Frequently asked questions

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Current practice uses preferred formulae for calculating greenfield runoff rates which do not have any site gradient terms in them. This results in lower greenfield flow rates than probably take place in practice for steeper sites. For the developer this implies larger storage volumes which are often more difficult to provide in steeper areas. However for planners and the Environment Agency and other authorities this provides an additional margin of safety for controlling downstream flooding. It is important to recognise that the type of event which actually gives the peak flow rate for a site (often very short) is of relatively little importance to the receiving catchment which are more critical to flooding from longer rainfall events. These longer events will still be effectively controlled by the storage volume and flow control provided. 

What is not acceptable when using the surface water storage volume estimation tool is the arbitrary modification of parameters such as soil type to produce higher rates of flow for the site to try and compensate for the assumed higher greenfield runoff rates for sites with steeper gradients.

BFI (base flow index ) is a parameter which was developed by the Flood Studies Report Supplementary Reports (NERC, 1975) and provides a measure of catchment responsiveness. The BFIHOST parameter is a revision of this value for the 29 HOST classes developed by the Institute of Hydrology study Hydrology of Soil Types (HOST) (Boorman et al, 1995). 

Like SPRHOST, the HOST soil category (and therefore the BFIHOST value) can be derived from sources such as geological or superficial soil maps in conjunction with IH126 (Boorman et al, 1995; https://www.ceh.ac.uk/data/hydrology-soil-types-1km-grid), the Flood Estimation Handbook (FEH) CD-ROM (withdrawn from sale in November 2015 and is no longer supported) or the Cranfield University National Soil Resources Institute (LandIS) for the predominant soil data (https://www.landis.org.uk/data/nmhost.cfm). 

However, the easiest source of information for BFIHOST for any location across the UK is the CEH FEH web service (https://fehweb.ceh.ac.uk/). For a ‘point’ location BFIHOST and BFIHOST19 values are provided. However, a check will show that the value reported is usually not the same as the stated value of BFIHOST for a given HOST soil class. 

The reason for both BFIHOST and BFIHOST19 being reported by the FEH Web Service is that a revision of BFIHOST values has been proposed to some of the soil classes in research carried out by CEH in 2019 (Griffin et al, 2019). This study also added a further soil class (resulting in 30 classes), but with some of the original classes being grouped to have the same BFIHOST value. The reason why both are reported is that there has been no definitive position taken, following this research, as to what value is the ‘correct’ one to use. By inspection, the reported values for BFIHOST and BFIHOST19 are often very similar and therefore the choice of using the value for either does not matter. However as the correlation equation for FEH Statistical equation is based on the original BFIHOST classes it is suggested that the BFIHOST value is used. 

In February 2025, the Environment Agency publicly announced that UK CEH have developed an update to the FEH Statistical Method, which will include a revised Qmed equation for ungauged catchments calibrated using new updated catchment descriptors for SAAR, URBEXT, FARL and BFIHOST. The update is anticipated to be released in Autumn 2025 when they will provide guidance on transitioning to using the new method and descriptors. 

SPR is a parameter which was developed by the Flood Studies Report (FSR; NERC, 1975) and varies with the 5 classes of Winter Rainfall Acceptance Potential (WRAP) SOIL type. The FSR maps were all paper based as they were produced in 1975 but have been scanned and are hosted by CEH here https://www.ceh.ac.uk/data/software-models/flood-estimation-handbook. The Wallingford Procedure (Kellagher, 1981) reproduced some of the FSR maps, one of which was the soils WRAP map. This was also in paper form, but is available in digital form from HR Wallingford here https://eprints.hrwallingford.com/37/. 

The SPRHOST parameter is a revision of this value for the 29 HOST classes developed by the Institute of Hydrology study Hydrology of Soil Types (HOST) (Boorman et al, 1995). The official position is that it is not the same parameter as SPR, but for the purposes of this tool and similar analysis, they are often treated as an update on providing a more accurate assessment of SPR. Therefore when using the IH124 formula for estimating greenfield runoff, it is preferable to use the SPRHOST value.

Appendix 5 of the Environment Agency document “Preliminary rainfall runoff management for developments” (SC030219 rev. E, 2013) should be referred to for more information on SPR values for estimating greenfield runoff volumes.

The HOST soil category (and therefore the SPRHOST value) can be derived from sources such as geological or superficial soil maps in conjunction with IH126 (Boorman et al, 1995; https://www.ceh.ac.uk/data/hydrology-soil-types-1km-grid), the Flood Estimation Handbook (FEH) CD-ROM (withdrawn from sale in November 2015 and is no longer supported) or the Cranfield University National Soil Resources Institute (LandIS) for the predominant soil data (https://www.landis.org.uk/data/nmhost.cfm). 

The CEH FEH web service (https://fehweb.ceh.ac.uk/) also provides soil data. Unfortunately the data obtained for a ‘point’ location is only BFIHOST and BFIHOST19 and does not report the SPRHOST value or the soil class. In addition a check will show that the value reported is usually not the same as the stated value of BFIHOST for a given HOST soil class. There is a correlation equation (SPR = 72.0 – 66.5BFI; from Boorman et al, 1995) for deriving SPRHOST from BFIHOST and this can be used in the tool, however as the IH124 equation was derived using SPR values it is recommended that the WRAP soil type data is used in preference to estimating SPR from BFIHOST data. 

Many studies have been carried out by CEH since the work on the IH124 equation and the much later FEH Statistical equation (Kjeldsen et al., 2008). The latest is the Environment Agency Report SC090031/R2 issued in March 2024. This report is another attempt at updating the runoff estimation for small sites to replace the original work of IH124, but the outcomes are inconclusive because there is virtually no data on greenfield runoff measurements which are of a suitable scale for development sites. Other plot scale research includes the ReFH2 method which developed a catchment scale model with a modification for use for plot scale analysis. The plot scale work was calibrated against a limited data set of sites with relatively moderate sized rainfall events. There is relatively little information provided on this work in the “The Revitalised Flood Hydrograph Model ReFH2: Technical guidance” (Wallingford HydroSolutions and UKCEH, 2015).

There has been continuous pressure to prevent the continued use of IH124 with subsequent equations giving more accurate answers. However the FEH statistical equation has stood up well against the further research. In practice it must be recognised that applying river derived formulae to development sites is far beyond their validity of use (and this is effectively confirmed by the latest research study) so both equations continue to be used, due to their ease of use, for the purposes of drainage design for assessing storage volumes for attenuation of surface water discharges. 

HOST, is the acronym for Hydrology of Soil Types, from a research study by Institute of Hydrology in 1995 (Boorman et al, 1995). This 1995 study classified the soils of the United Kingdom into 29 categories. These classes are based on a series of conceptual models that simulate the hydrological behaviours associated with the soils and it interprets soils physical properties and their effects on the rainfall runoff characteristics of soils. A research carried out by CEH in 2019 (Griffin et al, 2019) added a further soil class (resulting in 30 classes), but with some of the original classes being grouped to have the same values. 

For more information on HOST and the parameters BFIHOST and SPRHOST used in this tool see How to obtain SPR/SPRHOST value?and How to obtain BFI / BFIHOST value?

SOIL indices (1 to 5) are defined in the Flood Studies Report (NERC, 1975). The index describes the maximum runoff potential for five soil classes and was derived by a consideration of soil permeability and topographic slope.

SOIL type 1 is sandy highly permeable material with permeability reducing as the SOIL value increases. SOIL type 4 is heavy clay and 5 (which is rarely applied) is exposed rock. For each of these categories there is a value of SPR which is used in the IH124 equation. These are 0.1, 0.3, 0.4, 0.47, 0.53 respectively. 

There is often confusion as to the values of the 5 SPR values for the FSR SOIL classes. These were originally: 0.15, 0.30, 0.40, 0.45, 0.50. These values were also used in the urban runoff model correlation work carried out for the Wallingford Procedure (Kellagher, 1981). However as a result of further work carried out in the 1980s by the Institute of Hydrology these FSR parameters were revised to: 0.10, 0.30, 0.37, 0.45, 0.53 respectively. These values are to be used for the IH124 equation if the WRAP SOIL indices are used.

