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Source: City of Southampton

During the early hours and into rush hour on 24 September 2019, Southampton saw a band of very heavy rainfall (in excess of 32mm/hour). Crews from Balfour Beatty, our highways partner, were dispatched to assist where flooding was occurring, including at Millbrook Road West. They remain on standby while the threat of intense rainfall remains.

The issue of flooding at Millbrook Road West is not due to blocked gullies (gridded drain covers) or maintenance issues managed by our highways team as incorrectly reported.

This is a complex situation involving infrastructure owned by a number of other stakeholders, including Associated British Ports (ABP) and Southern Water, and not Southampton City Council.

The Millbrook Road West / Paynes Road junction, where the flooding occurred, is the point at which multiple surface water sewers draining large areas of Freemantle meet and flow into a pipe under the railway line. There are two surface water sewers flowing into a single access chamber (where flooding first appears). These merge into another surface water sewer, owned by Southern Water, before connecting into a pipe owned by Associated British Ports (ABP). This pipe is then drained by a nearby pumping station owned and manually operated by ABP.

This Southern Water sewer is deemed to have insufficient capacity. With the capacity issues in the main sewer leading to the pipe, water will be unable to drain away from gullies, and in many cases it will appear that water is surcharging and flowing out of the gully once capacity within the gully itself has been filled. This does not mean that the gully is blocked – in this location water simply has nowhere else to go.

We will continue to work with partners to try and find viable solutions to this complex issue that results in highly disruptive flooding at this location. In addition, Southampton City Council will be looking into viability for options to manage water elsewhere in the catchment in an attempt to reduce water volumes arriving at this critical location.

Background information

Millbrook Road West was historically the shoreline of the city, prior to reclamation of the docks by Associated British Ports (ABP). During its repossession, the point of water discharge, previously naturally into the Test Estuary, was realigned and diverted into a pipe (located underneath the railway line and follows the railway west) to allow building works to take place above ground. Water now flows along the underground pipe (owned and maintained by ABP) before curving southward and discharging close to what was the King George Dry Dock before it was permanently flooded.

With the low land levels here and the tidal nature of the Test Estuary, the channel near to the King George Dry Docks is often below high water. Tidal flap valves are fitted to try and prevent tidal water flowing into the pipe and to reduce capacity, however, this also reduces the rate at which water can discharge during rainfall, especially if the tide is also high. ABP own and operate a pumping station at the King George Dry Docks which is used to pump water out of the pipe against the tide and to move water out of the system to free up capacity for water arriving from the wider catchment area. Currently, the pumps are manually operated by ABP and are usually switched on following an alarm which is triggered when water levels in the pipe are increasing.

MIL OSI United Kingdom