How to size stormwater attenuation tanks

Posted by Callum Vallance-Poole, on June 24, 2024.

sizing stormwater attenuation tanks

Stormwater attenuation tanks play a critical role in managing rainfall and runoff in urban environments. This helps to prevent flooding, reduce soil erosion, and alleviate strain on drainage systems. Before these tanks can be installed effectively, careful consideration must be given to their size and capacity.

From geographical location and storm return periods to climate change adaptation and drainage layout, each aspect plays a vital role in determining the optimal tank size. By understanding these factors and working with experts like GRAF UK, developers and engineers can ensure that stormwater management systems are tailored to the specific needs of their project and the surrounding environment.

Rainfall Patterns

The location of a site greatly influences its rainfall patterns, impacting the size needed for stormwater attenuation tanks. We can estimate water volume and select the right tank size by analysing regional rainfall data, such as Manchester’s higher precipitation compared to London.

The storm return period, or recurrence interval, denotes the average time between severe storm events, guiding the design of stormwater attenuation tanks. This is typically seen as values of 1 in 30 years or 1 in 100 years. Engineers rely on historical data and predictive modelling to gauge the frequency and severity of rainfall, which is crucial for sizing and planning stormwater systems. This analysis helps anticipate extreme weather events and adapt designs to manage runoff effectively amidst changing climate conditions.

In adapting stormwater management to climate change, Defra’s guidance on peak river flow allowances offers a framework based on historical data and future projections. By integrating this guidance, planners can anticipate shifts in river dynamics and prepare for altered precipitation patterns. Advanced climate models aid in predicting changes in rainfall intensity and distribution, informing strategies such as increased storage capacity and enhanced drainage infrastructure to mitigate urban flooding and runoff impacts.

Catchment Area Analysis

The catchment area plays a pivotal role in determining the size and capacity of a stormwater attenuation tank. It refers to the total land area from which rainfall runoff is collected and directed towards the tank for storage and management. The size of the catchment area directly influences the volume of water that the tank must accommodate during rainfall. Therefore, accurately assessing the catchment area is essential for properly sizing the stormwater attenuation system to manage runoff and mitigate flooding risks effectively.

Calculating Runoff

Calculating runoff involves estimating the rainfall that will accumulate and flow into the stormwater tank during a storm. This calculation considers factors such as the catchment area’s size, the rainfall intensity, and the surface characteristics. Various hydrological models and methods, such as the Rational Method or the Soil Conservation Service (SCS) Curve Number Method, can be utilised to estimate runoff volumes based on these factors.

Groundwater Table Level

The groundwater table level significantly influences the operation and performance of a stormwater attenuation tank. During heavy rainfall, the groundwater table rises as rainwater saturates underground aquifers. This impacts the tank’s capacity and functionality by affecting how much water can be stored in the soil and tank.

Stormwater attenuation tanks are designed to manage increased water intake during rainy periods and mitigate high groundwater levels. These may include perforated pipes or infiltration trenches at the base of the tank to facilitate groundwater recharge and drainage. And overflow outlets or relief valves to prevent tank overflow during extreme rainfall events.


Efficient water drainage requires careful design and placement of drainage pipes within the tank. These pipes should be positioned at the right depth and slope for gravity-driven water flow to the outlet. The invert level of the drainage pipes should be strategically located at the lowest point of the tank to enable complete drainage without any residual water accumulation.

Invert Levels

The drainage invert level refers to the bottom of the drainage pipe within the stormwater attenuation tank. Proper drainage invert levels are vital for efficient water flow within stormwater attenuation tanks. They prevent water stagnation, minimise overflow risks, and ensure continuous flow to downstream drainage systems. Maintaining optimal levels prevents waterlogging, preserving the tank’s integrity and operational efficiency.

Drainage Layout Drawing

A detailed drainage layout drawing is crucial for effective stormwater management system design and operation. It visually maps out drainage components, flow paths, and discharge points, ensuring proper water management on-site. Tailored to project needs and site conditions, it outlines various drainage systems, like infiltration systems or attenuation tanks, depicting key components such as pipes, chambers, and outlets. The drawing may include measurements, elevations, and construction details to ensure hydraulic performance and compliance with site contours and regulations.

Soil Infiltration Rate

The soil infiltration rate measures how quickly water seeps into the soil, influenced by factors like soil composition and moisture. Engineers assess it through onsite tests like a percolation test, where water is added to the soil and its absorption rate is measured over time. These tests help determine soil permeability and inform soakaway systems’ design to manage stormwater runoff effectively.

Rainfall infiltration and accumulation in soakaway systems cause fluctuations in soil water saturation, affecting system performance.

Engineers use various modelling techniques and hydrological calculations to predict water saturation dynamics in the soil over time. Rainfall intensity, duration, frequency, soil properties, and vegetation cover are considered to simulate water movement through the soil matrix.

Allowable Outflow

Calculating allowable outflow in stormwater systems is crucial to preventing flooding. It defines the maximum rate of water discharge into sewers without overloading them. This calculation considers sewer capacity, pipe size, and flow velocity to ensure system reliability and compliance. Flow rates are typically measured in litres per second (L/s), reflecting the volume of water moving through the system over time. Stakeholders can and prevent urban flooding and pollution by setting proper flow limits and monitoring discharge rates.

The importance of effective stormwater management is crucial as urban areas face increased rainfall, urbanisation, and climate change. Sizing a stormwater attenuation tank is a critical step in mitigating the impact of heavy rainfall, reducing flood risk, and protecting infrastructure and the environment.

At Graf UK, we specialise in providing innovative stormwater management solutions tailored to your project requirements. Contact us today to speak with our team of experts for help with sizing and designing your stormwater attenuation tank.