The Great Flood: Alberta s biblical deluge of Jon Noad

Transcription

1 The Great Flood: Alberta s biblical deluge of 2013 Jon Noad CSPG October 22, 2014

2 Brief note Tens of thousands have been killed worldwide in floods this decade; we were relatively lucky in Alberta. EUROPE Many people are still homeless or affected by the 2013 Floods. USA INDIA SE ASIA The opinions in this talk are all my own, not my employer s. Thank you to the various news services whose images I have used (with their permission): Global News; Calgary Herald; Globe and Mail and others. Thanks also to Jerry Osborn of UofC for his insights into flood frequencies across southern Alberta. Map of major world floods, 2010 to 2014

3 Introduction On June 21 st 2013 over 100,000 people left their homes in Canada's biggest ever evacuation. The flooding in Alberta over the following week was the costliest disaster in Canadian history, estimated at more than $1.7 BB insurable. Today we will look at: How the flooding started What contributed to the scale of the flooding How did it impact Calgary and surroundings What happened geologically And we will ask: Was this really a biblical* deluge? What is the chance of history repeating itself? *CBC & Globe and Mail Map showing 32 declared States of Local Emergency, covering over 40% of southern Alberta

4 Overview of the Bow River Basin Most water originates above the Pacific Precipitation from 600 mm in mountain valleys to 300 mm on eastern prairies, 25 to 50% as snow. CANMORE CALGARY Note locations of Canmore and Calgary

5 Why did the flood happen? Biggest reason for flooding was the amount of precipitation. Up to 300 mm of rain in a day, plus a metre of snow melt (average 600 mm/year), normally semi arid (snow melt record was in 2012). High pressure system in northern Alberta blocked the passage of a low pressure system to the South. Easterly winds pumped humidity onto rising slopes of Rocky Mountain foothills. Saturated AND frozen ground, plus steep slopes exacerbated runoff. Rapid mountain erosion during the flood. Rainfall: June 20 th to 21 st, 2013 >20 cm >10 cm Mike Drew/Calgary Sun/QMI Agency

6 The impact of the jet stream June 21, 2013 Blocked jet stream leads to higher temperatures to North than to South Trapped area of low pressure in southern Alberta Moist channel of air is pulled up from the GOM by the low pressure and slams up against the Rockies: the Atmospheric River June 20, 2013 June 19, Weather map, June 19, 2014 (

7 The field sketch. HIGH Large stationary High traps Low over Canmore 30 cm+ rain in 24 hours ROCKIES LOW Damp air from East CALGARY Warm damp air from Gulf of Mexico Flooding

8 IMPACT ON TOWN AND CITY

9 Cougar Creek - Canmore Cougar Creek feeds into Canmore from mountains to NE. Fed as much as 10 Billion litres of water through a 23 m gap in less than 2 days. Alluvial fan was channelised in 1960s. Previously channels had swept across the fan, now they focus flow & erosion km Max 3 day rainfall (mm) AFTER BEFORE

10 Debris flow on an alluvial fan Channel became a debris flood overnight. Ate away gardens (up to 100 feet of erosion) House locations less than ideal, 44 houses damaged along the creek. The major problem was development on fan. $1.3 MM remediation project undertaken in November 2012 to restore banks km

11 Debris flows and floods in Canmore June 2013 event classified as a debris flood. Debris flows are triggered by landslides, which evolve into debris flows when they hit a channel. Succession of deposits over time, not the first or the last. Town of Canmore website

12 Disastrous consequences: Highway 1 and 1A washed away. Canmore completely isolated. Lost highways The Creek had catastrophic flood potential (1980 engineering report). Most flooding close to Cougar Creek km AFTER BEFORE

13 80,000 evacuated (see map). Downtown abandoned for a week or more. 60,000+ homes damaged. Stampede still went ahead less than 2 weeks later. Volunteer spirit was incredible they had to turn them away: City received Governor's Award. Impact in Calgary km Elbow River Pink shading shows flooded areas

14 Why so serious in Calgary? km Inglewood late 2013 Inglewood June 21 st 2013 River 14 feet above normal depth. Affected whole city. Downtown closed off. Biggest problem was river cutting across meanders. Point bars typically heavily populated. More on Calgary in following sections View of floodplain

15 From the air June 22 nd 2013 (unseen footage) Photos by Shane Pelachaty and Phil Benham

16 Wildlife & the Zoo Potential for hippos to swim out of the building, into the flooded zoo and then into Bow River The zoo allegedly planned to move the lions, tigers and leopards to holding cells at the Calgary Court Centre Over $60 MM of damage to Calgary Zoo Only zoo casualties were a peacock and fish km Breanne Waggott caught a fish outside her home in Bridgeland, several blocks from river Calgary Zoo from the air

17 SEDIMENTOLOGY

18 Crevasse splay deposits 0 km 20 Crevasse splays represent overbank flooding deposits Deposition dominated by silt adjacent to the Bow River. Up to 50 cm of deposition recorded. Little evidence of climbing ripples due to homogeneity of sediment. Excellent mud cracks. Almost no depositional evidence remains of floods, although many fallen trees remain.

