Somerset local SuDS design standards

Water quantity

Discharge destination:

L1: Discharge must be prioritised according to the following discharge hierarchy:

1. Rainwater re-use and recycling
2. Shallow infiltration
3. Discharge to surface water body
4. Discharge to surface water sewer / highway drainage
5. Discharge to combined sewer network

Guidance notes:

• This standard strengthens the position of the West of England SuDS Developer Guidance standards (Runoff destinations) within Somerset. 
• Inclusion of rainwater re-use and recycling features in the design (e.g. use of water butts or rainwater harvesting systems) will be viewed favourably as they meet wider LPA policies on reducing potable water demand.   
• For rainwater harvesting systems to be accepted in storage volume calculations, it must be demonstrated that they will have surplus storage capacity under wet conditions. If not, they should not be included in the calculations.  
• ‘Shallow infiltration’ includes shallow infiltration from surface SuDS features such as swales, basins and raingardens; shallow soakaways and permeable paving.  Deep bore soakaways are not an acceptable infiltration SuDS technique.  They do not represent a natural drainage process, and do not deliver the multiple benefits of SuDS.  The LLFA will strongly resist their use, and seek advice from the Environment Agency if they are proposed. 
• BRE365 infiltration testing must be undertaken to provide evidence that there is capacity for infiltration SuDS at full application stage.  If infiltration testing has not been undertaken at outline stage or has not met the BRE365 standard for whatever reason, we expect proof of concept to be demonstrated for an alternative drainage proposal.  Where infiltration is proposed, an assessment of the contamination risk should be made (and evidence of consent from Environment Agency, where required). 
• Written agreement in principle from the relevant authority to make a connection to a watercourse/ surface water body/highway drainage/surface water sewer network should be provided. This should include systems under other authorities remit.  
• If a drainage scheme needs to cross adjoining land to discharge, and that land is not owned by the developer, it will be a requirement that the developer negotiates and secures the necessary written agreements directly with that landowner and any other parties. Evidence will need to be submitted to the LPA.  
• Evidence that the receiving waterbody or system has the necessary capacity and is in appropriate condition to accept discharge.  
• Confirmation of consultation with the Somerset Consortium of Drainage Boards, where discharge to a watercourse is proposed within, or draining into, an IDB area. 
• For more information, see CIRIA SuDS Manual Chapter 3.

 

Runoff rates and volumes

L2: For all developments, the peak runoff rate from the development to any surface water body, highway drain or sewer for the 1 in 1-year, 1 in 30-year and 1 in 100-year rainfall event must never exceed the peak greenfield runoff rate for the same event. In some cases, it may be necessary to restrict rates further depending on local requirements.  

Guidance notes: 

• This standard strengthens the position of the national Non-Statutory Technical Standards for SuDS (S2, S3), and the West of England SuDS Developer Guidance standards (Peak flow control) within Somerset. 
• If it is not possible to meet this standard on a previously developed site, the LLFA consider this an exception and a detailed justification statement must be included in the sustainable drainage strategy which clearly describes the improvements that will be delivered both on runoff rates and the overall quality and benefits of the scheme.   
• Any alternative proposal will be expected to demonstrate the maximum achievable betterment on the existing runoff rate to be agreed in advance with the LLFA.  A minimum starting point is considered to be 50% but the LLFA reserves the right to insist on a greater betterment in runoff rates where there are known downstream flood issues.  The LLFA will also favour betterment of the SuDS scheme quality e.g. reduction in impermeable area, remediation of the site and management of smaller events, improvements in water quality and biodiversity. 
• For developments within the River Tone catchment, the maximum allowable runoff in any event is 2 l/s/ha for all events (or the 1 in 1-year greenfield runoff rate, whichever is lower). 
• Use of existing drainage system on a previously developed site will require CCTV survey to prove continuity and integrity of the system.  Designs should be supported by attenuation / pipe full capacity calculations or a hydraulic model.  
• Any methodologies used for the calculation of runoff rates and volumes should be justified, and the LLFA reserves the right to request that other methods are used where it deems appropriate. Use of Flood Estimation Handbook (FEH) methods are preferred but IH124 is considered acceptable.  
• It is understood that some guidance recommends minimum discharge rates of 5 l/s, to minimise use of small orifice openings that could be at risk of blockages. However, Somerset’s local standards require a minimum two stage treatment train incorporating filtration features to remove suspended matter (see L11), and suitable maintenance regimes, which should minimise this risk.  Therefore, the minimum limit of 5l/s does not apply in Somerset. 
• If a development proposes to discharge to an estuary or to the sea, the LLFA may consider a free discharge, providing that it can be demonstrated that this will not have a detrimental impact on flood risk and the scheme meets the requirements for water quality. This does not negate the need to provide multi-benefit SuDS.   
• For more information, see CIRIA SuDS Manual Chapter 24. 

L3: For all developments, the runoff volume from the development to any surface water body, highway drain or sewer in the 1 in 100-year, 6 hour rainfall event should never exceed the greenfield runoff volume for the same event.

