Study reveals new landslide risk factor

Study reveals new landslide risk factor

New research by NASA scientists revealed that flooding rice fields - a previously unknown risk factor - triggered a disastrous effect following the fatal 2018 Palu earthquake in Indonesia. Shortly after the extreme shaking, the solid ground turned into a landslide of muddy soil, increasing the death toll and worsening the economic impact. The study was published on September 27, 2019.

Soil liquefaction happens when the shaking from a massive earthquake rips through wet and loose soil, becoming more powerful than the friction that usually holds together the dirt particles. This results in the soil losing its structural coherence, making it flow like liquid. As the once-solid ground gets washed away, the establishments and structures standing above it will crumble, leading to landslides. Furthermore, heavy objects such as vehicles get submerged into the mud, while buried water and sewer pipes surface.

During the Palu earthquake, initial reports pointed out that most of the approximately 2 000 fatalities were from a tsunami. However, current surveys revealed that landslides triggered by soil liquefaction caused as much damage as the raging waves.

Researchers were left puzzled at first since soil liquefaction commonly occurs in flat surfaces with wet or sandy grounds such as coastal plains. They had thought flat landscapes was a factor because the water table-- the distance below ground where land absorbs water-- must be shallow, and that's unusual on a hillside. Palu's soil is sandy, but it's in a sloping valley that seemed to pose small threat. It was found that all landslides originated along a distinct line, and this surprised the researchers when they noticed it. As the scientists took a closer look, it showed that the line was an aqueduct.

JPL's ARIA team produced this Damage Proxy Map of Palu after the 2018 earthquake. The aqueduct is marked in blue; red and yellow pixels show likely damage. The terrain slopes uphill from left to right, with major damage below the aqueduct and little above it. Image credit: NASA/JPL-Caltech/JAXA

"So we started studying why the aqueduct is clearly defining the boundary between land sliding and no sliding," author Sang-Ho Yun said.

The Gumbasa Aqueduct was finished in 1913 to lessen the threats of famine by providing a consistent supply of water for local farmers. Only land downhill from the aqueduct is irrigated, and the water is not pumped uphill. Farmers practice wet rice cultivation below the aqueduct, which at one point the field becomes flooded during the growing cycle. This principal method of rice farming in tropical Asia increases the water table as time goes by to just below the surface of the ground. Farmers grow coconut palms father downhill, which needs less irrigation and do not increase the water table as much.

Widespread liquefaction below the aqueduct was revealed via maps. Several slides carried about 16 square km (6 square miles) of land far downhill, in some areas farther than 15 m (49 feet). The slides were slowed or stopped by the plantations of coconut palms. There was no liquefaction identified uphill of the aqueduct.

Main author Kyle Bradley said the conclusion is clear that "If there hadn't been intensive irrigation, the landslides wouldn't have happened." 

On a positive note, he added, "This is a human-caused hazard, and it can have a human solution. We can't mitigate the hazard of ground shaking on Palu, but the agricultural practices can be updated based on this new understanding."

The plantation played a significant and critical role in halting the slides, and the authors suggest that planting more trees, those interspersed with rice fields, in places that are extremely irrigated might lessen the chances of soil liquefaction.

Palu is not the only city on Earth where farmers are growing extremely irrigated plantations on wet and sandy slopes, said Bradley.

"If an engineer had gone to Palu and evaluated the system from first principles, they would likely have been able to identify this risk. I hope this study is an impetus for people to go and study these other places."

The research was published in the Nature Geoscience journal.

Reference

"Earthquake-triggered 2018 Palu Valley landslides enabled by wet-rice cultivation" - Bradley K. et al - Nature Geoscience - DOI: 10.1038/s41561-019-0444-1

Abstract

The death toll and economic impact of an earthquake can be greatly exacerbated if seismic ground shaking triggers landslides. Earthquake-triggered landslides typically occur in two different contexts: localized failure of steep slopes and resulting landslides that pose a major threat to life in areas below; and lateral spreading of nearly flat sediment plains due to shaking-induced liquefaction, which can damage large areas of critical infrastructure. Unexpected catastrophic landsliding triggered by the 28 September 2018 earthquake at Palu, Indonesia did not occur in either typical context, but produced both destructive outcomes. Here, we show that alluvial ground failure in the Palu Valley was a direct consequence of irrigation creating a new liquefaction hazard. Aqueduct-supported cultivation, primarily of wet rice, raised the water table to near ground level, saturating sandy alluvial soils that liquefied in response to strong ground shaking. Large-displacement lateral spreads occurred on slopes of 1°. Slopes steeper than 1.5° sourced long-runout landslides and debris flows that swept through villages occupying the gentler slopes below. The resulting damage and loss of life would probably not have occurred in the absence of a raised water table. Earthquake-triggered landsliding of gentle, irrigated alluvial slopes is an under-recognized, but avoidable, anthropogenic hazard.

Featured image credit: Pierre Prakash/European Union

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