Greenfield Runoff Rate (QBAR): How It Sets Your Attenuation Tank Size

The greenfield runoff rate estimates how rapidly water drains from a site before development. Knowing this helps estimate the flow before development. QBAR helps establish the allowed discharge for an attenuation design. The tank holds the difference between runoff and outflow. Reducing the allowed discharge usually requires additional storage, and QBAR alone doesn’t determine a tank’s volume.

This is important for a developer analyzing quotes. Two suppliers may quote the same nominal capacity. However, they may differ in terms of their respective outlet levels or available water depths. Consider the case of the accepted discharge basis of the drainage designer. Next, assess the case of available storage. Also, consider the scope of work, installation, and other costs. The examples provided concern England, where local acceptance and connection conditions still need checking.

What does a greenfield runoff rate measure?

Greenfield runoff estimates the undeveloped discharge of flow for a given region and frequency of rainfall. The results for the whole site are expressed in terms of discharge. A rate is expressed in discharge per hectare. These values don’t represent a storage volume. A storage volume calculation must account for the variation in runoff during the storm.

Natural drainage spreads rainfall between infiltration, evaporation and surface flow. Roofs and paving change that response. An attenuation tank for controlled stormwater discharge temporarily stores some of the resulting runoff, while an outlet releases water under the agreed conditions. Storage delays discharge; it doesn’t necessarily remove that water from the catchment.

Take a look at the units on a report. Let’s use units of measure to illustrate. If we assume a specific flow of 2 L/s/ha over 0.8 ha, the flow rate becomes 1.6 L/s. This example doesn’t derive the permissible rate of an English development. The report must state the area of the land region, the method and the event related to the outcome.

Common mistake: A common error is to send potential suppliers a QBAR of 2 without a unit or area. Prices based on 2 L/s for the entire site may be quite different from prices based on 2 L/s/ha. Ambiguity should be resolved before the module selection stage to avoid surprises at the layout pricing stage.

Reporting 2 L/s/ha requires a contributing area to show complete site flow. Considering an example area of 0.8 ha, flow will be 1.6 L/s. Quotations for the tanks require the separate routed storage volume, the levels of the outlets, and the water levels.

QBAR versus QMED: choose the greenfield calculation method

QBAR is the mean of annual maximum flows, and QMED is the median of annual maximum flows. Both of these parameters describe different summaries of a catchment’s flood behavior. According to England’s national standard, it’s permitted to use either the QBAR or QMED value as a surrogate to define the 50% annual exceedance probability greenfield flow; however, the acceptance of the method for the project still needs checking.

The UKSuDS greenfield estimation guidance explains the IH124 and Flood Estimation Handbook approaches and their limitations on small sites. The equations derive from catchment evidence. Applying them to a development produces a planning estimate, not a measurement of the site’s actual peak flow.

Agree the method and lead local flood authority (LLFA) requirements with the approving body before buying data or carrying an old calculation into detailed design. For example, Arun’s local drainage guidance expects the FEH statistical method and limits its acceptance of IH124 to specified planning circumstances before 19 June 2025. That’s a local example, not a rule for every UK authority.

Check the method: On the calculation sheet, specify the software version, method, dataset(s), soil descriptors and area units. FEH22 rainfall and the FEH 2025 statistical method are different inputs to the wider assessment. Mixing versions of these inputs may change the outcome of the estimate even when the site boundary remains the same. When evaluating the quotation, redesign risk exists when an assumed Statistical Method remains unaccepted. The acceptance criteria should be documented, as changes to the greenfield rate may alter the hydraulic model and cause the priced layout to be deemed invalid.

Set the permitted discharge before sizing storage

Permitted discharge represents outlet constraints in hydraulic models, and is specific to the project at hand. England’s national standards compare greenfield flows with a specific rate of 3 L/s/ha, using different conditions for the 50% and 1% annual exceedance probability events. The comparison uses different conditions for the 50% and 1% annual exceedance probability events. Local requirements, and the receiving connection, also affect the level of discharge.

Under national standard 3.18–3.20, the comparisons work as follows:

• For the 50% annual exceedance probability event, compare the equivalent greenfield flow with 3 L/s/ha and use the greater value as the national peak limit.
• For the 1% event, first compare the runoff volume discharged from the development to surface waters or sewers with the greenfield runoff volume for the 1% event of six hours.
• If that discharged volume is greater, the national peak limit uses the 50% greenfield flow or 3 L/s/ha, whichever is greater. If volume is no greater, it uses the 1% greenfield flow or 3 L/s/ha, whichever is greater.

Ensure both sides of a comparison are reflected on an equivalent area basis. Account for justified interception and infiltration reductions when comparing discharged volumes. In accordance with standard 3.27, a QBAR/QMED proxy may be utilized to establish a 50% baseline; however, the requirement to perform a volume test isn’t waived. More restrictive locally supported controls, in accordance with standard 3.10, take precedence.

