RISK-BASED FORECASTS OF COASTAL RESPONSE TO SEA LEVEL RISE IN SOUTHERN BRAZIL

Since large uncertainties concerning the precise values of model parameters for forecasting sea level rise coastal impacts exist, the stochastic modeling approach was adopted in this study through use of the Random Shoreface Translation Model (RanSTM) (Cowell et al., 2006). The model applies sediment-mass continuity principles during shoreface translation in response to sea level rise, so that it inherently involves predictions of coastal change based on a sediment-budget analysis for each specific site. A range of values of sea level rise for 2100 time horizon was chosen to deal with uncertainty associated with the published projections itself (Table 1). In order to evaluate the relative effects of regional variability on coastal response under climate change, simulations were performed for each pre-defined coastal sector(cell) on the basis of offshore slope and geometry, and sediment-budget factors (Table 1).

Table 1. Range of sea level rise rates and sediment-budget considered within model forecasts.

	Minimum	Mode	Maximum
Sea level rise (m)	0.181	0.892	1.13
Sediment-budget at Cell 3 (m3/m of coastline)	+1496	+1496	+3870
Sediment-budget at Cell 6 (m3/m of coastline)	-1660	-760	-360

¹ IPCC (2007) lower bound; ² IPCC (2007) upper bound with accelerated ice- flow (0.20 m), plus (0.10 m) southern Atlantic locally higher levels; ³ Climate Change Science Congress held in Copenhagen (2009).

Comparison of coastal sectors (Cells 3 and 6), which have strongly different shoreface slopes (the latter is steeper), illustrates the range of recession distances likely to be exceeded at any given probability level (from 0,01 to a 100%). The low gradient sector (Cell3) displays the highest recession distances (0.01%= 1148 m; 50%= 610 m; 90%= 285 m), and the steeper sector (Cell6), the lowest ( 0.01%= 429 m; 50%= 269 m; 90%= 164 m). Recession distances likely to be exceeded at different levels of risk by 2100, are compelling in demonstrating that the effect of differences in shoreface geometry is overwhelming in driving coastal recession compared to even strong effects related to the sediment budgets.