Does it work?

Mossaic interventions are designed to reduce the risk of rainfall-triggered landslides. However, it is difficult to establish what would have happened to a particular slope if a drainage intervention had not been carried out – this is the counter-factual problem in arguing for risk reduction.

A recent study summarises the landslide risk issues pre- and post-Mossaic implementation within six vulnerable hillside communities in St Lucia and Dominica. The study demonstrates the landslide risk reduction performance of the drainage design and construction approaches taken, and delivered by community members. Evidence from an additional five communities is reported in chapter 9 of our book and equally affirms the  lowering of  landslide  risk on hillsides following interventions.   Evidence of landslide risk reduction having been achieved in such locations is rare, which is why we believe the outcomes reported are significant.

Given that such interventions cannot completely eliminate landslide hazard, there remains the residual likelihood of instability on such slopes, which may be triggered by:

• more extreme rainfall events
• poor drain maintenance (blocked, overflowing or broken drains),
• poor maintenance or disconnection of household roof-guttering and grey-water pipes
• construction of new houses with no connection to drains

It is therefore important to collect evidence of post-project slope stability, particularly in relation to subsequent large rainfall events, but also other potential contributing factors, as an indication of the effectiveness (or limitation) of each intervention.