Although the uksuds greenfield runoff rate estimation and surface water storage volume design tools automatically populate the soil SPR from the FSR WRAP map, it is recommended that the SPRHOST and BFIHOST values are used instead using the HOST soil data or preferably from the FEH web service.

The minimum and maximum value for SPRHOST used in this tool is constrained to the minimum and maximum for SOIL at 0.1 and 0.6 respectively. 

The greenfield runoff rate, which is usually used for assessing the requirements for the limiting discharge flow rate (and subsequently the attenuation storage volume), for a site should be calculated for the whole development area as submitted for planning (including all houses, gardens, roads/paved surfaces, and all open space) that is within the area served by the drainage network and that could generate runoff to the proposed drainage system, even if this is only likely to occur during very extreme rainfall events (this may be the case for grassed/vegetated areas). 

Significant green areas such as recreation parks, general public open space etc., which are not served by the drainage system and do not play a part in the runoff management for the site, and which can be assumed to have a runoff response which is similar to that prior to the development taking place, can be excluded from the greenfield analysis. Open green spaces are useful areas for managing extreme events and therefore they would normally be included in the drainage analysis. A decision made to use only part of the site for assessing the limiting discharge rate will result in a smaller runoff rate and may therefore result in a greater storage volume being needed.

The exclusion of large green areas was important when using the previous (pre-2025) uksuds surface water storage volume estimation tool as it’s validity for use was limited to the paved proportion of the site being greater than 50%. This is no longer the case with the current storage tool.

Qbar is the peak rate of flow from a catchment for the mean annual flood (a return period of approximately 1:2.3 years). It was produced in report IH 124, Flood estimation for small catchments (Marshall and Bayliss, 1994) and is largely based on the work of the Flood Studies Report (NERC, 1975).

Controlling flow rates from sites is currently based on Qbar for deriving the discharge rates for the 1:1 year and 1:100 year return periods. When the statistical FEH equation is used to derive Qmed, a conversion factor is applied to obtain Qbar.

More information can be found in What method should be used for calculation the greenfield runoff rates?

IH124 equation:

Qbar = 0.00108 x (AREA)^0.89 x SAAR^1.17 x SPR^2.17

where:

• Qbar is the mean annual flood flow from a rural catchment (m³/s)
• AREA is the area of the positively drained site (km²)
• SAAR is the standard Average Annual Rainfall for the period 1941 to 1970 (mm).
• SPR is Standard Percentage Runoff coefficient for the SOIL category.

Qmed is the peak rate of flow from a catchment for the median annual flood (a return period of 1:2 years). It is the key parameter used by the FEH statistical equation (Kjeldsen, 2008) and ReFH2 (WHS and UKCEH, 2015) in deriving river flow rates.

More information can be found in What method should be used for calculating the greenfield runoff rates? 

FEH Statistical equation:

Qmed = 8.3062 x (AREA)^0.851 x 0.1536^{(1000 / SAAR)} x FARL^3.4451 x 0.0460^{(BFIHOST x BFIHOST)}

where:

• Qmed is the median annual flow rate; the 1:2 year event. (m³/s)
• AREA is the area of the catchment (ha).
• AAR is the standard average annual rainfall for the period 1961 to 1990 (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 IH124 equation uses parameters which are provided on paper-based information. In contrast the FEH based methods are all digital. 

In February 2025, the Environment Agency publicly announced that UK CEH have developed an update to the FEH Statistical Method, which will include a revised Qmed equation for ungauged catchments calibrated using new updated catchment descriptors for SAAR, URBEXT, FARL and BFIHOST. The update is anticipated to be released in Autumn 2025 when they will provide guidance on transitioning to using the new method and descriptors. 

Greenfield runoff is usually calculated as the peak rate of runoff for a specific return period due to rainfall falling on a given area of vegetated land. There are methods which predict only the peak rate of flow while others produce a runoff hydrograph.

Although greenfield runoff assessment is often a requirement for development sites, the approved formulae have all been derived from river catchment runoff data. The publication of ReFH2 and the ReFH plot scale method has been produced to try and make the predictions more relevant and accurate for site developments. 

The extrapolation of these formulae to site scales is justified on the basis of providing a consistent approach that can be universally applied, rather than giving an accurate assessment of the actual runoff rate from a site. No method would claim to provide an accurate assessment of site scale runoff. 

There are a number of factors which are not taken into account which can result in under-prediction of runoff rates from a site. These include:

• The vegetated land characteristics (whether it is treed, grassland, farmed or otherwise landscaped) is not used;
• The gradient of the site is not used;
• Rainfall intensity is not used.

However it must be stressed that although the formulae claim to give a greenfield flow rate, and that it might be much less than the value of the figure given, this does not invalidate the principle of controlling runoff to protect against flooding of the catchment downstream.

The Flood Studies Report (NERC, 1975) used a theoretical model to classify soils according to their hydrological performance. The factors used were the soil types, depth to impermeable layers, how often the soils are waterlogged and the slope. These factors are considered most important in determining the soils likely response to rainfall.

This classification was named the Winter Rainfall Acceptance Potential (WRAP) and a map was produced showing the distribution of the 5 WRAP classes across the whole of the UK. Each WRAP class has a Standard Percentage Runoff (SPR) value, which are described in the What are SOIL indices? question.

The Winter Rain Acceptance Potential map for the UK, produced by the Wallingford Procedure (Kellagher, 1981) is available from HR Wallingford at https://eprints.hrwallingford.com/37/.

Scanned versions of the original FSR paper maps of SOIL classes are available from CEH https://www.ceh.ac.uk/data/software-models/flood-estimation-handbook. There is a soil category referred to as ‘Urban’ on the FSR map that is used for city areas. It is assumed that the SOIL value is 4 for the purposes of the tools.

There are many methods that can be used for calculating greenfield runoff, however there are only two methods provided in the uksuds greenfield runoff rate estimation tool. These are:

• The equation derived in IH 124, Flood estimation for small catchments (Marshall and Bayliss, 1994);
• The FEH statistical equation by Kjeldsen et al. (2008).

In addition a default value of 3 l/s/ha can be used in the surface water storage volume design tool.

IH124 and FEH Statistical are both widely used though other options are available such as the ReFH2 tool by Wallingford HydroSolutions and UK Centre for Ecology and Hydrology (2015). 

IH124

Qbar = 0.00108 x (AREA)^0.89 x SAAR^1.17 x SPR^2.17

where:

• Qbar is the mean annual flood flow from a rural catchment (m³/s)
• AREA is the area of the site (km²)
• SAAR is the standard Average Annual Rainfall for the period 1941 to 1970 (mm).
• SPR is Standard Percentage Runoff coefficient for the SOIL category.

Information for SAAR and SOIL are available from paper maps which were issued by the Flood Studies Report (NERC, 1975; https://www.ceh.ac.uk/data/software-models/flood-estimation-handbook) and also subsequently in the Wallingford Procedure (Kellagher, 1981; https://eprints.hrwallingford.com/37/). 

The value for SAAR was reassessed in the Flood Estimation Handbook 1999 (Institute of Hydrology, 1999) as parameter AAR for the period 1961 – 90. This has subsequently been revised several times with the most current data given in the FEH22 rainfall model (available from the FEH Web Service at https://fehweb.ceh.ac.uk/). The differences between any of these values for SAAR (from FSR or FEH data) is likely to be small for most locations across the country. The advantage of using the digitally based information is the increased level of accuracy for a specific site. However this must be seen in the context of the derivation of the formula which uses this parameter value which was produced prior to the digital age. 

Similarly the value of SPR for the 5 SOIL types in the Winter Rainfall Acceptance Potential (WRAP) map can be replaced by the SPRHOST values for the greater differentiation of the 29 soil types based on IH126 Hydrology of Soil Types (Boorman et al, 1995) or from FEH (UKCEH FEH web service). Unlike the parameter SAAR, the two parameters of SPR and SPRHOST are said to not be exactly the same measure of the hydrological characteristics of soil. However the difference is considered to be sufficiently small to justify the substitution of the SPRHOST parameter value based on the advantage of being much higher resolution soil information. 

