The DWMP blog – Episode 21. An afterthought - strategy?

This is the latest in a series of blogs discussing the development of Drainage and Wastewater Management Plans (DWMPs).?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).

This episode is an afterthought to my blog after reflecting on how DWMPs would (or would not) have affected some recent drainage strategy projects that I was involved with.?It considers an area that also seems to be an afterthought to the published guidance on DWMPs.

The issue is the need, wish or opportunity for sewer diversions.

Most of our urban areas and their sewerage systems have grown organically as development has happened over the years.?The sewerage system that we have ended up with may not be the one that we would plan now if we were starting from scratch.?What potential changes should we consider when we develop improvement options.

In Episode 3, I have already discussed the need to create a map to show the dilution ratio at each current or potential discharge point to inform the strategy for abandoning or relocating discharges.?But what should we consider within the sewerage system itself that could allow us to gain benefit from changing the sewer layout?

The issues

What are the issues that we need to consider that could drive sewer diversions to change the system layout?

Current issues

Existing needs for diverting sewers would include:

• Significant sewers that pass under buildings.
• Sewers under airports, railways, streets of special engineering difficulty or alongside high-pressure gas mains.
• Significant sewers with little or no access for inspection or maintenance due to; heavy traffic, security restrictions or health and safety issues.

Future issues

DWMPs are long term plans looking at least 25 years ahead and so will consider future changes.?For sewer diversions these include:

• Sewers under planned greenfield or brownfield development sites.
• Sewers under or adjacent to planned major infrastructure projects (HS2 has led to large numbers of sewer diversions).

Rationalisation

This covers those bits of the sewerage system that are just not how we would ideally want them to be.

• Pumping stations that can be replaced by a gravity sewer to reduce operating costs, maintenance and carbon footprint.?It is common for infill developments to keep the sewerage within the development boundary and pump out rather than requisitioning a sewer to flow downhill across third party land.
• Potential shortcuts; perhaps arising from changes in road layouts or past rationalisation of treatment works.

The review of maintenance needs discussed in Episode 17 should already have identified low gradient sewers that currently require unsustainable maintenance regimes.

The ideal world

We could go even further and start with a blank map and imagine the sewerage system that we would build today if we were starting from scratch.?We will never, of course, build this ideal system but it might help us to see the smaller scale changes that could be cost beneficial.

The tasks

What do we need to do to provide us with this strategic view of sewer diversions?

We need a geographic view of the issues and opportunities.?A GIS layer of these colour coded as to current, future or nice to have and showing the significance of the sewer diameter.?Combine this with the map of dilution ratios to show the best places for discharges to the water environment and we have a useful tool to inform the development and assessment of improvement options.

Summary

The methodology for DWMPs was intended to encourage a more strategic, long-term view of drainage and wastewater planning.?To achieve this it took some of the detailed tools for strategic assessment from the Water Resource Management planning process and grafted them onto drainage planning.?However, it forgot to set the strategic scene through consideration of the big picture of what the future system should look like.

So, like this addition to my series of blogs, it appears that strategy was something of an afterthought in implementing the DWMP process.