Imagine a purchaser who has received a tank price before the outlet has been determined. One drawing shows the control as “5 L/s”, while the drainage drawing shows it as “2 L/s”. The tank in question may not be able to provide the required storage at the water level. Reducing the outlet to 2 L/s won’t by itself resolve the issue as it will also impact the amount of water retained during the storm and recovery afterwards. Ask the designer to redo the model. Provide the revised storage and levels to all bidders. Comparison of bids will only be useful when all bidders have priced the same revised storage and levels. The two values of outlet shown here illustrate variation. Neither of these represents a default design value.

The specific limit of 3 L/s/ha isn’t an absolute permission to discharge 3 L/s from every plot. For an assumed 0.8 ha area it corresponds to 2.4 L/s before applying the appropriate comparison and project conditions. This comparative analysis will need to remain separate from the discharge approval.

Define the site area and design storm inputs

A discharge calculation must establish the contributing area, design rainfall and receiving levels; the previous 2.4 L/s example doesn’t establish an allowed discharge.

The land that can contribute runoff will need to be included in the design, including permeable areas during extreme events. This design will need to consider limits on the receiving levels. Appropriate climate allowances for design rainfalls will be considered in the design. The greenfield baseline is calculated without that climate uplift.

Roof-only estimates may underestimate runoff from saturated landscaping and/or other connected surfaces. Identify the catchment limit in the survey and drainage plan. Depending on the evidence, infiltration may reduce the volume of runoff requiring discharge; our infiltration versus attenuation guide provides information on the temporary detention option and disposal to the ground.

“different critical durations may apply to different storage features used on a development.”

The explanation of the UKsuds storage-method covers hydraulic routing and a search across storm duration. Longer events may govern storage while shorter intense events may govern pipeline flows, so critical duration can differ. Model tailwater too: its effect on outflow should be applied when modeling stored water. A surcharged receiving pipeline may reduce outflow during the event while the tank stores water.

England’s standard 2 addresses retaining the first 5 mm for the majority of events through use, infiltration or loss to soil and atmosphere. The system needs mechanisms to retain the first 5 mm of water for the majority of events and must consider water quality, biodiversity and amenity.

In a system, the interception and detention functions of each design element need to be considered separately. Meeting the first 5 mm objective can’t be inferred from a large sealed tank, and an assumed 2-hour storm can’t establish the critical storage event. Always consider the complete drainage in detail before reaching conclusions and assuming a design element may fulfill the function of a storage solution.

Worked example: discharge limits and attenuation storage

The largest accumulated difference between inflow and outflow during the simulated event, allowing for other justified losses, is referred to as required attenuation storage. It shows stored water, not outlet losses. An example with a constant outflow illustrates that a percentage reduction in outlet rate doesn’t always result in an equal percentage increase in tank size. Actual design requires variable inflow and outflow and the hydraulic response of the outlet.

Picture an empty store for now. Let’s say it receives 20 Liters/second for 2 hours, which equals 7,200 seconds. For now, let’s also ignore infiltration. In this example, the outlet has full stated flow and lets the water through from the beginning of the storm. Incoming water is 20 × 7,200 ÷ 1,000 = 144 m³. At 5 L/s, the outlet removes 36 m³ over that period, leaving a peak of 108 m³. At 2 L/s, it removes 14.4 m³, leaving 129.6 m³. Decreasing the outlet by 60% increases storage by 21.6 m³, which is 20% in this example. These assumptions don’t give us a universal 20% for a different site.

Discharge-to-Footprint Sensitivity Table

Each line of the table assumes constant inflow and duration of rainfall. Footprint additionally assumes a 0.95 usable void ratio and 1 m active water depth. Values shown are rounded after calculation.

A real-world outlet typically relies on water head. An outlet rated at 5 L/s at its design head can allow less flow when the water head is lower. The constant outlet formula can underestimate storage. In this case, it doesn’t provide a conservative upper bound. Before each succeeding storm, check residual water and the drawdown. Also consider the maximum stored volume of a single event. Refer to national standard 3.22 for assessment and mitigation of potential blockages, with suitable protection of orifices smaller than 50 mm.

Holding a 20 L/s inflow for 2 hours, the ideal example rises from 108 m³ to 129.6 m³ when outflow falls from 5 L/s to 2 L/s. The 20% gain demonstrates sensitivity. The 20% increase demonstrates sensitivity, while the approved tank volume must come from the project’s hydraulic model.

Convert usable storage into tank footprint

Usable storage volume, effective void ratio and active water depth determine the footprint of the tank. To determine the footprint of a uniform rectangular tank, divide usable water volume by the void ratio and active water depth. For attenuation tanks, determine the location of the outlet and the maximum depth of water, because a nominal volume of the tank cannot alone determine the layout of the tank.

For a geocellular attenuation tank, using the illustrative 129.6 m³ requirement, a 0.95 void ratio and 1 m active depth gives 129.6 ÷ (0.95 × 1) = 136.4 m² before module rounding. A 0.8 m active depth case gives an effective area of 170.5 m². These checks are geometrical and don’t consider excavation limits and/or structural approval.

