Portsmouth was one of the first cities in the UK to benefit from a dedicated sewerage system in 1865, a time in which foul and surface water systems were combined because that type of system was adequate due to lower flows and less paved areas.

The City’s drainage network continued to develop over the years; it remained dependent on the use of combined sewers and the Eastney Pumping Station, however, and this eventually led to problems. During periods of high rainfall, incoming flows exceeded the capacity of the system, and excess flow was pumped from the Eastney pump station to 40,000m3 capacity storm tanks at Fort Cumberland; once the tanks are filled, flows are discharged to the sea.

This system did not work that well though, especially during big storms, as nearly 50% of the entire surface area of Portsmouth drains to Eastney, causing extremely large flows of more than 20,000 l/s during storm conditions. An exceptionally severe storm overwhelmed the Eastney pump station on 15 September 2000, causing the pump room to be flooded, which stopped the pumps. This resulted in more than 750 properties being flooded.

The Portsmouth Flood Alleviations Scheme was created to divert surface water to the sea instead of entering the combined sewer, thereby freeing up capacity in the existing infrastructure.

The Portsmouth Flood Alleviations Scheme included:

  • Substantial works such as new surface water pipes, culverts and swales were created, and several sections of the public sewer were remediated to divert runoff and tidal ingress;
  • A smart hydrometeorological monitoring system to enable prompt diversion of excess water during periods of high rainfall;
  • Real-time Sewer level monitoring; and
  • Intelligent rain gauges (smart sensors) combine with radio telemetry to inform a computer based catchment-wide water model

The system provides advance notice for staff at the Eastney pumping station that diverts large quantities of water to storage tanks during periods of heavy rain, thereby dramatically reducing the risk of flooding in Portsmouth.

The flood alleviation scheme has been a success and recently surface flow from 34 Hectares of paved area was diverted by the project, averting previous flooding.

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