From: Bob Older
Sent: 11 March 2021 23:40
Subject: MRA: March Meanderings
The October rainfall in 2020 was the highest in the past 30 years. While November and December rainfall was unremarkable, the addition of over 100mm in January led to a 4month total to end January of 399mm - just a few mm short of the exceptional figures of 2001 and 2003, and actually more than 2014 - the last really wet period we “enjoyed”. Those October rains set the scene for the rest of this aquifer recharge season and, on 1st February, many of the observation boreholes, (not those at the upper end of the valley) were at, or close to, their highest in the past 30 years. So, not in the least surprising that we have seen flow from Mobwell Pond all the way to Chalfont Park for the first time since July last year, beautiful views in Missenden Abbey grounds and prodigious amounts of groundwater flooding in the lower valley. Also not surprising that there has been no shortage of comment in the news and on social media in the last couple of months regarding the weather and conditions along the Misbourne, especially in Chalfont St Peter.
Of course, those who complain of local short-term road flooding due to blocked gullies certainly have a point and there is plenty of evidence of choked grilles. So do those who complain that the drains need more regular maintenance - a recent Thames Water’s statement at the beginning of the troubles mentioned removal of blockages, including fatbergs. Debate may continue as to whether the main drains are of sufficient size to accommodate the housing that has been added over the years or whether the problem is illegal connections of surface water drainage from roofs and paved areas into the foul drainage system. However, it is surely relevant that this same main drainage system down the valley was installed in 1954 when the Amersham sewage works, which used to discharge its treated effluent into the river - not pipe it down the valley - was decommissioned. The Thames Water tweet attached at the end of this letter is possibly the most honest comment on this arterial drain I have ever seen.
The first picture below shows groundwater in the woods beside the river downstream of Chalfont St Giles. Water is pouring into the manhole (as it was when i drew attention to a similar photo in 2013). The pipeline is well below groundwater level for several miles so any leakage into manholes or failed pipe connections will be inward.
[cid:1FA78386-3FB9-46A8-A08A-B664D186ED4D@home] [cid:F7BB8461-481D-4E92-B527-510C64F08FF3@home] [cid:E247A98A-6D14-4FBB-9C6E-220A40858EC2@home]
The second picture shows dilute sewage flooding in the centre of Chalfont St Peter. It can be seen bubbling up from a manhole (just under the tree) on the same main pipeline because it is surcharged - it just cannot cope with the flow. Also, just upstream - (3rd photo) - flow is being diverted from another surcharged manhole into the river! Not the whole story I’m sure but one of the many that has nothing to do with “The river bursting its banks”. (The River Chess has a similar story of sewage overspill and discharge into the river.)
Locally, it seems that there is less of an automatic call of “The river has burst its banks” every time there is a puddle than there used to be, so maybe people are more aware of things than a few years ago. The phrase “bursting its banks” only makes any sense in the context of a chalk-stream where flow has been artificially raised above the groundwater level, for instance by the construction of mill leats. It is therefore unusual to see water pouring out of the watercourse. We saw it once in Chalfont St Peter in 2001 when the bank by the Scout Hut was not quite high enough to prevent flow breaking over and joining the already groundwater flooded Scout Hut - yes, that is an artificially raised channel, possibly a remnant of the old mill? This year, similar occurred in the leat to Quarrendon Mill as it did, at a different spot, in 2014 in its quest to return to an original stream line.
The recent work to divert the river out of the Quarrendon Mill leat for a few hundred yards just downstream of the Amersham by-pass has provided lots of interest and correspondence The resulting shallow lake has attracted walkers and waterfowl. For various reasons, it seems that the notional vision of returning the river to its natural course was not/could not be achieved. Simply, it seems as an observer, that the new line would have had to be too far to the west and the bed too low for flow to return to the leat at the designated point.) Hence, a bund was needed to contain high flows. This new “bank" was “burst” in the recent high flows as the river attempted to really finish the job and revert to its course away from the mill! We understand, the bund is to be raised slightly and some other re-profiling carried out. This “problem” - if indeed it is one - is certainly not the one we were concerned about from this project! We wait and see.
Looking ahead, we note that this February’s rainfall was comparatively modest and March is normally uneventful so, unlike 2014, when February and April’s was exceptionally high, we are unlikely to see a repeat of an extended period of flooding this year but can hope for sustained flow from Great Missenden all the way through the Chalfonts into the summer and maybe beyond.
Some might argue for work parties again after the Coronavirus Roadmap Step 1(a) - anticipated 29th March, but probably after Step 2 - anticipated 12th April is more realistic. As i’ve said before, I’m not getting any younger (and my random bouts of dizziness don’t fit well with bobbing up and down clearing weed) so we really do need to find someone willing to take up the organising of our work parties and storage of the equipment ……… plus, maybe someone else willing to take on washing gloves and Hi-vis etc. Perhaps, after 13 years, we need a committee? - volunteers and suggestions please.
By the way, did you see the the Colne Valley and particularly kick-sampling for invertebrates in the River Misbourne on CountryFile - available here<https://www.bbc.co.uk/iplayer/episode/m000t18b/countryfile-colne-valley> or on BBC Catchup? (if using this link, there is a brief advert for the David Attenborough programme first which you can skip if you like)
I have just completed updating the website with the data up to February 1st. We now have records for a worthwhile period on show and the evidence of the cyclical nature of our complex river is clear to see - e.g.in the pages at Quickview<http://www.misbourneriveraction.org/node/18>.
Hopefully, the government’s roadmap will hold fast and so, quite soon, we will again be able to eee each other again by or in the river.
Copy of a message received from Thames Water pasted on Facebook: