Fluvial Design Guide - Chapter 10

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Case study 10.1 – Leigh barrier

The Leigh barrier was constructed in 1980 to provide flood alleviation for Tonbridge. It comprises an embankment dam across the valley of the River Medway upstream of Tonbridge, with a gated flow control structure to regulate outflow from the reservoir. The embankment dam is some 1300m long and up to 5.7m high. The maximum capacity of the reservoir is 5.8 million cubic metres, with an operating depth range of 4m.


The maximum volume that can be stored in the reservoir is approximately 15% of the volume of the 100-year flood. Operation of the reservoir to lower river levels in Tonbridge requires considerable judgement to ensure the storage volume is used effectively. Filling the reservoir too early in a flood event could result in the peak passing through the reservoir with no attenuation. On the other hand, delaying the storage of flood flows could result in only partial use of the reservoir and unnecessarily high flood levels in Tonbridge.

Operation of the barrier during a flood requires analysis of a wide range of data including rainfall records, weather forecasts, recorded flows upstream and downstream, observations of flood levels in the river system; as well as the flow into and out of the reservoir, and the reservoir level (which must not exceed a defined upper limit). In effect the operators have to predict the incoming flood hydrograph in order to determine the rate at which the reservoir can be filled.

In the severe floods of October 2000, the barrier was used very successfully but the operation required difficult decisions under pressure.

Leigh flood barrier in operation (October 2000)

Looking towards left abutment

Embankment beneath elevated highway

Review of operating rules

In 2005 Mott MacDonald was commissioned to carry out a study of the operating rules, which had remained unchanged since 1980. Development of new rules was timely, because the operating equipment on the control structure had been the subject of a major refurbishment in 2005.

The team used their mathematical model of the Medway to examine the results of a wide range of operating rules on various flood events, including the 2000 flood. Although it proved possible to develop complex rules that would result in very effective operation in the case of a particular flood, it was found that one set of rules was not universally applicable. It was therefore decided to produce a very simple operating procedure based only on one observed parameter – the water level in the reservoir – and to provide guidance for operators to allow them to deviate from the default rules in the light of prevailing conditions.

The model studies showed that these very simple default rules would allow the barrier to be operated effectively in a wide range of floods. Fine tuning of the operation to suit prevailing conditions in any particular flood event could be left to the judgement of the operators, supported by a range of guidance tools. Colour-coded guidance (see example opposite) has been produced to facilitate the job of the operators. The overall aim has been to provide a basic system that is very simple to follow, freeing up operator time for the fine tuning.

This graph below also illustrates the new default operating rules, which involve step changes in outflow as the reservoir fills. At particular water levels (when the reservoir is 50%, 67% and 85% full) a review of the operation of the barrier must take place, to determine the need (if any) for deviation from the default operating rules. Once the reservoir is full, the gates must be opened sufficiently for the outflow to equal the inflow.


The diagrams below illustrate the operation of the barrier in a large flood based on the default rules described above. On the left is a plot of inflow into and outflow from the reservoir. It can be seen that the operation is effective in achieving a significant reduction in the peak flow in the river through Tonbridge. The diagram on the right is the corres�ponding plot of reservoir level against time. It shows that the peak water level achieved in this flood is close to the maximum allowable.

Further information on the operation of the scheme in the 2000 floods can be found on the Environment Agency’s website (http://www.environment-agency.gov.uk/homeandleisure/floods/31716.aspx).

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