Appendix 5 of the Environment Agency document “Preliminary rainfall runoff management for developments” (SC030219 rev. E, 2013) gives an analysis comparing the two equations based on the substitution of the SPR parameter. As a result of this analysis, it is recommended that a lower bound limit to the value of 0.1 is used if used with the IH124 method.

The IH124 equation predicts a greenfield peak flow rate estimate for the mean annual flood called Qbar (a return period of approximately 1:2.3 years). This value is then factored by a growth rate parameter to give a flow rate for other return periods.

FEH statistical equation

The FEH statistical equation predicts the median annual flood called Qmed (a return period of 1:2 years).

Qmed = 8.3062 x (AREA)^0.851 x 0.1536^{(1000 / SAAR)} x FARL^3.4451 x 0.0460^{(BFIHOST x BFIHOST)}

where:

• Qmed is the median annual flow rate; the 1:2 year event (m³/s).
• AREA is the area of the catchment (km²).
• AAR is the standard average annual rainfall for the period 1961 to 1990 (mm).
• FARL is a reservoir attenuation function and is set at 1.0 and therefore has effectively been ignored. This means that areas which have water bodies which attenuate the runoff will over-predict the greenfield runoff rate.
• BFIHOST is the base flow index derived using the HOST classification.

In contrast to the FSR method of IH124, the ReFH based methods are all based on digital and higher resolution information. These formulae are generally regarded as being more accurate than the IH124 though this can only be assessed at the river catchment scale and not site scale. 

It is interesting to note that analysis for the FEH statistical equation peak flow formula found the use of BFIHOST (Base Flow Index) to produce a better correlation than the use of SPRHOST. As flood flows are not intuitively linked to river base flows this provides a warning to treat all these formulae with a degree of caution. 

To obtain the flow rate for any specific return period, a growth factor based on curves given in FSSR 14 (NERC, 1983) can be applied to both the FEH and the IH124 equations. In practice a hydrologist would carry out a detailed pooling analysis based on selecting similar catchments to obtain a likely growth curve for predicting values for higher return periods. However this requires a suitably skilled person to carry out such work. 

3 l/s/ha

A research study was carried out by HR Wallingford, “Storage requirements for rainfall runoff from greenfield development sites” (SR580; HR Wallingford, 2002), which looked at various throttle rates for limiting the discharge from the sites in the context of the peak flood flow of receiving rivers. In summary a throttle rate of 3 l/s/ha demonstrated that sufficient retention of surface water runoff was achieved to protect the river against flooding being exacerbated. However it recommended the use of IH124 for estimating greenfield runoff rates due to its ease of use to take account of the different soil type and runoff rates. 

However since that time, multiple alternative methods have been produced and recommended for estimating greenfield runoff rates in spite of the fact that these are all based on river flow data with catchments measure in tens of square kilometres. It is believed that the default value equivalent to Qbar or Qmed of 3 l/s/ha is adequate for general use for the purpose of estimating drainage runoff control, but that the alternative greenfield equations provided by the tools are suitable simple alternatives to use if the authorities or drainage design team prefer to base the estimate on a nationally derived greenfield equations. 

It should be noted that when the greenfield equations are applied with SOIL types 1 or 2 (or equivalent soil types for the FEH statistical equation) that Qbar and Qmed values are less than 2 l/s/ha. This value is considered to be a cut-off to avoid excessive storage volumes being calculated which are likely to based on storms with durations greater than 24 hours. 

ReFH2 plot scale equation

Information from ReFH2 is no longer used in the current surface water storage volume design tool, however if desired the value could be calculated within the ReFH2 software and then included within the surface water storage volume design tool as a “user specified” discharge flow rate. For information, some information is given here. 

The ReFH2 software has a module specifically developed for plot-scale application of the method. This method uses specific plot-scale equations for Time to Peak (Tp) and Baseflow Lag (BL) which are different to the catchment scale tool. The areal reduction factor for the rainfall is set to 1.0 and the winter design storm profile is recommended.

Tp is the unit hydrograph time to peak in hours and for plot scale applications is calculated by:

• Tp = a PROPWET^b AREA^c (1 + urbext₂₀₀₀)^d SAAR^e

BL is the baseflow recession constant or lag in hours and for plot scale applications is calculated by:

• BL = a PROPWET^b AREA^c (1 + urbext₂₀₀₀)^d BFIHOST^e

Tp and BL are evaluated by the software for a catchment area of 0.5 km². For greenfield sites, the urbext parameter will be set to zero. These values are then used to compute the design storm duration which is then applied to the site. The plot-scale models use AREA as an alternative to the catchment method descriptor DPLBAR (mean drainage path length), and also SAAR as an alternative to the catchment descriptor DPSBAR (mean drainage path slope). This is because DPLBAR and DPSBAR cannot be estimated at the plot-scale by the FEH catchment descriptor software. Because the formulae are applied at 50ha to define the design storm duration, the length of the event is often significantly longer than event durations calculated by other methods at site scale. This results in a lower intensity storm and therefore a lower predicted runoff peak flow rate than might give the actual peak runoff rate for a site. The recommendation for using a winter profile event, when typical events causing maximum peak runoff are summer events, is unclear.

The total site area is the whole area within the Planning “red line” boundary being considered for planning and development. The trend for some users or planners stipulating that only the paved and roof area should be used was originally driven by the incorrect practice of assuming no runoff taking place from grassed / vegetated surfaces. As the surface water storage volume estimation tool applies runoff from all grassed / vegetated surfaces, it is recommended that the whole site area is normally used for estimating the greenfield runoff rate which is subsequently used for the limiting discharge rate. 

The approach would be to either apply the greenfield criterion to the whole site or apply one of the previously developed site methods (relaxation or betterment) to the impermeable areas. 

The ReFH2 plot scale tool has a method for estimating flow rates from a catchment which is partly urbanised. However as runoff takes place at different rates and proportions and is a function of the size of the event, it is considered that a model should really assess the paved and grass / vegetated components of the site separately to arrive at an estimated site discharge rate. 

There are two methods of approach suggested. The first is to reproduce the previous drainage system in a simulation model and determine the flow rates discharged from the site for specific return periods (usually 1:1 year, 1:30 year (for sewerage systems) and the 1:100 year event). Constraints are applied such as not including surface water runoff flooding and ensuring the drainage models do not use a pressure head above ground level. Use of the outfall section of pipework and taking the pipe gradient of the final length of pipework as being equal to the pipe gradient is an alternative, possibly conservative, but less accurate approach.

Where details of the site drainage system and the properties it served have been lost or cannot be determined sufficiently accurately then a flow rate can be assumed by assessing the flow rate from the previously developed impermeable areas based on 100% runoff for rainfall of 35 mm/hr.

Both methods will result in very high rates of flow and should only be used where a ‘betterment’ factor is applied to reduce the flow rate to closer to greenfield flow rates.

Brownfield sites and previously developed sites are often permitted to use a less onerous discharge control rate than one set using a greenfield runoff equation. Reasons given have included the need to encourage the redevelopment of sites which may be more costly due to contamination and the precedent that the site may have had uncontrolled discharge in its previous state. There is an expectation that the greenfield limit of discharge should be used if possible, but that an alternative (more generous) limit can be given. This is reflected in the English SuDS Standards (Defra, 2015) but there is no prescriptive approach for setting what the discharge rate should be.

The surface water storage volume estimation tool provides two methods; one of which has been commonly applied as a principle (there must be a ‘betterment’ in the discharge from the site), and a new approach which is related to the greenfield flow rate which is based on factoring (‘relaxing’) the greenfield flow rate based on the two formulae (IH124 and FEH Statistical) used in the tool.

The ‘betterment’ method is based on assuming that the historic drainage system for the site was designed to serve 35 mm/hr for the existing site paved area. No allowance is made for climate change, no runoff takes place from the pervious areas and runoff from paved surfaces is assumed to be 100%. This equates to a flow rate of 97 l/s/ha from a site with 100% impermeable surface area. A ‘betterment’ factor is then applied which can be selected between 20% and 60%. Thus the maximum limiting discharge rate can be 58 l/s/ha. For sites with a lower impermeable fraction (say 50%) and applying the highest betterment target of 20%, this would result in ~10 l/s/ha.

