Updates to our tools in 2025

The greenfield runoff rate estimation tool has been refreshed in 2025 to make it more user friendly. The available greenfield runoff methods of IH124 and FEH Statistical remain the same however there is a move away from using HOST soil classes and towards using the FEH web service BFIHOST values. If the same input parameters are used the estimates should be the same as the previous tool. 

This surface water storage volume design tool has been completely revised in 2025 to be more technically robust.

There are three principal differences between the new tool and the previous tool. These are:

• Method of analysis for storage calculation;
• Runoff model assumptions; and
• Limiting discharge rates and Long Term Storage calculation.

Method of analysis for storage calculation

The method of analysis for the original storage tool was the use of correlation equations based on the results from a simple set of physically based models using FSR rainfall and soil types. Modifications were then made to the results to take into account current FEH rainfall models and other aspects such as rainwater harvesting systems.

The updated storage tool carries out direct hydrological and hydraulic calculation of runoff and routing from the FEH22 rainfall data which is provided by the user. In addition there are no adjustments made for addressing other site features such as use of rainwater harvesting systems

There is also no need now to ensure PIMP (percentage impermeable) is greater than 50% of the site area as all contributing surfaces have defined runoff factors and routed to and through the storage unit.

Runoff model assumptions

The current model assumes a 100% runoff coefficient from roof and paved surfaces and has also explicitly include runoff and routing from grass / vegetated surfaces. Two options are available for determining the percentage runoff from grass / vegetated surfaces as explained in What is the runoff coefficient for grass / vegetated area surfaces?  In summary, the model is either based on based on a fixed percentage equal to SPR or a ‘variable’ runoff model can be selected where the percentage runoff increases from 0% up to the SPR (or SPRHOST) value of the soil when rainfall depth is 40mm or more. This approximates to the formula in FSSR16 (1985). 

The previous storage tool assumed no runoff from pervious surfaces but made an adjustment to the paved area on the assumption that 30% of the pervious area drained to the system. The adjustment took account of the soil type.

Limiting discharge rates and Long Term Storage calculation

The current model sets the limiting discharge from the storage based on a calculation of the greenfield runoff rate or previously developed site flow rate. There is however scope to adjust these using relaxation factors on greenfield and betterment factors on previously developed flow rates, or setting a user defined flow rate. This is then checked to ensure it is not too low a flow rate to cause a blockage risk.

The previous tool could also limit discharge flow rates to the greenfield Qbar flow rate, but this was overridden if it was less than 2 l/s/ha. The previous tool also had the option of Long Term Storage, however this has been removed from the revised tool as this approach was limited to more complex designs which would likely use more sophisticated software. These updates improve the robustness of the tool and the results from the new tool will differ from the previous tool.

The spreadsheet tool was replaced by a new user friendly web tool in 2025. The new tool has expanded to include more infiltration design system types to be modelled (with the addition of soakaways with infiltration from the side only and planar infiltration systems). The new tool requires FEH22 rainfall to be used instead of the old FSR rainfall in the old spreadsheet tool. As such, the estimates may be different to those generated by the previous spreadsheet tool.

This is a new tool to the suite of uksuds tools in 2025. It is a tool built by HR Wallingford as part of the EC StopUP project. The SuDS performance evaluation tool (also known as the StopUP SuDS Tool) allows users to evaluate and report on a SuDS scheme design for its hydraulic (interception, volume losses, peak flows and flooding) and water quality performance through its ability to simulate both design storms and time series rainfall.

The StopUP project started in September 2022 and runs until August 2025. In addition to this the tool will be further developed between February 2025 and December 2026 as part of SuDS-iQ , a project led by Southern Water, delivered by HR Wallingford and funded by the Ofwat Innovation Fund. 

If you have any feedback or undertake any testing of the tool against other drainage models or observed data we would be grateful if you can share those with us at support.stopup@hrwallingford.com.

The StopUP project received funding from the European Union’s Horizon Europe research and innovation program under grant agreement No 101060428. The work was also supported by the UKRI Innovate UK [grant number 10052357].

There have been no changes to the 'Water quality assessment for SuDS developments (SuDS manual)' or the 'Water quality assessment for SuDS developments (WaND)' in 2025.