How the risk maps are built

Method

The maps follow the IPCC risk framework: the risk of harm from a hazard combines the hazard itself, the exposure of people and assets to it, and their vulnerability.

Each index follows the INFORM approach: indicators are rescaled from 0 to 100 between two goalposts (100 the worst situation in Sierra Leone), averaged with weights within each component, and the Risk index is the geometric mean of Hazard, Exposure and Vulnerability, so that an area only scores high when all three are high.

Indices are relative to Sierra Leone: they compare chiefdoms with each other and with other periods, not with other countries. A score of 0 does not mean that there is no hazard, only that the chiefdom has the lowest value of the country.

Goalposts are the 2nd and 98th percentiles of the values of all chiefdoms, over the baseline and every scenario and horizon together, so that a future period scores higher than the baseline only where values increase, and a few extreme chiefdoms do not squeeze the others. Counts of people are compared on a logarithmic scale.

Only the hazard varies with the scenario and the period, through climate indicators of the climate explorer (median of the climate models). Exposure and vulnerability describe the current situation.

Classes

• Very low (0–20)
• Low (20–40)
• Medium (40–60)
• High (60–80)
• Very high (80–100)

Risk run 20261005-144021, hazard drivers from climate run 20261005-143826.

Extreme heat

Prolonged high temperatures that cause heat stress, affect health, labour productivity, crops and livestock.

The hazard follows the climate scenarios and periods; exposure and vulnerability describe the current situation.

Hazard

Exposure

Vulnerability

Limitations

The hazard only uses days above 35 °C at 0.25 degree (about 28 km): it does not account for humidity, which makes heat more dangerous in Sierra Leone's humid climate (wet-bulb temperature and heat index are planned), nor for urban heat islands. Threshold counts depend on the bias correction of the climate data.

Riverine flooding

Rivers overflowing their banks after prolonged heavy rainfall, flooding homes, fields and roads.

The hazard follows the climate scenarios and periods; exposure and vulnerability describe the current situation.

Hazard

Exposure

Vulnerability

Map layers

• Flood depth, 100-year river flood: Water depth during a river flood that occurs on average once in 100 years, in the current climate (permanent water bodies and spurious depths left out). (Copernicus Emergency Management Service, Global River Flood Hazard Maps v2.1.2, CC BY 4.0)

Limitations

Flood extents come from a global model at about 90 m that only covers rivers draining more than about 3,000 km² and the current climate: flash floods, urban floods and small rivers are missing. Future change follows the wettest five days of the climate models, a climate driver rather than a modelled flood extent.

Drought

Long dry spells and rainfall deficits that reduce harvests of rain-fed crops, pasture and water supply.

The hazard follows the climate scenarios and periods; exposure and vulnerability describe the current situation.

Hazard

Exposure

Vulnerability

Limitations

The hazard uses rainfall indicators only (longest dry spell, total rainfall); evaporation and soil moisture (SPEI) are planned. Sierra Leone is humid: contrasts between chiefdoms mostly reflect the longer dry season in the north. Exposure only counts people and cropland, not livestock or water supply, and the cropland map misses much of the bush-fallow farming.

Landslides

Slope failures, rockfalls and mudflows, mostly on steep, deforested or cut slopes after heavy rain, as in Freetown in August 2017.

No projection available. Landslide susceptibility depends on slopes, soils and land cover, which climate projections do not describe; heavier rainfall will make landslides more frequent where they already occur.

Hazard

Exposure

Vulnerability

Map layers

• Landslide susceptibility: Landslide susceptibility from very low (1) to very high (5), about 1 km. (NASA LHASA global landslide susceptibility map v2, NASA open data (licence of the map file to be confirmed))

Limitations

The global susceptibility map is at about 1 km: it misses many local slopes, such as the cut slopes and informal settlements of the Freetown peninsula. National susceptibility maps will replace it when available.

Coming soon

• Coastal flooding
• Erosion