Sizing a stormwater attenuation tank means determining the storage volume required when the flow of stormwater entering the system exceeds the rate at which it can be discharged or infiltrated. The calculation is based on the contributing catchment area, runoff characteristics, local rainfall data, the selected design event and the allowable discharge rate. The required storage corresponds to the maximum difference between cumulative inflow and cumulative outflow during the critical rainfall event.
The general principle can be expressed as:
Storage volume = incoming runoff volume – discharged volume
The calculation method must always be adapted to the project location. Rainfall data, design return periods, climate change allowances and maximum discharge requirements may vary significantly between countries, regions and local authorities.
Stormwater attenuation tank sizing: 5 key principles
Catchment area: identify all surfaces that contribute stormwater runoff to the drainage system.
Runoff: assess how roofs, roads, car parks, permeable surfaces and landscaped areas contribute differently to runoff.
Design rainfall: use rainfall data representative of the project location and an appropriate design return period.
Allowable discharge: determine the maximum flow that can leave the attenuation system according to the receiving system and local requirements.
Storage volume: calculate the maximum volume that must be temporarily stored when incoming runoff exceeds the allowable discharge.
EN 752 and the principles of stormwater drainage design
EN 752 provides a common European framework for drain and sewer systems outside buildings and establishes general principles for their planning, design, operation and management.
For a stormwater attenuation project, the main parameters to consider include:
- the contributing catchment area;
- the type and permeability of the surfaces;
- the runoff characteristics;
- local rainfall data;
- the selected return period;
- the allowable discharge rate;
- the characteristics of the receiving drainage system;
- any applicable climate change allowance.
The storage requirement is determined by comparing the volume of runoff entering the system during a rainfall event with the volume that can be discharged during the same period.
EN 752 provides general design principles, but it does not replace national or local requirements. The design criteria applicable to a project must therefore be checked according to its geographical location.
Step 1 – Calculate the contributing catchment area
The catchment area is the total surface from which stormwater runoff is collected and directed towards the attenuation system.
A development rarely consists of a single type of surface. It may include roofs, roads, car parks, paved areas, permeable surfaces and landscaped areas, each contributing differently to runoff.
To estimate this contribution, a runoff coefficient can be applied to each surface.
A simplified relationship is:
Ae = A × C
where:
- Ae: effective contributing area;
- A: surface area;
- C: runoff coefficient.
For a site containing several types of surfaces:
Ae total = (A₁ × C₁) + (A₂ × C₂) + … + (An × Cn)
How do different surfaces affect stormwater runoff?
The runoff coefficient represents the proportion of rainfall that contributes to surface runoff.
Impermeable surfaces generally generate a greater and faster runoff response than permeable or vegetated surfaces.
Surface
Roof
Asphalt
Concrete
Impermeable car park
Landscaped area
The coefficients used for an actual project must be selected according to the applicable calculation method and local guidance. Soil characteristics, slope, surface condition, vegetation and saturation can all influence runoff.
For example, a 10,000 m² site does not necessarily have an effective contributing area of 10,000 m². If part of the development consists of permeable or landscaped surfaces, its hydraulic response may be significantly different from that of a fully impermeable site.
Step 2 – Determine the allowable discharge rate and time of concentration
The allowable discharge rate is the maximum flow that can leave the attenuation system. The lower the permitted discharge, the greater the temporary storage requirement is likely to be.
It is generally expressed in litres per second (l/s) or litres per second per hectare (l/s/ha).
The permitted value may be defined according to:
- the capacity of the downstream drainage network;
- the characteristics of the receiving water body;
- pre-development runoff conditions;
- flood-risk objectives;
- national, regional or local drainage requirements.
The attenuation system then operates as a temporary buffer:
Inflow > allowable discharge → water is stored
Inflow < allowable discharge → stored water is progressively released
What is the time of concentration?
The time of concentration is the time required for runoff from the hydraulically most remote point of a catchment to reach the point being considered.
It depends on factors such as:
- catchment dimensions;
- slope;
- surface characteristics;
- flow paths;
- runoff velocity;
- drainage network configuration.
The time of concentration helps characterise how quickly the catchment responds to rainfall and can influence the rainfall duration used for hydraulic calculations.
Step 3 – Use local rainfall data and design return periods Stormwater attenuation tanks must be sized using rainfall data representative of the geographical location of the project.
Rainfall intensity varies significantly between countries and even between locations within the same country. Using generic rainfall values can therefore result in an incorrectly sized storage system.
Design rainfall is commonly characterised using relationships between rainfall depth or intensity, storm duration and frequency of occurrence, often referred to as Depth-Duration-Frequency (DDF) or Intensity-Duration-Frequency (IDF) data.
These datasets make it possible to estimate the rainfall associated with different storm durations and return periods.
What is a rainfall return period?
A return period expresses the statistical probability that a rainfall event of a given magnitude will occur or be exceeded.
For example:
- a 10-year return period corresponds to an annual exceedance probability of 10%;
- a 30-year return period corresponds to approximately 3.3%;
- a 100-year return period corresponds to 1%.
A 100-year rainfall event does not mean that such an event occurs only once every 100 years. Two events of this magnitude can occur within a much shorter period.
The design return period must be selected according to the applicable standards, flood risk, type of development and local requirements.
Practical example: calculating the required storage volume
The useful storage volume of a stormwater attenuation tank is determined by identifying, for the selected design rainfall, the maximum difference between the cumulative volume entering the tank and the volume that can be discharged at the allowable discharge rate.
Let us take a simplified educational example for a 10,000 m² development.
Data
Roofs
Impermeable roads and car parks
Landscaped areas
Total area
Effective contributing area
Rainfall depth
Allowable discharge rate
Duration considered
1.Calculating the rainfall volume over the effective contributing area
40 mm corresponds to 0.040 m.
7,200 × 0.040 = 288 m³
The theoretical incoming volume is therefore 288 m³.
2. Calculating the discharged volume
For a constant allowable discharge rate of 10 l/s over 60 minutes:
10 × 3,600 = 36,000 litres = 36 m³
3. Simplified calculation of the required storage volume
288 – 36 = 252 m³
Under the assumptions used in this example, the required storage volume is therefore 252 m³.
Step
Effective contributing area
Rainfall
Incoming volume
Discharged volume
Simplified storage
Calcul
Assumption
40 mm
7 200 × 0,040
10 l/s × 3 600 s
288 – 36
Résultat
7 200 sqm
0,040 m
288 m³
36 m³
252 m³
Please note: this deliberately simplified calculation does not, on its own, represent the complete application of the rainfall method. In a real design, several rainfall durations are assessed in order to identify the one that produces the maximum storage volume.
Which parameters should be considered when sizing a stormwater attenuation tank?
The sizing of a stormwater attenuation tank depends primarily on the effective contributing area, the design rainfall and the allowable discharge rate. There is therefore no standard storage volume per hectare that can be applied to every project.
Two developments with the same total area may require very different storage volumes depending on their degree of impermeability, geographical location, selected return period and discharge constraints.
The hydraulic calculation determines the required useful storage volume. This requirement must then be translated into a physical solution: an above-ground attenuation basin, an underground attenuation tank, an Ultra-Lightweight Cellular Structure (ULCS) or another solution suited to the project.
To understand the overall purpose of these systems, see our complete guide to stormwater attenuation systems.
Sizing therefore defines the hydraulic requirement before the construction solution is selected. For a real project, the assumptions and results must be validated by the competent project engineer or design consultant.
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