Guidance notes:

• These standards strengthen the position of the national Non-Statutory Technical Standards for SuDS (S4, S5, S6), and the West of England SuDS Developer Guidance standards (Volume control) within Somerset. 
• The sustainable drainage strategy should demonstrate clearly that the volume generated in addition to the 100-year, 6-hour greenfield volume will be stored or infiltrated on site. 
• Any additional volume generated that has to be discharged (i.e. cannot be infiltrated) should be released at a low discharge rate (2 l/s/ha or the 1 in 1-year greenfield runoff rate) (long-term storage).  Alternatively, all runoff above the 1-year event can be released at this low discharge rate.  Further detail on storage volume estimation and design can be found in CIRIA SuDS Manual Chapter 24.    
• If it is not possible to meet this standard on a greenfield or previously developed site, the LLFA consider this an exception and a detailed justification statement must be included in the sustainable drainage strategy.  Any alternative proposal will be expected to demonstrate the maximum achievable betterment on the existing runoff volume to be agreed in advance with the LLFA, in addition to delivering a high-quality sustainable drainage scheme with multi-functional benefits. 
• For more information, see CIRIA SuDS Manual Chapter 24. 

 

L4: Attenuation storage must be designed with a sufficient drain-down time (to half-empty within 24 hours) following a storm event to allow for it to receive runoff from subsequent events.  

Guidance notes: 

• The sustainable drainage strategy should demonstrate clearly that the outlet control will allow attenuation storage features to drain-down in the required time. 
• For more information, see CIRIA SuDS Manual Chapter 24. 

 

L5: For all developments, post-development runoff rates and volume calculations must take into account future climate change and urban creep 

Guidance notes: 

• This standard strengthens the position of the West of England SuDS Developer Guidance standards (Climate change and urban creep) within Somerset. 
• The sustainable drainage strategy should explicitly show that appropriate climate change and urban creep allowances have been applied in runoff and volume calculations. 
• Climate change allowances must be in line with the latest Environment Agency climate change allowances for the relevant management catchment for the site:  
  • Across the majority of Somerset, the 'Upper End' climate change allowance should be applied.  
  • Within the Internal Drainage Board area (IDB), this must be designed to the 'Central' allowance, and test the sensitivity of the 'Upper End' climate change allowance.  
• All post-development runoff and volume calculations must allow for urban creep as per the table below (from LASOO non-statutory technical standards): 

Residential development density (dwellings per hectare)	Urban creep change allowance (% of impermeable area)
≤25	10
30	8
35	6
45	4
≥50	2
Flats and apartments	0

• For more information, see CIRIA SuDS Manual Chapter 10. 

 

Flood risk within the development:

L6: The drainage system must be designed so that, unless an area is designated to hold and/or convey water as part of the design, flooding does not occur on any part of the site for a 1 in 30-year rainfall event.

L7: The drainage system must be designed so that, unless an area is designated to hold and/or convey water as part of the design, any flooding within a 1 in 100-year plus climate change rainfall event is retained within the site boundary, and no flooding occurs in any part of: a building (including a basement); or in any utility plant susceptible to water (e.g. pumping station or electricity substation) within the development.

L8: Flows resulting from rainfall in excess of a 1 in 100 year plus climate change rainfall event, OR from overtopping or failure of a SuDS feature, must be managed in designated exceedance routes that minimise the risks to people and property.

Guidance notes: 

• These standards strengthen the position of the national Non-Statutory Technical Standards for SuDS (S7, S8, S9), and the West of England SuDS Developer Guidance standards (Flood risk within the development) within Somerset.   
• If it is not possible to meet these standards, we consider this an exception and a detailed justification statement must be included within the sustainable drainage strategy, outlining the reason why the standard cannot be met, and how the flood risk will be mitigated.   
• Where flows exceeding the drainage system are managed on site, a sustainable drainage strategy must explain where the water will flow/be stored, and how it makes its way back into the drainage network within the site. This should be demonstrated to an appropriate level of detail for the type of application:  
• Outline – statement and map showing flow routes 
• Full/Detailed – map indicating key ground levels and flow routes, design cross sections and depths of exceedance storage areas, modelling of flow routes. 
• Design must demonstrate that roads, car parks and public spaces will be used in a multi-functional way to manage exceedance flows safely. 
• Designed must identify and mitigate the risk of a surcharged / tide locked outfall on the drainage system.  
• All attenuation structures must be located outside of the area at risk from flooding in the 1 in 100-year plus climate change. 
• All flow control devices restricting the rate of flow should have a bypass feature to manage flows when a blockage occurs. 
• 1D or 2D modelling of flood risk may be required for ordinary watercourses within or adjacent to new developments.   
• For more information, see CIRIA SuDS Manual Chapter 24. 

 

L9: A minimum of 1.0m buffer must be ensured between the base of any infiltration SuDS component and the groundwater table.

Guidance notes: 

• This standard supports the Somerset Drainage Boards Consortium Surface Water Drainage Guidance – Infiltration Techniques and formalises advice from the Environment Agency. 
• If groundwater rises to the base of infiltration SuDS features such as soakaways, they will not operate correctly and there is a risk of flooding to the site.  
• An estimated groundwater table level at the site should be determined by trial pit or borehole investigations as close as possible to the location of the infiltration SuDS feature at full application stage.  Groundwater level should be monitored daily for at least one week between mid-November and mid-April, and the highest value taken.  If ground investigations have not been undertaken at outline stage, we expect proof of concept to be demonstrated for an alternative drainage proposal.  
• For more information, see CIRIA SuDS Manual Chapter 13.                                                                                                                                        Photo credit: The UK Groundwater Forum

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Water quality ⟹