People living below the poverty line in developing countries are often pushed to urban fringes and into unplanned settlements. Their houses are typically built on land where there is no municipal infrastructure and which may be prone to hazards such as landslides. At the very least, those landslides cause damage to livelihoods, homes and possessions. In the worst-case scenarios, they cost lives.

Researchers at the University of Bristol have developed a highly effective solution to this risk with MoSSaiC (Management of Slope Stability in Communities).

This unique science and engineering based methodology focuses on the efficient management of surface water and is delivered via a combination of community participation, local expertise and Disaster Risk Reduction funding.

Projects in Saint Lucia and across the Caribbean have already achieved significant results.

Densely populated hillsides that previously suffered frequent landslides triggered by moderate rainfall events have now been made safer. In 2010, these urban hillsides even withstood Hurricane Tomas, an extreme rainfall event that delivered over 500mm of rain in just 24 hours.

Overall over 800 homes in 12 urban communities have benefitted from MoSSaiC.

Not just safer...

Those communities have experienced many other indirect benefits too, most of which derive from improvements to surface water management and drainage infrastructure. For example, many households now have a more sustainable water supply through rainwater harvesting.

There have also been improvements in access to and from communities during the rainy season due to reduced surface water and associated debris on footpaths. This means fewer missed days of work and school, and less time spent clearing debris.

From models to actions

The research behind MoSSaiC, carried out between 2004 and 2011, was led by University of Bristol’s Professor Malcolm Anderson and Dr Liz Holcombe; first in the School of Geographical Sciences, then the Department of Civil Engineering.

The idea was built around CHASM, a software model previously developed by Anderson that simulates the effects of hydrological and geotechnical processes on slope stability.

Initial work extended CHASM to quantify landslide risk as well as predict landslide events by modelling post-landslide slope geometries, costs of debris removal and slope re-instatement. This allowed different slope stabilisation investment scenarios to be tested for multiple slopes against budgets and design lifetimes.

Encouraged by this development, in 2004 the Government of Saint Lucia asked Holcombe and Anderson to develop a new cost-effective strategy, incorporating CHASM, to reduce landside risk in its extensive informal urban communities.

“The problem Saint Lucia faced was that traditional approaches to slope stabilisation, such as retaining walls, were too expensive for such a wide-spread problem,” says Dr Holcombe.

“The communities’ own non-engineered walls were also ineffective. CHASM simulations confirmed the main mechanism destabilising many slopes was infiltration of rainfall and waste water. Urbanisation was also increasing the hazard by altering natural drainage patterns and micro-topography. Conventional wide-area landslide hazard mapping couldn’t account for these highly localised processes, so the root causes of the landslides were not being recognised or addressed.”

Working with the communities

Building on this analysis, Holcombe and Anderson worked with community residents, local contractors and government engineers to carry out detailed slope feature mapping.

CHASM showed that reducing infiltration at key locations could often improve slope stability for whole hillsides. Community-wide drainage networks were then designed that captured rainfall and surface water.

“The MoSSaiC methodology came about by integrating research local knowledge and expertise with proven local drain design and construction practices,” says Dr Holcombe. “This inclusive approach, and the evidence of its effectiveness, helped to drive local policy change. For the first time in this context, slope water management was recognised as a cost-effective means of reducing landslide risk for whole communities.”

The new MoSSaiC process of community mapping, stability assessment, drainage design and construction was replicated in other communities to deliver appropriate solutions tailored to the conditions of each slope.

The result of this pioneering work is that there have been no reported landslides in any of the communities where MoSSaiC interventions are present.