This research employed spatial analysis tools in Geographic Information System (GIS) to evaluate the spatial extent of future flood prone areas in the two case study Cities Porvoo and Loviisa.
A sum operation was run in the raster calculator to generate a flood prone area analysis with digital elevation model (DEM) and sea level results of the Regional Climate Model (RCM) as source data.
The following operation has been done:
Flood prone areas (neg. values = under sea level) = elevation model + relative land up lift (25 cm/100yr*) + eustatic sea level rise (15 cm/100 yr) – absolute sea level rise model
*Value 25 cm used in both case areas Porvoo and Loviisa
To change the DEM (now in N60 height system) closer to the NH60 the following sum operation can be done
(DEM + 11 cm*)
*Difference between N60 and NH60 in Helsinki is 11 cm
After the operation the result grid has been classified by the following way:
The result indicates that significant proportions of nowadays dry land might be part time flooded during occasional storm surges in the future in case of the high case scenario. This can be seen especially in the Laivasilta and Sahaniemi areas in Loviisa and in the east bank of river Porvoo. This might indicate need for land use planning to properly manage pressures caused by sea level rise in coastal areas. However, in case of low case scenario the land uplift will be more extensive than sea level rise and nowadays flood prone areas might be even less affected. Sea level change based on the three scenarios is no major risk for the case study areas but extreme events might cause problems.
DEM:
| Data layers | Digital elevation model of Loviisa and Porvoo | |||
| Source | City of Loviisa, city of Porvoo | |||
| Layer type | Raster – Grid | |||
| Scale | 5 m cell | |||
| Origin | Interpolated from 1 m contour lines | |||
| Projection | Finnish KKJ3 | |||
| Height system | N60 |
The following operation has been done:
Flood prone areas (neg. values = under sea level) = elevation model + relative land up lift (25 cm/100yr*) + eustatic sea level rise (15 cm/100 yr) – absolute sea level rise model
*Value 25 cm used in both case areas Porvoo and Loviisa
To change the DEM (now in N60 height system) closer to the NH60 the following sum operation can be done
(DEM + 11 cm*)
*Difference between N60 and NH60 in Helsinki is 11 cm
After the operation the result grid has been classified by the following way:
Areas with height:
| –24-0 cm | Low case scenario (land uplift more extensive than sea level rise, exposed areas) | |||
| 0-24 cm | Ensemble average scenario (sea level rise more extensive than land uplift, underwater areas) | |||
| 24-75 cm | High case scenario (sea level rise more extensive than land uplift, underwater areas) | |||
| 75-220 cm | High case scenario + storm scenario (worst case scenario, underwater areas) |
The result indicates that significant proportions of nowadays dry land might be part time flooded during occasional storm surges in the future in case of the high case scenario. This can be seen especially in the Laivasilta and Sahaniemi areas in Loviisa and in the east bank of river Porvoo. This might indicate need for land use planning to properly manage pressures caused by sea level rise in coastal areas. However, in case of low case scenario the land uplift will be more extensive than sea level rise and nowadays flood prone areas might be even less affected. Sea level change based on the three scenarios is no major risk for the case study areas but extreme events might cause problems.