Experimenting criteria for risk mitigation in fluvial-coastal environment

Abstract

Vanishing beaches and hungry water1 originated by different human pressure are a worldwide issue, with significant and sometimes unpredictable socio-economic and environmental impacts (Kondolf, 1997). The United States Environmental Protection Agency estimates that between 80% and 90% of
America’s sandy beaches have been eroding (EPA, 1994).
In most cases, single beaches may be losing only a few centimeters per year, but in some cases the problem is much worse: the coast of Louisiana is losing about 15 m of shoreline per year (NOAA, 2003).
In Europe about 40% of the shoreline is eroding, in some countries an erosion range between 50% and 70% was registered (Beachmed, 2008).
These problems should be tackled on a large scale and with a different approach. Rivers and coasts, i.e. the border between land and water, represent transition environments which are extremely sensitive to perturbations, both natural or anthropogenic. Each environmental component, whether a physical, biotical or anthropic element, mutually influences the adjacent one and thus triggers interactions. These complex processes do not follow the linear physics of causeeffect, but are dominated by a non-linear or chaotic physics.
Therefore, small modifications in the catchment due to a forcing could cause significant variations of shoreline, out of phase and also far from the river mouth, often with amplified feedback.
Historically, in planning and managing, rivers and coasts are considered as separated domains with very different dynamics. The river mouth represents the common point where waters and sediments coming both from land and sea
interchange. A modern approach should be based on the criterium of flumen unicum, whereby the river basin and the facing littoral are considered as a single indissoluble and continuous space-time system. Actually, both these environments are strictly linked to each other, from the farthest end of the watershed to the seabed down to 15 m depth, where the main geomorphic and sedimentary processes take place....