How Sea Level Rise May Hit You Through the Backdoor: Changing Extreme Water Levels in Shallow Coastal Lagoons.

Due to their choked geometry, coastal lagoons can attenuate extreme water levels compared to the open sea. However, this protective property is expected to decrease due to sea‐level rise. By studying idealized lagoons in a non‐dimensional parameter space, this study describes non‐linear interactions between tides, storm surges, freshwater fluxes into the lagoon, and sea‐level rise. The non‐dimensional numbers include lagoon geometry and forcing scales. The main objective is to provide an overview of potentially affected lagoons and to highlight the importance of attenuation changes due to sea‐level rise. Tidal and storm surge induced maximum water levels inside lagoons rise faster than sea‐level rise for most of the parameter space. Maximum water levels due to freshwater fluxes rise slower than sea‐level rise for strongly choked lagoons. For compound events, the response between rising faster or slower than sea‐level rise depends strongly on the lagoon geometry. Plain Language Summary: Coastal lagoons are known to reduce extreme water levels compared to the open sea due to their narrow and shallow connections to the ocean. However, the rising mean sea level will decrease this protective property. In this study, we investigate lagoons using a simple box model to explore how combinations of tides, storm surges, river discharge, and sea‐level rise will change water levels inside lagoons. The study aims to provide an overview of potentially affected lagoons and to highlight the importance of these processes. Due to sea‐level rise, water from the ocean can more easily flow into the lagoon. Therefore, our results show that maximum water levels inside a lagoon can rise faster than sea‐level rise. On the other hand, river discharge can exit the lagoon more easily at the same time. For this case, water levels inside a lagoon rise slower than sea‐level rise. For events where tides, storm surges, and river discharge are all present, the maximum water level may rise either slower or faster than sea‐level rise, depending on the properties of the lagoon. Further studies are needed to understand the implications for coastal communities. Key Points: Tides, storm surges, freshwater fluxes, and sea‐level rise interact non‐linearly on the water level in choked coastal lagoonsMaximum water levels inside choked lagoons can increase faster than sea‐level rise due to non‐linear attenuation changesMaximum water levels due to freshwater fluxes increase slower than sea‐level rise [ABSTRACT FROM AUTHOR]