Rubble Mound Structures Research Articles

A NEW DESIGN METHOD OF RUBBLE MOUND STRUCTURES

A new design method of rubble mound structures with stability and wave control consideration is proposed, by which the reduction of wave reflection and run-up and increase in rubble stability are assured under the given wave conditions. Wave control and stability increasing functions due to change of the slope shape of rubble mound structures are discussed on the basis of the experimental results for regular and irregular waves. The new design formula developed here considered the allowable percentage of damage and the wave grouping effects on rubble stability using a new assumption of the mean run-sum as an index of the irregular wave force. The run-sum is defined as the energy sum of a run satisfying a critical wave condition and the mean run-sum is the mean of run-sum for a irregular wave train.

Destruction Mechanism and Design of Rubble Mound Structures by Irregular Waves

Destruction Mechanism and Design of Rubble Mound Structures by Irregular Waves

REEF TYPE BREAKWATERS

Reef type breakwaters refers to a low-crested rubble mound breakwater without the traditional multilayer cross section. This type of breakwater is little more than a homogenous pile of stones with individual stone weights sufficient to resist wave attack. In recent years a number of low-crested breakwaters has been built or considered for use at a variety of locations. Most of these structures are intended to protect a beach or reduce the cost of beach maintenance. Other applications include protecting the water intakes for power plants, the entrance channel for small boat harbors, and providing an alternative to revetment for stabilizing an eroding shore line. In situations where only partial attenuation of the waves on the leeside of the structure is required, or possibly even advantageous, a low-crested rubble mound breakwater is a logical selection. Since the cost of a rubble mound increases rapidly with the height of the crest, the economic advantage of a low-crested structure over a traditional breakwater that is infrequently overtopped is obvious. Because the reef type breakwater represents the ultimate in design simplicity it could be the optimum structure for many situations. Unfortunately, the performance of lowcrested rubble mound structures, and particularly a reef type breakwater, is not well documented or understood.

Overspilling/overtopping of rubble-mound breakwaters. Results of studies, useful in design procedures

This paper presents an attempt to generalize the results from many series of model tests during which the amount of water overspilling/overtopping the crest of the breakwaters was measured. In one case, also prototype measurements were performed, for comparison between model and prototype measurements. All tests were made with irregular waves reproduced directly from natural wave records. The results are presented in a dimensionless form. The main parameters, which have been varied through the tests, are (1) breakwater type (both pure rubble-mound structures and rubble-mound structures with concrete superstructures have been tested); (2) significant wave height, H s; (3) wave period (mean wave period, T z); and (4) wind speed, U. The results presented may be used for preliminary design of similar structures.

EFFECT OF BROKEN DOLOSSE ON BREAKWATER' STABILITY

Although the use of rubble-mound structures for protection of coastal areas is common throughout the world and considerable hydraulic design data have been developed to aid the designer, very little data are provided on the structural integrity of individual armor units and the effect of broken units on the stability of such structures. The forces to which such structures are subjected are complicated and vary with type and geometry of the structure, depth of water, bottom configuration seaward of the structure, water level relative to the crown of the structure, and wave dimensions. Since all of these parameters are involved, accurate determination of wave forces cannot be calculated; and although it is not impossible to model the structural strength of armor units or measure forces on individual armor units, there are physical limitations which make the tasks extremely difficult. There is one way, however, that a breakwater stability model, as it exists today, can provide useful information on this subject and that is to purposely break given numbers of armor units and observe the effect on the overall stability of the structure. Such tests have been conducted at the U. S. Army Engineer Waterways Experiment Station for the Atlantic Generating Station (AGS) Breakwater in which specific answers were desired; thus, the data are limited to the dolos armor unit. The data do provide, however, significant indications regarding the type and extent of breakage (cluster or random) that is most detrimental to the overall stability of the structure; whether costly reinforcing steel is warranted to assure reduced armor breakage; and where necessary, data from this type study can be used as a basis for setting maintenance criteria that will prevent deterioration of the breakwater due to armor unit breakage.

