Plaque at Tower Gardens
The Environment Agency states that the normal water level – sometimes called the normal summer water level – in York is established at 5 metres Above Ordinance Datum (AOD). Ordinance Datum is the internationally recognised standard value for sea level. There is a gauge board at Ouse Bridge showing the height that recent major floods have reached. It is calibrated in feet and inches with zero set at normal water level.
York sits on the confluence of the River Ouse and its tributary, the River Foss. The interaction of the two rivers with the significant amount of rainfall the catchment attracts from the Rivers Swale, Ure, Nidd and Wharfe combined with the melting of accumulated hill snow from the Pennines makes the city particularly susceptible to flooding. Historical research has shown that the force of the rivers overflowing has been a serious concern to the citizens of York for many centuries.
1263 Flood water rampages down North Street as far as the junction of Bridge Street and Ouse Bridge.
1316 Floods hold York Castle to siege washing away essential earthworks.
1564 An ice jam,caused by huge chunks of thawing ice borne upon flooding waters smashed away parts of Ouse Bridge.
1625 and 1638 Highest floods in York ever recorded.
More recently there was serious flooding in 1947, 1978, 1991, and 1995. However, 2000 saw the worst floods on record for that century. The River Ouse rose to an astonishing 5.5 metres above its normal level, flooded 540 properties and put a further 320 seriously at risk. For the next three days the commercial and industrial life of the city was virtually brought to a standstill. When the waters subsided the damage at that time was estimated at well over £2 million.
More recently, following heavy rain which began on 21 December 2015, water levels in the Rivers Ouse and Foss rose rapidly. The Foss Barrier was duly lowered and the pumps activated. However, the water flows exceeded the capacity of the pumps and, in addition, water leaked into the pump house affecting the electrical operation. At 3pm on 26 December, a decision was made to raise the barrier and the pumps and power were turned off. This led to extensive flooding in central York requiring the evacuation of properties and businesses and resulting in major damage. An enquiry, public meetings and exhibition were held in 2016 resulting in a £45 million investment to upgrade the capacity of the barrier pumps along with numerous other flood defence projects to prevent a repetition of the catastrophic floods of Boxing Day 2015.
Due to the complex geography of the area, York is at risk from a variety of sources. This has called for a coordinated and innovative approach to controlling floods. Following the floods of 1978 a series of flood defence improvements were initiated and, since 1979, work on defending York from the destructive force of flooding has continued. A number of separate yet coordinated schemes in the city have now been implemented at a cost of around £10 million. These include a number of flood barriers to help direct flooding from the Ouse to the Foss. These are combined with arrangements to pump internal flows from sewage systems and internal watercourses to prevent pollution. Work has also been carried out to improve and use the upstream washlands, such as Clifton Ings, in order to reduce the frequency and extent of the flooding in the city. Steps have also been taken to protect fresh water treatment plants.
Because of the delicate relationship between the Foss and the Ouse, rising water in the Ouse can often result in a dangerous reaction in the Foss. A rapid increase in the volume of water in the Ouse would force the Foss back on itself causing it to overtop its banks and flood surrounding properties. It was this dramatic effect that contributed to the severity of the floods in 1947, 1978 and 1982.
Many conventional flood control systems such as channel improvements or bypass channels were inappropriate because of the nature of flooding on the Foss. The solution was a moveable barrier system – a large ‘turn and lift gate’ which, when in place, effectively isolates the Foss from the Ouse, stopping water from surging back upstream. Because this would also prevent water naturally flowing from the Foss into the Ouse, a system of high volume pumps was installed. When the barrier is lowered, the optimum level of water is maintained by pumping water around the barrier directly into the Ouse.
A flood wall was also built between the pumping station and the higher ground at Skeldergate Bridge helping to keep the Foss and the Ouse separate and protecting the access road to the pumping station. Another important factor was that the site is close to the city centre and this needed to be taken into consideration. Consequently special attention was paid to the visual impact of the structure. Natural materials such as sandstone were combined with glass to allow the barrier to blend into its surroundings. The scheme has received several awards including the Institute of Civil Engineers and the Brick Development Association. Total cost was £3.34 million.
