This document discusses alluvial fan flooding and earthquakes in Makarora Township, New Zealand. It summarizes that debris flows are sediment-water mixtures that occur after heavy rain in tectonically active areas, contributing to alluvial fan development. Dendrochronology of tree rings in the area indicate possible earthquake events in the 11th, 1445, 1717, 1826 and 1860 centuries. The nearby Alpine Fault poses risks of future M8 earthquakes. Hazard mapping was conducted and council responses included technical reports, planning changes, and refused subdivisions to address flood risks from stochastic events revealed by historical and geological evidence.

1. Alluvial Fan Flooding & the
Significance of Earthquakes
Dr Stephen Swabey
sswabey@umwelt.com.au 02 4950 5322

3. 0 10
km

4. DoC Centre
M akarora Village
M akarora Township
0 2
km Lake Wanaka

5. 1 N ovember 9 5 3
8 6 O ctober 12 0 0 7
0 0.2
km

6. Flaxmill Creek
alluvial fan

7. 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

14. 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)

17. 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

18. Active Faults
• Alpine Fault nearby
• Most recent large
event M6.2 SW of
Haast in 2001

19. 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!

20. 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

21. Council Responses
• Technical report
• District Plan change
• Subdivision consent refused

22. 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