The DWMP blog – Episode 2. How does a DWMP differ from a WRMP?

This is the second in a series of blogs.?If you haven’t already seen Episode 1, I suggest that you start there.

The process for producing Water Resource Management Plans in the UK is well respected and has many strong features.?These include:

• It is a mature process that has improved and stabilised over many rounds of planning.
• There is a standardised methodology used by all companies so plans can be compared.
• It requires long-term plans considering future changes and uncertainty.
• Production of the plans is a statutory requirement and they need to be reviewed by Government.

When devising a process for producing Drainage and Wastewater Management Plans it made sense to learn from these strengths and build them into the new guidance.?

Drainage and sewerage planning already had mature processes that had been defined and refined over the last 30 years.?These included the Sewerage Risk Manual, the Urban Pollution Management Manual and the Drainage Strategy Framework.?There were some gaps in these, and their implementation was not standardised across the industry but they provided a good starting point.?

Unfortunately the DWMP guidance set out to start afresh and incorporated much of the WRMP process wholesale including terminology and methodologies that were not appropriate for DWMPs.?We were in danger of losing the mature processes that we already had and replacing them with an immature implementation of WRMP processes.

The structures of the water resources and wastewater systems are different and therefore the required planning methodologies s are very different.

WRMPs are high level plans.?They consider service failure due to lack of capacity of water supply and the links between supply and each town and city.?A water company has perhaps a hundred water supply sites that are the key interactions between their operation and the water environment.?Most are linked together requiring a company wide, or region wide, plan.

DWMPs are detailed plans.?They consider service failure due to lack of capacity of wastewater treatment and the links provided by the sewerage systems to each street in every town and city. ?A water company has perhaps a thousand wastewater treatment works, ten thousand other discharges to the water environment and millions of individual potential locations of service failure due to flooding.?All of these are affected by climate change and other future trends.?Catchments are generally not connected to each other and investment needs can be determined individually.

The DWMP guidance borrowed a top-down planning process from WRMPs, that sets level of service targets for the company then divides these to set targets for each catchment.?In practice this was not used and targets were set bottom up for each street to give common levels of service across all areas.?The guidance needs to be updated to recognise this.

The DWMP guidance also borrowed the terminology of supply demand balance from WRMPs.?This caused considerable confusion as it is alien to drainage planners who refer to probability of exceedance and level of service.?Drainage planners struggled to accept that increased intense rainfall due to climate change was a “demand” issue or that building a bigger sewer was a “supply” solution.?Most companies moved away from this terminology and back to the conventional language of probability and capacity.?The guidance should be updated to follow this trend.

The funnelling process of moving from Generic options through an unconstrained list of options to feasible options and preferred options was also borrowed from WRMPs although with some changes in terminology. ?Even the original WRMP terminology is confusing

• An unconstrained option should be technically feasible but may be constrained by planning or environmental issues.
• An unconstrained option should be effective - that is, provide a defined capacity.
• A feasible option should not have any constraints that cannot be overcome.
• A preferred option is one selected for implementation.

The DWMP guidance introduced an additional layer of constrained options but this only served to confuse matters further.?The terms constrained and unconstrained are easy to misunderstand so I propose to use the word applicable.

There are four concepts here:

• Applicable - no constraints such as planning, environmental, customer acceptability or land use that would prevent use in this location.
• Effective in solving the problem.
• Can feasibly be built.
• Delivers best value.

In DWMPs there are very many issues and options to be addressed and detailed modelling is needed to prove effectiveness.?The options are therefore first filtered to those that are applicable before modelling them to determine if they are effective.?This is a different order from the WRMP process.

The funnel used in DWMPs was therefore as shown below.?The guidance should be updated to clarify the steps of this process.

Drainage and sewerage planning has previously not looked far enough ahead and the requirement to look 25 years ahead is welcome.?Part of the reason for the shorter-term view in the past was some unintentional disincentives built into the regulatory process.?These barriers have now been removed and the industry is ready to take on long term planning as required by the DWMP guidance.

Putting drainage and wastewater planning on a statutory basis is a positive step and I welcome its inclusion in the 2021 Environment Act.?However, the devil will be in the detail and it will be important to frame the regulations with sufficient flexibility that plans meet the needs of each location and each issue.?Simply imposing the current guidance as regulation would not achieve this.

So DWMPs can learn from WRMPs, but not as much as some people had thought.?Drainage engineers are already adept at planning for investment in pipe networks and perhaps the drinking water sector can learn from them on this.?If anyone needs help in developing guidance for statutory Water Distribution Management Plans, I am sure that the drainage community will be happy to help.