Paula Freire (Portugal) 1; Marta Rodrigues (Portugal) 1; André B. Fortunato (Portugal) 1
1 - Laboratório Nacional de Engenharia Civil
Agriculture is one of the most relevant economic activities in the Tagus estuary upper region, where a public irrigation perimeter (Lezíria Grande de Vila Franca de Xira) covering about 13,400 ha of low-elevation estuarine marginal terrains protected by a dyke is located.Two main natural hazards with potential to increase with climate change affect this area. First, during very low Tagus river discharges, salinity can propagate upstream and reach the main water abstraction station, limiting the availability of water with quality for irrigation (salinity below 1).
Secondly, estuarine high water levels forced by spring tides and severe storm surges can overflow and damage dykes leading to the inundation of agricultural lands inundation. This presentation discusses the level of risk associated with these two hazards and the possible effects of climate change. The water salinity and the inundation are assessed for different scenarios through numerical modelling and the likelihood is estimated based on relevant historical data and/or probability forecasts.
The consequences evaluated are: the percentage of time per week during which the salinity of the abstracted water is adequate for irrigation; the percentage of the area inundated and the percentage of the dyke length overflown. A consequence / probability matrix approach is used for the risk analysis. For the salinity concentration of the water, results show that consequences with very high severity (>50% of time with water inadequate for irrigation) have a very low likelihood with return periods over 100 years. An event similar to the worst recent drought that occurred in 2005 (10-50 years of estimated return period) has consequences with high severity (>25 to 50% of time with water inadequate for irrigation). These consequences may be exacerbated by sea level rise and more severe droughts in the future.
Independently of the likelihood, the severity of the inundated area consequence is always low (≤5% of inundated area). However, for the dyke overflown consequence, medium severity (>10 to 30% of overflown dyke length) can be reached for very low likelihood scenarios (return period over 100 years). If a sea level rise of 0.5 m is considered, the severity of both consequences is high (>10 to 30% of inundated area/ overflown dyke length).