design of ocean structures

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DESIGN OF RUBBLE MOUND BREAKWATERS

ABSTRACT
Breakwaters are an integral part of a port and harbours. In comparison with other breakwaters such as vertical wall type and composite types, rubble mound breakwaters are conventionally used in the places where abundant natural stones are available. In this paper, the design aspects of rubble mound breakwaters is detailed. The main criterion in the design is the stability of armour blocks for the design wave climate. The importance of various characteristic layers of a typical rubble mound breakwater is discussed.

INTRODUCTION
A harbour is a protected zone where the ocean going vessels are safely berthed. To create such highly tranquil area, the waters have to be either naturally protected or artificially protected from the severe environmental forcing such as wind-waves and storms. Due to the increase in the demand for safe berthing places both for commercial and fishing activities, there is a need to construct harbours at many locations with artificial protected environment. Such protection is highly rendered by breakwaters. There are many types of breakwaters which can provide proper functionality. In our environment, rubble mound sections provide a viable option for the breakwater construction.

The layout of breakwaters depends on the actual needs, benefits expected from the methodology, effect on adjacent shorelines and more importantly economy. In this lecture, rubble mound breakwater as an effective harbour creator element is considered. The design on the layout of breakwaters with particular reference to our Indian coasts is presented in this workshop. The design of cross-sectional details of breakwaters has been presented. Although the proper design of breakwaters would not guarantee the functional achievements of the breakwater, it requires great attention. This is because many of the breakwaters have failed due to the defective design in this respect.

1

STRUCTURE OF BREAKWATERS
Breakwaters can be constructed using rubble stones, pre-cast concrete units or blocks, rock-filled timber cribs and gabions, steel sheet pile, timber sheet pile, and grout filled bags and tubes. A typical rubble-mound breakwater cross section is shown in Fig. 1. An armour layer at the top protects the other layers beneath it from washing away. Thus, the armours have to be designed to withstand under severe environmental forces. And, basically an inner core layer of smaller size stones to prevent any sediments to seep through the breakwater section. Depending upon the requirements, there may be few (none to two) under-layers between armour and core layers. The stones in the under-layer are chosen in such a way that it will not fit through the voids of its immediate overlaid layer. The breakwaters are often provided with rubble toe protection that serves as a scour blanket to prevent undermining and thereby a reduction in lateral stability. Typically, toe protection is necessary on the sea side of the breakwater section where the environment forces high waves on the breakwater. The entire cross section would be placed over the filter layer blanket, which is laid on the seabed. The filter layer evenly distributes the entire weight of the structure into the seabed and hence, it is the foundation for the breakwater super structure. And also, the filter layer prevents the seabed materials seeps into the core layer and bigger stones to settle into soft sands.

2

DESIGN OF BREAKWATERS
Forces acting on Breakwaters
Wave forces.
The rubble mound breakwater can preferably used at exposed sites because of a rubble-mound structure's ability to withstand severe wave loads and to decrease wave reflection. Moreover, the risk of scouring and formation of strong rip currents along rubble mound is reduced. Most rubble mound breakwaters are designed with quarry stone as armor and it is heavy enough to be stable under a selected design wave height.

Current forces
Currents can exert forces...
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