Brisset e 20150708_1500_upmc_jussieu_-_room_103

Elodie BRISSET, Frédéric GUITER, Cécile MIRAMONT Tiané TROUSSIER, Yoann POHER, Rosine CARTIER

Edward ANTHONY

How humans feed flood:

lessons from the past

Flood hasards in mountains • Steep slopes (velocity) • Highly erodible geology (sediment load)

How humans have modified past flood response to climate ?

What this means for our future ?

Future projections in the Alps (Boroneant et al., 2006)

• “Frequency of extreme precipitation is likely to increase in a warmer climate” • “More moisture will evaporate over the sea […] facilitate the built up of intense

weather systems”

Land degradation

Studying the past

Environmental reconstructions

using lake sediment archives

Long-term perspective of present/futur

changes

• Past complex environmental dynamics • Trend and threshold of long-term evolution

Case study: the alpine Lake Allos

Methods: multiproxy analysis on lake sediment archive

Chronology: 15 14C ages AMS

Sedimentology of flood layers Lamina counting on smear-slides Grain-size measurements

Palynology Chemical extraction, pollen taxa identification

Results

Increase of number of floods

Increase of low intensity flood frequency

Increase of sediment yield during high and medium

intensity floods

Increase of sediment yield

Step change 2000 years ago :

Results

Increase of number of floods

Step change 2000 years ago :

Increase of sediment yield

Transhumant pastoralism

Mining

Deforestation

Gradual change in human impact

Results

Increase of number of floods

Step change 2000 years ago :

Increase of sediment yield

Gradual change in human impact

Long term human impact induced

an abrupt and lagged response of

flood increase

Vegetation did not act its

role of buffer of

precipitations

Holocene floods over the European Alps

Flood frequency in lake sediment archives a) Allos (this study) b) Anterne (Giguet-Covex et al., 2012) c) Ledro (Vannière et al., 2013) d) Southern Alps stack (Wirth et al., 2013) e) Northern Alps stack (Wirth et al., 2013) f) Mondsee (Swierczynski et al., 2013)

Higher mean flood frequency

and higher variability

Common pattern of accelerated

erosion during the Late Holocene

By changing sensitivity of landscapes,

human societies have amplified

geomorphic response of natural hazards

contact: elodie.brisset@gmail.com

IMPLICATION FOR FUTURE • Slope sensitivity to erosion has been a

decisive long-term factor modulating flood

• Vegetation management may be crucial to significantly mitigate flood risk in the course of future more frequent climatic extremes

Schematic alpine landscape evolution

Today

2000 years ago

7000 years ago