The DWMP blog – Episode 24. Integrated flood risk management

Urban flood risk is complicated because of the many factors and organisations that play a part.?This is particularly so in the UK where much of the drainage system is the responsibility of regional sewerage undertakers rather than local municipalities.

Water Company DWMPs have an important role in planning how to manage urban flood risk, but they do not cover the whole lifecycle of urban runoff and flooding.?They do not cover:

• How much water gets into public sewers – property owners and highway authorities are responsible.
• What happens to water that doesn’t get into public sewers –local municipalities are responsible.
• What happens to flood water from sewers in extreme rainfall – responsibility is unclear.
• Flooding from local watercourses – landowners are responsible.

However, local authorities have an overview role for understanding and managing local flooding and are provided with guidance for how to prepare a Surface Water Management Plan (SWMP).?My representation (simplified) of the SWMP process is shown below.

It has strong similarities to the DWMP process.

• Making the case matches with the Risk Based Catchment Screening
• Both involve establishing a partnership with other organisations.
• Both define the baseline risk in monetary terms and have increasing complexity of assessment depending on the size of the risk.
• Both develop and assess options based on whole life cost benefit.
• Both promote delivering options by the organisation(s) best able to deliver them.
• Both require a regular review of the plans.

Two different organisations are following similar methodologies to develop separate plans to manage urban flooding and, although not in isolation from each other, they are far from integrated.?Where do we start to integrate them??I think that DWMPs are the best starting point as although they are a recent introduction, they build on 50 years of sewerage modelling.?There are four things that we need to add to produce an integrated plan?

1. Modelling of overland flow that either doesn’t get into the sewerage system or that escapes from the sewerage system.?This is not currently required, although some DWMPs do consider the latter using 2D overland flow modelling.
2. Modelling of overland flow in extreme rainfall less frequent than an annual probability of 1:50.
3. Modelling of the detail of local watercourses, culverts, ponds and other drainage components which are only included where they interact with the sewerage systems.
4. Integrated development of options.

Modelling overland flow

A requirement for 2D overland flow modelling does impose an extra computational burden and additional work to ensure that the flow representation is realistic.?However, there are several approaches at varying levels of complexity. As for other aspects of the plans the level of complexity should be set depending on the magnitude of the risk.

• Detailed triangulated mesh – where a detailed ground model with ever smaller triangles closely fitted around every building and other obstruction.
• Simplified triangulated mesh – where the buildings and obstructions are simplified to equivalent simple shapes or where the mesh is allowed to leave gaps around them.
• Square grid 2D modelling – where the ground surface is represented as a regular grid of 0.5?m or 1?m pixels rather than as detailed triangulated mesh.
• Pseudo 2D modelling – identify the overland flow paths and represent these as notional 1D surface channels.?This can be combined with a simplistic flood spreading model at each model node to estimate the flood impact.

Modelling extreme rainfall

It is complex to represent in detail the interaction of piped drainage systems and overland flow in extreme rainfall events.?However, for an event with an annual probability of say 1:200 the contribution of the piped drainage system is small compared to the overland flow.?A simplified approach can therefore be used of ignoring the piped system (other than major culverts) for this assessment.?This then identifies areas where the urban landform should be adapted to control flood water or where property level protection will be required to protect against extreme conditions.

Modelling watercourses and culverts

Local authorities have a duty to record watercourses, culverts and other “above ground” drainage infrastructure.?This aspect therefore requires their input to the development of DWMP modelling.

Integrated development of options

Developing best value options requires really joined up action between sewerage undertakers and local authorities.?SuDS for new build, retrofit of SuDS to properties and to highways requires local authorities to act; but they may need some funding from the sewerage undertakers whose service levels it improves.

Reshaping the urban landform can only be done by local authorities in collaboration with landowners but they need to be incentivised to deliver this.

Should local authorities be able to constrain the options that sewerage undertakers select perhaps by requiring planning permission for pouring concrete or upsizing sewers.?Or maybe just a real desire to collaborate would be enough.