s104 suds guidance document

S104 SuDS Guidance Document Ponds & Wetlands (This document should be read in conjunction with S104 SuDS Technical appraisal form for Ponds & Wetlands)

Copyright © United Utilities Water Limited 2021 1

Comment no. Technical Guidance

High level considerations, location & layout

1 Wetlands can be considered for adoption where they form part of a wider SuD system, but only where their primary function is not to deliver a defined water quality standard. Wetlands are bodies of water with larger proportions of the surface area covered by aquatic planting. They also tend to have greater depth variations and may include shallow islands.

2 Maintenance considerations must include any specific features that are likely to pose difficulties and any associated mitigation measures that have been put in place – see chapter 32 of CIRIA C753 for guidance

3 Flood risk to exiting features: No surrounding properties or features should be at risk – see chapter 36 of CIRIA C753 for guidance

4 Topography: Steep sites can result in increased velocities resulting in risks to scouring, erosion, resuspension of pollutants and health & safety – see chapter 8.4 of CIRIA C753 for guidance. It may be difficult to site a pond on steeply sloping sites and ponds should not be sited on unstable ground. Ground stability should be verified by assessing site soil and groundwater conditions. Ponds should not be built on waste-fill materials, uncontrolled fill or non-engineered fill - see CIRIA C753 23.3

5 Shape: Ponds shall appear, in so far as is reasonably practicable, to be natural. They should not be constructed in symmetrical shapes, have any angular corners and the gradient shall be varied (for example a single gradient around the perimeter would not be acceptable) Ponds should be designed so that flow enters the pond and gradually spreads out, avoiding the creation of dead zones caused by corners. The inlet and outlet are to be located so that the flow path is maximised through the SuDS component to optimise the sedimentation process. Baffles, pond shaping and islands can be added within the permanent pool to increase the flow path length and maximise water quality treatment effectiveness.

Design requirements

6 The side slopes confirmed to be no greater than 1 in 3 between the aquatic bench and safety bench and no greater than 1 in 4 where mowing access is required. A 3.5m safety bench must also be provided with a slope of less than 1:15 although this will depend on land availability, designated access and the type of maintenance equipment required for the pond.

7 Permanent water depth: The top water level for the permanent pool is not related to a particular storm event and should be at the invert level of the outfall structure. The maximum permanent water depth should be between 0.6 and 1.2m, this is to avoid stratification and anoxic conditions.

8 Temporary storage depth: Where wetlands are used for flow attenuation, the maximum depth of temporary storage above the permanent pool should be limited; 0.5m is usually acceptable for small/medium sized ponds so that the risk to plant damage is low. The critical duration is the length of rainfall event that results in the greatest flow rate, flood volume or flood level. Return periods from 1 in 1yr to 1 in 100yr and durations from 15 minutes to 48hrs are often required to be assessed – see CIRIA C753 chapter 24.2.3 for further guidance.

9 Flow control diameter: This must be 100mm when open green SuDS components are utilised within the design

Ponds & wetlands


10 Pre-treatment / Sediment control: All ponds shall be designed with suitable upstream pre-treatment systems to reduce the risk of rapid silt accumulation. Where there is no upstream pre-treatment (i.e. source control components to deal with sediment), this shall be in the form of a sediment forebay with consideration to access for plant and personnel. For planted ponds, further considerations will be required regarding maintenance access (i.e. sediment removal) to avoid damage to any planting. The plan area of the forebay must be at least 10% of the total pond area and can either be a separate basin or formed by building an earth berm, stone or rock filled gabion or rip-rap across the upstream portion of the pond. Each forebay should be accessible and easily maintained and a fixed sediment depth marker is recommended to monitor silt levels over time. Where it is not feasible for SuDS components to be installed to provide pre-treatment, a catch-pit may be used. Suitable access shall be provided to the catch-pit to allow for sediment removal and cleaning (multiple catch pits will not be accepted for maintenance reasons). For systems with multiple inlets, pre-treatment should be provided at each inlet that is likely to contribute to a significant sediment load. Consideration can be given to a concrete lined forebay as this would also protect from erosion and can facilitate desilting (without damage to a liner for example). See chapters 23 & 28.4.8 of CIRIA C753 for further guidance.

11 Representing component for hydraulic calculations: For conveyance it must be represented as part of the online piped network. If labelled as a storage component then it must be explained how this is linked in the model as part of the online network.

12 Discharge velocities must be acceptable and the designer should provide suitable flow spreading and energy dissipation to prevent disturbance to the permanent pool volume – see CIRIA C753 23.8.1 for guidance. Any erosion protection must be reflected in the hydraulic model by applying a headloss of 0.5 at the point of outfall. The same headloss value of 0.5 needs to be applied for catch pits where applicable.

13 Exceedance flow velocities: No surrounding properties or features at risk. See chapter 36 (table 36.1) of CIRIA C753 for guidance on acceptable velocities for exceedance

14 Lining considerations: Ponds will always be lined unless constructed in an area of clay soils that have been confirmed to be suitable to retain the water. Below is a list of information/considerations required when lining a pond;

Specification of chosen liner required. Acceptable materials can range from butyl rubber, bentonite sheeting or clay - see CIRIA C753 section 23.9.1

Benching should be considered to ensure that layers of saturated sub-soils do not slip off the liners in very wet conditions

If a clay liner is to be used, the thickness of clay should be 0.5m-1.0m to ensure complete impermeability If an impermeable membrane is proposed, cover must be provided and noted to protect it during

desilting / maintenance works If a liner is used in high groundwater scenarios, there is a risk that it could float, this needs to be

considered. Base levels of components below groundwater levels must be avoided as achieving a reliable seal with an impermeable liner can be difficult. It must be noted that a >5% in liner leakage test compromises operation. For further guidance see chapter 8.3 of CIRIA C753.

For all brownfield sites, unlined ponds or wetlands should not be used unless it has been clearly demonstrated that there is no risk of groundwater pollution.

15 Materials: This drawing must confirm the materials that are proposed. Whilst there are no unique materials that are used in detention basins, typically you may expect the construction to involve subsoil, topsoil, geotextile, geomembrane and aggregate. See CIRIA C753 chapters 23 (Ponds & Wetlands), 30 (Materials including specification for liners – see 30.5.4) and 31 (Construction) along with chapters 21 (Soils) & 22 (Materials), of CIRIA C768 for further guidance.

Ponds & wetlands


Ponds & wetlands


Construction Construction of Ponds & Wetlands should be in accordance with the guidance provided in the appropriate chapters of CIRIA C753 (23.11) and CIRIA C768 chapter 36 along with CIRIA C753 Appendix B: Construction assessment checklist. Below are a few visual images extracted from CIRIA C768 detailing correct behaviours/considerations as well as what can also go wrong;

Ponds & wetlands


s104 suds guidance document

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