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Drainage considerations in site development - SuDS

Looking to develop a new or existing site brings numerous requirements to identify and mitigate against unforeseen risks which, if dealt with from the outset, can be accommodated within or even be beneficial to the overall value of the opportunity.

Flooding is most commonly seen as a consideration for the potential risk of damage to a site. However, it also drives the requirements set by Planning Authorities in conjunction with guidance from agencies such as Defra to manage how land use changes affect downstream flooding. Any change that can increase the immediacy and volume of water which leaves a site (green or brownfield) can carry negative implications in higher peaks of flow.

Understanding these impacts is a beneficial consideration generally but as of 6 April 2015, SuDS are a requirement in the planning process for all new developments in England for areas affected by flooding or within CDAs (Critical Drainage Areas) and for any major developments generally.

SuDS basin

Sustainable Drainage Systems (SuDS) offer a means to potentially mitigate the impact of development on handling rainfall by preferably encouraging water to be infiltrated to Ground Water on site or at least delay the release to surface water bodies or drains. There are however some considerations before choosing the most appropriate approach such as:

| flood risk - site susceptibility to surface or groundwater flooding which might impede drainage strategies
| topography - the topography of the ground influencing drainage direction for drainage and natural depressions
| contamination - ground contaminants could be given a pathway to reach surface water bodies or protected groundwater resources.
| ground stability - Improper loading of the ground could also bring ground stability issues both relating to slope stability or sub-surface cavities.
| permeability - Superficial and bedrock implications on permeability of the surface affecting absorption of water
| existing infrastructure - The presence of existing storm drain and other public infrastructure.

SuDS can bring potentially cost saving solutions such as handling the capacities and implication of discharge to storm drains and critically providing options where the development area within the site is below the level of the nearest established drainage systems. SuDS can often be delivered in a means that improve the landscaping and ecology of a site. Any ability to absorb surface water also has the benefit of preventing the discharge off site of many surface contaminants which are mobilised particularly in the first flush of rainfall.

Implementation of SuDS can draw on numerous methodologies from permeable paving and parking surfaces to swales, bio-retention features and infiltration basins. Often the solution is a hybrid of elements both SuDS based and releasing to local water bodies or even storm drains. Whilst most images of SuDS suggest ground surface based solutions, even a development entirely comprising of building can use a "blue roof" strategy to mitigate runoff and can do so at considerably lower cost than strategies such as basement tank construction.

At Centremaps, we're always keen to provide our clients with the information they need to most effectively qualify risks and the options to mitigate those and best realise potential opportunities. Working with GeoSmart to make available their data and reports fits well with our principles in relying on experts who carry out such work every day and can deliver the efficiencies associated along with close working relationships with and understanding of Local Authorities and their planning teams.


Introducing SuDSmart reports:

SuDSmart reports from GeoSmart are quick and simple to order alongside all your other site data information from www.centremapslive.co.uk.

The reports are designed to provide an initial assessment of the site potential bringing together valuable information. It is important to stress that there is unfortunately no instant complete solution to sign off planning authority requirements. Typically, the report would lead to a SuDS design by a landscape engineer or similarly qualified professional. Once a site design has been agreed, a maintenance plan will be required to demonstrate who will be responsible for the long term adoption of any implemented controls (i.e. owner/occupier/local council/utility).

GeoSmart have developed three report levels (SuDSmart standard, pro and plus) which go from unifying data advising the conditions predicted at a site through assessing the likely implication of the intended change of use with supporting calculations to proposing a strategy for implementing SuDS or otherwise.

In addition to having a team of specialist hydrologists with over 15 years experience and great working relationships with the Environment Agency and Local Authorities across the UK, GeoSmart have been able to deliver client savings in time and cost by developing in house datasets which are also available to licence.

 

The datasets behind SuDSmart: 

 

GeoSmart Datasets:

SuDS Infiltration Suitability and GroundWater Flood Risk, included in all SuDSmart reports and also available as spatial datasets:

The SuDS Infiltration Suitability Map (50m grid)

GeoSmart SD50 dataset - SuDS infiltration rates across Great Britain qualified to a 50m grid

Ranks areas as high, medium or low suitability dependent on various factors including slope, flood risk, groundwater susceptibility and superficial and bedrock geology.

