Case study: Vulnerability assessment, Terceira Island

Location of the case study

Terceira Island, PT

Description of the application

Due to sea level rise (SLR), some freshwater springs could disappear below the seawater line. The fresh water lens underneath the island will shrink due to SLR. Many of the springs that used to flow year round are already becoming ephemeral and they flow mostly in winter and much less or not at all in summer. This is in part explained by the land use change that took place in the last decades. The water that used to be stored in the peat soil of the forests is now released much faster to the ocean, since the trees and turf soil have been removed to make place for pasture. Some of the springs occur were ‘hanging or perched aquifers’ encounter a fault or impermeable layer. The perched aquifers have a decreased flow in periods of drought. The tides have a large effect on flow in the aquifer: tidal pumping is felt up to hundreds of meters from the coast. These movements bring in seawater that reduces the size of the fresh water lens by mixing and dispersion. SLR and increased storm frequency will make this effect just worse. SLR‐related flooding of the low areas (Lajes basin, the harbour) will cause surface and groundwater salinization. Up‐coming of salt water by pumping in the Lajes basin could increase as well.

lessons learned

The case study projected a 3D problem on a 2D plane. To have a more realistic mathematical description of double density flow in a fractured aquifer, the third dimension should be taken into account as well as other factors ignored here such as layering, matrix porosity, spatially and temporally varying recharge and many more. Although characterizing and modelling a fractured aquifer, as the one underneath Terceira, is a complicated and expensive matter it may be worth the effort since climate change may cause periods of drought and reduce the traditional sources of freshwater on the island, such as perched aquifers, springs and wells in the Lajes basin. Terceira is too far from the mainland to import water and the only sources of fresh water have to be found on or below the island itself.

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Implementation costs (Show all)

>1.000.000 €

Served purpose of implementation (Show all)

Strongly Positive

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Vulnerability assessment

Last modified: Sept. 13, 2016, 4:41 a.m.