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Surface water storage volume estimation updates

Wednesday, January 18, 2017

A range of improvements have been made to the storage volume calculator along with the addition of FAQs for all tools. 

The major changes are:

  • Users can now use flow information, generated from the ReFH2 plot scale tool, to provide an alternative set of input data for estimating storage requirements.
  • A contribution of pervious area runoff has been included for estimating storage volumes. This has been brought about to address the growing trend of specifying that the limiting discharge allowance should be only based on the impermeable area of the development which results in considerably larger estimates of the volumes of storage. Although pervious runoff from developments is not explicitly part of accepted analysis procedures to date, this aspect has been alluded to in the SC030219 Environment Agency guidance documentation in the assessment of Long Term Storage volumes. A more detailed discussion on this topic is planned to try and encourage a consistent national approach is maintained. 
  • Volumes of storage calculate for small developments are now usually quite a lot less than the previous version of this tool due to a revised approach for dealing with the minimum low flow rate of 2l/s/ha.. This has been brought about because the early guidance in SC030219 assumed flow rates as low as 1l/s/ha could be applied. 
  • Along the same lines, the use of 5l/s as a minimum flow rate is now regularly being challenged with lower flow rates being requested. The tool now therefore allows flows to be as low as 1l/s. 

Tool change modifications

There are three categories of changes made to the storage volume calculator. These are:

  • Error corrections
  • Alignment with SuDS Standards
  • Refinements to the tool capability

Error corrections

In some instances control flow rates were being incorrectly reported, but this did not affect the calculation of volumes.

The initial representation and the calculations of storage when infiltration and rainwater harvesting inputs were used has been changed as the original assumptions are not considered to be appropriate. The details of the changes are explained below. 

Alignment with SuDS Standards

The tool was implemented to reflect the Environment Agency document “Preliminary rainfall runoff management for developments (SC030219)”. In it the minimum flow rate for Qbar was limited to 1l/s/ha. This has always been at odds with the position taken in the SuDS Manual and the SuDS Standards where 2l/s/ha is normally the minimum requirement. Changes have been made to this effect. This means that the maximum possible amount of storage (worst case is Soil type 1) is around 600m3/ha if a site is fully paved, before factors such as climate change etc. are applied. This does mean that some sites, particularly those with Soil type 1 and 2, will be predicting smaller volume requirements now.

Refinements to the calculator 

Quite a significant number of changes have been made to the tool; some of which are based on requests, and some to provide a better prediction of the storage volumes required. These are discussed below.

  • Minimum discharge flow rate
    The minimum discharge rate of 5l/s has been modified as a user definable value. A value down to 1l/s can now be entered. (The practicalities of using such low flow rates should be considered carefully by the design engineer).
  • Definable Qbar value
    Qbar has been definable for some time and this is unchanged. It is worth noting that the calculation of Long Term Storage remains unchanged related to soil type; so, for example, if Soil type 1 is used, but a Qbar (or minimum discharge flow rate) is selected which is much more generous which might be equivalent to Soil type 4, it is quite likely that the storage volume will be dictated by the Long Term Storage volume, because this will still be based on the SPR value for Soil type 1 unless you also edit the soil type. The decision to over-ride the value of Qbar should therefore be taken with care. 
  • Minimum site area
    The minimum site area is set to 0.02ha and the minimum percentage paved is still applied so the minimum paved area is 0.01ha. This is effectively equivalent to one big house or two small ones. In most situations this results in a zero storage requirement or very close to it. 
  • Infiltration / rwh
    If infiltration / rainwater harvesting is used for all site runoff it will still calculate the storage for the minimum site area of 0.02ha. If infiltration /rwh is used to serve paved areas such that the paved proportion is less than 50%, the net site area is reduced so that the proportion of paved surface with runoff is 50% of the effective site area for the purpose of calculating storage. This means that the estimate of Qbar and limiting discharge flow rates will also all be reduced from the values that would have been calculated based on the ‘Area positively drained’.
  • Pervious area contribution
    Even though the tool uses 100% runoff and zero for pervious runoff, it is thought that the runoff assumptions for the long duration 100 year events used for the storage calculation is not conservative in its prediction of runoff where there is a significant proportion of the site which remains pervious. A new term has therefore been added with a default contribution of 30% of the pervious area being assumed to contribute runoff and this is done by using the SPR value of the soil type. This contribution effect tends to zero as the paved proportion of the site tends to 100%. This 30% default value can be set to the range of 0% through to 100% based on an assessment of the pervious areas which are considered to continue to contribute runoff to the drainage system. 
  • Attenuation storage volumes
    Calculations have been modified to take account of discharge rates where discharges at low return periods are defined by minimum flow rate conditions, but higher flow rates are allowed for the 30 and 100 year return periods. 
  • Long term storage
    The recent change to 100% runoff from paved surfaces for calculating long term storage has reverted back to 80%. Long term storage volumes are set to zero where the limiting discharge rates are governed by the 2l/s/ha rule, or when the site limiting discharge flow rate governs the runoff.
    Long term storage is reduced or zero if some runoff is addressed by infiltration or rainwater harvesting (rwh).
    Long term storage calculations now take account of discharge rates where discharges at low return periods are defined by minimum flow rate conditions, but higher flow rates are permitted for the 30 and 100 year return periods. 
  • Climate change default
    The climate change default has been set to 40% (up from 30%) based on the EA guidance first issued in February 2016. A link to this guidance can be found here.
  • FEH / FSR default
    This default remains unchanged. It should be noted that rainfall information has changed with the issue of FEH13 in November 2015. A facility has been provided to enter a value of FEH13 rainfall depth for the 100 year 12 hour event to produce a revised estimate of this factor. 

Send us your feedback

We are keen to receive feedback on these changes, and particularly if there are any concerns over the results obtained. Feedback on the storage volumes predicted by the tool compared to values obtained using detailed simulation modelling would be particularly appreciated. Similarly points requiring further FAQs or clarification on existing FAQs are also welcome.

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