When the River Ouse reaches 7.4 metres AOD, the duty officer is alerted. When the Ouse reaches 7.8. metres AOD, the barrier is lowered taking four minutes. Visible and audible alarms warn any boats. Once the barrier is in place the flow from the River Foss is transferred around the barrier and into the Ouse by up to eight pumps. These pumps automatically maintain the water level of the Foss at around 6.5 metres AOD. When the flood subsides and the level of the Ouse drops to 6.5 metres AOD, the levels on either side are equalised, the gate is opened and the pumps shut down.
Flood prevention measures
A variety of other solutions apart from the Foss Barrier have been constructed:
- North Street: In 1992-3 a series of flood gates and walls were constructed to protect this important area and also a main trunk sewer which can be isolated and pumped. The architectural landscaping of the pumping station has been located in a secluded corner of the gardens and flood walls have been placed alongside the road leaving riverside views unaffected.
- Lower Ebor Street: Badly flooded in 1978 and again in 1982. A combination of flood walls with steel trench sheeting has been constructed as well as earth embankments. Valves have been installed to isolate sewage, incorporating a small pump to evacuate sewage when river levels are high.
- Holgate Beck: Upstream tributaries of Holgate Beck were diverted to discharge flow directly into the Ouse downstream of York to prevent flooding in the Acomb area of York. Upstream of York, where Holgate Beck joins the Ouse, a two-pump station was built to control levels.
- Lower Bootham: The flood in this residential area caused £1.2 million worth of damage to 134 properties here. The flood alleviation scheme to Lower Bootham was installed in 1983 compromising a 650 metre earth floodbank, combined with a 280 metre concrete wall, providing a defence 460 millimetres higher than the highest floodwaters of 1982. The local sewage network has been modified and isolated at three key points to allow sewage to be pumped during times of high water levels. The scheme received an award from the Institute of Civil Engineers for its outstanding excellence in concept, design and execution.
- Acomb Landing: This is the site of the water treatment works supplying fresh drinking water to the city. After the 1982 flood, a reinforced retaining wall was added to the existing embankments significantly raising the level of protection.
- Clifton Ings: This is a natural flood plain upstream of York which can store 2.3 million cubic metres of water. In 1982 at a cost of £1.25 million, the existing flood banks were raised to provide greater storage. Sluice controls for letting water in and out of the Ings were constructed. This system is effective for medium-order floods of up to 4.27 metres AOD. For higher-order flooding, the site is designed to let banks overtop allowing the full capacity of the site to be used.
- Leeman Road: In 1978, 225 houses were seriously flooded. In 1980 a flood bank was constructed in front of the houses and the sewerage system improved allowing sewage to be pumped out when the river levels are high. During the 1982 flood, high winds blowing over Clifton Ings generated large waves which overtopped the Leeman road defences. The flood bank was raised in response to this.
Causes of flooding
The debate on causes of floods covers numerous issues including climate change, global warming and upland farming management. Encouraging habitats such as upland bogs and moors, woodlands, wetlands and species-rich grasslands can act as giant sponges reducing run-off. Research by various universities and research bodies is producing valuable evidence towards the debate.
‘Hard-engineered’ flood defences such as flood walls, embankments and barriers will continue to be key methods of flood control. But they are expensive and not always environmentally friendly as they rarely improve the ecological or aesthetic value of the river corridor according to the Forestry Commission Forest Research on Flood Risk Alleviation. Flood defence engineers are now shifting their attention to the potential of using ‘soft-engineering’ techniques for more sustainable development. These concentrate on techniques to reduce run-off, woodland being one such solution. A case study by the Forestry Commission on the Rivers Laver and Skell catchments west of Ripon shows that floodplain woodland alleviates downstream flooding.
The Flood Risk Management Research Consortium (FRMRC) by Imperial College and Bangor Universities has worked on a case study at Pontbren in mid-Wales. Land use management intervention in catchment areas studies land use and permeability by enhancing vegetation such as woodland and moisture retention of soils. Work by Professor Jane Rickson and her team at Cranfield University is on soil erosion and conservation using a rain simulator which shows run-off. Many soils are compacted by machinery and farm animals so do not absorb rainfall. Drainage channels are cleared causing run-off and certain crops such as Maize grown in defined rows can also increase run-off.
© Pat Hill