Top five SUDS issues raised by FSA’s following BAU site inspections

Scottish Water has identified various issues during ‘construction’ inspections on SUDS type assets.

In order to streamline this process, Scottish Water has detailed the top five current issues and offers this as a guide to developers to assist with the construction of SUDS type assets, eradicating construction issues that would hinder the vesting of that asset. 

On all occasions, the developer shall be wholly responsible for the design and construction of sewerage infrastructure, including SUD systems, to serve the proposed development.

The developer and/or his designer shall certify that their design complies with this specification and accept liability for compliance through their professional indemnity insurance. These responsibilities/liabilities shall not be discharged to Scottish Water following a satisfactory audit of their design and all elements of the design will comply with Sewers for Scotland v4.0.

Scottish Water hopes that by detailing the issues below, in future, they will be minimised for all involved.

If you have any further queries, please contact Richard Binnie, Scottish Water Vesting FSA Team Leader at Richard.Binnie@SCOTTISHWATER.CO.UK who will be happy to support and advise.

1. Wrong type of liner specified and incorrectly installed in the construction of a pond or basin. During the construction of a pond the membrane liner shall be at least 0.75 mm thick, under laid by 50mm of sand to prevent damage. The liner shall have a slope of 2% towards the drain down outlet. The liner material shall be a single layer of butyl compound or polythene. If polythene is used, it shall in addition be protected by a geotextile fabric. A design risk assessment shall be submitted to seek Scottish Water’s approval where the designer determines a liner is not required. Where detention basin liners are to be used in areas of high groundwater, the basin shall have a base level 500mm above the maximum groundwater level, the liner being installed in such instances to prevent infiltration of groundwater through the base of the basin. The use of a basin liner should be carefully considered in all instances.
2. Pre cast concrete headwalls set too high at the outlet. If the design of the inlet pre cast concrete headwall is replicated at the outlet with an area of flow dissipation rising to the outlet invert this in effect creates a dam rather than an outlet preventing the basin from emptying and allows amounts of stagnant water to build up. Although pre cast concrete headwalls are accepted it is imperative that the design, construction and operation of these headwalls allow the basin to empty.
3. Precast concrete headwalls cast with significant slopes. This prevents a slope of 1:4 being created around the entire perimeter of the asset which will create difficulties with operation and maintenance procedures in future ownership. Although pre cast concrete headwalls are accepted it is imperative that the design and construction of these headwalls allows for safe operation and access.
4. Inlet and outlet too close together. Depending on the design this may leave a large part of the basin potentially unused reducing catchment opportunity. The alignment of the inlet and outlet shall maximise flow detention times and minimise the risk of flows short circuiting and outlets shall be located as far from the inlet as reasonably practicable.
5. Lack of access to outfall for inspection and future access. The outfall is often located some distance from the asset and across land with significant vegetation growth, fields with grazing animals or where arable planting may occur and the slope of the water course is often not accessible. Consideration must be given to access all points of the outfall pipe and headwall for maintenance purposes in the future including safe access to any flap valve at the outlet. This may include burying manholes. If there is a requirement to bury manholes where ploughing operations are likely to take place these shall be constructed in accordance with Scottish Water’s requirements. They shall only be allowed where indicated on an approved layout/longitudinal section and shall be located using marker/indicator posts at boundary fences or similar positions, giving the distance to the buried manhole. Where Scottish Water has agreed for manholes to be buried, covers shall be set 600mm below ground level and suitably covered and taped with waterproof material.