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What happens when river levels rise in York
The River Foss and the River Ouse meet in York. The Foss is a highly-urbanised catchment which responds quickly to high levels of rainfall. The Ouse is a more rural catchment that responds to heavy rainfall from the Pennines and can therefore be slower to rise.
Historically, rises in levels of the River Ouse would force water back up the Foss (a tributary of the Ouse) causing it to overtop its banks and flood surrounding properties. This dramatic effect contributed to the severity of the floods in 1947, 1978 and 1982.
History of flooding on the Foss Barrier
Following the floods of 1982, work was carried out in York to prevent and reduce the risk of flooding in the city, including the construction of the Foss Barrier in 1987. The barrier has protected the city on a number of occasions, including the floods of 2000, 2007 and 2012 and forms part of York’s city-wide defences.
How the barrier works
The barrier is a 16.5 tonne gate which is normally held in a horizontal position above the river allowing boat traffic to pass underneath it. Its primary purpose is to prevent high levels from the River Ouse from entering the River Foss. When the River Ouse is high, the barrier is lowered and it forms a seal with the river bed and prevents water ‘backing up’ from the Ouse into the Foss.
When the barrier is down, water cannot discharge from the Foss into the Ouse, so it must be pumped around the barrier into the Ouse to prevent it backing up.
Timeline of events on Boxing Day: Saturday 26 December 2015
Significant rainfall fell on Christmas Day and Boxing Day over the Swale, Ure, Nidd, Ouse and Foss catchments which were already saturated. River levels on the Foss and Ouse were already elevated and responded quickly with the Foss reaching record levels. Without the Foss Barrier and other local schemes, flooding would have been much worse.
On Monday 21 December 2015 the Foss Barrier was lowered to deal with rising levels on the River Ouse, in the normal way. On Saturday 26 December 2015 there was further heavy rainfall across Yorkshire which resulted in increasing river levels on the Ouse and a rapid increase in river level on the River Foss. Approximately 600 properties were flooded.
A Flood Alert (meaning flooding is possible, be prepared) for the River Foss was issued at 07:07am on Saturday 26 December 2015.
A Flood Warning (warning flooding is expected, take immediate action) was issued for Huntington Road and Foss Islands at 10:41am on the Saturday 26 December 2015.
A further 5 Flood Warnings to cover the whole of the area impacted by the River Foss were issued at 16:51pm on Saturday 26 December. These informed that the pumps on the River Foss were struggling to cope with the volume of water.
The decision to issue 6 Severe Flood Warnings (warning of severe flooding and danger to life) was made at 18:45pm on 26 December, when the decision to also lift the barrier was made.
The police made the decision to evacuate high priority residents at 21:00pm and started knocking on doors.
We warned around 2,300 properties at risk in the Foss catchment. This includes homes and businesses whose landlines are registered to our Floodline Warnings Direct service and Extended Direct Warnings customers in the area (who are registered to receive warnings to their mobile phone numbers).
This infographic shows what happened to river levels on both the Foss and Ouse over the Christmas period, before and after the barrier was raised. The technical hydrograph provides a more detailed picture of the same information and is aimed at a technical audience.
Why we raised the barrier
The barrier was raised on Boxing Day evening, to allow the River Foss to flow into the Ouse. The River Ouse was at a similar level to the River Foss by this time, but was rising more slowly. At this point, if the barrier had remained closed, then levels on the Foss through York would have risen more rapidly.
Water, which had already entered the workshop at the barrier, started to rise at a faster rate. It then entered the electrical switch rooms, situated on the ground floor, where the control panels for both the pumps and the barrier are housed. We are currently looking into how and why the water entered the facility.
If the barrier had been left in the down position once the pumps were turned off because water had entered the electrical switch room, we would have had no means of raising the barrier quickly. This would have resulted in very rapid and even more widespread flooding.
Raising the barrier gate significantly slowed the rate at which the River Foss rose. This provided more time for the emergency services to begin evacuation and for residents to evacuate their properties in the rapidly worsening situation. As soon as the barrier was lifted the electrical supply to the site was isolated.
The extreme flows on the River Foss caused flooding of properties on Huntington Road. The road was already flooded before the barrier was raised.
All 8 pumps were fully working on Boxing Day, they were just overwhelmed by the quantity of water. They continued to work until we took the decision to turn them off. By raising the barrier we were able to slow the rate of rising water as the water could then flow freely into the Ouse.
What we did next
Environment Agency staff and our contractors worked around the clock to get the Foss Barrier operating after the high river levels flooded the electrical switch room and affected the power system.
A Chinook helicopter was used to lift equipment onto the Barrier’s pump-house roof on Monday 28 December. This equipment enabled us to lower the barrier and restart 4 of the pumps at 00:50am on Tuesday 29 December. The army also assisted with the installation of a temporary bridge to route electric cabling across the river on the morning of Wednesday 30 December. This meant we were able to start the remaining 4 pumps and the barrier was fully operational.
The current situation at the Foss Barrier
Environment Agency staff are working in the area, checking defences, clearing blockages and our incident room team continue to monitor the weather situation for further rain. The barrier and pumps are operating as designed now.
Find out if you are at risk of flooding You can also contact Floodline on 0345 988 1188