This site has been developed to support planners and developers to obtain site-specific guidance in terms of:

Tools for assisting in the outline design of SUDS, in particular:

• Storage requirements for drainage systems
  
  
• The drainage issues and SUDS components which are likely to be most appropriate.
  
  

Tools for assisting with detailed design aspects of drainage, and also the evaluation of the hydraulic and water quality performance of drainage systems:

• Assistance with assessing joint probability drainage issues
  
  
• Assistance in designing soakaway systems
  
  
• Evaluating the hydraulic performance drainage systems
  
  
• Evaluating the water quality treatment effectiveness of drainage systems
  
  

The availability of the storage assistance and guidance in drainage design is important for the following reasons:

1. Storage provision for drainage schemes is needed in virtually every development. The cost of drainage is largely a function of the storage requirements and this tool enables this to be estimated quickly and freely without the need to resort to experts and the use of drainage software.
   
   
2. There has been great variability in the design criteria for drainage schemes requested by the Environment Agency and local authorities. These tools allow all parties to use an objective and consistent output to initiated discussions and submit planning applications.
   
   
3. There is general understanding in the industry that SUDS are a good idea. However detailed guidance on their application for specific sites is very welcome as many developers and some local authorities are still unfamiliar with the use of SUDS.
   
   

An equivalent tool for use in Ireland can be found at www.irishsuds.com.

The storage tool
The storage tool is based on work initially carried out for the Environment Agency to provide a simple methodology for estimating storage requirements for drainage systems. The methodology is available in the Defra / Environment Agency document “Preliminary rainfall runoff management for developments” (2005), W5-074/A/TR1/1 rev. D. This was subsequently simplified to produce a spreadsheet tool for internal Environment Agency use.

The current national procedure for estimating rainfall and river flow in the UK is the Flood Estimation Handbook (FEH). This superceeded FSR when it came out in 1999. However FEH is a digitally based methodology and has unique data values for a number of parameters associated with each square kilometre across the country. FSR was not a digital system and rainfall is a simple function of two principal parameters, M560 and Rainfall Ratio “r”. This tool therefore is based on FSR but also includes a correction factor (which is location specific) to take account of the difference between FSR and FEH. As with the rest of the tool, approximations and assumptions are being made in the use of this factor (which is automated, but can be over-ridden), but it does ensure that the tool is not considered to be obsolete in applying FSR rainfall parameters.

More detailed information on elements of the tool is available when using the tool and in the glossary. In addition the criteria in the current version of the Defra / Environment Agency guidance document “Preliminary rainfall runoff management for developments” (2005), W5-074/A/TR1/1 rev. D is available here.

A link to the CIRIA SUDS manual is provided here. Chapters 3 and 4 cover issues relating to criteria for design of drainage systems.

The site specific guidance
This tool aims to provide a bespoke report which provides initial guidance on the drainage design in terms of the use of the appropriate use of SUDS for the development site. More information on best SUDS practice is available from a number of sources, principally the CIRIA SUDS Manual (2007) and Use of SUDS in High Density Developments, HR Wallingford, (2005).

This tool takes into account:

• The type of development,
  
• The ownership of the system,
  
• The country in which the development is taking place,
  
• The size of the development,
  
• The soil type,
  
• Whether the land is contaminated,
  
• The groundwater level and risk to the aquifer,
  
• Whether it is greenfield or brownfield development,
  
• The topographic location (floodplain etc),
  
• Whether it is in a particularly water scarce area.
  

Detailed design tools
Various spreadsheet tools have been developed to assist with detailed design of aspects of the drainage system. These tools consist of:

• Joint probability tool
  This tool aims at addressing the problem associated with the dependency between urban runoff events and that of the catchment in which it is located. Where a SUDS system has a pond or basin located in or close to the floodplain has to take into account potential high river levels when extreme events take place on the site. The critical duration event for the drainage system is often fairly similar to that of the catchment (between 6 and 24 hours or more). Clearly the event dependency is quite likely to be high and if the discharge from the site is constrained by high river levels, then careful consideration of the hydraulic performance of drainage systems for extreme events needs to be made. A proper analysis on dependency needs examination of long records and simulation of the drainage systems, but this tool can provide some indication of return periods where dependency between events needs to be taken into account. Guidance is provided on the process of deriving dependency functions and typical dependency values are suggested.
  
  
• Infiltration tool
  A spreadsheet tool is provided that enables the sizing of infiltration soakaways or infiltration trenches for any location in the UK. This tool is based on the method provided in CIRIA report 156.
  
  
• Evaluating the hydraulic performance of drainage systems
  Forming part of the Environmental Assessment toolkit, this tool is aimed at providing a sustainability measure of the hydraulic performance of the drainage system. The performance indicators are based on a comparison between pre and post development response of a site to rainfall with the assumption that post-development runoff should replicate as closely as possible the pre-development state.
  
  5 measures are used based on hydraulic model outputs from time series rainfall:
  
  
  1. Peak flow rate for extreme events
     
  2. Peak flow rate for frequent events
     
  3. Volume of runoff for extreme events
     
  4. Volume of runoff for frequent events
     
  5. Annual volume of infiltration
  
  
  Peak flow rates and runoff volumes are determined from a ranking of all events.
  
  Annual volume of infiltration is determined from a continuous rainfall series.
  
  Both the pre-development and post-development states of the site are modelled using standard drainage modelling tools. In the UK this is most likely to be InfoWorks CS or Micro Drainage.
  
  It should be noted that the accuracy of greenfield runoff, particularly for relatively small events is likely to be very approximate. Accepting this limitation, the tool does provide an objective basis for evaluating the hydraulic performance of a drainage system in terms of its complete hydraulic performance and not just in terms of its level of service.



• Evaluating the water quality performance of drainage systems
  Forming the second part of the Environmental Assessment toolkit, this tool is aimed providing a qualitative assessment of the treatment effectiveness of the surface water drainage system. The performance indicator score is a function of site area, percentage of impervious area, land use types, drainage outfall discharging boundary and the drainage treatment components.
  
  This tool is not based on comparing the runoff water quality with greenfield conditions, nor does it attempt to compute pollutant concentrations. It assumes that runoff is only contaminated from paved surfaces. Runoff to infiltration is excluded from this calculation.

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