The Groundwater Flood Risk Map (5m grid)

GeoSmart GW5 - groundwater floodrisk qualified across Great Britain to a 5m grid

Allows assessment of groundwater impact, at a level specific to the property. It advances insight to reveal risk of groundwater flooding rather than potential susceptibility.

 

Additional datasets included in all SuDSmart reports:

 

Site location

SuDSmart location map

A map and aerial imagery revealing the context of site

Site topography

SuDSmart site topography to identify slope and depressions

The slope of a site and any depressions and the site's height in relation to surrounding features.

Surface Water Features

SuDSmart surface water features

Picking up moving and static water bodies nearby

Risk of flooding from surface water

SuDSmart risk of flooding from surface water

Area of risk for surface (pluvial) flooding

Risk of flooding from rivers and sea

SuDSmart risk of flooding from rivers and sea

Areas of risk for flooding

Sewer Features

SuDSmart existing sewer features available surrounding the site

Preliminary records of all sewer features surrounding the site which may aid or be affected by changes in site run-off

Source Protection Zones

SuDSmart source protection zones

Any protected areas as delineated by the Environment Agency where water quality could be impacted by proposed or historic land use and contaminated land

 

The SuDSmart report levels

SuDSmart:

The standard SuDSmart report explains the findings of the above and also provides an executive summary of the SuDS suitability and proposes some next courses of action

SuDS suitability statement

SuDSmart sample (4MB)

 

SuDSmart Plus:

The SuDSmart Plus report adds to the standard report by taking into account storage, volume and peak flow rates. These are considered in absolute terms and in considering the implications of change in site use and any regulations and directives including those to accommodate climate change.

SuDSmart runoff and volume summary

The full calculations behind this table are include in the Appendix B, visible both in the Plus and Pro samples

SuDSmart Plus sample (4MB)

 

SuDSmart Pro:

Finally, the SuDSmart Pro report introduces additionally a strategy for the site including a potential SuDS scheme options layout.

 

A combination of SuDS features are proposed (see figure below), comprising swales, permeable paving, an infiltration basin and bioretention area to reduce surface runoff, and infiltrate 64 m3 (equivalent to the 1 in 100 year, 6 hour rainfall event, including 30% allowance for climate change). Parts of the site are at risk of fluvial flooding which may affect the SuDS design. The site has permeable underlying gravel but the potential for high groundwater levels at the southern end. If infiltration SuDS are unfeasible, attenuation storage features and a controlled discharge (<5 l/s) to the River Rea, 50m to the south, may be considered. Once mitigation measures are put in place, the development is considered extremely unlikely to increase flood risk.

 

Proposed potential site utilisation to meet runoff requirements through SuDS

SuDSmart Pro sample (4MB) 

 

Follow up recommendations:

Beyond the reports, site investigation will be required in order to confirm anticipated capacities for ground infiltration, surface water bodies to absorption of run-off and sewer suitability and condition.

Additional considerations: The final design capacity for an infiltration SuDS system depends on the Site constraints and the following assessments are required:

  • Site topography and drainage routes, confirmation of sufficient depth to the high water table, review of ground stability and minimum soakaway separation from adjacent buildings, confirmation that contaminated land will not constrain infiltration on the Site, flooding constraints in the vicinity of the Site, calculation of runoff rates and volumes and assessment of SuDS options and drainage strategy.
  • Environmental and ecological considerations: The Site is not located within a Special Protected Area (SPA) or a Site of Special Scientific Interest (SSSI).
  • CDM considerations: If your development is defined as ‘Construction Work’ under CDM 2015, you or the organisation that is having the work carried out will be defined as ‘the Client’ and have specific duties under the Regulations. A full list of CDM considerations and our Terms and Conditions can be found on our website, the links can be found in section 14 at the back of this report.
SuDS infiltration assessment

 

The full details of the GeoSmart range of products including SuDSmart, FloodSmart and EnviroSmart available from centremapslive can be found here.

There can be a lot to consider so please don't hesitate to get in touch and have an expert talk you through the options.

 

Useful reading:

 

Webinars:

Webinars are available, get in touch to express your interest