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BOROUGH ENGINEER AND SURVEYOR'S SERVICE
DRAINAGE DIVISION.

MUSWELL STREAM FLOOD RELIEF SCHEME
FACT SHEET.

These notes will give a brief outline of the work behind the scheme and why it is necessary.
In a catchment area stretching from near Muswell Hill to Green Lanes near the North Circular Road, flooding has occurred regularly to up to 100 properties at a time, following heavy summer thunderstorms.
Some of the properties have been flooded several times a year resulting in considerable damage to residents' furniture, carpets etc., and obviously causing upset, disturbance and hardship.
The cause of the flooding is the lack of capacity of the public surface water sewers during and after heavy storms.
If much larger sewer pipes were laid the problem of very high flows of water would only be transferred to a point further downstream, which in this case would mean flooding would occur elsewhere.
The answer to this problem is to build two large retention tanks which can hold large volumes of water. During a storm, water from the surface water sewers in the area flows into the tanks through very large pipes. A small outlet pipe at each tank allows just the right amount of water to flow out of the tank which will not cause the existing pipes to overflow.
During a typical storm it would take about ¾hr to fill the Woodside tank, but it would take up to 5 hrs to empty.
The effect of a storm, the peak of which only lasts for a short period, would then be reduced considerably.
Location of the two tanks was determined by many factors, but primarily by the suitability of open land. Knocking down, say, 60 houses to build the tanks would not have been very popular with the residents and the cost would have been enormous.
Having decided to build the tanks at Woodside Park and Albert Road the catchment area was analysed to study the effect of the tanks, if they would work and also how big they should be.
When designing a scheme such as this it is important that careful estimation is made of how much rain will fall in an area whilst a storm travels over it. Since the intensity of some storms is greater than others, i.e. more rain falls in a given time period, this also has to be given, careful consideration.
Once the size of the tanks is finalised the design of the tank structures is carried out. Construction using reinforced concrete is the best method in terms of durability and constructional and maintenance costs, so the tanks are designed using this method. Since the tanks have to be below ground level the design is quite complicated and has to allow for the various pressures exerted by the ground, and the very large volumes of water likely to enter the tank.
The tanks are basically concrete boxes with concrete columns inside supporting a concrete roof. A smaller, but still very significant, part of the works is the pipework associated with the tank which includes heading (tunnelling) under High Road from Woodside Road to Berkshire Gardens in Enfield for the outfall pipe from the tank. The attached drawing shows the heading line from MH9 to MH1.
Finance for the cost of the scheme comes from Thames Water Authority and Haringey Council act as their agents, i.e. they do the work on behalf of TWA.
So it can be seen that before construction works actually start, many months of research, analysing, planning and designing, by Engineers, has to take place.
The attached fact sheet should give a good idea of the size of the works involved in constructing the Woodside Park retention tank plus some interesting comparisons.