An Integrated GIS Method to Evaluate Susceptibility and Hazard of Sea Level Rise in Coastal Plains: the Versilia Plain

Giuliano Gallerini (1), Mauro De Donatis (1), Sergio Silenzi (2), Saverio Devoti (2),
Marco Fulvio Nisi (2), Massimo Gabellini (2)

(1) Environmental Sciences Dep. (Scientific Campus), Urbino University (IT)
(2) ICRAM, Central Institute for Marine Research, Roma (IT)

A great number of scientific evidences suggest an accelerated sea level rise in the next 100 years related with current climate changes.
Eustatical and geo-morphological researches on Italian coastal zones show that many plains are susceptible of flooding as results of Relative Sea Level Rising (RSLR) (e.g. in Aminti et al. 2001; CENAS 1998; Colantoni et al. 1997; Marini et al. 2000; Nisi et al. 2000; Nisi et al. submitted).
The need for a prevision of impact on coastal plain areas becomes evident, considering the demographic increase in the coastal regions.
Aim of this study is to develop a preliminary methodological guideline for the evaluation of the Integrated Hazard and Risk of coastal areas to relative sea level changes, which would provide indications for a compatible territorial management through editing thematic maps.
This method, developed in G.I.S environment, will forward the future physiographical scenarios of the territory in 25, 50 and 100 years time (with optimistic, intermediate and pessimistic scenarios). Moreover, it evaluates the susceptibility of an area to possible damaging events resulting from RSLR.
We adopt a multidisciplinary approach, utilized for the pilot studies carried out in the plains of Versilia (NW Tuscany), a particularly vulnerable area to RSRL because of its morpho-altimetric asset and subsidence rates. This includes geological, topographical, geomorphological, hydrological, geo-hydrological and Land Use surveys, forecasts of ground level changes and accelerated beach erosion with respect to sea level rise. All data are processed through Geographic Information System (GIS).
In particular, we process and edit the following map-layers (GIS):

The main outcomes of this integrated approach are: hi-resolution land zonation of potential future assessment of the coastal plain; the evaluation of Risk connected to RSLR; and planning of best land use, sedimentary deficit, coastal management.

Figure 1 - Methodological scheme for RSLR Risk evaluation and study area

The methodological approach described above has been applied through four GIS elaborations (Microstation Geographics software, by Bentley Systems).
The topographical basis used for surveys, analysis, GIS and "SeaLevel" applications is the Regional Technical Map (CTR) of Regione Toscana, with a nominal scale of 1:10.000.
The first elaboration was aimed at obtaining a provisional Digital Terrain Model (DTM) of pessimistic, intermediate and optimistic scenarios for the years 2025, 2050 and 2100 (for a total of nine different DTMs), by considering sea level rise and differential rates of subsidence.
The second elaboration was finalized to obtain Susceptibility maps, one for each forecasting period. The result was achieved by using parametric maps, and integrating them in Homogeneous Territorial Unit (HTU) synthetic maps. The HTU maps were processed through a GIS overlay operator, according to the following:

The results were subdivided into an appropriate number of classes and, using thematic re-symbolization tools, translated into Susceptibility maps.
Third, we applied again GIS overlay operator between the Susceptibility map and the Specific Hazard Map to obtain an Integrated Hazard map.
The last elaboration provides an estimate of Risk. The map of Risk is extracted through intersect between the Integrated Hazard map and a modified Land Use map. The Land Use map has been modified by assigning to each relevant element a normalized score, based on its economical value.
The results obtained develop some considerations.
The potential average shoreline retreat is absolutely relevant, ranging between 21 m of the optimistic scenario for the year 2025 and 364 m of the pessimistic scenario for the year 2100. In particular, the tourist area of Viareggio would seem to be subject to a high probability of submersion, unless remarkable preventive interventions are made. Moreover, wide portions of the versilian backdune areas turned out to be highly exposed to flooding and to sea ingressions as well as to the worsening of both the meteorological events and wave phenomena.

Figure 2 - Detail of the pessimistic scenario for the three periods

It is however important to underline that the results obtained from any model concerning climatic changes must be dealt with caution because of the uncertainties regarding the effects of global warming and the results of the models in general. Also, in the case of the Versilian plain, the reliability of results decreases as the predicted time interval increases. This is essentially due to the fact that the values of subsidence for compacting of organic soils are analyzed large temporal intervals (more than 25 years); such values represent in fact an important share in the total RLSR rates, but the data they originate from is not sufficient to determine the time of the end of phenomenon in function of the sediment's thickness.
By applying the precaution principle the maps created can be used for a fast identification of the high risk coastal areas, as well as for planning nourishments and other coastal defense works, or for deciding possible restrictions to the construction of new infrastructures or homes in the areas characterized by the greatest hazard classes.
This parametric method, supported and integrated by G.I.S., offers a particular advantage to calculate new different scenarious and risk degree, implementing new previsional data with the procedures and analysis tecniques developped.

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