Why was the reservoir water lowered?
A report and letter were filed in 2019 by the Dam Supervisor with The Environment Agency advising Liberty Lake Leisure Ltd that it would be prudent to lower the water level by one metre in order to reduce pressure on the dam, avoiding its collapse, with Oliver Dowden MP and other relevant stakeholders aware of this precautionary action. Liberty Lake Leisure Ltd is required by law to conduct a Section 10(2) Report engineering survey with the Environment Agency. This is traditionally only needed every 10 years but in Aldenham’s case, due to the concerns of continuing movement of the dam, it has been requested every 5 years. Here are the key findings from the covering letter and report, which are also available to Hertsmere Borough Council.
Through the on-going maintenance and repair taken on by Liberty Lake Leisure Ltd at personal cost, the reservoir has been:
Adequately maintained and is generally in a satisfactory condition
The overflow arrangements are satisfactory and adequate to pass the design flood outflow
The margin between the top of the dam and wave wall and the overflow level is more than the margin recommended in “Floods and Reservoir Safety”
There are effective means of lowering the reservoir level
No movement of the surrounding land has been noted which might affect the stability of the reservoir
There are trees and bushes on the lower part of the slope and some additional useful work has been done to cut this back
The crest was seen to be in a satisfactory condition although there has been significant movement in the past and there is evidence of continuing movement via cracking in the tarmac which is of concern.
The Council have recently resurfaced areas of the crest path and this process is likely to be a continuing requirement on an annual basis
However, if movement continues to occur, it may be necessary to undertake a permanent lowering of the lake level to reduce loading on the embankment
He goes on to suggest that Liberty “may consider a temporary or permanent reduction in water level to reduce the load on the existing structure.”
‘Whilst all dams move, the settlement and movement profiles at Aldenham are quite different and after decades of investigations by many, I personally don’t think the mechanism of the “failure mode” is understood. I believe the movement patterns will continue and could accelerate with time. And thus I believe it prudent to allow for significant capital investment, either to rebuild or replace the embankment.” It is also fair to assume, though it’s not in the letter, that the recent near-collapse of a dam of similar age and structure at Whaley Bridge in Derbyshire has sharpened people’s focus considerably, not least the insurers’.’
Managing the perceived risk
The owners, taking on this advice seriously in turn have said, in a letter to Oliver Dowden MP and others, that “As owners of the reservoir property we need to manage our perceived risk based on our available and limited resources. We have therefore considered it prudent to reduce the level of the water in the reservoir initially by only a metre but subject to further assessment by our supervisor.”
Regretfully, the lowering of the water and refusal of planning permission for investment in the area has resulted in Aldenham Sailing Club leaving the site and looking for a new location.
Statement from Aldenham Angling Club
“So as the media is commenting on the situation at Aldenham Reservoir on our behalf, we thought we would explain our views. The lowered water level has increased fishing potential and as many of the older lads have suggested, it used to fish better at the lower level, which it is currently being proven with 30+ catches in the past 6 weeks alone….With regards to the fence, this will benefit the club. We had installed 15 signs to try and keep the public to the footpath, to avoid general public walking in on our anglers during a global pandemic, but sadly these got ripped off by the public. Anglers are constantly having to clean up their swims after public have picnics on our fishing platforms and leave their rubbish, making it very unsafe for the fish and other wildlife, so the fence will also stop this. We are saddened to hear that the Sailing Club will be leaving the reservoir but wish them all the best in finding a new water to sail on. They have been fantastic neighbours and we will miss them greatly. The owners have tried to create the funds for the dam wall many times.”
— Aldenham Angling Club
See the full statement on the Aldenham Angling Club’s Facebook page here
For further information, we have posted a Notice of Provisional Designation of Aldenham Reservoir as a High Risk Reservoir from the Environment Authority, Reservoir Safety Team, dated 28 May 2019, which was served on Hertfordshire County Council [HCC].
Following this Notice, HCC terminated its lease on the Reservoir in the same year and did not wish to renew its lease which it was entitled to.
HCC could not or did not defend the provisional designation and therefore a Notice of Confirmed Designation was served on Liberty dated 22 April 2016.
You may note that it says, and we quote,
