Sea Level Rise Weakens the Natural Protection against Storm Surges in Shallow Lagoons

Sea level rise (SLR) increases the likelihood of storm surges by shifting their frequency distribution to higher base levels. Shallow lagoons are located along many coasts, providing natural protection against storm surges by significantly reducing the surge heights inside the lagoons compared to the open coast.

In this study, we investigate the effect of SLR on storm surge heights by using a numerical model of the Western Baltic Sea with a resolution of 200 m. We find that SLR linearly increases storm surge heights at most areas of the open Western Baltic Sea coast, e.g., a storm surge of one meter height in the present time would increase to 1.2 m height at an SLR of 20 cm. For shallow lagoons, on the other hand, the results suggest surge height increases of up to 30% additional to SLR, e.g., at an SLR of 20 cm, the surge height inside the lagoon would increase to 26 cm. We investigate this behavior in further detail with a box model to study the parameter space using the following lagoon parameters: lagoon surface area, depth and width of the connection to the open water, friction, and varying surge shapes and heights.

We find that the increase is largest for lagoons where the ratio of the width of the connection to the area of the lagoon is close to ~10⁻⁵ m⁻¹. The additional surge height decreases with increasing depth of the connection. We further find that surge height increases with surge height itself, e.g., higher surges will increase more in the future than lower surges. We find increases up to 30% for lagoons in the Baltic Sea and for lagoons in the Gulf of Mexico up to 10%.

In summary, our results highlight that additional (non-linear) surge height increase needs to be considered when evaluating and planning future coastal protection measures in shallow coastal lagoons.