The ‘relaxation’ method only permits a maximum of 3 times the greenfield site limit of discharge and this might therefore approximate to 10 l/s/ha depending on the assumed soil type. Often the soil type in an urban area is assumed to be soil type 4 on the basis that the site no longer represents the typical greenfield classification that it might have been due to compaction.

It can be seen that betterment is generally more generous than the relaxation method and therefore effectively there is a continuous range for setting the limit of discharge from greenfield flow rates through to around 50 l/s/ha.

Two further points can be made; firstly that previously developed sites are often small and therefore the limit of discharge is not based on these methods, but by the minimum flow rate for blockage risk, and secondly, that these flow rates (either minimum flow, betterment or relaxation) will result in short critical duration events and relatively small storage volumes.

Previously developed sites are sometimes termed brownfield sites. However brownfield is also used to denote polluted or contaminated sites. The term “previously developed” is now generally being used to refer to non-greenfield sites and the different hydraulic criteria is often applied for the site discharge control. A site which has previously been developed with a drainage system serving buildings or other constructed elements would be categorised as previously developed. In addition, sites where buildings which have been removed, but where the site has not been reinstated to greenfield conditions, may be considered to be in this category, but there may be time limits associated with this.

The current position on redevelopment of previously developed sites is that surface water runoff management should provide “betterment” (assuming there was little or no runoff control from the site previously); usually of the order of 30% or 50% of the existing flow rate is expected, though using the greenfield rate is always an aspiration. Although this does not often result in a significant discharge constraint (and therefore small storage volumes), it should be noted that government planning guidance does look to reduce flood risk. In some instances this will point to the need to impose greater constraints than meeting a limited “betterment” target.

It should be noted that although betterment has generally been used in the past, there is now general awareness in the Environment Agency and other authorities that reducing flood risk to the receiving watercourse, this approach is largely ineffective.

Whatever degree of betterment is used, it is important to note that compliance to Interception criterion is equally valid to apply to previously developed sites. This will achieve significant environmental benefits even if flood risk is not effectively addressed. 

Betterment is achieved by demonstrating that flow rates discharged from the site are less than those that took place previously on the site. Although this should preferably be based on assessment of the historic drainage system, a simple approach based on assessing the runoff from impermeable surface for a rainfall of 35 mm/hr is provided by the surface water storage volume design tool. 

However Sewerage Undertakers, if their drainage system is in receipt of the runoff, may arbitrarily stipulate the limiting discharge requirement based on their assessment of increased flood risk to the network. 

It is desirable to apply greenfield criteria flow rates to previously developed sites. However there is often a more generous allowance given for these sites. The surface water storage volume design tool has three options for assessing a flow rate. These are:

• A ‘relaxation’ factor between 1.0 and 3.0 which is applied to the greenfield flow rate; and
• An assessment of the flow rate from the previously developed impermeable areas which were drained based on a rainfall of 35 mm/hr with 100% runoff. This flow rate is factored by a ‘betterment’ factor which ranges from 0.2 to 0.6.
• A user specified value, which may be the previously developed flow rate calculated using a simulation model with a ‘betterment’ factor applied (see “How is the peak runoff rate from a previously developed site calculated?”), or could be an agreed flow rate with a Sewerage Undertaker, the Local Authority, or other asset owner.

This flow rate is then usually applied to the critical duration 1:100 year event to calculate attenuation storage volume.

The runoff coefficient for roof and paved surfaces is 100% within the surface water storage volume design tool. 

The surface water storage volume design tool estimates an attenuation storage volume required on the site. The estimate is based on the difference in the rainfall runoff flow being routed to the storage and the smaller allowable rate of flow out of the storage.

FEH22 rainfall and a climate change allowance factor is required to determine the design rainfall depth. Runoff takes place from contributing paved, roof and grass / vegetated surfaces with the percentage of runoff being lower and the speed of routing being slower for the grass / vegetated surface areas. 

The allowable flow rate out of the storage is based on a calculation of the discharge flow rate from the site, which will depend on whether the site is greenfield or previously developed. The design flow rate is increased if the flow rate is too small to design for without causing a blockage risk to the throttle. The throttle from the storage is represented by an orifice with the diameter calculated to deliver the design flow rate through the orifice at the maximum depth of the storage.

The storage can be set to various shapes and porosities. The model is then run for a range of storm durations from 15 minutes to 48 hours to arrive at the design water depth for the design return period based on the critical duration event.

It is important to recognise that the concept of a single storage unit at the downstream of a large drainage network is not regarded as good practice (SuDS Manual; Woods-Ballard et al, 2015). Emphasis on source control and volume reduction across the site is an important principle. 

The estimated total storage volume should be distributed across the site using source control SuDS (to deliver Interception) connected, for larger sites, to downstream conveyance and storage components.

The tool designs the storage to the design return period, however if this is less than 1:200 years, the model also provides results for the more extreme return periods. In this case the model makes two assumptions about the storage design:

• If the sides are sloped, the extra storage beyond the design return period continues at the same slope up to 1.5 times of the design height, and then beyond 1.5 times the design height the storage is assumed to have vertical sides.
• If the storage has a porosity <100%, the porosity <100% is used for the storage up to 1.5 times the design height but beyond 1.5 times the design height a porosity of 100% is applied.

Every drainage unit on a site, whether it is a conveyance or storage system, will have a critical duration event which will result in the highest velocity or largest volume of storage respectively when being designed or evaluated using standard design rainfall events. It is therefore important to run a range of duration events to ensure the ‘worst case’ condition is found for each drainage element on the site.

There are two methods available within the surface water storage volume design tool to determine the runoff coefficient from grass / vegetated surfaces.

1. Fixed percentage equal to SPR. This is the simplest approach where the percentage of runoff is fixed for all rainfall events regardless of return period or duration.
2. Fixed percentage based on rainfall depth and SPR. This is a more accurate method as the percentage runoff is based on the total rainfall depth of the rainfall event which is a simple approach to take account of increased saturation of the soils throughout the event and therefore more runoff from larger events. The percentage runoff equation is:
   1. 
   2. 

Where: 

1. RD is the total event rainfall depth (mm).
   1. SPR is the standard percentage runoff.

Both approaches require a Standard Percentage Runoff (SPR) value. The SPR value varies depending on soil type and can be derived from either the WRAP soil type or estimated from a BFIHOST value. See  How to obtain SPR / SPRHOST value?.

It is recognised that very short but very high intensity rainfall will probably generate more runoff that this formula would give. However there are many uncertainties associated with predicting pervious runoff (particularly topographic gradient) and these must be considered by the user in using this and other similar tools.

There is a practice by some engineers of designing drainage systems using 84% or 75% as the runoff coefficient from impermeable surfaces and no runoff from pervious areas. This was derived from the original Wallingford Procedure (Kellagher, 1981) runoff model (the Old PR equation) based on incorrect assumptions. Runoff may not take place from small rainfall events, but it will take place for very large events and therefore an appropriate runoff coefficient for grass / vegetated area surfaces should be applied. 

The total site area is the whole area within the “red line” boundary being considered for planning and development. This includes any areas of the site that will not drain to the drainage system that is being designed as they naturally have a separate point of discharge to the environment. This area is not used in the storage calculations, but is there for information and should be equal to or greater than the total roof, paved and grass / vegetated areas.

The total roof, total paved and total grass / vegetated areas are all the areas that could generate runoff to the proposed drainage system, even if this is only likely to occur during very extreme rainfall events (as may be the case for grassed/vegetated areas).

If, as part of the drainage proposals, any areas are ‘disconnected’ e.g. fully drained to an infiltration component for the storage system design return period, then these can be classified as ‘non-contributing’ areas.

The area used in the greenfield runoff calculation to set the discharge flow rate from the site is equal to the sum of the total roof, total paved and total grass / vegetated areas. The trend by some authorities to only use the impermeable area for assessing the limiting discharge rate for sizing the storage system is incorrect (and results in onerous storage volumes).

The areas used in the storage volume design are equal to the total contributing areas adjusted for urban creep.

Urban creep is defined as any increase in the impervious area that is drained to an existing drainage system without planning permission being required and therefore without any consideration of whether the receiving sewerage system can accommodate the increased flow. It can include, the construction of patios, conservatories, small extensions and additional paved driveways.

