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Alluvial Fan Flooding & the Significance of Earthquakes

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A presentation to the Floodplain Managers Association of NSW

A presentation to the Floodplain Managers Association of NSW


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  • 1. Alluvial Fan Flooding & the Significance of Earthquakes Dr Stephen Swabey sswabey@umwelt.com.au 02 4950 5322
  • 2. 0 10 km
  • 3. DoC Centre M akarora Village M akarora Township 0 2 km Lake Wanaka
  • 4. 1 N ovember 9 5 3 8 6 O ctober 12 0 0 7 0 0.2 km
  • 5. Flaxmill Creek alluvial fan
  • 6. Debris Flows & Flooding • Sediment-water mix (various proportions) • Requires available sediment source (weathering, erosion, mass movement) • Not mobile until heavy rain experienced • Most active in tectonically active settings • Major contributor to alluvial fan development
  • 7. What Is Risk to Makarora? • How frequently is alluvial fan active? • Debris flows destroy trees • So, date tree establishment periods • Incremental cores, tree-ring counting (dendrochronology) • Main tree species: – Matai (very old, large) – Beech (younger, fast growing)
  • 8. Possible Disturbance Events • ~11th century earthquake (??) – one matai tree • ~1445AD earthquake (?) – one matai tree • 1717AD earthquake – no beech trees older • 1826AD earthquake – SW portion of fan • 1860AD fire (Hassing, pyromaniac explorer) • Post-1861AD logging • Earthquake events typically 8 m Alpine Fault movement, caused by ~M8 earthquake • Large-scale Pipson Creek activity post-dates 2001AD Alpine Fault M6.2 earthquake
  • 9. Active Faults • Alpine Fault nearby • Most recent large event M6.2 SW of Haast in 2001
  • 10. Future Events • Four M8 earthquakes in last 900 years • No M8 earthquake since 1717AD • Alpine Fault rupture has conditional probability: – 0.54-0.87% in 1 year; – 10-16% in 20 years; – 24-35% in 50 years; and – 41-56% in 100 years • 2001 M6.2 event was just a wake-up call!
  • 11. 1994 flood mapping 2006 flood & debris flow 1999 flood zone 2006 hazard zone Hazard Zone • Using Pipson Creek as indicator of what may happen in M8 earthquake • Combined flood and alluvial fan hazard zone • Does not include mass movement and liquefaction hazards • General scale – not suitable for site use
  • 12. Council Responses • Technical report • District Plan change • Subdivision consent refused
  • 13. Conclusions • Stochastic events can be significant for flood risk • Historical and geological evidence should be sought to understand if such risk exists • Acknowledgements: – Otago Regional Council for funding – Dr. Gretel Boswijk, University of Auckland – Clint O’Brien, Gary Charteris & Ian (Mo) Turnbull