Demonstrating the value of beaches for adaptation to future coastal flood risk

Abstract

Cost-effective coastal flood adaptation requires a realistic valuation of losses, costs and benefits considering the uncertainty of future flood projections and limited resources for adaptation. Here we present an approach to quantify the flood protection benefits of beaches accounting for the dynamic interaction of storm erosion, long-term shoreline evolution and flooding. We apply the method in Narrabeen-Collaroy (Australia) considering uncertainty in different shared socioeconomic pathways, sea-level rise projections, and beach conditions. By 2100, results show that failing to consider erosion can underestimate flood damage by a factor of 2 and maintaining present-day beach width can avoid 785 million AUD worth assets from flood damage. By 2050, the flood protection and recreational benefits of holding the current mean shoreline could be more than 150 times the cost of nourishment. Our results give insight on the benefits of beaches for adaptation and can help accelerate financial instruments for restoration.

Fig. 1. Key steps for estimating the flood protection services provided by beaches.

From right to left. Step 1: Nearshore wave downscaling. Step 2: Integration of erosion on topo-bathymetries due to sea-level rise. Step 3: Modelling of surf-zone morphodynamics and hydrodynamics. Step 4: Integration of storm erosion on long-term topo-bathymetries. Step 5: Modelling of coastal flood propagation inland. Step 6: Calculation of flood damage to assets. Step 7: Calculation of the flood protection benefits of beaches in terms of avoided flood damage.

Fig. 2. Representation of the flood protected area and value at Narrabeen-Collaroy.

The flood protected area and value are the magenta and purple shaded regions, respectively, in planform and profile sketches. Turquoise and orange areas denote the flood extent from a 30-year total water level (TWL) event with and without coastal erosion, respectively. a Storm erosion due to the 30-year event on an uneroded beach (present). b Storm erosion due to the 30-year event on the beach already eroded by a recent previous storm (present). c Storm erosion due to the 30-year event on the beach already eroded by sea-level rise (SLR) (2100). d Storm erosion due to the 30-year event on the beach already eroded by sea-level rise and a recent previous storm (2100). Note that in a reduced TWL due to storm erosion leads to lower flooding than if no erosion is considered; and in b the initial erosion condition of the beach is such that regardless the reduction in TWL, flooding is higher than if no erosion is considered.

Fig. 3. Geographic location of the Narrabeen-Collaroy beach system.

Narrabeen-Collaroy is a 3.6 km long embayed beach that is located in 20 km north of Sydney (Australia). The beach system is bounded by Long Reef Point to the south and Narrabeen Headland to the north, with beachfront houses and apartments on most of its backside.

Fig. 4. Dynamic and static results at Narrabeen-Collaroy.

Results of the application of the dynamic and static approaches (turquoise and orange, respectively) are provided for 2020, 2050 and 2100, for two sea-level rise (SLR) confidence scenarios (medium and low confidence, M and L, respectively), two emissions scenarios (SSP2-4.5 and SSP5-8.5, boxes and circles, respectively) and three SLR trajectories per each confidence scenario and SSP associated with three percentiles of the distribution (5th, 50th, and 95th, horizontal lines of the boxes and circles for the SSP2-4.5 and SSP5-8.5, respectively). a Total water level in a profile located in the centre of the beach (m). b Flooded area for the beach in storm conditions (ha) measured from the 2018 mean high-water line. c Flood damage for the beach in storm conditions (million AUD). d Flooded area for the beach in poststorm conditions (ha) measured from the 2018 mean high-water line. e Flood damage for the beach in poststorm conditions (ha). Note that to improve visualisation, in c and e y-axis is broken and results below 75 million AUD are represented in a distant-proportional distorted scale. The plots were created using MATLAB R2022a.

Fig. 5. Attribution of the variance of the Narrabeen-Collaroy results to their sources of uncertainty.

Break-down of uncertainty (%) in the total water level (a), the flooded area for the beach in storm conditions (b), the flood damage for the beach in storm conditions (c), the flooded area for the beach in poststorm conditions (d), and the flood damage for the beach in poststorm conditions (e). Variance fractions are attributed to the approach (dynamic or static), the emissions scenario denoted here for simplicity as SSP (SSP2-4.5 or SSP5-8.5), the sea-level rise (SLR) percentile (trajectory associated with the 5th, 50th or 95th percentiles), and interactions (excluding interactions between SLR components due to their dependency). Results are shown for 2020, 2050 and 2100 for two SLR confidence scenarios (medium and low confidence, M and L, respectively). The plots were created using MATLAB R2022a.

Fig. 6. Benefits of maintaining the present-day coastline at Narrabeen-Collaroy.

Results consider storm and poststorm conditions (magenta and purple, respectively). a Flood protection benefits in terms of avoided flood damage (million AUD). b Recreational benefits in terms of avoided loss of recreation (million AUD). Results are shown for 2020, 2050 and 2100 for two sea-level rise (SLR) confidence scenarios (medium and low confidence, M and L, respectively), two emissions scenarios (SSP2-4.5 and SSP5-8.5, boxes and circles, respectively) and three SLR trajectories per each confidence scenario and SSP associated with three percentiles of the distribution (5th, 50th, and 95th, horizontal lines of the boxes and circles for the SSP2-4.5 and SSP5-8.5, respectively). Note that to improve visualisation, in a results below 50 million AUD are represented in a distant-proportional distorted scale. The plots were created using MATLAB R2022a.

Fig. 7. Benefits of maintaining the present-day mean coastline at Narrabeen-Collaroy and benefit-cost ratio.

The benefit-cost ratio is a first-pass evaluation of the trade-off between these benefits and the cost of beach nourishment to counteract sea-level rise (SLR) erosion. a Flood protection benefits in terms of avoided flood damage (million AUD). b Recreational benefits in terms of avoided loss of recreation (million AUD). c Cost-benefit ratio considering only flood protection benefits. d Cost-benefit ratio considering only recreational benefits. e Total cost-benefit ratio. Results are shown for 2020, 2050 and 2100 for two SLR confidence scenarios (medium and low confidence, M and L, respectively), two emissions scenarios (SSP2-4.5 and SSP5-8.5, boxes and circles, respectively) and three SLR trajectories per each confidence scenario and SSP associated with three percentiles of the distribution (5th, 50th, and 95th, horizontal lines of the boxes and circles for the SSP2-4.5 and SSP5-8.5, respectively). Note that to improve visualisation, in a and c results below 50 million AUD and 10, respectively, are represented in a distant-proportional distorted scale. The plots were created using MATLAB R2022a.