Coastal Engineering Journal

This study examines the characteristics of random wave runup on models of seawalls located close to shorelines. Some of the seawalls are fronted by artificial reefs. The measured representative runup values are compared with predictions from various formulae. The applicability of a prediction method for composite slopes is also investigated. This prediction method is based on runup curves, derived from regular wave experiments, which use an imaginary uniform slope to represent the real compound profile.

The main results of the study are as follows: (1) the Rayleigh distribution provides a generally satisfactory representation of random wave runup; (2) the runup formulae reviewed in this study generally predict smaller values than the measured representative runups; (3) the prediction model based on the imaginary slope and associated curves, derived from data on regular waves, gives an approximate upper limit to the measured mean of all runups from random waves and an approximate lower limit to the measured average of the highest one-third of the runup values.

The position and configuration of the coastal shelf of the northern Bay of Bengal amplify the storm surge height extraordinarily. The low-lying coast of Bangladesh is severely devastated when extreme surges hit the land. However, the influence of track of a cyclone on surge height is not well known. A calibrated numerical model is used to simulate series of storm surges for a severe cyclone by shifting its track position and angle. The cyclonic wind field has been generated using the extreme parameters adopted from historical measurements. Surge heights simulated by the two-dimensional model are compared at selected locations along the coast. It was found that the cyclone with a landfall location at Barguna, a place between the Sundarban and the Meghna Estuary, develops the highest surge of 12.0 m (PWD) at the northeast of the estuary. The magnitude is further amplified to 13.0 m (PWD) when the surge coincides with the high-tide condition; a surge of similar magnitude attacked the coast of Bangladesh in 1876. Change in track orientation shows little influence on surge heights at the eastern coast. However, relatively higher effect was found in the western coast. Surge height distribution has also been presented with respect to local land or embankment elevation for the extreme event. A flood map is produced showing inundation depth in the aftermath of the extreme surge attack. The coastal areas at the northern tip of the upper bay and the western banks of the Meghna Estuary are found to be the most vulnerable locations.

A horizontal two-dimensional finite element model is developed in order to estimate the process of scour around a large-scale cylinder due to waves. The present model differs from previous models in the sense that the wave model is based on an elliptic mild slope equation and the sediment transport induced by the steady streaming is considered. The current induced by the gradient of radiation stress is considered and calculated using a depth integrated shallow water equation. The contributions of the Lagrangian drift velocity to the scour is also considered in this model. The model is validated against a few cases where experimental data are available. The comparison of the calculation results with the experimental data indicates that the present numerical model predicts the scour around a large cylinder reasonably well. The effects of Keulegan–Capenter (KC) number, the grain size of sediments and the model scale on scour around a large cylinder are also investigated.

Observed profiles of tsunami caused by the 2003 Tokachi-off Earthquake at 10 offshore wave gauges and 23 coastal tide stations were compiled and analyzed. Comparison of offshore wave and coastal tide data was conducted in wave-to-wave analysis bases and spectrum analysis bases. The tsunami amplification ratio between offshore station and coastal tide station varied due to seabed topography induced natural frequency difference. The frequency spectrum response was obtained at those harbors. The correlation analysis among the sea-surface fluctuation, on-off-shore current and long-shore current was also conducted at offshore wave stations.

This paper describes a model of the net sediment transport rate under linear and arbitrary nonlinear oscillatory flows in the sheetflow regime. The model is based on a two-phase flow description. The turbulent diffusivity was formulated to take into account phase-lag effects for different wave conditions and sediment properties. An introduction of a modifier to the turbulent diffusivity produced asymmetry in the concentration profiles, which qualitatively agrees with temporal and spatial distributions of the concentration profiles measured during laboratory visualization experiments. The model can predict transport rates in the sheetflow regime for combined waves and co-directional currents and for waves alone. The model is validated for a wide range of sediment median particle diameters, wave periods, and maximum flow velocities. Comparisons with experimental data on transport rates under linear and nonlinear waves, including velocity and acceleration asymmetric oscillations, were made and showed good agreement. Other comparisons encompassing combined waves and currents were conducted, also with good agreement.