Imagine a site team with a fixed elevation of the finished surface. A lower module footprint wouldn’t necessarily represent a smaller excavation. However, if the outlet isn’t deep enough, some of the space for stored water wouldn’t be usable during the design event. The designer must determine if the lower water storage volume would be available during the design event. When comparing modules, the designer and contractor should consider working clearances, bedding, cover and space for connections. These spaces should not be considered usable water storage. This doesn’t mean that a smaller module would be able to be installed without difficulty in the available location.

For further design and construction of the tank, provide anticipated fill and drawdown levels, and describe the hydraulic and structural loading conditions for attenuation tank installation. Proposals should define and identify dead volumes and access ways and describe how these affect usable storage. Prices shouldn’t be compared based on nominal volume without those deductions.

At a void ratio of 0.95, lowering the active water depth from 1 m to 0.8 m increases the expected footprint from 136.4 m² to 170.5 m² with 129.6 m³ of liquid. While the footprint of the tank increases by 25% from this illustrative design, the remaining space for bedding, cover, construction and clearance must be checked before accommodating the increase.

Engineer-to-Supplier Design Handover

The handover from engineer to supplier defines the approved tank hydraulics. Identify the owner and revision of each input and state what would require the designer to recalculate. Purchasing, construction and operating agents can check exclusions, installation and access respectively. Check the positioning of product access and controls prior to fixing the order.

This matrix will help you classify changes. The responsibility column shows a potential project assignment to discuss at the next meeting and confirm in the appointment, and doesn’t suggest a supplier automatically bears design responsibility. Other related submission documents are found in the design-suds Supporting Documents guide.

For the receiving connection, planning calculations and land rights are separate. Leeds’ guidance on discharge points, for example, asks for written agreement where offsite pipes and privately owned receiving systems are involved. Confirm the applicable requirements for the actual location.

When not to order the tank yet

Hold the product order when the outlet rate is still provisional, the quoted volume includes unavailable storage, or the receiving level hasn’t been considered. An installed attenuation tank cost comparison should use matching design assumptions. Buying extra modules doesn’t resolve an inaccessible control or an unapproved connection.

Let’s say there are two bids, each for 130 m³. The first of the two defines capacity between the working levels, and includes the control chamber. The second uses the module’s nominal capacity, and doesn’t include control. Financial analysis may focus on the lower headline price figure. The contractor, however, has to deal with the missing chamber, and potentially, a larger excavation. Request the two service providers to price the same volume, working levels, and scope of service. If the designer changes those parameters, the same change and revision must be sent to both suppliers and the client.

Following the designer’s resolution of hydraulic and structural queries, request a detailed product layout against that information, and clarify each supplier’s services and exclusions in the quotations. If the evidence isn’t available, conditional quotations can be provided to assist in the preparation of a preliminary budget. However, such quotations shouldn’t be considered as approvals for the work.

What changed in 2025 for greenfield runoff estimates?

There have been several notable updates in 2025. As such, it’s prudent to implement version control on certain assessments. Three separate 2025 changes make version control important: England published new national SuDS standards, UKSuDS updated its estimation tools, and the FEH statistical method changed. FEH has also updated its statistical methods. While older estimations of QBAR may still be used to inform a planning decision, it should be ensured that the methods and scope reflect those accepted by the authority.

The first publication date of the 2025 edition in the government publication record for non-statutory standards is 19 June 2025. These most recent revisions (30 July) linked to the previous version of the standards. These aren’t a new set of standards. These standards outline requirements for England within the wider planning and approval processes.

UKSuDS describes March 2025 changes to its storage tool, including explicit modelling of grassland runoff and hydraulic routing. Separately, the UK Centre for Ecology & Hydrology’s FEH update covers the 2025 statistical method, including QMED and associated adjustments. Naming the software alone leaves these choices unclear.

It’s a procurement risk to procure outdated inputs, as suppliers may charge based on the wrong usable capacity. Keep the revision sheet along with the calculation report, accepted discharge, rainfall data, and descriptors of the site soil, as well as the authority’s response. Local resources, such as the Essex Design Guide, are helpful in providing context, but their date and application need checking alongside the national guidance. Changing the methodology of the calculation is a valid reason for review. Not all changes to calculations require a larger storage capacity.

If an earlier quotation assumed 108 m³ and the approved model now requires 129.6 m³, identify the changed inputs before ordering the additional 21.6 m³. What are the changed inputs justifying the additional 21.6 m³? An increase in rainfall or a change in the outlet constraints may warrant an increase in the usable capacity. A usable-depth change can also alter the layout; the numerical difference alone cannot identify which component needs redesign.

Send Storm Manage the designer’s discharge basis, required storage, operating levels and tank installation constraints through our form. Hydraulic acceptance and the designer’s responsibilities must be clear.

Frequently asked questions

References & Sources

• Defra: National standards for sustainable drainage systems, standards 2, 3 and 7
• Defra: publication date, scope and update history
• UKSuDS: greenfield runoff rate estimation
• UKSuDS: methods and storage estimation questions
• UKCEH: Flood Estimation Handbook and 2025 statistical update
• Arun District Council: local extreme-rainfall design guidance
• Leeds City Council: local flood-risk development control standards