Abstract
Navigable floodgates such as flap gates and vertical lifting gates are attracting attention as effective countermeasures against tsunamis and storm surges, particularly after the Tohoku tsunami of 2011. A narrow gap between adjacent gate units would inevitably cause an intrusion of seawater into the port basin, which cannot be simply simulated using common tsunami models because the gap is too small to be reproduced in the model. The present study examines the water flow passing through a gate gap by applying a 2D-3D hybrid hydrodynamic model to derive the hydraulic head-to-inflow discharge conversion formula, which is defined as Q = α√(2gh)dW, where Q is the discharge, g is gravitational acceleration, h is the hydraulic head, d is the water depth, and W is the gap width. The coefficient of discharge α can be estimated by a linear regression, which varies with the gap width between the relevant floodgates. For example, the relationship 0.15h + 0.25 was derived for the case where W = 0.3m. The proposed model was verified through case studies in which a time series of hypothetical water levels composed of a Level 1 tsunami and tide or that of a storm surge alone was simulated for a medium-sized port. With this proposed method and given the water level outside the port, the water level rising over a port basin enclosed by floodgates and breakwaters can be calculated using a spreadsheet or other simple computational means that do not require complex models based on computational fluid dynamics.