To allow for future urban expansion within the development, a 10% increase in roof and paved surface area is often suggested if there is no alternative value stipulated by the local planning authority.

The premise often given for setting the limit of discharge to be the same as the greenfield rate is the assumption that the hydrological response for the original state of the site is the ‘best’ way to protect the environment and people downstream. In fact this is a fallacy for many reasons and these include:

• The extra runoff volume generated by the site will enhance flooding downstream;
• The timing of the runoff might be better to realise it earlier at a higher rate of flow;
• The application of the greenfield formula used, if applied to the river catchment at the location of the site, would give a much lower limit of discharge rate;
• The assumption that the greenfield formulae predicting runoff based on river flow analysis can be applied to a small site is clearly wrong (and research tends to indicate that these approved formulae significantly under-predict the peak runoff rates);
• There is no research that conclusively demonstrates the assumed benefits of applying specific ranges of limiting discharge to reduced downstream river flooding. 

Having said this, it is important to recognise that increase in volume and rate of runoff is probably detrimental to many smaller streams and rivers and that a consistent universal methodology should be applied to drainage design. 

However it is therefore important to recognise that, whatever formulae is used, the accuracy of its prediction of the greenfield flow rate is of limited importance. To try and limit the continuous debate on this issue, a pragmatic limiting discharge of 3 l/s/ha is now put forward as being a sensible value to use. However the option of using Qbar (or factored value of Qmed to obtain Qbar) is also available. 

Current practice, and the SuDS Standards, are largely based on the research carried out by HR Wallingford in 2002. This research (SR580; HR Wallingford, 2002) established that, for flow rate control on its own to be effective, the limiting rate of discharge needed to be 3 l/s/ha or less. A control rate less than 3 l/s/ha was shown to generally result in sufficient stored surface water remaining on the site during the peak period of flooding of the adjacent river for the catchments tested to limit the additional runoff produced by the site from exacerbating the flooding in the receiving river. 

The reason for 2 l/s/ha being set as a minimum was to try and ensure excessive storage did not get designed (a cut-off rate to avoid the lower flow rates resulting from the use of FSR soil types 1 and 2 in the greenfield equations), and to use it as the discharge rate for Long Term Storage (see What are the standards for volume control and runoff rates? ) if the 1:100 year greenfield discharge rate was used for the greenfield runoff. 

One of the largest uncertainties in the design of infiltration systems is the infiltration coefficient, as this may reduce over time, particularly if effective pre-treatment is not included within the design and/or system maintenance is poor.

To account for this, a factor of safety is introduced into the design which reduces the infiltration coefficient. The factor to use depends on the consequences of failure and total areas to be drained.

The following equations are used for the different infiltration systems to estimate the height or base area of the infiltration system:

• CIRIA Report 156, 3D infiltration system equation:
  • Rectangular soakaway with infiltration from sides and base
  • Ring soakaway with infiltration from sides and base
  • Infiltration trench with infiltration from sides and base
• CIRIA Report 156, 3D infiltration system equation, where area of base is set to 0:
  • Rectangular soakaway with infiltration from sides only
  • Ring soakaway with infiltration from sides only
• CIRIA Report 156, planar system equation:
  • Planar infiltration system with infiltration from base only

The following equations are used for the different infiltration systems to estimate the time for half emptying:

• CIRIA Report 156, 3D infiltration system equation:
  • Rectangular soakaway with infiltration from sides and base
  • Ring soakaway with infiltration from sides and base
  • Infiltration trench with infiltration from sides and base
• CIRIA Report 156, planar system equation:
  • Planar infiltration system with infiltration from base only
• BRE365 equation:
  • Rectangular soakaway with infiltration from sides only
  • Ring soakaway with infiltration from sides only.

The concept of infiltration is aimed at trying to prevent any runoff taking place from sites when there are small rainfall events. The aim is to minimise the discharge of polluted runoff from entering streams and rivers, particularly in summer periods when they have low flows and the water is warm; conditions which are already stressful for flora and fauna. The emphasis is on achieving no runoff for small rainfall events which are less than 5mm. A small study was carried out by HR Wallingford on Interception (Supermarket representation for SuDS Guidance, Interception storage analysis (2014) using time series rainfall analysis to show how SuDS with little or no infiltration capability could deliver Interception. The three key points to note are:

• Evapotranspiration in summer is a key mechanism for preventing runoff using soil storage;
• Very low rates of infiltration can be effective in significantly enhancing prevention of runoff;
• An Interception criterion of 5mm of rainfall does not mean that rainfall from every event up to 5mm in depth will be prevented from runoff as antecedent conditions for some events will be particularly wet. A probabilistic approach is therefore needed for defining and assessing compliance.

Detailed time series runoff analysis need not be carried out if this is considered to be too complex, and simple rules of thumb can be used. These are based on showing that losses based on using evapotranspiration and marginal infiltration are greater than average daily rainfall (annual rainfall / 365) using summer conditions. Guidance on this is provided in the SuDS Manual.
Rainwater harvesting is another mechanism by which Interception can be delivered. See “How can Rainwater harvesting be used for surface water management?”

The water quality model within the SuDS performance evaluation tool is not physically based but uses a pollutant wash-off model combined with average event mean concentrations reflective of different land use types. The efficiency of SuDS to remove pollutants reflects observed removal rates for typical components (based on measured influent and effluent concentrations published in summaries of data held within the BMP database (https://bmpdatabase.org).

There are a default set of pollutant profiles provided. However to use a different set of buildup / washoff factors, users can download the pollutant profile (as a CSV), modify it and re-upload the profile.

The SuDS performance evaluation tool is a simple volume balancing tool evaluating each SuDS component individually. The SuDS are represented as simple storage reservoirs which use up to three storage layers (‘drainage’, ‘soil’ and ‘surface’) and transfer water between the layers using simple rules.

Unlike more complex drainage software, the lateral routing through the SuDS and through connecting pipework are not represented and the hydraulic influence of downstream SuDS is simplified.

To model the hydraulic performance of the SuDS the following simplifying assumptions are made:

1. The cross-sectional area of the unit remains constant throughout its depth except for:
   1. Ponds and basins, where the cross sectional area of the surface is assumed to increase linearly from the bottom to the top;
   2. Swale geometry is more complex and allows for a sloped base and sides.
2. Flow through the unit is one-dimensional in the vertical direction.
3. The layers act as simple reservoirs that store water from the bottom up.
4. Inflow to the unit is uniformly distributed into the soil layer (or drainage layer where there is no soil layer).
5. Flooding occurring at the final SuDS unit stores the water above the SuDS unit with a continuation of the same plan area.

Two mechanisms of evaporation are represented by the tool:

• Evaporation from the depression storage of the upstream catchment areas draining to the SuDS; and
• Evapotranspiration from the soil layer of the SuDS units.

Evaporation is only applied for time series rainfall and no evaporation or evapotranspiration is applied for design storms. 

The rate of evaporation is based on monthly temperatures using the 1985 Hargreaves equation (Allen, et al., 1998). The parameters required from the user are the latitude and the daily minimum and maximum air temperatures for each month. The default parameters provided in the tool are for London obtained from https://weatherspark.com/countries/GB/ENG and therefore should be updated for other locations accordingly.

The interception criteria within the Welsh SuDS standards (2018) and CIRIA SuDS Manual (2015) is for zero runoff for the first 5mm rainfall for 80% of events during the summer and 50% in winter.

There are a number of technical issues in assessing the performance of a continuous time series of rainfall for meeting the rule of retaining the first 5mm of rainfall for all rainfall events. 

The reason for this is that some drainage systems (large ones and those with highly constrained limiting discharges), can still be draining down when the next event commences. It is difficult to therefore determine whether at least 5mm has been retained for these events. Fortunately the proportion of rainfall greater than 5mm is normally less than 50% (based on the default 9 hour inter-event period), and for 10mm the proportion of larger events is quite small.

Therefore the performance evaluation tool currently evaluates the number and percentage of rainfall events complying with the zero runoff (interception) criteria for events by rainfall depth bands. The bands are: 0-2 mm, 2-5 mm, 5-10 mm and 10 mm+.

