During the initial project steps, community residents and government technical teams create detailed maps of slope features such as previous landslides and altered slope geometry (cut and fill), loading (buildings), drainage, and vegetation. Engineering judgment and scientific tools are used to interpret the effect of such features on slope stability and confirm local landslide causes.
Typically, the dominant instability mechanism in these densely constructed communities is infiltration of rainfall and household water into slope material. Based on that information, drains are designed to intercept overland flow of rainfall and capture household water (roof water and grey water), thus reducing landslide hazard. Donors and governments cannot build drains for all houses in all communities. Even in the vulnerable landslide-prone communities selected for Mossaic projects, it is not possible from a budgetary or political standpoint to fund every drainage intervention that might be beneficial. For each of these communities, a slope process zone map, initial drainage plan, and drainage prioritization matrix are developed (chapter 5 in our book) enabling broad priorities to be established. Once the design and specification of the drainage plan is complete, the cost of the interventions can be estimated, and the works bid out to local contractors in the community (chapter 6 in our book).
In areas of high housing density, the capture and controlled drainage of water from houses is a vital element of the final drainage plan for landslide hazard reduction. Increasing the housing density and volume of publicly supplied water discharged onto a slope can result in a corresponding increase in the number of days the soil is saturated per year, if there is no drainage. This level of saturation is significantly reduced by capturing roof water and gray water and routing the water to newly constructed (or existing) drains. Having confirmed the alignment of drains within the community, then houses to be connected to drains can be determined—prioritizing zones where household water significantly contributes to surface water infiltration and slope instability