Climate change increases the risk of landslides

Gilles Grand­jean. Land­slides are multi-factori­al pro­cesses: they occur when the slope is steep enough and the geo­logy favour­able. Oth­er factors can also play a part: rain­fall, for example, sat­ur­ates slopes and encour­ages their destabil­isa­tion. So, as rain­fall increases, so does the num­ber of land­slides, accord­ing to a cor­rel­a­tion that can vary spa­tially¹. As cli­mate change affects pre­cip­it­a­tion (by increas­ing the fre­quency of extreme events), it also has an impact on the risk of land­slides. How­ever, it remains very dif­fi­cult to estab­lish the dir­ect role of cli­mate change in the occur­rence of landslides.

Firstly, because we don’t have com­pre­hens­ive mon­it­or­ing of land­slides. On the very large land­slides that move reg­u­larly, such as Super-Sauze or La Clap­ière in the French Alps, a large num­ber of sensors are installed: lasers, tele­meters, radar, weath­er sensors and so on. How­ever, many oth­er moun­tain slopes are not equipped. Moreover, land­slides are extremely com­plex pro­cesses. We use digit­al mod­els to under­stand them bet­ter. Dur­ing a land­slide, the phys­ics of mater­i­als can be dif­fi­cult to ana­lyse: the same event can be sub­ject to the phys­ic­al laws of brittle, vis­cous or even flu­id mater­i­als, which adds com­plex­ity to our models.

Here again, it is dif­fi­cult to give a pre­cise answer because of the lack of data. There are no glob­al data­bases that exhaust­ively record all land­slides. In France, the Land­slide Data­base (BDM­vt) suf­fers from this prob­lem, par­tic­u­larly in unin­hab­ited areas where these events are rarely recor­ded. Teams are work­ing on the use of satel­lite imagery to sys­tem­at­ic­ally detect land­slides, but fur­ther devel­op­ments are still needed.

On the oth­er hand, in small areas where sensors have been installed, we are see­ing the effects of cli­mate change. We are see­ing an increase in the fre­quency of grav­ity instabil­it­ies, which are very closely linked to pre­cip­it­a­tion. We are also see­ing the base of gla­ciers melt­ing as a res­ult of rising tem­per­at­ures, releas­ing a lot of sed­i­ment – a col­lec­tion of mater­i­als such as gravel and sand – which accu­mu­late in the sur­round­ing tor­rents. When it rains heav­ily, the sed­i­ment-laden water rushes down the slopes, caus­ing much more dam­aging floods down­stream and increas­ing the risk of cre­at­ing logjams.

The sci­entif­ic com­munity is cur­rently look­ing into this sub­ject. Moun­tain areas are gradu­ally adapt­ing to changes in cli­mate: slopes are being mod­i­fied by erosion, plant spe­cies are chan­ging, and so on. But it is chan­ging so rap­idly that we fear that the slopes will not have time to adapt. This could lead to cata­stroph­ic events, includ­ing land­slides. On the oth­er hand, the dir­ect impact of human activ­it­ies may lim­it the nat­ur­al adapt­a­tion of moun­tain regions.

The risk of land­slides is also affected by human activ­ity. Slopes can be weakened by the con­struc­tion of infra­struc­ture (for example, if they are cut into to build a road) or by land clear­ance. Anoth­er effect that can be amp­li­fied by human activ­ity is bank erosion. When erosion is severe, riverb­anks can become unstable and small land­slides can occur. Through a regress­ive effect, they can gen­er­ate a lar­ger land­slide across the entire slope.

Finally, risk is defined as the com­bin­a­tion of a nat­ur­al phe­nomen­on and the vul­ner­ab­il­ity of inhab­ited areas: even if a land­slide occurs, without infra­struc­ture or people, the risk is zero. By build­ing more infra­struc­ture in moun­tain­ous areas, vul­ner­ab­il­ity is mul­ti­plied, and the risk is increased.

Gen­er­ally speak­ing, the risk of land­slides is not going to decrease, due to the increase in the num­ber of infra­struc­tures in moun­tain areas. But it is impossible to draw any gen­er­al con­clu­sions. In a study pub­lished in 2018, we assessed future risks in Pyrenean and Alpine val­leys on the basis of IPCC scen­ari­os and socio-eco­nom­ic scen­ari­os developed with loc­al stake­hold­ers².The res­ults are very var­ied: the risk increases in some areas, but not every­where. It all depends on the ori­ent­a­tion of the val­ley, its lat­it­ude, its vul­ner­ab­il­ity, etc. Assess­ing the impact of cli­mate change requires stud­ies spe­cif­ic to each valley.

 The sci­entif­ic com­munity is heav­ily involved in these issues, par­tic­u­larly with­in the pri­or­ity research pro­gramme and equip­ment (PEPR) Risques coordin­ated by BRGM with CNRS and Gren­oble Alpes Uni­ver­sity. We are also work­ing on moun­tain risk pro­jects, such as IRIMONT and VIGIMONT, with the ANR, devel­op­ing early warn­ing sys­tems in particular.

There are solu­tions to reduce the risk, but there are also prob­lems. For example, some nature-based solu­tions involve leav­ing nat­ur­al areas around rivers to absorb floods, or replant­ing slopes, which can lim­it the devel­op­ment of eco­nom­ic activ­it­ies. The oth­er prob­lem con­cerns the vul­ner­ab­il­ity of infra­struc­ture. When works are car­ried out to reduce the like­li­hood of land­slides, this can encour­age more to be built. But these struc­tures – fences, retain­ing walls, etc.- require main­ten­ance. In the absence of main­ten­ance, and if the infra­struc­ture fails, the risk of land­slides is even great­er than before because of new con­struc­tion. What’s more, these struc­tures are some­times adap­ted to a past cli­mate and the stand­ards no longer cor­res­pond to cur­rent or future cli­mat­ic hazards.

Interview by Anaïs Marechal