The reason for these bands are that it would be anticipated that nearly all rainfall events less than 2 mm will meet the zero runoff criteria as depression storage largely prevents runoff occurring for smaller events, therefore this is not a good measure of interception. Events greater than 10 mm are likely to be outside the range of events which are likely to be wholly retained on site. As such, the 2-5 mm and 5-10 mm bands may be of greater interest in terms of identifying whether the SuDS system delivers effective interception.

As the compliance to meeting the 5mm Interception rule is a probabilistic one (to cater for very wet periods), compliance is assumed to have been met if 50% of winter events and 80% of summer events in the 2-5mm band result in zero runoff from the site. 

However, based on a knowledge of the polluting effects of the runoff and the receiving waters characteristics through the year, users could choose to raise the probabilistic compliance or look for the 5-10mm band to meet the compliance values.

When rainfall takes place on greenfield sites there is, for the majority of rainfall events during the year, no discernible surface water runoff to receiving water bodies. The rainwater normally evapotranspires from the wetted soil. In winter the rainfall also contributes to groundwater recharge. 

However, impermeable surfaces generate runoff from virtually all rainfall events, and this can have a negative impact on the morphology and ecology of receiving water bodies. Interception is aimed at trying to replicate the minimal runoff volume characteristics of greenfield runoff. In most urban locations in the UK more than 50% of all rainfall events are less than 5mm in depth, therefore preventing runoff from these events is very significant in terms of both hydrological and pollution impact reduction on receiving waterbodies.

The total annual pollution load from runoff is closely correlated to the total volume of runoff. Therefore prevention of runoff from the majority of all small rainfall events and also reducing runoff volume from larger events contributes very effectively to reducing the pollution load passed to receiving waterbodies. This is particularly important during the summer months when dilution flows in receiving watercourses is normally low.

Although Interception is largely a water pollution reduction criterion, application of Interception across a catchment served by a combined sewer system, should significantly reduce spill frequencies and volumes at overflows.

The Interception criterion has rarely applied and this is mostly due to the difficulties in carrying out the analysis needed. The performance evaluation tool has been provided to address this problem. It not only provides the tools to model SuDS appropriately, but it also provides ten pre-loaded present day continuous rainfall series which can be used so that most locations can apply a suitable series for assessing the performance of the drainage system.

In theory, the design should consider change in rainfall due to climate change. However in practice adjusting time series is technically challenging and the use of present day series will generally be appropriate in that future summer rainfall is likely to have fewer and smaller rainfall events. Therefore compliance to a present day series is likely to be a conservative metric for assessing a drainage system for Interception for most drainage systems. For more information see Should time series rainfall be adjusted for climate change?

A high level of Interception provided for some parts of the site should not be considered as adequate compensation for a low degree of interception provision for other locations. Compliance should be required for the whole site, or at least paved areas, for it to be considered effective. As the assessment should be made at the site outfall, this trade-off is avoided when using this tool.

Interception mechanisms are based on runoff retention. This can be achieved using rainwater harvesting, or using soil storage and evaporation. Either infiltration or transpiration rates can dispose of the runoff from minor events to enable the next event to be captured. Infiltration rates of soils can be very low and still be effective at providing interception.

This criterion reinforces the importance of using vegetative and soil based SuDS components.

Although the SuDS Manual (Woods-Ballard, 2015) has rules of thumb for designing to meet Interception, the existence of this tool (and the capabilities of other tools for running continuous rainfall), means that use of a continuous rainfall series to assess the drainage system performance should be carried out for most drainage systems. 

The runoff coefficient for roof and paved surfaces is 100% by default within the SuDS performance evaluation tool. However, the user can choose to reduce the runoff coefficient for the paved surface to as low as 85%, which may be more representative on older paved surfaces.

There are two methods available within the surface water storage volume design tool to determine the runoff coefficient from grass / vegetated surfaces: fixed percentage runoff and variable percentage runoff.

Fixed percentage runoff

This is the simplest approach where the percentage of runoff is fixed for all rainfall events regardless of return period or duration. However, as the percentage runoff value is likely to be low for time series rainfall and higher for design storms it is recommended that if this method is used that the value will need adjusting depending on the type of rainfall simulated. 

The following values are recommended:

• For design storms a value equal to 0.5 to 1.0 times the soil parameter SPR (standard percentage runoff ) is recommended. As SPR for soils range from 10 to 60%, a value in the range 5 to 60% is recommended.
• For TSR a value in the range of 0 to 0.2 times SPR is recommended. As SPR ranges from 10 to 60%, a value in the range of 0 to 12% is recommended.

Variable percentage runoff – design storms

An equation is used to calculate a single percentage runoff value which is used throughout an event but which varies between events. i.e. each event in a design storm duration-depth matrix will have different percentage runoff values.

The equation is based on the event rainfall depth (RD) and soil SPR parameter. The greater the rainfall depth and the higher the SPR value the greater the percentage runoff.

There is no influence of antecedent conditions on percentage runoff.

The equation used is 

Where:

RD is the total event rainfall depth (mm).

SPR is the standard percentage runoff.

Variable percentage runoff – time series rainfall

Antecedent conditions are used to adjust the percentage runoff value at each timestep throughout a time series rainfall. The parameter describing the antecedent conditions, NAPI, increases during/after recent rainfall and decays during dry spells and wet periods. A higher NAPI value results in increased percentage runoff.

The antecedent conditions (NAPI, Normalised Antecedent Precipitation Index) is calculated for each timestep. 

The equation, for percentage runoff at each timestep is:

The equation for NAPI on a sub-daily timestep is: 

Where:

dt is the timestep in seconds;

k is the decay factor which is set as 0.8 ;

NAPI(t) is NAPI at the current timestep;

NAPI(t+dt) is NAPI at the next timestep;

P is the rainfall during the timestep;

E is the evaporation during the timestep.

If NAPI is calculated at less than -5 mm , the value is capped. There isn’t a cap on the maximum value of NAPI.

It is best to use rainfall time series which best reflects the hydrological characteristics at the site. The best approach to achieve this is to obtain observed rainfall timeseries from a local rainfall gauge. To represent the inter-annual rainfall variation you should ideally use a time series of a minimum of three years but ideally ten years.

However, where you don’t have access to this the tool has a number of pre-loaded rainfall time series from rain gauges you can choose from. It is sensible to consider both the geographical distance between the rain gauge and the site location but more importantly which rain gauge location has the most similar hydrological parameters to the site (e.g. whether the location is likely to experience a lot of long duration, low intensity rainfall, or short, high intensity storms).

A few of the parameters from the old Flood Studies Report can be used to compare rainfall characteristic similarity between the pre-loaded rain gauge time series with your site location. They are the annual average rainfall (SAAR), the 5 year 60 minute rainfall depth (M5-60) and the ratio of the 5 year 60 minute rainfall depth to the 5 year 2 day rainfall depth ('r'). You should determine the values for your site by checking the maps linked to within the tool (and accessible here https://eprints.hrwallingford.com/37; or for a more up to date SAAR value you can use the FEH web service https://fehweb.ceh.ac.uk/ data). Then compare them with the values provided for the pre-loaded time series.

In the UK, winters are predicted to become wetter and the summer drier, whilst in all seasons the most extreme events become more extreme. Whilst uplifts on design storms is well established, easier and more important adjusting time series to reflect a future climate is more technically difficult as it is not appropriate to simply factor a whole time series by an uplift factor.

There are methods and tools available to adjust a present day time series for future climate (e.g. UKWIR’s RedUP tool (UKWIR, 2022 & 2025)) but the use of a present day series will generally be appropriate in that future summer rainfall is likely to have fewer small rainfall events. Therefore compliance to a present day series is likely to be a conservative metric for assessing a drainage system for Interception for most drainage systems.

Climate change projections indicate that severe rainfall will have higher intensities. A climate change factor is therefore a requirement for design to reflect anticipated changes in the future climate. Whilst all countries within the UK apply an uplift factor to the design rainfall intensities, the guidance varies in each of the four countries of the UK. The guidance and links below are accurate as of October 2024.

It should be noted that using present day rainfall and then factoring the resulting storage volume by the climate change uplift value will significantly under-size the attenuation storage required.

