At the end of November 2023, Tim Carty – the Chairman of Milborne Port Parish Council – attended the briefing given by Lorne Thomson of Yeovil Rivers Community Trust, hosted by Milborne Port Climate and Nature Action Group.
Firstly, a note of thanks to Lorne Thomson of Yeovil Rivers Community Trust for his talk yesterday evening on flood risk and Natural Flood Management techniques, and also to Sarah Warren of the Milborne Port Climate Action Group for setting up the meeting. Much appreciated.
I thought it might be useful to feed back some of the takeaways I noted in relation to our role as Parish Council.
YRCT have until now been a relatively small scale charity, overwhelmingly reliant on volunteering and charitable funding. They have in recent years upscaled significantly, and now have one full-time and six or so part time employees, with charitable monies being supplemented by grant funding. They focus on awareness; strategy; co-ordination; and implementation and maintenance of (thus far small scale) natural flood management systems within the Yeovil area.
For those interested, here is a link to YRCT’s website: About – Yeovil Rivers Community Trust (yrct.org.uk)
A bit of local background and specifics. Temperatures within the local area have risen a couple of degrees since forty years ago. Humidity is higher. Rainfall has increased, but more importantly it has been more concentrated and intense. Droughts are more frequent. Typical daily rainfall in 1980 would rarely exceed 20mm or so. It is now much more usual to see rainfall of 35mm in a day, and this is often concentrated into just a few hours. The impact of heavy rainfall events is enhanced when the ground is either already saturated, or when it is very dry after a period of drought.
This is generic background against which the May flooding has been a major ‘event’ in public consciousness. On 9 May 2023, there was very heavy rainfall, with the epicentre likely between Yarlington and the A303, with serious floods in the Cadburys, the Camels and various other communities to the north and west of Milborne. The nearest meteorological centre was Yeovilton, which recorded 35.8mm of rain for the day. There is no reliable reading for the rainfall at the absolute centre (or exactly where that was) but it is estimated that the rainfall could have been up to 130mm. This type of event is likely to happen with increasing frequency over the next twenty years.
The takeaway from this was something of an unpleasant surprise for me. My reading had been that Milborne, with a sizeable part of its community close to the Gascoigne River, has an obvious exposure to flooding, and we have indeed experienced some of this over recent years, and going back in time, there has been a history of floods. Much of the village is formally within a flood zone. However. I had understood that, by flushing out a number of our culverts and keeping these better maintained, Wessex had done a pretty good job of ensuring we have a robust defence, which is why we had not experienced the kind of issues that nearby communities experienced. It turns out that this is mere fantasy. Our successful avoidance of a significant flood event of late has, more or less, been a result of not being unlucky and being the place that gets hit. The works and maintenance have helped markedly, but we would still be clobbered if we were unlucky in the future.
The basics of mitigation through Natural Flood Management (NFM) seem to be fairly straightforward. Within the community itself, and equally importantly just below it, we want the river to be unobstructed so that the water has no problem getting through the village and out the other side. Above the community you want the river to have numerous opportunities to be slowed, for any diverted water to be retained for periods and not return immediately to the channel, and you want to hold up water from any side watercourses from entering the channel for as long as possible during intense rainfall. There are numerous low-tech, low-cost techniques for achieving this, nearly all of which will tend to benefit and enhance existing land use (Hinged Trees; Run off pathways; shales; bunds; swales; and – on side streams- leaky dams). If locations for such works can be identified then YRCT can assist in planning and implementation, and they have a history of working these issues through with landowners. Lorne said there is no real point in getting worked up about things unless you have landowners’ support, and this is easiest when there is an indirect benefit to them.
A further point with regard to mitigation in new developments. It is becoming normal to see the LFA ask for permeable surfacing for the hard surfacing, and this works very well. Essentially it will allow the first 5mmm of heavy rainfall through into the underlying terrain and this makes a major difference in reducing the impact. Thereafter you’re on your own, but as you can see from the numbers above, in the new normal of 35-40mm daily precipitation you are taking out over 10% which makes a difference. However, Lorne noted that it only works if it is a) properly installed; and b) properly maintained. He did a survey in Yeovil a couple of years ago, and by his read over half of the permeable surfaces had either ceased to be so, or had never been permeable in the first place due to poor installation. New developments’ surface water arrangements are now required to be calibrated to cope with daily rainfall of 100mm with a further 45% adjustment for climate change.
Lorne said that in the wider picture, addressing issues close to headwaters, on communities like ours, makes a substantial difference. Ultimately, the Somerset Levels have two large problems. Firstly, they are not meant to be as they are, as we have sculpted much of them away from the marshland they are meant to be, and Nature will have a tendency to ignore our efforts. Secondly, the Bristol Channel has one of the highest tidal flows in the world, which means that twice a day, the door on the catchment basin gets closed shut and the river system cannot get out. Particularly bad during spring tides. Hence, whilst not directly relevant to us, anything done up at our level to reduce and slow the off-run from major rainfall events into the rivers will help downstream.
You can find the slides that accompanied this briefing here.