Journal of Waterway, Port, Coastal, and Ocean Engineering

The role of hydrodynamic sources of damping in regulating the flowinduced vibration response of flexible cylinders has frequently been a source of disagreement between investigators in the field. A hydrodynamic damping model is proposed in this paper for finite length cylinders and for cables, which effectively behave as if they were of infinite length. Measured and predicted damping for a strumming cable in a sheared flow are presented. The experimental results are from a flow‐induced vibration experiment conducted in a mill canal with a width of 58 ft (17.7 m). Variable headgates allowed a controllable horizontal shear to be produced. Hydrodynamic damping is shown to be one to two orders of magnitude more important than structural damping in sheared flow conditions.

The effect of increasing water depth on the performance of deep water tendons of tension leg platforms (TLP) is examined. Equations to determine limits on water depths for tendon systems based on strength and hydrostatic collapse requirements are presented. Relations between tendon tensions and reactions on a TLP hull are derived. Conditions for which neutrally buoyant tendons can be utilized are presented. For very deep water, the limiting diameter‐to‐thickness ratio (D/t) is presented for different tendon variables. For a water depth of 5,000 feet, the D/t ratio is limited to about fifteen due to hydrostatic collapse constraints. Composite tendons can provide reduced pretension, improved restoring force, and reduced drag forces.

A new hydrodynamic force model for the prediction of forces on seabed pipelines has been verified. Comparisons of forces predicted by the new model with those measured in full‐scale laboratory and field investigations demonstrate the accuracy of the model, in terms of general force characteristics, peak force values, and spectral energy content, for both regular and irregular waves, with or without superimposed currents. Pipeline motions calculated from predicted forces are in good agreement with motions calculated from measured forces. Similar comparisons using the conventional Morison equation show its deficiencies and demonstrate the considerable improvements attained by the new model. Two effects included explicitly in the new force model, related to the wake behind the pipeline and the time dependence of the force coefficients, are supported by measurements.

The PIPESTAB Pipe‐Soil Interaction Project developed a model that predicts the soil resistance to lateral motions of untrenched submarine pipelines. The model improves upon the typical coulomb friction estimation by including soil strength information and pipe displacement history in the resistance prediction. An extensive data base containing results from monotonic and cyclic tests of full‐scale pipe sections on five distinct soil conditions forms the foundation for the model. Pipe‐soil resistance is far more complex than simple friction. Test data convincingly shows a marked dependence of lateral resistance on pipe penetration and soil strength. The improved empirical model captures this dependence by estimating the lateral resistance using two terms, a sliding resistance component plus a soil passive resistance component. The empirical model incorporates all significant trends observed in the test results and is suitable for use in pipeline dynamic response predictions. The testing program significantly extends the industry's data base, especially in the areas of cyclic loading and correlation of soil parameters to measured resistance.

This paper discusses the stability of a class of rubble mounds referred to as reef breakwaters. Reef breakwaters do riot have a multilayer cross section like conventional breakwaters but are a homogeneous pile of stone. Individual stone weights are similar to those used in the armor or first underlayer of traditional breakwaters. Findings are based on a study which includes an extensive series of physical model tests conducted with irregular waves. An unusual method to quantify reef stability is presented. The method is unusual because it measures stability in terms of reduction in crest height due to wave attack. Reduction in crest height has less scatter than damage measured by number of stones displaced. Such an approach directly links stability to the primary performance characteristic of wave transmission. A stability model is developed which can accurately predict damage to the reef from very minor to very severe levels. The model also works for both submerged and subaerial mounds and for a wide range of wave conditions including those with heavy overtopping.

Monochromatic wave overtopping over the crest of an impermeable coastal structure located on a sloping beach is predicted numerically by expanding the numerical model developed previously for predicting wave run‐up on such a structure located on the horizontal seabed. The expanded numerical model predicts the temporal variations of the velocity and depth of the flow over the crest of the structure from which the average overtopping rate per unit width is computed. The model accounts for the effect of wave shoaling on the sloping beach in front of the structure located in relatively shallow water. The computed average overtopping rates are shown to be in agreement with available small‐scale test data for which smooth impermeable structures were fronted by a 1:10 slope. The numerical model also predicts the decrease of wave reflection due to the increase of wave overtopping. However, more detailed measurements will be required to further calibrate and evaluate the numerical model which may be extended to examine the armor stability of overtopped breakwaters.

In the early 1980s, a number of research projects were conducted to evaluate the potential impact on the Illinois Waterway of proposed increased diversions from the currently authorized rate of 3,200 cfs $(90.6m^3/s)$ to 6,600 cfs $(186.9m^3/s)$ and 10,000 cfs $(283.2m^3/s)\text{.}$ This article summarizes some of this research. The following effects of diversion are identified: (a) Increased scour and deposition of sediment in the upper reaches of the waterway; (b) a probable increase in backwater sedimentation rates; (c) no significant changes in sediment transport in the lower reaches except those associated with naturally increased inflow (however, the effects on resuspension, scouring, and deposition of sediments in the lower reaches are not known); (d) no significant increase in bank erosion rates except within the constricted reaches; (e) increased flood damage in the Alton pool compared to the upper reaches; (f) an increase in groundwater levels near the Illinois Waterway, making higher drawdown available for pumping; (g) increased pumpage costs for the drainage districts, ranging from 2–50% or more; (h) improvement in water quality in the upper reaches but no significant alteration of the osmotic balances; and (i) adverse effects on vegetation, including bottomland species of trees, on some species of invertebrates, on feeding grounds of migratory individuals, and on algal blooms (because of the availability of clear water).

The transformation of monochromatic and directionally‐spread irregular waves passing over a submerged elliptical mound was studied in a controlled laboratory experiment. A directional spectral wave generator was used to generate waves with equal peak frequencies and spectral energy, along with monochromatic waves of equivalent significant height and period. Spectra with both narrow and broad frequency and directional spreads were generated. Results indicate that monochromatic waves provide a poor approximation of irregular wave conditions if there is directional spread or high wave steepness.