The floods of December2015 in northern England

Name Peter Spencer, David Lindsay, Guy Dixon (JBA), Matthew Parkes

Job title Advisor, Modelling & Forecasting, Inland HydrologyDate 30 August 2016

Aerial photographs © Environment Agency and Peter SmithAerial Photography

What was significant about these events?

17,500 flooded properties

Affected much of northern England

Some locations were flooded more than once

Record 24, 36, 48 hour UK rainfalls

New highest recorded flows in England(~1,700m3/s) on Eden, Lune, Tyne

Highest-ever recorded flows at many rivergauging stations

Some locations had their highest floods in thehistorical record

December 2015 – Flooded Properties

Cumbria & Lancashire

Cumbria 6870

Lancashire 2340

Kendal 2,140

Carlisle 1,930

Keswick 730

Cockermouth 630

Lancaster 360

Croston (on 26th) 340

Total 17,500

How do the December 2015events compare with others?

2015 more widespread & more severe

Properties flooded in Cumbria

2005: 2,500

2009: 2,240

2015: 6,870

December 2015 – Flooded propertiessGreater Manchester Merseyside & Cheshire

Greater ManchesterMerseyside & Cheshire

GMMC 2,500Salford 750

Radcliffe 670

Rochdale area 230

December 2015 – Flooded propertiessYorkshire & Northumberland, Durham, Tees

Yorkshire 5,800(80% in West Yorkshire)

York UA 630

NDT 300

Tadcaster

Mytholmroyd

Leeds

Weather sequence

Northern England - around twicethe normal rainfall in November

By the start of December groundwas wet & lake levels were high

Named storms in December:Desmond (4&5th), Eva (23&24th),Frank (30-31st)

5th & 6th, mainly Cumbria, Lune,Wyre, Tyne

Continuing rainfall in December

On 25th & 26th, mainly Lancashire& Yorkshire

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Rydal Hall - daily rainfalls Nov & Dec 2015

Storm mechanisms

“Warm conveyor” – prolongedmild & moist southwesterlyairstream

Significant orographicenhancement

Prolonged and heavy rainfall

Strong winds (Desmond & Eva)

Storm mechanism in Desmondsimilar to Jan 2005 & Nov 2009

Desmond and Nov 2009 lastedfor around 36 hours

Overview - 5th & 6th December 2015

Affected most of Cumbria (except far west &southwest), and the Lune, Wyre, Tyne

Serious flooding in Carlisle, Cockermouth,Kendal, Keswick, Lancaster – and many villagesin NW & NE England

New flood defences overtopped at Carlisle,Keswick, Cockermouth

Wyre Flood Storage Basins completely filled

57 Severe Flood Warnings issued for Cumbria &Lancashire

Rainfall - 5th & 6th December 2015

Rainfall : 5th & 6th December 2015

Station Catchment \Town

12 hour 24 hour 36 hourRainfall

(mm)ReturnPeriod

Rainfall(mm)

ReturnPeriod

Rainfall(mm)

ReturnPeriod

Northwest

Skelton Eden 75 130-150 138 >1000 183 >1000

Thirlmere u/s Keswick 196 >1000 325 >1000 401 >1000

Honister Derwent 209 250-600 341 >1000 367 >1000

Kentmere u/s Kendal 106 25-75 183 60-100 225 100-200

Turnerford Lune 68 20-30 110 75-100 137 150-300

Abbeystead Wyre 66 10-15 97 20-40 115 25-60

Northeast

Chirdon North Tyne 56 30 94 100 122 180

Garrigil South Tyne 112 130 151 160 179 180

Overview - 25th & 26th December 2015

Rainfall less than 5th & 6th DecemberLancashire and Greater Manchester - mainly theRibble, Douglas, Roch & Irwell catchmentsYorkshire – mainly Calder, Wharfe, Aire, Ouse

FloodingLancashire - Ribchester & Whalley, St Michaels,Wigan and CrostonGreater Manchester – Radcliffe, Rochdale, SalfordSome part of Cumbria (again)Calderdale (Mytholmroyd, Hebden Bridge),Tadcaster, York, parts of Leeds

Rainfall - 25th & 26th December 2015

Rainfall : 25th & 26th December 2015

Raingauge Catchment 12 hour 24 hour 36 hourRainfall

(mm)ReturnPeriod

Rainfall(mm)

ReturnPeriod

Rainfall(mm)

ReturnPeriod

Northwest

Great Harwood Ribble 61 18 95 89 103 75

Common Bank Douglas 53 15 73 30-40 81 30-40

Holden Wood Irwell 71 25-35 109 50-120 128 50-160

Cowm Res Roch 70 15-20 107 40-70 116 30-50

Northeast

Scargill Nidd 42 4 77 20 82 15

Thruscross Wharfe 55 10 102 60 108 45

Hydrometric aspects – flow stations

In Cumbria & Lancashire42 stations had faults, 32 of thesefloodedRecovered most peak levelsmissed in the event

