Fwd: MRA: May Marshes

Fwd: MRA: May Marshes
Dear All
Good flow from Mobwell Pond all the way to Denham is usually cause for celebration, but with pumps and tankers still present, sewage pollution still blighting significant lengths, roads closed and much of the floodplain still being, well, a very soggy floodplain, it’s at best a bittersweet situation.
 
Much has been written and broadcast elsewhere about sewage in our poor little river and its effects on businesses and homes so perhaps we don’t need to go on about it here too much.  Suffice to note that we were showing leaking manholes at Chalfont St Giles to Thameswater engineers in 2014 after previous sewer surcharging and they are unchanged today.  Sustained pressure will be needed to secure the overdue maintenance of the sewer system and to plan innovative alternatives to road closures etc if this appalling situation is to be avoided in the future.
 
Claims in circulars distributed locally of the wettest “winter in 248 years” appear to have been teasing the facts a little.  Historical rainfall records in UK do go back 248 years – https://www.metoffice.gov.uk/hadobs/hadukp/data/download.html – although continuous, individual sites records did not begin till 1873, e.g. at Oxford.   For SE England, higher “winter” rainfall totals (Dec, Jan, Feb) than 2024’s are listed in 2020, 2014, 2001 and several other years back to 1877.  Adding Autumn (Sep, Oct, Nov), totals for 6 months were also higher in 1961 and 2001.  To satisfy the media search for “a wettest ever” headline it seems Sky News needed to go to the sum of 18 months in a row and this claim spread. The same records show this February as the wettest for SE England since 1951, but only if you forget this was a Leap Year.  Otherwise, 2020 and 2014 are trumps. Our local records show similar.  Nevertheless, it has obviously been exceptionally wet.  5 of the 10 highest annual totals have occurred in the past 25 years compared with the other 5 in the preceding 125 maybe giving a further indication of climate change – more figures below.
 
Given the impact of this year’s weather, it is again perhaps surprising to see that monthly readings of groundwater levels have, at most, only slightly exceeded previous notable years – again see below.  Maybe a more extreme picture will emerge when the more detailed information is released by Affinity giving hour by hour observations as the peak was probably mid month.   Clearly though, after the wettest 18 month total rainfall on record (see above!), the ground is utterly saturated and new rain is not being absorbed.  This has been leading to repeated disruptive flooding, for example in Chalfont St Giles, and prolonged groundwater ponding, for example behind Amersham Memorial Garden, and many other properties in the valley.  Any heavy rain (as 4th/5th May) quickly leads to repeat sewerage and drainage issues.   No-one can control the rainfall or the natural groundwater levels in the aquifer and any mitigation, if possible, of issues arising, for example at Mill Lane, Chalfont St Giles will need careful engineering and significant funds.
 
MRA’s agenda has previously generally been about trying to tackle low flow, but the Chiltern Chalkstreams Project “Mending the Misbourne” workshops and recent events remind us of wider issues.  Rather than a strap-line of “Volunteers Working To Keep The River Flowing“, maybe we should change to “Volunteers Working To Keep The River Healthy” – there’s a challenge!  The second workshop – held yesterday – was focused on identifying clear, practicable, priority tasks or projects for investigation, dvelopment and, crucially, funding.  Further work streams are anticipated to prepare “shovel ready” initiatives by the end of the year.  However, surely some significant work to seal leaking sewer manholes and pipes will be put in hand immediately conditions allow?
Meanwhile we could start thinking about some gentle clearance of fallen branches through the Chalfonts and perhaps some work to make the most of the comparatively rare good flow into Great Missenden, all assuming negligible residual pollution.  Before the worst of the recent flooding, the EA were suggesting a volunteer activity to make short term improvements between Amersham By-pass and Quarrendon Mill and asking for our help.   Let’s have more discussion and maybe planning for some work parties at a “not a meeting” from 7.15pm on Wednesday 22nd May at The Greyhound – all welcome.  Also to discuss arrangements for Chalfont St Peter Feast Day and other events.
PS: Some more figures.
 
A) Seasonal totals scatter diagram.  (Note: the many “winter totals” exceeding 2024 and the variation in the highs and lows) 
 
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B) 25 year rolling average of annual rainfall.  (Note: selecting 25 year period gives a fairly clear rising trend.  Using say 5year periods highlights other notable periods; using longer periods, emphasises a long term rising trend.
 
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C). Graph of Groundwater Levels at Chalfont St Giles compared with other notable highs.  (Note the same plots for other locations show a broadly similar picture – see our website)
 
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D) Flood plain above Chalfont St Giles
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E). Sewage laden river approx a mile downstream of Amersham Balancing Tanks – half a mile above Chalfont St Giles
 
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