The DWMP blog – Episode 11. Are we following the roadmap?

If you haven’t already seen the earlier episodes in this series, they are all here (https://tinyurl.com/MartinOsborneArticles) I suggest that you start from Episode 1 (https://tinyurl.com/DWMP-blog).

Appendix G of the draft version of the DWMP guidance, published in April 2018, set out a roadmap for future improvements to the methods This was not included in the final guidance, but it is useful to review where we have got to and what we still need to achieve.?

The roadmap set out 8 areas of improvement.?This episode of the DWMP-blog will explore what the roadmap proposed, how the approaches set out in these blogs meets that and the remaining challenges.

Methods for assessing collapses and blockages

The DWMP guidance stated that current planning approaches should be used for collapses and blockages but was concerned that it might be necessary to define more complex approaches.?I do not think that this is the case.?

Current region wide programmes but with an interface to plans for individual catchments are adequate approaches for dealing with these risks.?Best practice is in the Sewerage Rehabilitation Manual from the 1980s for sewer collapses and UKWIR Economic level of service for sewer blockages from 2010 for blockages.

Use of stochastic rainfall

There are two approaches to representing the long-term patterns of rainfall.?

• A stochastic approach that uses a statistical analysis of random patterns – such as design storms.
• A brute force approach that uses long records of rainfall to represent the random patterns.

The guidance misuses the terms by proposing a move towards a stochastic approach but defining this as using long records.?It borrowed here from Water Resource Management Planning that typically uses records representing 200 years of rainfall.?However, this is feasible as it only needs to be analysed at a weekly or monthly timestep.?DWMP models need to analyse at minute-by-minute timesteps to show the rapid changing conditions in sewerage systems so using long records is not feasible.

In practice, we need to move in the opposite direction.?For assessing overflow spill and impact we do use long continuous rainfall records representing 10 or 20 years.?This is a computationally intensive and a significant barrier to detailed analysis.?We need to develop a method to represent the whole pattern of these timeseries with just a few stochastic (statistical) events.

Several methods have been proposed for this including modified design storms or synthetic “envelope” storms built from the full timeseries.?We need more work to prove these approaches.

Simplified hydraulic models

Sewerage system simulation models are computationally intensive and so not well suited to broad scale assessments across multiple catchments.?However, we can develop simplified ways of using those models.?The discussion above on new approaches to timeseries rainfall is part of this development.

The need to plan across catchments has two aspects.

1. Assessing the balance between wastewater system discharges and absorptive capacity of the environment.
2. Transfers of flows between catchments.

A key factor for the first aspect is the dilution ratio between dry weather flow in the sewer and dry weather flow in the watercourse.?Plotting this as a thematic map for every discharge or potential discharge would allow a rapid assessment of which discharges needed to be reduced or abandoned and which could be kept or increased.?Simple pollutant decay models for the watercourses would allow this to be quantified.

For the second aspect we would expect inter-catchment transfer flows to be limited by excluding surface water, by overflow discharge or by attenuation storage.?An initial assessment can therefore be made using existing models but for steady state conditions for various multiples of dry weather flow; which is quick to analyse.

Putting these two techniques together gives a powerful approach to strategic planning.?

Tools for assessing risk in small catchments

I have already argued in Episode 4 that the tools for small catchments should be the same as for large catchments, but with more efficient ways of building and verifying them.?More work is needed to develop these techniques.

Additional metrics

Development of additional metrics and planning objectives to support long-term planning.?

The roadmap proposed additional metrics and planning objectives including monitoring compliance with treatment works flow permits and overflow discharge permits.?In practice most companies will monitor these but because of lack of pressure from the environmental regulator they do not necessarily enforce compliance.?Inclusion of compliance data for these in the DWMP reporting would be a useful incentive.

Investment planning tools

I discussed in Episode 5 the use of a cost benefit approach for all investment planning.?This provides a robust method to balance investment proposals within catchments and between catchments up to company level.?This should become the standard approach to investment planning rather than setting arbitrary performance targets and developing least cost solutions to meet them.?

Most companies have tools to implement a cost benefit approach and could readily extend it to integrate with DWMPs.?The gap is in having an agreed framework for costing the benefits of reduced flooding and environmental impact.?Some companies have this but there is no standard approach nor general acceptance that this is required.

However, we have identified the need for standard tools for funnelling options from generic to preferred within individual catchments.?Most companies have developed tools for this that range in complexity from simplistic to barely usable.?We need to share experience to develop a core functionality that can be adapted by each company to meet their own way of working.

Spatial visualisation

Development of a standard methodology for spatial visualisations of DWMP outputs.

The gap here is in a method for representing the broader picture with thematic maps of risk, investment and cost benefit.?Methods using grid squares or hexagons have been proposed; but I have always found that using the existing definitions of postcodes is a very powerful and easy to use approach to thematic mapping.?We need to agree on the approach and standard thematic colours.

Summary

The approaches that I have already set out would achieve most of the ambitions of the DWMP development roadmap.?There are some areas where more work is required, but for most, widespread adoption of existing best practice is all that is required.