20-Apr-2024

FoI 24-5446
Water Discharge at Gt Missenden

The request -

Dear HS2,

Water from the Siltbusters at Gt Missenden has been discharging into the River Misbourne since December (possibly intermittently) – see this page Would you please respond to the following Environmental Information Regulation requests ?

Under what circumstances do the Siltbusters discharge water via the overflow pipes ? Does this indicate that drainage ponds 1 and 2 are full ?
For how many days (in the last month) has this discharge occurred ?
Is the South Heath Culvert currently in use (i.e. draining the North Portal to Drainage pond 1) ?
The S17 application for the North Portal (Drainage -
https://pa-csb.buckinghamshire.gov.uk/online-applications/files/AAA1F2B2BAD63FD8B77C5ABDE5D60CD2/pdf/PL_23_1591_HS2-DRAINAGE_MITIGATION_REPORT-4726062.pdf )
makes no reference to overflows from the ponds, other than figure 3 shows a spillway ‘to convey excess flows downstream’. What steps will be taken to ensure that water drained from the tunnel portal cannot find its way into the Misbourne, via the culvert under the A413 ?

The response

Delivered on 05 March 2024 - last day permitted -

when "the available freeboard within attenuation ponds decreases."
20 days from mid-December to mid-January (so approx 2/3rds of the time ?)
No - so current overflow doesn't include drainage from the N. portal cuttings - which will presumably not improve the situation. Where does the water from the NP end up ?
"the link you provide (to the drainage report) as this part of the request does not allow HS2 Ltd to access what is contained therein." OK, so the Bucks planning site does have some off days, but the link does appear to be working; better try again - see HS2 covering letter . Here is a local copy of the report.

Summary

The N. Portal drainage report claims that the ponds are designed for a 100 year storm, + 40% . Discharge from the Siltbusters indicates that the ponds are nearly full, even though they are not yet receiving water from the North Portal via the conduit. Did anyone notice the 100 year storm ?

Better ask them why - see next month's exciting episode ...

Follow up - 9-Mar-24 (FOI 24-5493)

"Thank you for your reply to 3 of my 4 questions. I am surprised that you were unable to access the document referred to in question 4, as it formed part of your S17 submission to Bucks council. It is labelled 1MC06-CEK-TP-REP-CS03_CL05-000006 and I have attached a copy. My question related to figure 3 , and I have added 3 more :-

What is meant by ‘convey excess flows downstream’ ? There is no existing stream at this location.
The siltbusters currently discharge water into the drainage ditch beside the A413, which in turn discharges water directly to the river Misbourne via a culvert. What future measures will be taken to prevent drainage from the N. Portal (which may be contaminated by railway operations) from following the same path to the Misbourne ?
The figure claims that the ponds are designed for a hundred year storm + 40%. Your reply to question 2 confirms that the siltbusters discharge when the pond freeboard decreases, and question 3 states that the S. Heath culvert is not yet in operation. Why has the pond freeboard decreased, in the absence of a 100 year storm, and before the S H Culvert is providing additional input from the portal ?
Might the pond level be allowed to increase, or might the overflow be removed by tanker, to avoid contributing to the flood and groundwater levels downstream from Amersham, and the pollution incident in the Chalfonts ?

Response ! (16-Apr)

Excess flow will run over the top, presumably into the A413 drainage ditch ?
The design has not been completed. It is not clear from the S17 (PL/23/1591/HS2) if the drainage report was for approval, or for information.
The current drainage ponds are temporary ! We can charge again to dig some more !!
Are the temporary ponds much smaller than the final versions ?
You must be joking.

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