Dili­gence re­quired to pre­vent land­slides in quake af­ter­math


About a month af­ter the April 25 earth­quake hit Nepal, a land­slide oc­curred about 10 kilo­me­ters north of Beni, the head­quar­ters of Myagdi dis­trict.

The land­slide, which con­sisted of loose, fine sed­i­ments, blocked the Kali­gan­daki River, caus­ing the river’s wa­ter to ac­cu­mu­late in a reser­voir be­hind the land­slide dam. The wa­ter over­topped and breached the nat­u­ral dam, send­ing a flood of more than 2 mil­lion cu­bic me­ters of wa­ter down­stream. For­tu­nately, swift gov­ern­ment ac­tion to evac­u­ate vil­lages at im­me­di­ate risk pre­vented hu­man ca­su­al­ties, and the land­slide dam was breached in less than 16 hours. Had the lake that formed up­stream be­come larger, with the river wa­ter re­tained for a few more days, the re­sult­ing flood would have caused cat­a­strophic dam­age down­stream.

It is im­por­tant to note that this land­slide oc­curred as a re­sult of nat­u­ral pro­cesses that were fur­ther ag­gra­vated by the earth­quake. And learn­ing from other earth­quakes in the Hindu Kush Hi­malayan re­gion, this is not a sur­pris­ing event af­ter a mag­ni­tude-7.8 earth­quake.

Af­ter Wenchuan and Kash­mir

In the af­ter­math of the mag­ni­tude-8.0 Wenchuan earth­quake in Sichuan, China, in 2008, sci­en­tists recorded more than 12,000 land­slides along the re­gion’s river val­leys. The earth­quake and nu­mer­ous strong af­ter­shocks trig­gered many large land­slides that blocked rivers, re­sult­ing in the for­ma­tion of more than 30 lakes. One high-risk event was the for­ma­tion of a land­slidedammed lake near Mount Tangjia, Sichuan, which blocked the Jian River. More than 200,000 peo­ple living down­stream and at im­me­di­ate risk were evac­u­ated, and the gov­ern­ment con­structed a sluice to safely drain the lake and pre­vent large-scale dev­as­ta­tion.

An­other ex­am­ple is the mag­ni­tude-7.8 Kash­mir earth­quake in 2005 that af­fected In­dia and Pak­istan. The tremor trig­gered sev­eral thou­sand land­slides — mainly rock falls and rock slides — around the epi­cen­ter in Pak­istan. Sev­eral of th­ese land­slides dammed rivers, form­ing lakes up­stream — in­clud­ing the Hat­tian land­slide ap­prox­i­mately 32 kilo­me­ters southeast of the me­trop­o­lis of Muzaffarabad — and pos­ing a risk to thou­sands of peo­ple. Ob­ser­va­tions on the ground, com­bined with a com­par­i­son of satel­lite im­agery be­fore and af­ter the earth­quake, sup­ported the hy­poth­e­sis that land­slide haz­ards in­creased in the Neelum and Jhelum val­leys af­ter the 2005 quake.

Im­me­di­ately af­ter the earth­quake in Nepal, a NASA-led in­ter­a­gency team of in­ter­na­tional ex­perts, of which Ici­mod was a core mem­ber, col­lab­o­rated to map land­slides in the earth­quake-af­fected area. The team has iden­ti­fied more than 3,000 land­slides — a ma­jor­ity of which are con­cen­trated along river val­leys.

Weak Mon­soon, No Com­fort

The earth­quake and re­peated af­ter­shocks have not only trig­gered thou­sands of land­slides, they have also weak­ened the soil and desta­bi­lized slopes in Nepal’s steep moun­tain val­leys. It is an­tic­i­pated that mon­soon rain­fall could fur­ther weaken the slopes, which could re­sult in more land­slides. The in­ten­sity and amount of mon­soon rain­fall are cru­cial fac­tors that could in­flu­ence the oc­cur­rence of th­ese land­slides.

