Carshalton Ponds Refurbishment

A little background, Carshalton Ponds were last fully drained, cleared of silt and repaired in 1996. Carshalton Ponds take water from ground springs, road runoff (through gullies), and through an augmentation supply. The augmentation supply is a requirement of the Sutton and East Surrey Water Company under their licences to abstract water locally, whereby they pump water from The Wandle, near Watermead Lane (Middleton Road), upto the culvert under Honeywood Museum.

The Issues In recent yearshas been the silt build up in the upper pond to the point where if there is any low flow in the ponds it can be seen above the water level.

Maintaining water and water flow in the ponds can be complex because the ponds were designed to capture the spring water, but not let water flow back into the ground when the springs were not operating. This is achieved through the channels that run around the outside of the ponds and through a series of concrete plugs in the base that are designed to lift when water pressure dictates.

There is a build up of silt particularly in the upper pond that is unsightly when it is above water level. The pond’s water has in past years changed to some odd colours due to low flow, heat and oxygen levels, not helped by the amount of organic silt. These are artificial ponds that are shallow with concrete bases, so flow is slow in parts of the ponds, the water warms up quickly, and the amount of wildfowl plus road runoff creates quite a bit of silt.

The chanels around the edge of the top pond are damaged. This means that when the springs are not working water is able to flow back into the ground, which reduces the flow into the rest of the Wandle. The channels in the lower ponds are largely intact although there are some ponting requirements to make it fully water tight.

The seals between the concrete sections of the lower pond, and also around the plugs, need inspecting and if necessary repair to prevent further water loss from the River Wandle flow.

For this project the Parks Team has received £433,000 of Council capital funding to carry out works to the pond.

The Parks Team is working with specialist project managers in the Council’s Resources Team to deliver the project, and a specialist engineering contractor has been appointed to support the production of the specification and to advise on any required consents.

A tender exercise has taken place to appoint a main contractor for the project and Land and Water Ltd. have been appointed.  A pre- commencement meeting is being arranged after which the Contractor will provide a full updated programme of works. The start date for the works is still to be agreed although we anticipate being on site in November, and we also presently estimate a timescale of approximately 16 weeks duration for the work.

The funding for the project allows for all required works (silt removal and channel repairs) to be completed in the top pond (Honeywood) but based on tendered rates the works in the lower pond might need to be reduced depending on final amounts of silt removed against estimates, and any additional works that affect the contingency sum. 

There are currently no works planned for the canal area in Grove Park.

Local groups and stakeholders have and will be consulted along the way, and meetings have taken place with The Environment Agency and with Sutton and East Surrey Water on site. The Environment Agency has said that a Flood Risk Activity Permit (FRAP) is not required for this work.

The Sutton and East Surrey Water Company has agreed that for the duration of the project they will divert the Augmentation flow to just below the cascade in The Grove Park. There is a gauging station just below there that measures the flow of the water in the Wandle at that point. This will allow for the ponds to be drained down.

We expect the main contractor to carry out fish removal as far as is possible for the bigger fish that do not move naturally downstream as part of the work. We have purposely arranged for the works to start in the Autumn to avoid, as far as is practicable, the bird nesting season.

Although the Augmentation flow will be diverted. If anything compromises the diverted flow (such as a burst to the pipe) a flow will be fed into the upper pond from the south of the area. While this is unlikely we are aware of the requirement to ensure that the Wandle is kept flowing.

If the springs should start to flow the water will need to be moved out of the ponds into the river. It is unlikely given the very dry summer that we have had that this will be an issue, however we need to be ready.

Heavy rain will introduce road runoff into the ponds that again will need to be moved into the Wandle.

The above issues will be discussed with the main contractor, prior to commencement. The Environment Agency will be monitoring the works, and we are aware that any water moved into the Wandle must be clean or filtered so as not to let the works in the pond stir up silt that then pollutes the Wandle downstream. The Contractor is a specialist in their field, also working on projects for the EA.

Silt has been taken in the past and tested showing there is no pollution present. Silt testing will need to be carried out again, prior to work starting, to ensure that this is still the case, if it is not then this will affect where the silt can be disposed of, and could increase prices.

Any increased prices will need to come from within the project (reductions in other works), or potentially from the contingency sum, as there is no additional funding

Using dewatering pumps a temporary dam will be created to the lower ponds to isolate the lower pond from the canal in The Grove. Any fish will be relocated and silt will removed from both ponds and collecting channels.

The planned date for repairs which includes the collecting channels and pond bases will then take place in early December, with completion planned for mid January.

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