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Taiwan Tectonics & Rivers (Model river response to equakes Yanites…
Taiwan Tectonics & Rivers
Jhuo-Shuei River
Water Q vs sed conc (1994-1998)
Correlation btwn discharge & sed flux
--> coincides with sediment conc.
Typhoon causes water Q to peak
Not continuous ↑, instead
Sudden ↑
slowly tapers off
Interp
:
Big flood - causes ↑ water discharge in system
Lotsa sed mobilised & exported
Sed conc. peters
off to background later in season
Why?
Sediment in system ↓ (unless another typhoon mobilises more sed)
Lotsa gauging stations
Try to understand
risk
associated w variable flow of Taiwanese Rivers
Allows analysis of typhoon Q vs sed flux
i.e. do typhoon events drive sed flux ?
Typhoons
& sed
Typhoon mobilises sed available in system
However, there must be a limit
Sed depletion ?
Begs Q
How is system replenished with sed ?
Typhoons cannot flux out sed indefinitely...
Sed flux = f(typhoon Q)
Since sed flux = f ( energy in system to mobilise sed )
Chi-Chi & Landslides
Dadson
Chi-Chi Equake
Triggered lotsa (> 20 000) landslides
Typhoon Toraji
~30 000 landslides
~ 56 % Chi-Chi landslides = reactivated
80% Toraji landslides
occurred in areas
NOT
previously failed
Preconditioning
Extreme events precondition substrate for failure
How ?
Loss of cohesion & frictional strength of hillslope
due to strong seismic ground motion
Evidence
80% Typh Toraji landslides
--> occurred in areas previously not failed
Surface area disturbed by landsliding in Typhoon Toraji
↑ with proximity to Chelungpu Fault
Effects Equake vs Typhoon
Equakes
condition system for failure
introduce more sediment
Typhoons
mobilise & generate more sediment
River response to faulting
Fault displacement of land surface
--> affects river profile
River meets 'lip'
where fault steepening creates break in river profile
River will either:
Incise through steepening
Alter its course
Cut through steepening
Knickpoint forms to ↑ stream power
Model river response to equakes
Yanites 2010
Upper reaches steepen
∴ below fault reaches disequilibrium as reach = too flat
thus system migrates upcatchment
T1. Surface rupture - landsliding
Whole system downcuts & retreats upstream
Right after equake
T2
Proximal
KP propagates upstream
Distal
Channel widens (incision limited0
T3
Proximal
Bed alluviated - river widens
Distal
Bedrock incision restarts
T4
New equilibrium profile develops