Hillingdon Council’s proposals for improvements to Ruislip Lido include raising the water level to enable the reintroduction of swimming and boating. The Council seems to have overlooked the reason these activities ceased in the early 1990s due to the advice of their Engineer that the water level of the Lido should be reduced to provide flood protection to properties in the Flood Plain of the Cannon Brook downstream of the Lido.
Here is the Engineer's report in full, with areas of interest highlighted.
Utilisation of Ruislip Lido as a flooding prevention facility
Members will be aware of proposals to lease the recreational facilities at Ruislip Lido to Eau Naturelle Limited*. During the negotiations with Eau Naturelle it has become apparent that the potential for minimising flooding in the area immediately below the reservoir dam cannot be maximised unless certain measures are taken to control water levels in the Lido. The matter is further complicated by the very considerable silting of the reservoir that has resulted in depths of saturated silt up to 2 metres thick which inhibits the use of the area for boating activities and is a potential danger to users of the lake.
Historically the lake has been allowed to find its own level and for the past forty years, at least, has been utilised for water sports. The surface water run-off from the whole of Northwood passes through the lake, the level rises rapidly in times of storm and the lake overflows into the Canon Brook, a main river under the control of the National Rivers Authority. The speed at which the overflow commences varies according to the severity of the storm and the water level in the lake before the storm starts.
Flood prevention measures are assessed against the likelihood of varying volumes of water falling in a storm. The National Rivers Authority normally aim at providing protection against the 1 in 100 year storm but in the case of Canon Brook will find it impossible to protect against more than the 1 in 50 year storm, even if no water overflows from the Lido, ie, the level of the water before the storm is sufficiently low to accommodate all the storm without running over the spillway.
Negotiations for the letting and future management of the Lido for leasing require the water level to be allowed to rise to the level of the spillway, ie for there to be no apparent protection from flooding at all. Whilst in the past that high level has been attained quite often, in recent years with changing climatic conditions the level has continued to reduce to that which we see today. It is not suggested in the letting arrangement that anything should actually be done to increase the level above this, but consequently nothing would be done to prohibit the water rising to its hitherto levels if the climate changed.
For information, during the past 15 years two instances of serious flooding have occurred in the area south of the reservoir. In 1977 and 1988 houses were inundated with flood water to depths approaching 1 metre above floor level and the safety of the residents was threatened. It is known that during the 1988 storm the reservoir spillway was surcharged to a depth of at least 2/3 metre and high volumes of water from the reservoir contributed greatly towards the damaging flooding.
The depth below spillway that would prevent this type of flooding up to storm frequency of 1 in 100 years is the depth at which, due to the spell of exceptionally dry weather, the lake has remained for many months. This depth is 1.5 metres below spillway and the engineering recommendation would be that the lake level be returned to that level after any storm by use of the drainage sluices which exist at the bottom of the lake.
This would be contrary to the requirements of the leisure users and the proposed lease. The lake would then become virtually useless for water-borne activities. This problem could be resolved by removing the silt and resealing the lake bottom where necessary, hence providing a sufficient depth for sailing and other leisure and amenity uses while keeping the water level at -1.5 metres.
The cost of this work would be considerable and would vary according to the methods of disposing of the silt and sealing the lake. It is possible that grant aid would be available from the Ministry of Agriculture, Food and Fisheries and that contributions to flood relief and the maintenance of water-borne activities could be forthcoming from the National Rivers Authority.
It must be emphasised that had the dam not existed the flood protection which it potentially provides would cost many millions of pounds to construct and the Council therefore possesses a flood prevention asset of great value.
Should the de-silting of the lake prove to be impossible to fund, it is hoped to make other proposals which would also safeguard the downstream residents without losing the recreational value of the area. Up until that time the risk as it exists at the moment would remain.
Investigations are being carried out into the problem by the Engineering Consultancy and further reports will be made to this and other Committees. 2 April 1992
*At the time this report was written the Council was in negotiation to lease the whole of Ruislip Lido to a company called Eau Naturelle. These negotiations subsequently fell through and the Council has since maintained management control of the Lido.