Surface Water Drainage Impact Assessment

Where a development requires a connection to the surface water sewer network there is a requirement to undertake a DIA. The purpose of the DIA is to establish whether the effect on the receiving network either presents no significant additional risk or presents a risk that can be effectively and realistically mitigated. Anglian Water will raise an objection in planning to any development proposing a connection for surface water when the impact on the sewer network has not been evaluated.

Regulation and governance

Surface water drainage management is primarily governed by the FWMA (2010) in which the LLFA have the sole responsibility to approve the onsite drainage strategy for all major developments as part of the planning permission. This approval extends to the red outline boundary of the development, with additional requirement for the drainage strategy not to cause flood risk elsewhere. It therefore means surface water drainage is often done on an incremental basis and not catchment wide down to outfall. The LLFA however, has to work in collaboration with other Risk Management Authorities (RMA) such as Water and Sewerage Companies, Environment Agency and Highway Authorities to manage flood risk within the wider catchment and river basins.

As such, unless the proposed drainage strategy is part of a wider strategic development, the abutting network to a new development might not have included the flows from the adjacent development, as this will have been approved separately by the LLFA.

Therefore, the developer is required to undertake such analysis as is necessary to devise a drainage strategy that poses no flooding risk to receiving networks within and beyond red line boundary of the development site.

Developer’s obligation

A DIA will be required to establish whether there is residual capacity in the receiving network to accommodate the additional flow and if not, the extent of network reinforcement required to accommodate the additional flow. As the developer is responsible for providing the appropriate surface water disposal infrastructure, work to determine the feasibility of a connection to a public sewer is considered to be part of the developer’s evaluation of surface water disposal options to a consented outfall.

In order to progress a DIA, the developer will need to engage with Anglian Water to establish an agreed methodology and parameters for the assessment.

DIA Process

The DIA process will commence once the developer has submitted a PPE. However; a DIA for a surface water connection could involve varying degrees of complexity depending on the size of development, the discharge rate required and the sensitivity of the proposed receiving network. The proportional change in the existing operation of the network will be a defining factor to establishing the appropriate methodology and complexity for the DIA.

Relatively small increases in predicted flow may be satisfactorily assessed using a desktop approach with some supporting manual calculations. More complex predicted impacts will require detailed analysis, up to and including hydraulic modelling.

While Anglian Water can undertake detailed analysis, our resources are limited and often committed on our investment programme. Therefore, we would encourage developers to utilise their own resources to undertake such detailed analysis that may be required, to minimise avoidable delay. Analysis completed by the developer can be submitted to Anglian Water for review and approval.

Site specific details will influence the selection of the assessment method, so a pre-application consultation with our DS Growth Planning team is recommended to ensure the correct information is submitted at PPE.

In general, the process would be expected to progress in the following steps:

i. Pre-application to confirm relevant information
ii. PPE to provide the developer with sewer network asset data, necessary to represent existing capacity
iii. Developer incorporates sewer network data into the SW design hydraulic calculations
iv. Developer submits hydraulic calculations to Anglian Water for checking and approval

If required….
v. Developer submits outline proposals for mitigating predicted impact or network reinforcement
vi. Anglian Water will consult with its operational teams and specialists on the viability of proposed mitigation measures
vii. Developer funds network reinforcement under the appropriate section of WIA1991

Sewer network asset data

Data will be provided from Anglian Water hydraulic models. It will be limited to pipe dimensions, levels, connectivity and pipe sub—catchment information. The data can be supplied in either CSV or shapefile format.

It is important to note that much of this data will not have been directly verified and may be either inherited from earlier data sets, inferred or derived by engineering judgement. The data will be supplied with notification to identify its provenance and the level of confidence that should be placed in it.

Please note that where existing data on an asset that is critical to the DIA is of low confidence, we may require a survey to confirm that data before accepting the results of any analysis based upon it.

Anglian Water will not supply results of our own analysis, as this will have been undertaken to different criteria to that required for new development.

Hydraulic analysis techniques

The scale and complexity of the required drainage strategy will determine whether detailed hydraulic modelling is necessary for the DIA. Small developments, presenting a modest increase in additional flow may be adequately assessed using manual calculations.

Although hydraulic modelling will not be appropriate for all sites, for complex developments with multiple drainage strategy options, modelling provides the basis for iterative assessment. There are benefits in adopting the modelling approach when evolving a complex strategy, as it is both repeatable and auditable, while maintaining the assumptions relating to our existing network operation.

Anglian Water will provide design hyetographs to the Developer based on the existing sewered catchment. These can be used to verify the design input in the models from the onsite drainage design. Anglian Water will provide a flood hydrograph to the connection point using the Revitalised Flood Hydrograph (ReFH2) method.

We will use auditable GIS data to calculate the catchment area and the percentage impermeable area or PIMP (also known as the percentage urbanisation) to adjust the catchment characteristics and hence the outflow hydrograph. By using the aerial mosaic images, the PIMP of the existing catchment is adjusted and similar to designing a new network, a conservative approach may be adopted. In most cases this will not require extensive data and flow surveys. This is a sensible approach that will enable a quick way to model the impact on our existing network, with all the unknowns.

Evaluating and approving analysis

Once the developer has completed the analysis it should be submitted to the DS Growth Planning team for evaluation and approval.

The submitted analysis will need to be accompanied by records of any survey work undertaken to verify asset data, along with flow survey details, if this was required as part of the agreed DIA methodology.

Review of the DIA is part of the PPE and is included within the PPE fee. The review findings and any recommendations will be provided to the developer in the form of an addendum to the PPE report.