Minimum Information

Minimum information required to prove the existence of an existing connection to the public sewer

Any new connection of surface water from re-development areas to the existing public sewer has the potential to increase flood or pollution risk to the existing network and cause disturbance to our existing customers. As such any proposed connection to the existing network should be accompanied by evidence to prove the existing surface drainage arrangements, together with hydraulic assessment of the impact of the resultant flows for at least the 1:30 year return period +climate change. Referred to as a DIA, this should be able to confirm that there would be no detriment to the existing operation of the network and where appropriate, the DIA should demonstrate an improvement in the network operation.

In addition to the above, surface water drainage designs are in most cases narrowly focussed, only considering drainage design within the development red line boundary. Hence, unless the development is part of a wider catchment or strategic development, the receiving public sewer network will not have been designed to accommodate the additional flow. A DIA is therefore required in all circumstances to prove there is capacity for the new flows from currently undrained areas. No consideration will be made for unrestricted discharge as this will not be in line with the current design standards.

The following is the minimum information required to prove an existing connection to the public sewer network:

1. Full site survey of existing pipes/chambers including at the point of connection to the public sewer
2. Existing impermeable areas (roofs and hardstanding) and how these are drained into the pipes on site, i.e. gullies, channel drains and downpipes
3. CCTV survey proving 1 and 2, and the status of the existing onsite network
Provision of the above does not discount the need to follow our current surface water drainage policy. In all cases any approval to connect to the public sewers will be accompanied by a DIA and the intrinsic requirement not to increase flood risk to the existing network. The DIA takes account of the flows within the network for the upstream catchment to the connection point and the impact on the downstream network.

The following information will not be accepted as evidence to prove adherence to the surface water hierarchy:

• Use of generic desktop soil maps. Surface water hierarchy evidence should be based on site specific investigations
• Evidence of clay soils does not imply SuDS cannot be used. SuDS does not require infiltration throughout. Different solutions can be applied on any site which will not necessarily take up a lot of space but provide wider benefits.
• Evidence of made ground or contamination. SuDS can be shallow and when used cumulatively can provide wider benefits including retaining the first 5mm of rainfall within the site

Minimum information required as evidence to request connection of non-public flows to the public sewer

Anglian Water like any other Water and Sewerage Companies (WaSC) are responsible for the adoption and maintenance of public sewers and lateral drains. Public sewers with respect to surface water drainage are defined as sewers that drain roof and appurtenant yards. Flows from areas such as footpaths and highways are classified as highway runoff and are the responsibility of highway authorities. Similarly, flows from greenfield areas or gardens is classified as land drainage is usually the responsibility of the landowner or Internal Drainage Board where one exists.

WaSC are identified as a Risk Management Authority (RMA) within the Flood and Water Management Act (2010). The FMWA sets out the duties and responsibilities of RMAs in managing flood risk within developments and wider catchments. Among their duties an RMA has to work collaboratively with other statutory bodies in managing surface water flood risk.

Thus Anglian Water works collaboratively with Highway Authorities and LLFA in managing surface water flooding. Some key areas involve highway drainage improvement works and surface water flood alleviation schemes by LLFA. In both cases, the flows from these areas are classified as non-public flows and Anglian Water are not duty-bound to accommodate such flows within the public surface water network. By law the surface water network is not meant to accommodate non-public flows.

However, in the wider duties of working collaboratively to manage surface water flood risk, Anglian Water does cooperate with other RMAs to facilitate and provide solutions that have wider benefit to the overall surface water management within catchments. It is therefore incumbent on us to set out the evidence required when such non-public flows require a connection to the public network.

The following is the minimum information required to prove the need and wider benefits resultant from a connection of non-public flows to existing public sewer network:

• Technical Drainage report outlining the current wider surface water flooding problem within the area or catchment.
• The Technical Drainage report should highlight the before and after scenarios for the following critical design events; 1-in-1, 1-in-5, 1-in-10, 1-in-30 year. 1-in-30 year is the critical design for stormwater network where no flooding should exist.
• The wider benefits of the proposals to the existing public sewer network, including social benefits to the wider community.
• For highway improvements; consideration of SuDS solutions will be key to accommodate any new highway drainage into the public network.

Further information on the use of ReFH2 for hydraulic analysis

The ReFH2 is a design rainfall-runoff methodology for producing rainfall hyetographs of different return periods with corresponding design flood or hydrograph. It is commonly used for small catchments of up to 0.5km2, on small sites to inform pluvial flood risk and drainage design. The model can be used for both greenfield and brownfield sites.

ReFH2 uses catchment descriptors to describe the climate, drainage characteristics and soils for the site together with rainfall model data to produce a rainfall-runoff model. It can be used in ungauged catchments and since this is a design application, it can be used with confidence to model the impact of new flows to an existing catchment including the downstream network for different return periods associated with the new design.

It is important to note that there is peak flow as well as well as design rainfall intensity for each return period.

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