Question

Briefly explain what are coastal engineering structures and the differences between the two categories.

Provide a brief history of the structure Ripraps and Gabions (including its category)  

How widely is the structure used?

Answer 1

Coastal engineeering structures are the structures that are constructed near to the coatal areas for different purposes. Different types of structures are constructed under different circumstances but the criteria to be used for the selection and design of specific type of coastal structure must be authentic and comply with the standards. There are various set of criteria that need to be considered in the selection and design of coastal structures- a)structural stability criteria associated with extreme environmental conditions which may cause severe damage to or failure of a coastal structure and b) functional performance criteria. The two categories of coastal structures include:

a. offshore structures- located in the deep water and are subjected to forces due to short crested multi directional waves, which are predominant apart from other forces due to wind, ocean currents etc. The different types of offshore structures are gravity type structures, pile supported structures, floating structures and submarine pipelines.

b. near- shore structures- are subjected to various marine environmental forces due to waves, winds and currents. The wave forces are the dominant forces and are decisive in the design of near shore coastal structures. They can be grouped into- Rigid structures which include sheet piles and walls; semi- rigid or composite structures consisting of Caissons or cells on rubber base and flexible structures consisting of rubblemound breakwaters and sea walls. Rubblemound structures are the most commonly applied type for breakwater and seawall.

Riprap is a man placed rock or other material used to armor shorelines, streambeds, bridge abutments, pilings and other shoreline structures aginst scour and water, wave or ice erosion. The word rip-rap was connected with a nautical word, rip-rap which meant 'a stretch of rippling water, often caused by underwater elevations. The word rap also meant blow or strike. People may have started to call it rip rap because the waves constantly blow into or strike the rocks. It is belongs to the category of near shore structure. Riprap is used where a structure or shoreline is continuously exposed to rushing water along a shoreline to limit erosion associated with wave action. Riprap works by absorbing and deflecting the impact of a wave before the wave reaches the defended structure. The size and mass of the riprap material absorbs the impact energy of waves, while the gaps between the riprap traps and slows the flow of water, lessening its ability to erode soil or structures.

Gabion is a cage, cylinder or box filled with rocks, concrete or sometimes sand and soil. It belongs to near shore structure. Gabions protect slopes and streambanks from the erosive forces of moving water. Rock filled gabion baskets or mattresses can be used as retaining walls for slopes, to armor the bed and/ or banks of channels or to divert flow away from eroding channel sections. The purpose of a gabion revetment is to provide short term protection from backshore erosion by absorbing wave energy along the dune face. Their application is restricted to the upper part of sandy beaches, since they are not sufficiently durable to withstand regular direct wave action. The word gabion derived from Italian word 'gabbione' meaning big cage. About 7000 years ago, early gabion type structures protected the banks of the Nile. In the medieval era, gabions were employed as military fortifications. Later they were used for structural purposes in architecture. Evidently, Leonardo da Vinci used gabion for the foundations of the San Marco Castle in Milan. In recent history, civil engineers have used gabions extensively to stabilize shorelines, riverbanks, highways and slopes against erosion.