Surface water storage volume estimation

The Surface water storage volume estimation tool provides both greenfield flow rate estimates for setting limiting discharge rates as well as surface water storage attenuation volume estimates for use in initial drainage design. The tool is designed to be used with a minimum of information by users who need not be drainage engineers.


The Surface water storage volume estimation tool uses a storage assessment method developed by HR Wallingford based on correlations between storage requirements and hydrological and hydraulic characteristics of sites. This methodology is based on the premise that the flow rate discharge constraints for storm water runoff from the site are defined by the greenfield runoff rates for the 1 year, 30 year and 100 year return periods. The tool is flexible in allowing the use of three different methods for estimating greenfield runoff rates along with various options for assessment of storage volumes using variations in the design criteria (e.g. minimum design flow rates from the site, requirements for control of runoff volumes for the 1:100yr, 6 hour event). 

The drainage design criteria applied are in line with best practice in the SuDS Manual and the SuDS Standards in England, Wales and Northern Ireland.

The methodology takes into consideration the partial use of infiltration or rainwater harvesting along with whether or not permeable areas contribute runoff. It also makes allowances for different hydrological regions, climate change and other factors.

When to use this tool?

The Surface water storage volume estimation gives a quick estimation of storage volume requirements for a site, which can be a major cost component. This will assist with ensuring that initial design of site layouts include sufficient space in which to locate the storage volumes. It will also assist with Planning applications as all the assumptions on flow rates and other design inputs are listed in the one page reports.

How to use the tool?

The Surface water storage volume estimation tool is an online tool, which is free to use for all registered users. 

To start using the tool, the user simply clicks the location of their site on the map which then pre-populates a number of data cells, such as the latitude and longitude of the site, soil type and hydrological characteristics. The user needs to provide data such as site size and site development characteristics such as impermeable area. All these values can then be edited by the user. The map values and users edited values are displayed side by side to enable comparison. The volume estimates of the default parameters and the edited ones are also reported to show the impact of making these changes. 

Users can then calculate the greenfield runoff rate using one of three approved methods: IH124 and the FEH statistical tool. Information on flow rates can also be entered based on outputs from the ReFH 2 web-based tool if this has been used to estimate greenfield runoff rates instead.


The Surface water storage volume estimation tool provides estimates of the required volumes of storage needed on a site along with recommendations on the limits of discharge that should be applied. Limiting discharge rates are modified based on a range of decisions including minimum flow rates, and where use is made of infiltration and rainwater harvesting. 

The results report on both the default volume estimates based on the GIS data obtained by the map-click, as well storage volumes based on user edited values. All data and other parameters used in the calculation are reported in a one page report for each greenfield method used. 

As the storage tool has been developed based on correlation analysis it is important that the results are cross-checked during detailed design to establish more accurate estimates of the storage volumes required.


The Flood Estimation Handbook (FEH) 2013 rainfall data is the industry standard estimation of design rainfall. 

The data can be found on the FEH Web Service at The FEH Web Service is a product of the Centre for Ecology & Hydrology (CEH), and is developed, maintained and made available by its agent Wallingford HydroSolutions (WHS).

The Flood Estimation Handbook, produced by the Institute of Hydrology in 1999 effectively replaced Flood Studies Report in the UK. 

The following QMED equation is from a revision to the statistical approach in 2008 (Kjeldsen et al, 2008).

Qmed = 8.3062 x (0.01 x AREA)0.851 x 0.1536(1000 / SAAR) x FARL3.4451 x 0.0460(BFIHOST x BFIHOST), m3/s


  • Qmed is the median annual flow rate; the 1:2 year event.
  • AREA is the area of the catchment in ha.
  • SAAR is the standard average annual rainfall for the period 1941 to 1970 in mm.
  • FARL is a reservoir attenuation function and is set at 1.0 and therefore has effectively been ignored. This means that areas with water bodies which attenuate the runoff will over-predict the greenfield runoff rate.
  • BFIHOST is the base flow index derived using the HOST classification.

The Institute of Hydrology carried out a number of studies on revising the runoff equations produced in the original Flood Studies Report (1975). IH124 was specifically produced to address the runoff from small catchments. (Institute of Hydrology, 1994).

Although shown to be slightly less accurate than more recent FEH based methods, it is still considered to be an acceptable approach for assessing greenfield runoff rates. The IH124 equation estimates Qbar with the following equation:

Qbarrural = 0.00108 x (0.01 x AREA)0.89 x SAAR1.17 x SPR2.17, m3/s


  • Qbarrural is the mean annual flood flow from a rural catchment (approximately 2.3 year return period).
  • AREA is the area of the catchment in ha.
  • SAAR is the standard average annual rainfall for the period 1941 to 1970 in mm (SAAR 41-70). SAAR 61-90, which was analysed for FEH for rainfall from 1961 - 1990, is virtually the same and can also be used.
  • SPR is Standard Percentage Runoff coefficient for the SOIL category.

The ReFH 2 tool provides flow rates and volumes for greenfield and developed sites. The UKSuDS tool for storage estimation uses the ReFH 2 peak flow rates for greenfield runoff along with a few other parameter values. The storage analysis in the UKSuDS tool is not based on the volumes of runoff that can be predicted by the tool for both the predevelopment and post development states. 

Long Term Storage is the term given to the volume of temporary storage which needs to be provided for the additional volume of surface water runoff that is generated by the development that is greater than the volume of greenfield runoff. The greenfield runoff volume is calculated using the 1:100 year 6 hour event. This volume is the amount that can be discharged at the 1:100 year greenfield runoff rate.

The additional runoff volume should be discharged from the site at a flow rate less than 2l/s/ha for this event. As critical duration events for the design of the site storage system will be much longer than 6 hours, the Long Term Storage volume is not calculated using the 1:100 year 6 hour event, but needs to be assessed using the critical duration event.