Exposure to Severe Weather Events: A review of recent experience in the Western Cape

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Exposure to Severe Weather Events A review of
recent experience in the Western Cape
Ailsa Holloway, Gillian Fortune,
Disaster Mitigation for Sustainable Livelihoods
Programme University of Cape Town September, 2009
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Presentation Focus
Southern Cape 2006

• Role of ex post analysis in managing climate risk
• Overview of recent severe weather in Western Cape
• Reflection on severe weather consequences for
  flood risk management.
• Unpacking causal chain between development
  and severe weather impacts

Knysna 2007
Port Shepstone 2008
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Managing Risk Role of Ex Post Analysis

• Risk is a probalistic concept about a future
  negative outcome (it has yet to happen)
• Ex post disaster event research analyses actual
  events that have happened.

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It provides useful insights about

• What/who resists and what fails in a severe
  weather event. The what, the where and the
  when and the who
• How much this costs
• The causal chain that leads to the loss (ie the
  mix of risk factors) the why and the how
• Ex post research studies actual disaster events
  to guide risk averse development

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Severe Weather Events for 2003, 2004, 2005, 2006,
2007 and 2008
2003
241 mm
218mm
2004
228mm
2005
324mm
2006
150 mm
2007 - June
206 mm
2007 - Nov
2008 - July
93 mm
2008 - Nov
150 mm
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Severe weather events - costly for W. Cape
Not adjusted for inflation
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Focus on Cut-off Lows Affecting Eden ie Prov.
Depts and Municipal Impacts
Severe weather losses 2003 - 2008
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Provincial losses are substantial (2003, 2006
2007 weather events total R 1.2 bn)
2007 losses up by more than five times March 2003
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A Review of Ten Damage-Bearing Severe Weather
Events Since 2003

• Cape Town August 2004 and 2008
• W. Coast June 2007, July 2008
• Winelands/ March 2003, April 2005,
• Overberg November 2008
• Eden December 2004, August 2006
• November 2007

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Cape Town August 2004 and 2008
A little history Image from 23 June, 1904 62.5
mm fell in one hour
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100 years later5-9 August 2004 (2 large cold
fronts)

• Resulted in flooding in Maitland, Woodstock,
  Pinelands, Athlone, Bonteheuvel, Nyanga, Guguletu
  and Somerset West (Mainly Salt Water Catchment)
• Peak rainfall intensities over 3 hr durations
  exceeded 100 year storm Athlone (67mm)
  Pinelands (62 mm)

Source CoCT in 2004 Storm Report on
www.egs.uct.ac.za
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Cape Town Superstorm, 30-31 August 2008
100mm
24 hour rainfall, 30 August 2008 (46 August
monthly rainfall 24 hrs) Source Thea Beckman,
UCT
Storm surge gt 5 m and wave heights gt 7 metres
Images www.news24.com
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West Coast June 20076-11 June (Cut-off low),
25-26 June (Frontal system)

• First event rainfall over Bergriver
  Municipality gt 130 mm (but total rain 280-318mm)
  Swartland (120mm) Saldhana Bay (110 mm)
• Second event Rainfall over Witzenberg
  Municipality gt 50 mm
• Combined totals for June 2007 200 mm greater
  than mean historical average

Roode Els Berg 289 mm Assegaibos 328
mm Ceres 394 mm Hawequas 219
mm Jonkershoek 198 mm
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West Coast and Western Winelands 4 9 July 2008
(Cut off low)

• Most striking feature long duration of heavy
  precipitation.
• Wettest July in recent history for Ceres, Roode
  Els Berg
• July 2008 totals were 300 (Roode Els Berg), 145
  (Assegaibos) and 200 (Ceres) of July mean
  rainfall

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2003 and 2005 Winelands and OverbergCut-off Lows

• 22-25 March 2003 Montagu floods
• 241 mm in Montagu,
• (Kingna River mainly responsible for
  flooding)
• 160 mm in Swellendam
• 10-12 April 2005, Bredasdorp flood 228 mm rain

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11-13 November 2008 Cut off Low Overberg and
Winelands

• Meterologically, less significant than 2007
  cut-off low during the same month.
• However, still significant losses and research
  now ongoing

Korinte-vetdam 281 mm Hawequas Forest 250 mm
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Eden - December 2004, Jul/August 2006, November
2007 Cut-off lows

• 21 23 December 2004 (Heidelberg 168 mm, Knysna
  219 mm)

Flood line inside a house on the Duiwenhoks River
Heidelberg Municipal Chalets 22 Dec, 2004
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Jul/August 2006

• George Rainfall
• 31 Jul 4 Aug 2006 328 mm
• PLUS
• 21 24 Aug 2006 118 mm

Total 446 mm
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2006 Cut-off low profiled exposures generated by
non-perennial floodways that flash-flood in a
severe weather event
Source Xavier Middleton
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It also indicated the role of increasing
urbanisation in George Municipality from 1957 to
2004
1957
1985 to 2004
Increased by 728.28 hectares from 1957 to 1985
Source Xavier Middleton
Increased by 7,488.4 hectares from 1985 to 2004
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21-22 November 2007 Cut-off Low
This 3-day event created new rainfall records for
areas it affected Contributed over 70 to
highest November total rainfall in the last 30
years Assegaibos 500 and George 530,
respectively of the November mean.
Wolwedansdam 316 mm George 333 mm
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Institutional risk factors (ie under- investment
in municipal maintenance)
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Investments in municipal maintenance are
protective and reduce losses
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To conclude

• Severe weather is a reality for us in the Western
  Cape no region can be complacent.
• There has been impressive improvement in managing
  severe weather since 2003 especially in
  mobilising following warnings.
• However still a long way to reducing climate
  risk developmentally..