aggregated over the lifespan of the road – in this case, 30 years – during which time multiple heavy rains would be expected to affect the road.

Note that the exact frequencies of these rain events are unknown; however, the probability of a specific type of rain event occurring at least _'n'_ times during the 30-year lifecycle of the road can be estimated and used in the calculations of costs and ALF. For example, the probability of an "extreme intensity" 10-year rainfall occurring at least four times during the lifecycle of the road is 35%. With this information, a risk averse decision maker may decide to include, in the cost estimations, the recovery expenses associated with at least four of these extreme events. For a less cautious decision maker, conversely, this risk level may not be high enough to consider additional recovery expenses in the cost estimations. In other words, the costs and expected losses associated with future rain events depend on the selected probability threshold which, in turn, represents the risk aversion level of the decision maker.

The losses associated with each predicted hazard yield _sums_ of the number of people affected by disruptions. The hypothetical unit is akin to accumulated 'person-disruptions' over the lifespan of the asset, which may count multiple disruptions that affect some of the same people.

# A. Road Upgradation Options

Road upgradation can be undertaken up to varying degrees, using diverse materials that have both cost as well as climate resilience implications. The scenarios listed below identify the structural characteristics of the road under each upgradation option, specified at a road segment level, vis-à-vis the base scenario which consists of the road remaining entirely in gravel but with regular maintenance.

Scenario 0 represents the Status Quo (SQ) , wherein no upgrading is undertaken and the practice of grading and re-graveling as needed (typically every 1-3 years) to repair damaged road segments is continued. Additionally, three upgraded scenarios are considered.

Scenario 1 represents a Medium Design (MD) , wherein road sections with gradients between 9-15% are resurfaced with a bituminous surface coating (BTC /'chip-seal') and sections with gradients of 15% or higher are concrete cement-paved. Sections with gradients of less than 9% are left graveled. Scenario 1 also constructs lined drains for all segments with 9% gradients or above (all sealed sections).

Scenario 2 represents a High Design (HD) that improves upon the medium design by extending chip seal resurfacing to any sections with gradients exceeding 6% up to 15% and also concrete cement-paving segments with gradients exceeding 15%. Scenario 2 constructs lined drains for all segments with 6% gradients or above (all sealed sections).

Lastly, Scenario 3 takes a different approach by sealing roads only if they are both long and steep. This Sealed Long, Steep Sections (SLS) design aims to upgrade only those slopes most susceptible to run-off damages due to extended lengths coupled with gradients of 6% or above. Where there are long, sloped road segments, water often flows along the road and accumulates, causing drainage problems and quick deterioration of graveled surfaces. These represent some of the most problematic sections of the East Road and would be targeted under this scenario for priority sealing. Road segments that are steep but short (less than 250 m) would be left graveled.

For all improved scenarios, concrete cement is proposed for any upgrading over 15%, since chip-seal does not typically work well on high gradients.

Using Global Positioning System (GPS) tracking records for the Malaita East Road from Atori to Dala, 5,856 road segments with gradient details were delineated and mapped using geographic information system (GIS) mapping software. These segments were mapped for the current road and also used to model surface conditions defined by the proposed upgrading projects.

14