Improve peak flow estimates by:Use of modelled top-end ratingsConsistency between stations alongthe same watercourse

Uncertainties remain due to:Inability to gauge such eventsInability to measure floodplain flowsChanges in channel morphology

Gauging stations with highest ever flow inDecember 2015

NE 25-29th Dec

NW Dec

NE 5&6th Dec

Rivers : 5th & 6th December 2015River Station Rank Peak flow

(m3/s)Peak as ratio

of QMEDIndicative

return period

NorthwestEamont Pooley Bridge 1 268 4.45 350Petteril Harraby Green 1/2 110 3.26 175Caldew Cummersdale 2 279 1.75 20Eden Sheepmount 1 1900 3.09 300Greta Low Briery 1 350 3.15 175Derwent Ouse Bridge 1 450 4.36 500Derwent Seaton Mill 1/2 600 2.93 300Kent Victoria Bridge 1 403 2.80 150Leven Newby Bridge 2 224 3.09 150Lune Caton 1 1740 2.39 100NortheastTyne Bywell 1 1730 2.04 95-190South Tyne Haydon Bridge 2 915 2.02 50-80North Tyne Reaverhill 2 716 2.12 50-75

Rivers : 25th & 26th December 2015River Station Rank Peak flow

(m3/s)Peak as ratio

of QMEDIndicative

return period

Northwest

Yarrow Croston 1 79 2.34 150

Calder Whalley Weir 1 501 2.94 450

Roch Rochdale 1 93 2.13 80

Roch Blackford Bridge 1 192 2.67 150

Irwell Irwell Vale 1 177 2.04 70

Irwell Bury Ground 1 284 2.54 100

IrwellManchesterRacecourse

1 500 1.86 50

Northeast

Nidd Hunsingore 1 297 2.47 80-100

Calder Mytholymroyd 1 276 3.26 100+

Ouse Skelton 2 544 1.69 40-50

Aire Armley 1 350 2.49 200+

Aire Lemonroyd Weir 1 266 1.72 30-50

Wharfe Tadcaster 1 480 2.23 100

Catchment rainfalls and flood volumes

Return periods of raingauge data higher than riversBut averaged rainfall return periods over large catchmentsgenerally similar to river return periods

River Station Rainfallreturn period

River returnperiod

Eden Sheepmount 170 300

Derwent Seaton Mill 220 300

Lune Caton 230 100

Douglas Croston 140 150

Flood volumes

With initially wet catchments and long duration storms,flood volumes and percentage runoffs were very high.Many percentage runoffs in Cumbria were around 90%.

North west EnglandAMAX where maxima on 5&6th December

North west EnglandAMAX where maxima on 25&26th December

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Irwell at Manchester RacecourseAnnual Maxima (provisional) from 1941

Appleby

Historical flooding76 floods from 1800 to 19761.7m above previous gaugedhighest (Jan 2005)442m3/s, ~return period 200Flooded properties:

Dec 2015: 176Nov 2009: 4Jan 2005: 53

One location was flooded threetimes in December

Eden at Carlisle : Highest events from 1848

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Flood Alleviation Schemes built at Carlisle, Keswick,and Cockermouth after the 2005 & 2009 floods.

Design return periods were quoted as 200, 75, and100 years respectively.

All three schemes were overtopped in 2015.

Detailed flood estimates made using the standard UKFlood Estimation Handbook (FEH) with updates.

Statistically, the chance of such events is pretty remote

So are our methods up to the job?

Questions after the events on 5&6th Dec 2015

1. Are there factors leading to:Short-term “bunching” of flood peaks

Long-term changes affecting flooding

2. Should we make more use of long-termapproaches such as historic data & continuoussimulation?

Questions after the events on 5&6th Dec 2015

Questions after the events on 5&6th Dec 2015

3. FEH methods - The statistical methods in theFEH can use data from anywhere in the UK.

Should methods make more use of local data?

Do we select the right stations?

A pooling group should behydrologically similar

Stations are selected forpooling groups mainly oncatchment area & SAAR

So selected stations can havevery different floodcharacteristics (e.g. QMED)

And very differentpermeabilities (SPRHOST)

4. Are very large floods different?Flood runoff?

Flood routing?

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Example lake routing

Questions after the events on 5&6th Dec 2015

5. Is Cumbria different?Is transfer of flood data fromother locations appropriate?

Are rainfalls different?

Is Cumbria showing the effectsof climate change first?