Journal of Waterway, Port, Coastal, and Ocean Engineering

Existing floating breakwaters of different types are examined and the performance of each studied. An important parameter identified with the performance characteristics of floating breakwaters is the width to wave length ratio $(W/L)\text{.}$ Earlier studies revealed that to achieve a transmission coefficient $\left(K_t\right)$ less than 0.5, $W/L$ ratio should invariably be greater than 0.3 for any type of floating breakwater. Hydraulic model tests reported herein suggest that fixing a row of pipes with certain length and interval at the bottom of a trapezoidal float reduces $W/L$ ratio to around 0.15 to achieve a transmission coefficient less than 0.5. This essentially brings down the width requirement of the floating breakwater by half, reducing the cost. Nondimensional graphs indicating the variation of transmission and reflection coefficients with respect to relative depth $(d/h)$ relative width $(W/L)\text{,}$ wave parameter $(H_i/g{T}^2)\text{,}$ and gap to diameter ratio $(b/D)$ are presented in this paper. The experimental results for $K_t$ are compared with the results already reported by past investigators on similar studies.

This paper describes changes to the flow pattern around a horizontal circular cylinder immersed in shallow open‐channel flows, with different gap openings between the cylinder and the bed. Experiments show that the low flow depth causes backup of the upstream flow depth due to choking of the flow. This phenomenon causes the occurrence of a weir‐flow condition at the cylinder, where a large drawdown forms. The net result is an increase of the drag force. When a gap is present between the underside of the cylinder and the bed, the oncoming flow divides itself into two components as it approaches the cylinder. The study presents a unique relationship describing the amount of gap flow as a function of the undisturbed flow depth and gap size. Changes to both the drag and lift forces exerted on the cylinder for different flow depth and gap size combinations were measured and reasons for these changes discussed.

A very simple, conceptual explanation of the cause of down‐drift offset inlets is presented. Sheltering of a time‐varying wave climate by the inlet's ebb‐tidal shoal causes the offset. Wave sheltering is defined to include all of the nearshore wave transformations that occur across the ebb‐tidal shoals, including wave refraction, diffraction, shoaling, breaking, and other forms of attenuation. Sheltering tacitly includes the effect of the shoal on the directional spread of incident wave energy. This explanation of down‐drift offsets is better than the oft‐quoted explanation of refraction around the ebb shoal for two reasons: (1) The present explanation, sheltering of a variable wave climate, specifically includes the factor of time, which is obviously important in geomorphologic feature development; and (2) there is little justification for explaining the current reversal only in terms of wave refraction. All that is needed to create a down‐drift offset inlet is an ebb shoal and a wave environment with a dominant direction of littoral drift and periods of reversal.

The relationship between truncated Fourier series solutions of several different long‐wave evolution equations is explored. The models are all designed to represent the same physics in the asymptote of small nonlinearity and frequency dispersion, and yet give different numerical solutions for given values of dimensionless parameters. It is found that the problem of obtaining a steady solution in a coupled‐mode model of shallow‐water wave evolution is related more to the problem of properly choosing the corresponding time‐dependent evolution equation than to the problem of truncating the infinite series representation of the solution to that equation. In the process, a particular lowest‐order coupled‐mode model is related to a particular modified form of the Korteweg‐deVries equation. It is shown that the existing lowest‐order model may be inherently inaccurate in the case of relatively high waves, due to the nonphysical behavior of the form of the nonlinear term inherent in that model. Finally, solitary and cnoidal wave solutions are given for the entire family of modified Korteweg‐deVries equations, and their properties are compared.

Assuming that fluids are incompressible, inviscid, and homogeneous, natural linear surface waves in a rotating system can be decomposed into different frequency components. For each frequency component, solutions can be further decomposed into modes of an infinite series in the vertical direction. A depth‐integrated two‐dimensional model equation is derived to include Coriolis effects for waves of each mode propagating over variable depth topography in rotating fluids. The model equation reduces to the mild‐slope equation for waves in irro‐tational flows when Coriolis effects are ignored. It also reduces to an equation for waves in a rotating fluid of constant water depth. With respect to the Coriolis factor, propagating and evanescent modes of frequency components are distinguished. Analytical solutions for waves of the zeroth‐mode around a circular island on a paraboloidal water depth are obtained. Effects of the earth's rotation on wave response around a circular island are also discussed.

The hydrodynamic forces on taut nylon cable segments with PET shrouds in steady uniform flows are determined experimentally. Commercial 1.25 L (polyethylene terephthalate (PET) bottles are used as shroud material; the cable used has a diameter of 2.1 cm. The range of flow velocity is from 0.25 to 1.35 m/s. Both inline and transverse force coefficients are given as functions of the reduced velocity for three test models: Bare cable; cable with 50% area perforated shrouds; and cable with nonperforated shrouds. Comparisons of the dominant vibrating-force coefficients indicates that a cable with 50% area perforated shrouds may be a good candidate for fish and kelp aggregation devices.

Experimental studies were undertaken to investigate phreatic surface motion in rubble-mound breakwater models. This motion was assessed as a function of a number of governing variables, including armor unit type, geometry of the armor layer, breakwater slope, and core permeability. Tests were conducted in a two-dimensional wave flume using regular waves. Both the maximum and average phreatic surface elevations were found to increase with increasing wave height, increasing wave period, and steeper slopes. The setup within the core was also significantly higher for tests with spheres in the armor layer, due to the increased permeability normal to the slope. Increasing core permeability resulted in sharp rises in the maximum and average phreatic surface elevations.