Subsurface Contributions to Compound Flooding in Coastal Areas of Tha Sala District, Nakhon Si Thammarat Province, Thailand

Compound flooding — the concurrent or sequential occurrence of multiple flood-generating mechanisms — poses escalating risks to low-lying coastal communities globally. Whilst surface-water drivers such as storm surge, river discharge, and extreme precipitation have received substantial research attention, the contribution of shallow groundwater dynamics and subsurface stratigraphy to compound flood severity remains poorly characterised in tropical monsoon coastal settings. This study investigates subsurface contributions to compound flooding in Tha Sala District, Nakhon Si Thammarat Province, southern Thailand — a Gulf of Thailand coastal plain characterised by rapid urbanisation, intensive agriculture, and recurrent seasonal inundation. An integrated field investigation combining electrical resistivity tomography (ERT), continuous groundwater monitoring across twelve observation wells, and coupled hydrological–groundwater modelling (HEC-RAS 6.1 / MODFLOW-2005) was conducted over two monsoon seasons (2023–2024). Subsurface profiling revealed a laterally discontinuous clay–silt confining layer at 2.0–8.5 m depth that significantly impedes vertical drainage during flood events. Groundwater level monitoring demonstrated that antecedent soil moisture saturation precedes all identified compound flood episodes, with the water table rising to within 0.3–0.8 m of the surface during combined rainfall–tidal events. Modelling results indicate that groundwater exfiltration and impeded infiltration collectively account for 22–38% of total floodwater volume during the three major compound flood events recorded in the 2023 monsoon season (August–October). These findings demonstrate that subsurface processes constitute a quantitatively significant and systematically under-represented component of coastal flood risk in tropical low-land settings, with direct implications for flood hazard mapping, drainage infrastructure design, and early warning system development in Nakhon Si Thammarat Province and analogous coastal plains across Southeast Asia.