12 December 2001
DESIGN OF MARINE IMPROVEMENT PROJECTS
INTRODUCTION TO COASTAL PROJECT ELEMENT DESIGN. The
CEM Section VI-2 contains sections on the introduction to coastal project element
design, types and functions of coastal structures, site specific design conditions,
materials and construction aspects, fundamentals of design, reliability based design of
coastal structures, design of specific project elements, and designing for repair,
rehabilitation, and modification.
TYPES AND FUNCTIONS OF COASTAL STRUCTURES. Coastal
structures are used in coastal defense schemes with the objective of preventing
shoreline erosion and flooding of the hinterland. Other objectives include sheltering of
harbor basins and harbor entrances against waves, stabilization of navigation channels
at inlets, and protection of water intakes and outfalls. An overview of the various types
of coastal structures and their application is given in CEM Section VI-2. Overall
planning and development of coastal projects is covered in CEM Section V.
Breakwaters. Primary applications of breakwaters are to provide
protection against waves for shore areas, harbors, anchorages, and basins, and to
enable maintenance-dredging operations. A secondary purpose is beach erosion
Jetties. Jetties are devices parallel to a navigation channel used to
protect the channel from shoaling with littoral drift and to stabilize the entrance to a tidal
inlet. They may also provide wave and wind protection and direct or confine the flow of
river or tidal currents. Sand bypassing of jettied inlets is often necessary to preclude
erosion of the downdrift coast.
Revetments, Bulkheads, and Seawalls. These structures are used to
protect embankments or shore structures from eroding or from damage due to wave
attack or currents and to retain or prevent sliding of land. Revetments are generally
rubble construction. Seawalls and bulkheads are generally more rigid structures
constructed of steel, concrete, or timber. Another design for seawalls is to use Igloos,
patented by Nippon Tetrapod (Figure 7-1). Igloos can be used as space-saving wave
absorbers or breakwaters. Prefabricated concrete units have been used successfully
as wave-dissipating walls in harbors.
Groins. Groins are used to protect the coast from erosion and to retard
or control littoral transport to stabilize a beach. Groin fields should generally be filled
with imported material to preclude erosion of the downdrift coast.
Headlands. Headlands are high, steep-faced border points of land
extending into the ocean or other body of water. Large segments of shorelines can be
stabilized by construction of artificial headlands.