England

The climate change allowance for peak rainfall intensity depends on the location of the site and the lifetime of the development. There is Environment Agency guidance (https://www.gov.uk/guidance/flood-risk-assessments-climate-change-allowances#peak-rainfall-intensity-allowance ) and an interactive map (https://environment.data.gov.uk/hydrology/climate-change-allowances/rainfall ).

Wales

The climate change allowance for peak rainfall intensity depends on the lifetime of the development but is the same across Wales. The Welsh Government guidance is accessed at https://www.gov.wales/climate-change-allowances-and-flood-consequence-assessments. 

Scotland

The climate change allowance for peak rainfall intensity depends on the location of the site. There is Scottish Environment Protection Agency (SEPA) guidance (https://www.sepa.org.uk/environment/land/planning/guidance-and-advice-notes) and an interactive map (https://scottishepa.maps.arcgis.com/apps/webappviewer/index.html?id=2ddf84e295334f6b93bd0dbbb9ad7417)

Northern Ireland

A single 20% rainfall uplift for the whole of Northern Ireland is recommended (https://www.infrastructure-ni.gov.uk/publications/technical-flood-risk-guidance-relation-allowances-climate-change-northern-ireland).

The Flood Estimation Handbook (FEH) 2022 rainfall data is the UK industry standard estimation of design rainfall. 

The data can be found on the FEH Web Service at https://fehweb.ceh.ac.uk/. 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).

For rainfall data, purchase a ‘Point’ data point.

The official position taken by the Environment Agency has been:

“We do not normally count the storage provided within water butts or rainfall harvesting measures towards the attenuation storage requirements as there is no guarantee that these devices would be empty at the time that a rainfall event occurs”. 

There has not been any more recent information from the Environment Agency which changes this position that is known of. This is a reasonable position to take for standard rainwater harvesting design as it takes no account of the relationship between the demand and supply of water to the storage unit. For a complete explanation of designing rainwater harvesting systems for surface water control reference can be made to BS 8515 “Rainwater harvesting systems - Code of practice” (BS 2009, revised 2013). This document is obsolete as it has been replaced by BS EN 16941-1 so it is no longer available. However BS 8515 document did provide a method of analysis and design for storing roof runoff to provide stormwater management. There is also more information in the report by HR Wallingford “Stormwater Management using Rainwater Harvesting” (SR736; HR Wallingford, 2012).

In spite of the historical official position by the Environment Agency, water companies have been making claims recently of significant benefits associated with the use of water butts to help alleviate flooding and sewer spills. These are either just storage or slow-draining 200 litre storage units. 

RWH units can be designed to provide some storage and some attenuation with slow drain-down using a small orifice, or can be designed to retain all the runoff if certain design rules are followed. The problem with using RWH to provide distributed attenuation storage include:

• The unit cost of storage is much greater than a large basin downstream;
• The ownership with a householder presumes suitable maintenance and no changes made to the operation of the unit;
• Using RWH units to drain down effectively contradicts the main purpose of such units which is to conserve water resources and minimise the volumes of potable water needing to be treated;
• The unit rate of flow control will be of the order of 50 to 100 l/s/ha (which is a high limiting discharge rate) due to the requirement to meet the minimum size of the orifice to avoid blockage risk. 

The main design principle which allows rainwater harvesting systems to be designed to provide full retention of the surface water runoff is based on regular daily demand and that the demand is greater (on average over a period of time) than the supply to it. There is uncertainty associated with the calculations for both the demand and supply elements and these issues are discussed in the HR Wallingford report (SR736; HR Wallingford, 2012).

RWH, whether designed for water conservation only or stormwater management as well provides benefits in delivering Interception for all connected surfaces where demand for the system is regular and consistent throughout the year. Where the supply yield to demand ratio is much greater than 1.0 (and the system is not actively managed) then it will be ineffective for surface water management, but it can still be considered to provide Interception as it will prevent the first 5mm of most rainfall events from creating runoff.

The general position taken on minimum flow rate in the past has been to use 5 l/s to avoid throttle sizes from being too small and thus create a risk of blockage. However as most development sites are small, this flow rate often does not really provide the degree of throttling of flow that is thought to be needed.

The water companies Design Construction Guide (Water UK, 2023) allows adoption of throttle orifices down to a 50mm diameter, but limiting flow rates of 2 l/s and even 1 l/s are now often stipulated. 

The blockage risk is very much a function of both the orifice size and the solids (twigs etc) that can be transported in the surface water runoff. Therefore drainage systems using standard gullies and pipe systems may well convey debris in the flow and have a high risk of blockage, while flows discharged from permeable pavements will not have any debris washed through it from the surface. Appropriate risk assessment is therefore essential based on the drainage system characteristics and the throttle size being applied. Where a small orifice is being proposed and the risk of blockage high, the repercussions of flooding and maintenance requirements to address the blockage then need to be explicitly considered.

There is a option within the uksuds surface water storage volume estimation tool to override low discharge flow rates calculated using the greenfield methods if they are too small to design for without a risk of blockage. The user simply eneers the minimum orifice diameter or flow rate that would be acceptable.

This FAQ only comments very briefly on a couple of issues; for more information the user must study the appropriate current documentation.

The SuDS design standards is different in each of the four countries of the UK due to the devolved responsibilities for drainage.

Broadly it is considered that the Welsh Standards (Welsh Government, 2018) are the truest reflection of the principles in the SuDS Manual (Woods-Ballard et al, 2015). The English standards (Defra, 2015) are least prescriptive, but are technically incorrect in some details such as the reference to ‘…the same event’ as the duration of the event for the greenfield formulae is not known. Furthermore the guidance effectively permits the use of 1:100 year greenfield flow rates for the 1:100 year event without constraining this to only discharge of the greenfield volume. The Welsh Standards correctly details this aspect which is in line with the recommendations of original research (SR580 HR Wallingford, 2002) and the SuDS Manual (2015). The Scottish standards (SUDSWP, 2016) was originally focused on water quality, but also includes hydraulics.

However it is important to recognise that the English Standards are non-statutory and that the Lead Local Flood Authorities (LLFAs) and local planning authorities often stipulate the correct approach. However, due to the wording of the current standard, there are many cases where the 1:100 year greenfield flow rate is applied without addressing the volumetric constraint required.

The uksuds surface water storage volume estimation tool has decided that the volumetric criterion is so rarely applied that the limiting discharge for the greenfield flow rate is set at Qbar or 3 l/s/ha in the surface water storage volume design tool. 

The return periods that are traditionally set for UK drainage design criteria are:

• 1:30 years: roads should not flood for events up to this magnitude
• 1:100 years: runoff discharged from the site should be managed on site and discharged to meet a stipulated flow rate.

In addition to these, there are a number of additional criteria which are normally used, including:

• 1: 1 year: Where SuDS are being designed for water quality performance, there are a range of methods that are used, but it is quite common to assess their hydraulic characteristics using a 1:1 year event;
• 1:1 and 1:10 years: Lower return periods than the traditional 1:30 years are considered where flooding can be tolerated to designed depths and durations at specific designed locations (such as remote areas of car parks or playgrounds);
• 1:100 years or greater: requirement for safe access / egress on roads in locations such as floodplains up to this magnitude;
• 1:100 years or greater: no flooding of properties or critical infrastructure up to this magnitude, with a suitable freeboard specified that is related to the uncertainty associated with achieving that freeboard;
• 1:100 years: The management of runoff should be achieved within the site rather than flooding being passed overland to other areas. Flood flows can be safely routed through the site dictated by the topography and pond at suitable low points;
• 1:200 years or greater: Locations adjacent to the sea (and rivers in Scotland) use higher return periods to take into account the greater consequence of flooding from the sea or embanked storage or flash flooding;
• Extreme return periods: There are always exceptions where higher return periods are to be considered. For example nuclear sites or locations in the flood route of dams located upstream. 

It is important to recognise that design criteria can be relaxed as long as the implications are clearly understood and designed for and agreed with the relevant stakeholders. 

In designing or assessing the performance of drainage systems, the critical duration event must be considered and this duration storm will vary depending on the drainage element being considered.

