The Foss Barrier consists of a retractable floodgate and pumping station owned and managed by the Environment Agency. It straddles the River Foss in York, just above the confluence (where two rivers join) with the River Ouse. The barrier’s purpose is to prevent water from the Ouse flowing upstream into the Foss, raising water levels and flooding properties along the River Foss. The pumps next to the flood gate move water around the barrier via a side culvert so that it can continue to drain into the Ouse. This prevents water levels on the Foss rising when the barrier is closed. The Ouse’s normal water level at York is approximately 5 metres AOD (Above Ordnance Datum). When the river level reaches 7.5 metres AOD, the barrier is lowered into the Foss, shutting it off from the Ouse. The electrically driven barrier takes less than five minutes to lower into position. The barrier operates as a ‘turn and lift’ gate (see below) so it opens and closes in a similar way to a garage door. The pump-house attached to the barrier maintain the water level in the Foss at around 6.5 metres AOD. When the flood subsides and the level of the Ouse drops back to 6.5 metres, the levels in both rivers are equalised so the barrier opens, and the pumps shut down.
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History of the Barrier and the 2015 Boxing Day Flood
After many years when the River Foss breached its banks and flooded surrounding properties, the York Water Authority began working on the Foss Barrier after floods in 1982. It was first put into operation and tested in 1987.
Before construction, an analysis was carried out to determine the realistic maximum flow rates of the River Foss during an event. This indicated that a flow of 30 m3s−1 (metres cubed/tonnes of water per second/ cumecs) was very unlikely to be exceeded. As a result the pumping station capacity was set to 30 cumecs.
The barrier was officially opened in 1988 and operated successfully for almost 30 years until Boxing Day 2015. On the 26th December 2015, whilst the barrier was closed, extremely heavy rainfall in the River Foss catchment area led to a previously unprecedented discharge rate of approximately 40 cumecs. This overwhelmed the pump’s capacity at the barrier and water levels on the Foss began to rise by roughly 10 m3/s.
Whilst the 16.5 ton barrier remained closed, the water levels in the Foss continued to increase which led to the difficult decision to open the barrier. If this decision had not been made, the rising water could have damaged the electricity supply for the whole station and would have meant the barrier would have been stuck down with the pumps out of action. With the barrier closed and the pumps out of action, there would have been even worse flooding on the Foss. Therefore, the decision was made to open the barrier whilst it still had a working power supply.
As a result of the Boxing Day event, roughly 600 properties including the pumping station were flooded. In an independent report published in 2016, it was found that this decision to open the Foss barrier reduced the severity of flooding along the River Foss. Since then, the barrier has undergone major improvements whilst remaining fully functional. These improvements should be completed by summer 2022.
Improvements
Following the 2015 Boxing Day event £38 million was allocated to upgrade the barrier and the surrounding infrastructure. The improvement with the highest priority was to increase pumping capacity at the barrier. A similar study to the one conducted for the original barrier was carried out and found that the capacity should be raised to 50 cumecs. The higher discharge rates are mainly due to climate change and land use changes which result in water reaching the river more quickly due to there being a greater area of impermeable surfaces in the Foss catchment to allow for infiltration to occur. This causes rivers to fill up more quickly than in the past. A higher pumping capacity has been achieved by installing eight new 500 kW pumps of the same size into the existing holes of the old pumps. Using pumps of a different size would have caused significant delays to the project. Modern technology allowed the same number and size of pumps to be used whilst still increasing the maximum pumping capacity by more than 65%. Each pump weighs 6.5 tonnes and has variable speeds to give operators more control over water levels on the Foss. In comparison, the old pumps could only be switched on or off and had no variable speeds. Work began in September 2016 to replace the pumps with each pump taking a week to install. The barrier was operational with maximum pumping capacity by the winter of 2017