Save Danes Moss

Answering the question of why Danes Moss North does not flood
Danes Moss Wet Woodland

1,253,500 is a very big number.

It’s an important number, however, because according to data in the outline planning application for Danes Moss North, that’s how many cubic metres of peat [1] lie beneath the surface of the site.

Our community group is known for flagging up the huge amount of carbon locked up in all that peat (at least 57kg per m³ or about 72,000 tonnes [2] in total) but the pickled plant remains that make up most of its bulk are also full of tiny cavities. Add those to the spaces between the bits of preserved vegetation and it’s easy to understand why peat is so light when you buy it from the garden centre – studies show it is around 80-90% holes!

At Danes Moss North those holes are full of water which means that when pools appear at the surface, over one billion litres [3] are held in the peat below. And, because the peat sits in a vast clay bowl with only two low points that let it out, all that water is held back and released very slowly into the rivers Bollin in the north and Dane to the south. In drainage engineering this is called attenuation, and it helps to prevent sudden and catastrophic downstream flooding during intense rainfall.

NATURAL DRAINAGE

As the name suggests, Danes Moss North would drain naturally into the River Bollin via streams. In fact, until the early 19th century, this is almost certainly how it was drained. Since then, however, we have built a railway and roads, and quite a lot of buildings that prevent a direct connection between the Moss and the Bollin. Drainage patterns are very different now.

TODAY’S UNNATURAL DRAINAGE

Overall, the Danes Moss North site slopes from Congleton Road playing fields in the west towards Turf Lane in the east, with a few minor ups and downs in between. 

When the peat is full of water it flows in a shallow ditch [4] from the playing fields, towards the end of the footpath into the site from Moss Lane, where it turns northwards to flow alongside the path. Just before it reaches Moss Lane the ditch turns south-east and flows beneath the footpath from where it emerges to flow along the boundaries of houses on Moss Chase. From the end of Moss Chase, the ditch continues towards Cheshire Demolition’s yard where it enters a culvert, emerging between Turf Lane and the rail line before turning northwards to run parallel to the railway.

Watercourse across Danes Moss

Just south of Star Lane the ditch enters a culvert where, according to information in the Phase 1 reserved matters application, it enters a culvert that flows beneath the football ground and the Moss Estate.

How might the drainage system work if a road, supermarket, school, and 950 houses are built?

DEVELOPMENT AND FLOODING?

Nobody knows how it would work with any certainty because the reserved matters applications for Phases 2-10 contain very little information [5].

The application for Phase 1 is slightly different because a technical note on surface water drainage is included and it tells us that the road would take up 3 hectares of land (that’s 30,000 m²), and that all the water from the paved surfaces would drain into a concrete attenuation tank [6] located next to the rail line. There is no mention of what would happen to the water held in the 70,000 cubic metres of peat beneath the road which would need to be permanently drained to allow construction to take place.

Proposed attenuation tank on Danes Moss

The attenuation tank is supposed to do what the peat does at the moment – slow down the water flow before it enters the existing drainage system – and under normal conditions it probably could do that. Normal conditions are, however, becoming more unpredictable and extreme due to climate change and named storms are a feature of today’s weather patterns. Storms Ciara, Dennis, and Jorge passed over on consecutive weekends in 2020, and Elin and Gerrit [7] hit Macclesfield over Christmas 2023 – each storm dumping between 30-50mm of rain on the town. That’s about 40 litres of rain per square metre, or 1.2 to 1.5 million litres [8] that would drain off the proposed spine road.

The attenuation tank may be able to cope with a single severe storm, but multiple events would almost certainly overwhelm its capacity to control floodwaters. Would it then back up into the proposed residential areas? Or burst onto the rail line? We don’t know, and the lack of information in the reserved matters applications suggests that the developers don’t know either.

References

[1] Phase 2 Site Investigation Report: RoC Consulting, 15 th March 2017; para.6.1.40 (Area of development site assumed to be 54.5ha)
[2] Derived from Peat Bogs and Carbon: A critical synthesis; Lindsay R., University of East London; June 2010: sub-section 5.2.3.2: Table 6
[3] 1m 3 =1000 litres: water content of peat assumed to be 80% (Peatlands regulate the Water Cycle in our Landscapes; Lennartz B. et al, University of Rostock, June 2021)
[4] Drainage Technical Note; RoC Consulting, 12 th March 2019
[5] See, for example, letter of objection dated 11 September 2023 from Cheshire East Council Flood Risk Officer to CEC Planning Officer re: application 19/1796M
[6] Drainage Technical Note; RoC Consulting, 12 th March 2019; Appendix C – Surface water drainage layout; drg. No. D105 rev.02 – Reserved matters application 19/1796M
[7] UK Met Office; https://www.metoffice.gov.uk/ binaries/ content/assets/metofficegovuk/ pdf/weather/learn-about/uk-past-events/ interesting/2023/ 2023_12_storm_gerrit.pdf accessed, inter alia, February 2024
[8] Save Danes Moss Community Group: approximate calculation based on Met Office data and information contained in Drainage Technical Note; RoC Consulting, 12 th March 2019

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