19 Crevasse splay case study - Inglewood Around 300 m from river Around 8 feet above normal river level Current ripples show flow away from river Deposits only 1 cm in thickness, comprising laminated silty mudstone BOW RIVER BLACKFOOT TRAIL DEERFOOT TRAIL Flow Flow Flow

20 Creating braid bars (1) km

21 Creating braid bars (2) km Summer 2012 November 2013

22 Flooded point bars km Inglewood GC Inner river bends inundated. Most of the worst flooding occurred on point bars, due to short cutting Houses flooded to 15 feet depth in Calgary and other towns. Floods also picked out former oxbow lakes. Manmade chute bars Oxbow

23 River bend erosion km Outer bends eroded up to 60 m into the banks of the river - Inglewood 31 km of river paths destroyed Map shows how outer banks were eaten away As of September 26 th 2013 Paths remain closed Paths reopened

24 Effect of river scour km River scoured bridge foundations leading to collapse Led to a second evacuation Trees destroyed on banks and on braid bars Little recovery since 2013

25 Almost no net deposition Sedimentological summary Makes you realise how significant 50 cm of crevasse splay deposits really are Scour had much greater impact At bridge shore ends Around bridge supports Many pathways still closed Trees broken and bowed, but not thought to have any long term impact

26 FLOOD STATISTICS

27 Comparison with historical floods Bow River has continuous record from Three largest floods in 1879, 1897, 2013, all in June. Calm period from 1932 to June 21 st 2013 Bow River 5x and Elbow River 10x average discharge (2005 was only 3x). Bow River max discharge was 30% greater than average Niagara Falls Instantaneous max. discharge m3/s FLOOD CALM FLOOD 1897 photo: looking west from south bank of the Bow River near Langevin Bridge (Glenbow Museum)

28 Flood data from other Southern Alberta rivers Red Deer Bow Oldman Milk Discharge in m 3 /s There are serious floods every five years or so: Magnitude varies Which basin varies too

29 Other studies Golder and Associates 2010 study, like larger ones done previously for the province, show that the Bow River in Calgary was hit with estimated peak flows in both 1879 and 1897 that were 30 per cent higher than the city experienced in 2013 Highest flow on June 21, 2013 of 1,740 cubic metres per second was slightly more than the downward-revised 1-in-100 year event that Golder Associates predicted and 39 per cent smaller than what the Province s 1968 study said was probable once a century Worst floods were in 1879, 1897, 1902 Studies show diminishing estimated 100 year flood levels! Provincial studies 1879, study 1977 study 1983 study 2010 study Cycle - years Max discharge (m 3 /sec)

30 The next Calgary flood? Theory that lack of cyclonic storms from 1932 to 2005 not true Data shows that storms often centred over Milk and Oldman Rivers There is approx. a 63.4% chance of one or more 100-year floods occurring in any 100-year period And a 26% chance in 30 years of a 100 year flood 2013 flood was only 60% of highest recorded discharge P90* 200 m 3 /s P10* 584 m 3 /s P1* 2093 m 3 /s Suggests that this was around a 70 year flood Bow River at Banff *Extrapolated values Bow River at Calgary

31 GOING FORWARD

32 How to prevent it happening again? Residents got a look at the new Canmore flood mitigation project, which include a debris net and a steel (submarine) mesh on the banks of Cougar Creek. Following the massive flooding that swept through Calgary and southern Alberta in June, the Tory government announced it would buy out flood zone properties in communities where the flooding occurred (floodway) and ban new development in the areas. Examples of maps showing how the flood-prone areas are delineated. This affects insurance, building and house prices (squares are 1 mile). MY HOUSE

33 Dry dams and tunnels Overall Calgary flood mitigation projects totalling $830 MM, although real total nearer to $6 BB Great Calgary Flood Tunnel 6 km storm water diversion tunnel taking water from Glenmore Dam to Bow River Passage runs below 58 th Avenue, at peak flow of 500 m 3 /second Estimated cost of $200 MM to $300MM Nenshi: "science fiction" Dry dam for Elbow River near Bragg Creek Off stream storage site near Springbank Road to divert water from the Elbow River Southern diversion on Highwood River, 7 km long

34 Conclusions Cost to region around $1.9 BB+ insurable, estimated more than $4.7 BB in total. Overall more costly than Katrina (World: 3 rd ) Lack of preparations despite 2005 flood cost the region dearly. Large proportion of damage due to sewage back up in cellars. This was no 100 year flood, possibly only 60% of potential maximum discharge. 50% chance of 2005 flood within 25 years. Need to remember that fans are fans. Revisit dam management during flooding. New study needed including data from all previous work. Continuous monitoring. BE PREPARED!

35 THANK YOU, AND STAY DRY!

36 Ongoing monitoring 400 hydrometric stations, 600 meteorological stations Weather forecasting and modelling (Environment Canada, NOAA) River Forecast Centre, 24/7 operations triggered by significant rainfall or rain on snow events: Advisories Stage 1 alerts Stage 2 flood event