Flood risk assessment requires a quantitative understanding of hazards and vulnerability. In the coastal built environment, the human's vulnerability to combined hazards due to the floodwater and winds is an integral component of flood risks. The present study aims to reveal the human vulnerability to storm-induced coastal flooding, focusing on New York City during Hurricane Sandy. We develop a physics-based model to quantify individuals' physical vulnerability, both adults and children, to compound hazards of floodwater and winds. The model accounts for the failure of individuals caused by physical instability due to slipping and toppling. The governing equations consider the balance between the driving and resisting forces and moments applied to an individual concurrently exposed to floodwater and winds. We first calibrate the model using existing measurements in the literature and then implement it to study the vulnerability of New York residents in Manhattan to coastal flooding during Hurricane Sandy. Model results indicate that when combined floodwater and wind hazards were at their highest-level during Sandy, the majority of flooded areas were mainly a hazardous zone for adults and either a failure or drowning zone for children. About 5.4% and 47.4% of the total flooded area became a failure zone, and 19.9% and 42.4% became a drowning zone for adults and children, respectively. We conclude that winds can have a significant impact on the physical instability of individuals. For example, model results for children show that neglecting winds results in a reduction of 97.7% in the area of the failure zone.