Ex­perts have pre­dicted be­low nor­mal rain­fall dur­ing the 2015 mon­soon. Con­sen­sus on this was de­vel­oped at the sixth South Asia Cli­mate Out­look Fo­rum, based on an ex­pert as­sess­ment of pre­vail­ing global cli­mate con­di­tions and fore­casts from dif­fer­ent cli­mate mod­els from around the world. Cur­rently weak El Nino/South­ern Os­cil­la­tion (ENSO) con­di­tions are pre­vail­ing over the Pa­cific Ocean. The ENSO is a global cli­mate phe­nom­e­non that has a sig­nif­i­cant in­flu­ence on the year-to-year vari­abil­ity of the mon­soon over South Asia. Ac­cord­ing to the In­dian Me­te­o­ro­log­i­cal Depart­ment, the rain­fall this mon­soon over the whole of In­dia is ex­pected to be about 12 per­cent less than the long pe­riod av­er­age from June through Septem­ber. Ex­perts have also in­di­cated that across Nepal the prob­a­bil­ity of be­low nor­mal rain­fall this mon­soon is around 45 per­cent, with the like­li­hood of above nor­mal rain­fall around just 20 per­cent.

So how could a weak mon­soon af­fect the po­ten­tial for land­slides in Nepal in the af­ter­math of the earth­quake? Let’s look back at the Wenchuan earth­quake for more in­for­ma­tion. Be­fore the Wenchuan earth­quake, the av­er­age in­ten­sity of rain­fall needed to trig­ger de­bris flows was around 80 mil­lime­ters in 24 hours.

Re­searchers have found that af­ter the earth­quake, as lit­tle as 30 mil­lime­ters of rain­fall in a 24-hour pe­riod re­sulted in de­bris flows, with the mag­ni­tude of de­bris flows much larger than those be­fore the earth­quake. In Nepal, this could in­di­cate that even if the amount of rain­fall this mon­soon is less than nor­mal, as has been pre­dicted, there still re­mains a high risk of slope fail­ure and land­slides be­cause of the ef­fect the earth­quake has had on the coun­try’s steep moun­tain­ous ter­rain.

Stay­ing Pre­pared

Thus, it is im­por­tant that ef­forts be made to in­crease pre­pared­ness, and that mit­i­ga­tion mea­sures are put into place to help min­i­mize the ad­verse im­pacts of pos­si­ble land­slides. Ap­pro­pri­ate mon­i­tor­ing of river val­leys and their trib­u­taries is also re­quired, in­clud­ing in rivers that have been af­fected by land­slides like the Marshyangdi near Lower Pisang and in the Budi Gan­daki, which have been deemed safe for now, but may change un­der mon­soon con­di­tions. For this, the Depart­ment of Hy­drol­ogy and Me­te­o­rol­ogy has dis­patched field teams to af­fected ar­eas to en­sure that ex­ist­ing real-time wa­ter level and rain­fall sta­tions are in op­er­a­tion, which will be vi­tal for pro­vid­ing early warn­ing dur­ing the mon­soon pe­riod.

En­hanc­ing dis­as­ter pre­pared­ness, im­prov­ing the im­me­di­ate re­sponse, and ‘build­ing back bet­ter’ dur­ing re­cov­ery, re­ha­bil­i­ta­tion and re­con­struc­tion is the fourth pri­or­ity for ac­tion of the Sendai 2015 Frame­work for Dis­as­ter Risk Re­duc­tion en­dorsed by 187 coun­tries around the globe, in­clud­ing Nepal and other coun­tries in the Hindu Kush Hi­malayan re­gion. The frame­work sees dis­as­ter as a crit­i­cal op­por­tu­nity to build back bet­ter — in­clud­ing through the in­te­gra­tion of dis­as­ter risk re­duc­tion in devel­op­ment mea­sures — in or­der to make na­tions and com­mu­ni­ties more re­silient to dis­as­ters.

The cur­rent pri­or­ity of the gov­ern­ment is re­con­struc­tion and re­ha­bil­i­ta­tion, for which the iden­ti­fi­ca­tion of safe set­tle­ment ar­eas and haz­ard-prone ar­eas is es­sen­tial. The in­ven­tory of land­slides, their cat­e­go­riza­tion ac­cord­ing to the risk, and land­slide sus­cep­ti­bil­ity zon­ing is es­sen­tial for the re­lo­ca­tion of in­hab­i­tants and re­set­tle­ment and con­struc­tion. For long-term risk iden­ti­fi­ca­tion, haz­ard zon­ing, proper land use plan­ning, and “build­ing back bet­ter” could help avert fu­ture dis­as­ters and sup­port more sus­tain­able devel­op­ment. Shrestha is the pro­gram co­or­di­na­tor of the HYCOS Ini­tia­tive un­der River Basins Pro­gramme at the In­ter­na­tional Cen­tre for In­te­grated Moun­tain Devel­op­ment

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