The Effect of Waves on Rubble-Mound Structures

For thousands of years breakwaters have been built at or near the coast to protect harbors or coastlines from wave attack. One of the earliest known harbor protection schemes was devised in about 2000 B.C. for the Port of Pharos on the open coast of Egypt; it had a rubble-mound breakwater approximately 8500 ft long composed of large blocks of stone with smaller stone filling the spaces between blocks (Savile 1940). Until the development of experimental laboratory techniques to investigate the effect of waves on breakwaters, these structures were designed primarily from experience gained from other similar structures. It is the purpose of this review to discuss various aspects of the hydrodynamics of wave attack on such structures and the relation of certain analytic considerations and experimental results to the design of a rubble-mound. A breakwater built as a rubble-mound is constructed by placing material of various sizes layer by layer (or unit by unit) until the desired cross-section shape is achieved. Generally, the units are not structurally connected, so that the integrity of the rubble-mound depends on features such as the weight of the material, the interlocking nature of the material, and the cross section of the structure. Usually the structure is built with material graded from smaller sizes in the core to larger material armoring the face against wave attack. The armor layer may be composed of quarry-stone, if it is available in the required sizes and is economically feasible to use. When these conditions are not met, specially designed concrete units for armoring the face of the rubble-mound have been developed that tend to interlock better than rock when properly placed; hence, it may be possible to use armor units lighter than the required quarry-stone. Over the years numerous geometric shapes have been developed for such armor units, with each shape generally introduced in an attempt to improve on the interlocking characteristics of its predecessors. To mention only a few, various names used for different units are: tribars, tetrapods, quadripods, and dolosse. A brief description of two of these is presented; for a more detailed discussion of shape along with drawings of the units the interested reader is directed to CERC (1966) and Hudson(1974). Tribars, which consist basically of three circular cylinders connected by a yoke of three cylinders, are usually placed in a uniform geometric pattern on the face of the rubble-mound. Dolosse are shaped like the letter H with the vertical legs rotated 90° to each other, and are generally placed randomly on a rubble-mound face. It is the effective interlocking of dolosse that leads to the use of random placement techniques. Obviously an important aspect in the design of a rubble-mound is its stability under wave attack. This subject is discussed in detail, along with descriptions of the basis for certain design approaches currently used. The support of these design criteria as well as their limitations are discussed with reference to available experimental data. Three other aspects of the effect of waves on rubble-mounds are treated in this review: wave run-up, transmission, and overtopping. Run-up is defined as the vertical height above still water level to which waves incident upon a structure can be expected to travel up the face of the structure. Wave run-up is important in defining both the amount of wave energy transmitted over and through permeable rubble-mounds and also the quantity of water that may be expected to overtop the structure. In each of the following sections the discussion is directed toward understanding the fluid-mechanic aspects of the various problems and the features and the shortcomings of analytical and experimental models used in connection with the design of breakwaters constructed as rubble-mounds.

DESIGN OF FILTER SYSTEM FOR RUBBLE-MOUND STRUCTURES

The rubble-mound seawalls, groins, breakwaters and ripraps around oceanfall pipelines are still the most common type of shore-protection structures currently in use. Major reasons include: easiness to construct and repair, flexible with respect to settlement, favorable wave-energy dissipation, fitness for any water depth and any foundation, and because of its economical nature if rubble-stones are readily available. However, the importance of a filter system in the stability of such structures must be emphasized. More recently, the submerged rubble-mound structures have become very popular due to aesthetic reasons. A filter system is considered a must to protect such a structure against erosion as a result of wave overtopping. Following a brief review of the problems, general discussions are made of the specific causes of failures including piping, overtopping, and nonstability. The physical factors that would affect the filter design are discussed in detail. Methods for design of filter system is presented including those applicable to the gradation of filter layers and to the plastic filters. The design procedure involves (a) making mechanical analysis of the backfill and base material, (b) estimating sizes of voids in the rubble-stones, (c) designing filter by Terzaghi criteria as revised by U.S. Army Waterways Experiment Station. A sample design of graded filter system behind rubble-mound seawalls is given. The procedure for the selection of plastic filters is also included. A comparison of the merits and demerits of these two types of filter systems will be made. This comparison will take into account the model test results conducted in the Look Laboratory. However, the readers are cautioned about the scale effects of the scour characteristics particularly on those tests with light-weight model sediments. This paper is intended as a general guideline for practicing engineers who will be responsible for design and construction of rubble-mound marine structures.