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This morning, local environmentalist Max Wallis of Friends of the Earth Barry and Vale offers his extensive appraisal of the 23rd December flooding that devastated much of Penarth's peripheries.

The views expressed are not necessarily shared by Nub News.

The rainfall on 23rd December was particularly intense in the eastern Vale from Penarth to Wenvoe, so we can see it as a test of the authorities' preparedness.

We have a good measure of the rainfall from Dwr Cymru at Cog Moors. The council and Natural Resources Wales (NRW) say the rain was on already saturated ground, but that's not unusual for winter-time.

The floodwater on Lavernock Road past Cosmeston (Photo 1) was certainly "extreme" in comparison with any year in local memory, but was the rainstorm "extreme" as the VoG says?

Dwr Cymru's graph (Photo 2) compares two rainstorms in 2020: the total was not much less on 28 February 2020, but that on 23 December was concentrated.

Previous episodes of high rainfall over 12 years have reached about 40mm, but over 12 hours. That on 23 December was rather higher but unusually short in duration, most arriving in four hours. This meant that immediate run-off was relatively more significant than groundwater levels filtering through soils over several hours. Thus flood levels dropped extraordinarily quickly once the rain stopped at 17.00.

The VoG Cabinet has a report on Monday 8th February. Like the main website, this says that intense rainfall caused internal flooding, in particular houses in Dinas Powys, Sully and Penarth"

"We have a statutory duty to log all instances of flooding and investigate where necessary. The Council has appointed a specialist consultant to assist in investigating the flooding as part of the section 19 report under the Flood and Water Management Act 2010."

Runoff caused flooding of homes never before affected, on Penarth's Augusta Crescent and Lavernock Road, Cosmeston, and on Sully's Arlington Rd estate.

Both estates were built in the 1980s with roof and roadway drainage drainage into the sewer. They both look designed to maximise storm run-off down the road-ways rather than into soakaway green areas. Numerous householders have paved over frontages with scarcely permeable materials. In comparison, the Culverhouse Cross Bellway-HTV estate was built a few years ago with separate rainwater drainage - its attenuation pond failed the test in overflowing to add to flooding on the main road.

The Vale Council has to investigate the causes – and decide was it responsible as the drainage authority, was NRW responsible as authority for flood and river planning or was Welsh Water responsible for sewer inadequacies? It is fortunate the Vale have engaged an independent specialist.

NRW has overall flood planning duties. Their just-issued Flood Map Wales purports to add surface water flooding and climate change into the previous Development Advice Maps (DAMs) which have covered river and tidal flooding. The new maps cover climate change by adding 30% to rainfall figures, but surface water runoff is problematic.

The deep blue blocks (Photo 3) are the two Cosmeston Lakes, the Sully Brook is the 'main river' which can be traced from where it crosses Mile Rd (between the two Lakes) back to the overflow car park field (opposite the Schooner Inn) to the top right corner where it changes to purple and crosses under Lavernock Rd (continues in next map).

The map predicts the Brook would flood over this field and the bottom of the golf course, providing flood-storage. Note that the purple colouring from surface water and ditches is minor – yet on 23rd December the whole section of Lavernock Rd was streaming with 30-50cm water. This includes the uncoloured section outside Lower Cosmeston Farm. Though Cosmeston carpark is also uncoloured (unaffected) the Lavernock Rd flood in fact mounted the slope into the carpark, completely flooding it deep enough to pour into the Lake.

It's evident the NRW map is based on the Brook overflowing, including onto the roadway, yet the actual flooding came mainly from surface water, including from the Cosmeston estate.

Or did it? The roadway water is supposed to go into the drains, including new drains on the rebuilt section of Lavernock Rd outside the Park. Evidently much didn't. It bubbled up from drains on Fort Road and elsewhere. The sewer couldn't take it. Nor could the sewer take the run-off from the Cosmeston housing. As far as the Flood Map Wales applies, it would be to a more extended storm, not that of 23rd December.

There's also something wrong with calling the Sully Brook a "main river"; after all, it's little more than a ditch in the field opposite the Schooner. The standard data give its catchment as 246.5 ha, for the whole of Penarth south west of the railway and west of Plymouth Rd. Yet half this area drains into the St Cyres-Cogan Hall Farm branch of the Brook, flowing to the west of Cosmeston Lakes, left hand (west) side of the second map (Photo 4). Lower Penarth hardly drains into the Brook as it's largely piped underground.

This map shows its track from Cwrt-y-Vil playing field via the Athletic field (which might have underground drainage into it) and thence to emerge at Charteris Close. There little scope for run-off to reach it. The Close flooded on 23 December but from run-off.

When the storm ended, it quickly disappeared, some down the drains, other down the passage to Forrest Road into the road-drains there. The pattern of flooding (medium and low risk) pictured along the old watercourse from Cwrt-y-Vil via Forrest Rd did not occur. The branch of the ditch through the Westbourne Rd allotments also did not flood; this drains the Victoria Sq area, but nowadays there is just an old well there.

There is of course need to plan for both types of storm, like the 28 Feb and the 23 December ones. Indeed, standard rainstorm planning calculates rainfall runoff for a range of deruarysign-storms (FEH 1999 and 2013 models). Cambria consultants have done this for the planned new housing at Cosmeston (see Table, Photo 5) in order to choose attenuation pond capacity for the worst case.

The tabulated rainstorms are quite intense; the one-hour storm would be over twice the peak 15min intensity of 23rd December. They include an extra 30% for future 'climate uplift', though in some ways current storms are thought to already include much of the 30%.

See that the 4-hour peak storm of 45mm on 23rd December was just over half the 85mm in the Table (240 min). Without uplift the 4-hour total is 65mm. Thus the 23rd December rainstorm cannot be described as "extreme"; rather, the authorities have to plan for much more. In a 12-hour storm, 96mm total rainfall has to be accommodated, over double the more intense part of 28th February.

Because the longer storm has more scope for attenuation through infiltration, it's likely that the shorter storm like 23rd December is the more dangerous. If limited capacity of the sewer is at issue, again the shorter storm is the more dangerous.

Friends of the Earth Barry & Vale asked in 1999 for a policy on Sustainable Drainage Systems (SuDS) in the development plan, including restrictions on converting front gardens to hard-standing. The Vale Council was not interested. They continued to be reluctant, but now have a standard planning condition asking developers to 'investigate' SuDS and provide surface water drainage systems separate from the sewers in larger developments. The Council has however done nothing towards retrofitting the Sully and Cosmeston estates with SuDS drainage and use green areas for soakaways toobviate what hit us on 23rd December.

The Council has engaged a specialist independent consultant to work in partnership with Natural Resources Wales and Welsh Water to compile the statutory report. The consultant is tasked with compiling information on location and extent of flooding; summarise the rainfall event; make findings on the cause and recommend actions by three bodies.

Residents are invited to complete the incident form on the VoG website. The evidence should not be just those whose homes suffered, but from others who saw and experienced the flooding. We'd like the consultant to cover the lack of SuDS policy and retrofits, to question why Dwr Cymru approves house-building when their sewer has insufficient storm capacity, and question the Vale's approvals of house-building with insufficient storm-water capacity.

The Cabinet report is to go to a Scrutiny Committee on 16th February – people should submit their flooding-incident forms and evidence to [email protected] (HERE) in the next few days, and inform their local Councillors in time for the meeting.