Abstract: Nonlinear interaction between astronomical tides and storm surges can alter the process of a coastal storm surge and its extremes, inducing uncertainty in assessment of its hazard risks. Focusing on the Pearl River estuary, this study develops a numerical model that couples astronomical tides, storm surges, and their interaction based on the unstructured-grid model SCHISM. We reveal the modulating effect of tidal timing on storm surge amplification through ensemble simulations of Typhoon Mangkhut (2018) under different tidal timing scenarios. In the inner bay and near the estuary mouth, low-tide scenarios generally generate higher surges in the range of 0.2 - 0.3 m roughly. And, we also estimate the return periods of storm surge extremes under different tidal scenarios, based on the 20-year long simulations of separate extreme typhoon events and their Pearson-III distribution fitting. The results indicate the differences become particularly significant for high return periods, where high-tide scenarios may lead to an underestimation of surge heights. This study demonstrates the crucial role of tidal timing in storm surge risk assessment, a key factor of engineering design of coastal disaster prevention and mitigation.

Key words: Pearl River estuary, storm surge, astronomical tides, numerical simulation, extreme value statistics, return period, risk assessment