and Guidelines: Local planning and water management authorities provide specific guidelines on stormwater management. The attenuation tank must comply with these regulations to meet legal and environmental standards

The Role of Flow Control Devices

Flow control devices are installed in the last chamber of the attenuation tank before the water is discharged into the local authority manhole or watercourse. These devices regulate the flow of water to ensure it is released at a controlled rate, preventing downstream flooding during periods of heavy rainfall.

When water enters the flow control chamber, it passes through the flow control device, which restricts the flow based on the pre-set discharge rate. At low flow rates, water passes through the device unrestricted. However, at high flow rates, the device creates a vortex that controls the flow to the specified discharge rate. This ensures that the water is released gradually, reducing the risk of overwhelming the drainage system and causing flooding.

By controlling the flow rate, these devices force water to back up into the attenuation tank during periods of heavy rainfall. This backup allows the tank to store excess water temporarily, preventing it from being released all at once and overwhelming the downstream drainage system. The stored water is then gradually released at a controlled rate, ensuring that the drainage infrastructure can handle the flow without causing flooding.

In summary, the size of attenuation storage tanks is calculated based on factors such as rainfall intensity, impermeable surface area, discharge rate, soil infiltration rates, climate change allowances, and local regulations. 

Flow control devices play a vital role in managing the release of water from the tank, ensuring it is discharged at a controlled rate to prevent flooding.