The simple assumption is normally made that the return period of the rainfall is the same as the design level of protection against flooding for site drainage systems. This is reasonably valid where the area is dominated by paved surfacing and there is limited influence from either uphill areas or downstream water levels.

In the case of large sites there may be outfalls at more than one location; either in the form of receiving sewers or receiving waters. In these situations the predevelopment or greenfield subcatchments must be considered, and also whether further downstream the flows all pass into the same catchment. In principle the fact that there is more than one outfall should not result in a greater flow rate or volume being discharged from the site compared to what would have been discharged for a single outfall for the whole site.

Designing for exceedence is the assessment of the impact of a larger (more extreme) event than that used for designing the drainage components on the site. There is no prescription usually for what the assessment return periods should be, though embankments involved in storing significant volumes of water should be assessed using guidance associated with the management of reservoirs.

The method of analysis for assessing exceedence requires information on topography, finished ground levels, floor levels, structures which would be impediments to flows (buildings, walls) and possibly an assessment of the risks of future changes on the site which might create an obstacle to flows. The assessment can range from indicative flow routing using the level and structures information through to detailed 2D modelling. Depth of flooding is useful to predict related to return period and related to critical levels such as floors and any critical infrastructure. In addition it is useful to establish the duration of flooding especially at locations which might be planned for temporary flood storage if the design criterion is not particularly infrequent. For instance a car park might flood for return periods greater than a 1:5 year return period. The depth, duration and location of flooding would be important to establish in this situation.

Flows generated on the site and also flows which might come onto the site from other areas need to be considered.

In addition to the assessment of extreme rainfall conditions generating such flows, it is also important to assess flooding from other causes. These include failed drainage systems (blocked outlets, collapsed pipes) and even burst pressure mains, particularly where a water supply main within a site is reasonably large. Failure of risk of dams, reservoirs and even canals which might have an impact on the site must also be explicitly considered.

Source control is the control of runoff at or near its source, so that it does not enter the drainage system or is delayed and attenuated before it enters the drainage system. 

SuDS that can deliver ‘source control’ are fundamental elements of a SuDS scheme. ‘Source control’ SuDS components can deliver one or more of the following:

• capture, store and reduce surface water volume (e.g. rainwater harvesting systems, green roofs);
• capture, store and treat surface water (e.g. pervious pavements, detention basins, bioretention systems)
• capture, convey and treat surface water (e.g. swales)
• capture, store and infiltrate surface water (e.g. soakaways, infiltration trenches)

It is preferable to implement such systems at or very close to the source of the runoff. The same design objectives should include consideration of the full range of quantity, quality, amenity and biodiversity objectives and criteria for best practice SuDS design.

Interception is the prevention of runoff from the site for the majority of small (frequent) rainfall events (or for the initial depth of rainfall for larger events). When rain falls on greenfield sites, it produces no discernible runoff for small events. In contrast, rain falling on impermeable surfaces causes rapid runoff for almost all events which can lead to erosion and ecological deterioration of receiving waterbodies. In addition, runoff from paved surfaces are contaminated with urban pollutants which are then discharged into the environment with every runoff event. By designing systems to prevent runoff from small rainfall events, receiving waterbodies are protected and pollutants are trapped in components potentially allowing time for them to degrade and/or storing them for future removal. 

By capturing and storing or conveying runoff in soil or aggregate systems, source control components are allowing runoff from small events to be harvested, infiltrated and discharge volumes reduced. This minimises runoff for most small events. 

The concept of Treatment Volume originated with SEPA and rarely gets referred to or used now. The concept of treatment storage is to provide a body of water in which dilution and partial treatment (by physical, chemical and biological means) of surface water runoff can take place. This is defined as the dry-day volume of water (therefore not including the additional water temporarily stored during attenuation of runoff) being equal to 15mm of rainfall (the England definition) or the formula Vt (in Scotland, see the SuDS Manual (Woods-Ballard, 2015)). The Scottish method is also used in Northern Ireland. 

Treatment storage can theoretically be reduced where Interception storage is provided, or where the runoff from contributing paved surfaces has had some pretreatment such as passing through a swale; however there are no agreed rules or guidance.

The concept of treatment volume has been applied to SuDS units other than ponds. However this is rarely a requirement of design and the basis for its application for structures other than ponds is tenuous.

Infiltration should be used wherever possible, subject to the need to protect groundwater against polluted runoff. As all runoff is to some degree contaminated, protection is provided by ensuring that some treatment is provided by SuDS systems and that the base of all infiltration units should be at least 1.0m above the highest expected groundwater level. This is often difficult to establish, so groundwater levels should be measured through at least one winter period and a judgement made as to how much higher it might get in a really wet winter period.

Where there are important aquifers, or the runoff is particularly contaminated, there may be a need to prevent the use of infiltration.

The critical duration event for streams and rivers range from around 4 hours for very smallest of catchments, through to 8 to 12 hours for many small rivers, going up to 3 days for the largest of rivers such as the River Thames.

As the critical duration for storage systems for developments can range from 15 minutes for very small sites through to more than 24 hours, there are often instances where the receiving drainage system or river has a similar critical duration to the site drainage system. Where the site is relatively flat the receiving waters may provide a hydraulic constraint due to elevated water levels. Similarly discharge to the coast may be influenced by tide-locking. In these situations, the joint probability of the behaviour of the receiving waters and the site drainage system needs to be considered. The term used is “dependency”. The dependency is high where a large event is likely to result in an extreme response in both systems.

To avoid overly complicated analysis, the initial check is to apply the design criterion (usually the 1:100 year event) and assume total dependency (both systems having a 1:100 year response) and see what the implications are on storage or whatever performance metric is relevant. Where the consequences are not significant in terms of additional volume required, then there may be no need to carry out a more detailed evaluation. 

The 1m separation distance between the base of an infiltration component and the highest expected groundwater level is provided to protect the groundwater from pollutants. Firstly it ensures that the groundwater level is considered so that infiltration is an appropriate hydraulic option. However there are other aspects that are also being addressed by this criterion.

Groundwater observations are often limited and predicting extreme levels can be uncertain. It therefore provides a margin for uncertainty. If groundwater rises above the base, the storage capacity of the infiltration component will be reduced and the performance of the system will change from that for which it was designed.

One metre of underlying soils provides protection of the groundwater from contaminants which may find their way into runoff entering the component. Groundwater protection is extremely important as it is a very valuable resource and, once it becomes polluted, it is extremely difficult if not impossible, to remove pollutants. Urban runoff is usually contaminated with a range of pollutants and even roof runoff has pollutants. Contaminants may include accidental spills or intentional disposal of chemicals. The unsaturated zone help adsorb these pollutants on the soil particles before it passes to the groundwater. 

With the runoff proportion from impermeable surfaces being between several or many times greater than natural soils (depending on soil type) during a significant rainfall event, the control of runoff using a high discharge rate (such as the 1:100 year greenfield flow rate) is only going to have limited benefits as the extra volume of water generated and released from the site is likely to result in greater flooding in the drainage system or river downstream. Controlling runoff volume without control of flow rate may have a local impact, but the attenuation process as it passes downstream would result in flood levels which would be largely unchanged. It can be seen that in spite of the greater emphasis having been placed on flow rate control for the last 30 years, that it is volume control that is perhaps the more important of the two hydraulic control criteria when looking at catchment scale impact.

The current criterion for greenfield volume control is linked to the use of the 1:100 year 6 hour event. This has been selected based on ensuring a simple and consistent approach made across the country. The reason for 6 hours being specified is that small catchments and their streams are likely to be more sensitive to changes in urbanisation and the shortest critical duration of these catchments is likely to be of the order of 4 to 8 hours. 

In terms of the practicalities of design, there is a need to control both the discharge of the defined greenfield runoff volume at the 1:100 year greenfield runoff rate as well as controlling the discharge of the additional generated runoff at a much reduced flow rate. To achieve this, attenuation storage design is required to effectively provide two storage units with different flow rate controls. Many sites will generate much more runoff volume than the greenfield runoff volume based on the 1:100 year 6 hour event. This additional volume can be considered to be a separate storage system which has been termed Long Term Storage and this stored water is only discharged at a much reduced rate of discharge. The reduced flow rate has generally been limited to 2 l/s/ha.