• Issue 36

  • May 2018

The Source


Deliverable Surface Water Drainage Solutions Are Key To Robust FRAs

There is a strong media, and therefore public, perception that delays in the commencement of housing construction following the granting of planning consent are due to developers’ land banking.  In our experience however this is due to the time it takes to discharge planning conditions and, with respect to our areas of expertise, those related to flood risk and surface water drainage.

Having reviewed numerous Flood Risk Assessments (FRAs), Lead Local Flood Authority (LLFA) consultation responses and resultant planning conditions it very much appears that FRAs are routinely viewed by all parties as a box ticking exercise with minimal consideration being given to the deliverability of surface water drainage proposals which of course apply to all sites, whether in Zones 1, 2 or 3.

Factors which have contributed to this situation are pressures on the LPAs and LLFAs, lack of clear national guidance and, fundamentally, a solely hydrology based approach to many FRAs without the appropriate attention being paid to the development drainage proposals and the identification of deliverable surface water drainage solutions.  Little or no thought is often given to the drainage and attenuation methods which are routinely covered by a generic list of ‘gold standard’ SUDS measures. This box ticking approach is then carried through to the planning consultation process and can typically be reflected in a similarly generic list of conditions with absolutely no reference to the FRA.

As infrastructure designers to the development industry we therefore regularly find ourselves required to discharge drainage conditions armed with an inherited FRA identifying in broad terms only a drainage solution which in a number of cases has proven undeliverable.

We are of the firm view that the consultancy industry needs to address these deficiencies for there to be any hope of an improvement within the planning system.  After all, the quality of the LLFA response will very much be dictated by the quality of the submitted FRA!

Experience in the provision of detailed drainage and level designs for developers and sound engineering common sense supported, as required, by hydraulic modelling expertise are therefore key skills for the preparation of FRAs for residential schemes.

The sources of flood risk related to a development site can be categorised as;

  • Risk from external sources
  • Risk from development drainage proposals

Continuing the theme above we set out below our approach to development drainage proposals, often the sole source of risk on Zone 1 sites.

The fundamental requirement is to ensure that the development does not result in an increase in runoff rates and hence an increased flood risk offsite.  What seems to go unrecognised is that this is an approach that has been adopted by the development industry for some 20 years. Initially surface water was attenuated on site within pipe systems followed by the increasing introduction of other techniques such as above ground basins, with infiltration to ground as a preference being a constant throughout.

For greenfield sites, run off rates are calculated via industry standard software and identify the Q1 (1 in 1 year), Qbar (approx. 1 in 2.3 year), Q30 (1 in 30 year) and Q100 (1 in 100 year) rates based on the developable area.  There is an aspiration to assess brownfield sites on a greenfield basis but we believe that where connectivity of existing drainage systems to outfall can robustly be established there is a clear case for increasing proposed rates above greenfield values.

The NPPF requires that surface water be held on site for all events up to the 1 in 100 year event plus allowance for climate change.  If we are to attenuate up to this standard then we should be entitled to discharge up to the 1 in 100 year flow rate and this is recognised in the March 2015 Defra non-statutory technical standards for sustainable drainage systems and long established Environment Agency guidance.

We often review FRAs which propose to limit flows to the Qbar rate, with a resultant significant increase in attenuation and development costs, particularly if attenuation is to be provided in a pipe or tank system although in this regard we of course recognise there may be circumstances where a reduced rate is appropriate in the context of a specific scheme. As a result, many FRAs commit to the provision of attenuation volumes which are disproportionate to the development proposals and which as a result compromise the number of houses which can be delivered.

As noted, the greenfield runoff rate assessment is based on the total developable area i.e. that runoff which would have been generated on a greenfield basis by the area to be occupied by the development.  In the developed form, rainfall onto roads, car parks, drives and houses will be substantially directed to the surface water system whereas that falling onto garden and POS areas contained within the development area can be considered as held onsite.  However, we often see FRAs proposing to reduce the greenfield runoff rates calculated above to apply only to the impermeable area. As a result, greenfield runoff rates are typically halved with an increase in the volume of attenuation required.  This approach again compromises the development proposals.

Once the greenfield runoff rates have been established the required attenuation volumes can be identified based on the developable area.  It does appear to us however that it is considered by many that having established the rates, an outfall location and an attenuation volume supported by no more than a schematic drawing then the FRA work is complete. Often no regard is paid to level constraints, achievability of the required attenuation, adoptability of the drainage system by the relevant Sewerage Undertaker and overall deliverability.  In one case we identified that the surface water drainage solution proposed in an approved FRA could not be delivered due to the requirements to significantly raise levels. As a result the outline consent was potentially compromised.

The work required here should apply equally in support of an outline application as that for a detailed application since clearly deliverability needs to be established from the outset. Any other approach can only serve to undermine the planning process.

In conclusion it is our view that an FRA should clearly identify the method of drainage and attenuation underpinned by a robust assessment as to deliverability, and adoptability and all of course meeting the requirements of the NPPF. We accept of course that pressure will remain on the planning system at least for the foreseeable future and that generic conditions will continue to be imposed.  We do believe however that FRAs prepared to the appropriate standard will provide a sound platform for taking forward consultations and approvals with the LLFA and for ultimately improving timescales for the discharge of planning conditions and commencement of construction on site.

Director – John E Lees BSc CEng MICE MCIWEM

Lees Roxburgh