Water & Wastewater Treatment Magazine
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Databank Sewer flooding performance is strongly influenced by weather conditions, and the indications are that climate change could bring about a greater number of extreme rainfall events. As of last year, Met Office records showed that there had been 17 record-breaking rainfall months or seasons since 1910, and nine of those had taken place since the turn of the mil- lennium. Nonetheless, the water and sewerage companies in Eng- land and Wales have overseen decreases in both internal and external incidents over a prolonged period of time. The total number of internal sewer flooding incidents in 2018-19 was 3,252; by contrast, that figure was almost 12,000 in 1992-93. The 2016-17 period brought a spike, with internal incidents up to 5,522, but de- creases in the two subsequent years have brought the overall trend back on track. The number of incidents of sewage flooding onto gardens and private land has fallen consistently over recent years, dropping from 40,767 in 2013-14 to 23,489 in the latest results. There is reason to believe there could be further improve- ments. In its assessment of com- panies' plans to reduce sewer flooding in AMP7, the Con- sumer Council for Water said it is "generally encouraged by the overall commitment to sustainable drainage systems, innovation such as smart sewer schemes and that some companies are leading on groups to work in partnership with other stakeholders". Ofwat's PR19 methodol- ogy requires companies to use the number of internal flooding incidents per year – including the impact of se- vere weather – as one of their common performance com- mitments, while the number of external flooding incidents per year is one of its long list of asset health metrics with standard definitions. SEWER FLOODING United Utilities had 1.88 internal sewer flooding incidents per 10,000 connections The industry average is 1.29 incidents of internal sewer flooding per 10,000 connections Anglian Water had 0.4 internal sewer flooding incidents per 10,000 connections Properties flooded internally per 10,000 connections to sewers Water & sewerage companies in England and Wales External areas flooded with sewage per 10,000 connections to sewers Water & sewerage companies in England and Wales Number of properties affected by internal sewer flooding in England and Wales Number of external sewer flooding incidents in England and Wales Source: CC Water and Water UK 2015-16 2016-17 2017-18 2018-19 Anglian Water 0.81 0.94 0.57 0.4 Dwr Cymru Welsh Water 0.87 1.07 1.07 0.8 Hafren Dyfrydwy N/A N/A N/A 0.47 Northumbrian Water 1.4 0.99 0.78 0.94 Severn Trent 1.22 1.29 0.92 0.96 South West Water 1.56 1.21 1.07 0.74 Southern Water 1.91 1.7 1.45 1.52 Thames Water 1.79 2.47 1.56 1.73 United Utilities 3.4 6.41 3.01 1.88 Wessex Water 0.83 0.64 0.61 0.74 Yorkshire Water 1.84 1.83 1.85 1.75 AVERAGE 1.69 2.23 1.42 1.29 2015-16 2016-17 2017-18 2018-19 Anglian Water 9.8 9.7 8 6.2 Dwr Cymru Welsh Water 22.3 17.2 16.3 15.7 Hafren Dyfrydwy N/A N/A N/A 7.6 Northumbrian Water 9.4 6.1 5.5 5.8 Severn Trent 14.6 12.8 7.8 7.5 South West Water 19.8 16.8 13.9 15.4 Southern Water 24.7 22.7 19.4 20.6 Thames Water 11.1 8.7 5.1 3.9 United Utilities 12.1 12.9 10.5 9.4 Wessex Water 12.8 11.6 10.2 9.5 Yorkshire Water 16.4 15.7 17.4 16.2 AVERAGE 14.1 12.5 10 9.3 Source: Water UK Source: Water UK Source: Water UK 3,252 Properties internally flooded by sewage in England and Wales in 2018-19, down from 3,560 a year earlier 23,489 Areas externally flooded by sewage in England and Wales in 2018-19, down from 25,045 a year earlier www.wwtonline.co.uk | WWT | OCTOBER 2019 | 31 SEWER NETWORKS 2013-14 4,801 2014-15 4,333 2015-16 4,153 2018-19 3,252 2017-18 3,560 2016-17 5,522 Number of external sewer flooding incidents in England and Wales 2013-14 40,767 2013-14 38,240 2013-14 34,735 2013-14 31,083 2013-14 25,045 2013-14 23,489