Shoreline Position as a Geoindicator: definition and application to the Rio Grande do Sul Coast, Brazil

Beaches are highly dynamic environments and, thus, very complex. With respect to the impacts generated by a global climate change, the main expected phenomenon is the readjustment of the shoreline by erosive processes, which are favoured by the sea level increase, the greater cyclogenesis intensity, hydrological cycle alterations. It is of paramount importance for coastal settlements and policymakers to know whether local shoreline is advancing, retreating or stable, especially when making decision and environmental planning concerning intervention strategies adoption. Changes in the shoreline position can be drastic and costly since they directly affect transportation routes, coastal installations, communities, and ecosystems. For instance, in the south coast of Rio Grande do Sul (Brazil), at Hermenegildo Balneary, has been reported constant recession rates of 0.5 m/yr (Calliari et al., 2006), which is not much high, but enough to reach the coastal settlements, causing infrastructure damages and losses. Esteves & Santos (2001) and Teixeira (2005) conducted interviews with local people and concluded that the total average cost of construction of protective structures in this balneary is about U$550.000,00.

There have been some good efforts to know the Rio Grande do Sul shoreline behaviour over the years, and, despite of methodological differences, and thus its different results, it is already known that the coast tendency is mostly erosive. However, how these research results can be fast transitioned into operation, i.e., into relevant information for the coastal management?

Some geological processes, which can affect the environment on daily, seasonal, or decadal time-scales, can be described in terms of geoindicators, under a new framework developed to assist in monitoring and reporting environmental change. Geoindicators are defined as magnitudes, frequencies, rates or trends of geological processes and phenomena that occur at or near the Earth's surface and that are significant for assessing environmental change over periods of 100 years or less (Berger & Iams, 1996). Since the shoreline position meets this definition, the main objective of this work is to establish it as a geoindicator considering its local and regional relevant attributes, which is useful to be applied when making decisions that aim the maintenance of coast dynamic equilibrium and people safety.

To find such objective, it was constructed a table describing and specifying some shoreline characteristics: description, its significance and root (or cause), the environmental where applicable, types of monitoring sites, spatial scale, method and frequency of measurement, monitoring limitations and possible thresholds. Now, the geoindicator will be tested using the available data for the entire coast of Rio Grande do Sul. Finally, the results obtained from this test will be represented cartographically in order to facilitate the general public understanding.