Effects of multiple flood drivers on urban flooding due to joint coastal and fluvial mechanisms

Urban flood probability and severity due to joint coastal and fluvial mechanisms are investigated in this research. A focus is placed on dependency analysis between various drivers such as tides, surges and water discharges. Such assessment allows to better understand conditions that control urban flooding and to estimate combined effect of multiple source flooding.

Cork City, a coastal city in the south of Ireland, is used to investigate effects of correlated drivers. Here, probability and severity due to tides, surges and river flows occurring individually or jointly are estimated taking into account interactions and dependencies between these various flood drivers. Bivariate and trivariate joint probability approaches that incorporate these dependencies are used to determine joint exceedance return levels of water elevations. Subsequently, a numerical model of Cork Harbour, MSN_Flood, comprising a cascade of four nested high-resolution models is used to perform simulations of flood inundation under numerous hypothetical coastal and fluvial flood scenarios.

Results show that tide-surge interactions modulate phase and magnitude of their waves, and the interactions have damping effect on the total water level. In contrast, dependency between the surge residual and river flow amplifies the risk of flooding.

Mechanism of flooding plays a crucial role in flood characteristics with distinctive differences in flood wave propagation pattern and geographical extent of inundation. Fluvial mechanism in Cork area is a critical driver of flooding. While coastal defense system is capable of protecting against 1 in 200-year coastal driven flood, the river flow exceeding 50-year return period may solely cause flooding. The most severe floods in Cork City result from extreme river flows combined with moderate-to-high sea water levels.

Estimating the potential risk to flooding and understating flood-controlling conditions aids the flood risk assessment and flood prevention schemes.