The Cavern and completion of the town drainage scheme

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WRITTEN QUESTION TO THE PRESIDENT OF THE ENVIRONMENT AND PUBLIC SERVICES COMMITTEE BY THE DEPUTY OF ST. JOHN

ANSWER TO BE TABLED ON TUESDAY, 9th NOVEMBER 2004

Question

Would the President advise members –

( a ) how many occasions in 2004 to date has the Cavern not been able to cope with heavy rainfall and has

overflowed into the sea?

( b ) when the town drainage scheme will be completed and whether plans are still in place to separate

rainwater from the sewerage system?

Answer

1. Since 1st January this year to date, therehavebeenthree occasions when the Cavernhasbeenunable to contain all of the excess flow duringtimesofheavy rainfall, and there has been a resultant discharge to sea.

T  hese spills occurred on 7th/8th July, 19th August and 11th/12th October. In all cases, the flows in the

sewerage system were very high following prolonged periods of extremely heavy rainfall. The main pumping station at First Tower was working at the design capacity transferring flow to Bellozanne for treatment, with the excess flow spilled to the Cavern, but the duration and intensity of the rainfall on these occasions was such that the ability of the Cavern to store all of the excess flow was exceeded and flow spilled to sea. The storage capacity of the Cavern is 25,000 cubic metres or 5.5 million gallons.

J u ly 2004 was the fourth wettest July on record, and on 7th/8th July, some 51 millimetres of rainfall was

recorded, which was the second wettest July day recorded at Maison St Louis in 110 years.

O  n 19th August, the east of the Island experienced a localised and extremely intense rainfall event, centred on

St Helier, with some 24 millimetres of rain falling in a very short period of time. The intensity of the rainfall was such that the flow entering the Cavern backed up in the feeder system, triggering the emergency level sensor at the Weighbridge. Although the Cavern continued to receive the majority of the excess flow, this sensor allowed some flow to spill out to sea for approximately 45 minutes during the peak of the storm before closing again. The system contained the remaining high flows until the storm abated.

O  ctober 2004 was the wettest October on record since records began some 110 years ago and on 11th/12th

October, some 60 millimetres of rainfall was recorded, again making this one of the wettest days on record.

A  lthough the rainfall events on these three occasions exceeded the ability of the Cavern to store all of the

excess flow, it is important to note that excess flows were successfully contained in the Cavern during rainfall events on 63 other occasions since 1st January 2004, thus preventing spillage to sea on all of these occasions.

2. The replacement,reconstructionandupgrading of the sewerage system inSt Helier, such asthe recently completed scheme in Gloucester Street, is anongoing process which will continue for manyyears to come.

W  ork on the construction of an overflow chamber, link sewers and a shaft in Phillips Street is programmed to

commence in spring 2005. These works will complete the project to divert excess flows from the foul combined sewers into the Cavern during times of heavy rainfall, thus significantly reducing the risk of flooding in the central area of St Helier. The sewage will be held in the Cavern until the storm has passed and will then be returned to the main foul sewerage system for treatment at Bellozanne.

T  he Committee is continuing the process of separating surface water from the foul combined sewerage

system where this is practically and economically feasible. The scheme which is nearing completion in the Union

Street area and the works undertaken in Wellington Road this summer, are examples of this type of project.

F u rther water separation schemes are included in the current five year sewers capital programme but the

extent of the work that can be undertaken each year is severely limited due to the significant reductions in the proposed capital funding allocations for the sewers programme to be made available to the Committee during this five year period.

A  t some stage the issue of funding for the sewerage network needs to be resolved. I intend to discuss options

for funding with the Presidents as part of the 2006 budget allocation.