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Laboratory study on the shoreline changes behind a coastal structure by K. Mizumura and N. Shirashi
Coastal Engineering, 7 (1983) 175--176
175
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Discussions LABORATORY STUDY ON THE SHORELINE CHANGES BEHIND A COASTAL STRUCTURE by K. Mizumura and N. Shirashi 1 -- DISCUSSION RICHARD SILVESTER
Department of Civil Engineering, University of Western Australia, Nedlands, W.A. 6009 (Australia) (Received September 15, 1982)
The siltation and associated erosion t h a t is predicted from the offshore airport in Osaka Bay introduced groynes and closely spaced offshore breakwaters as means of stabilizing the coast. The present writer has discussed the inefficiency and uneconomic nature of these two types of structures (Silvester et al., 1980). Certainly if offshore breakwaters are employed, the groynes appear unnecessary. The offshore breakwater concept has been p r o m o t e d by Toyoshima (1974) who concludes that the main problem to be solved is their undermining by waves (Toyoshima, 1976). There is the additional mechanism of scouring during storms when the water forced into the leeward side of the structures must pass through the gaps at high velocity. This removes a good proportion of the sand accreted in the form of spits or even tombolos. Experience has shown that seawalls are still necessary on the centre line of the openings. The alternative of spacing headlands at greater distances apart permits zeta shaped bays to form between them. These shorelines provide the material required by storm waves to form an offshore bar. When more modest waves in the form of swell arrive, t h e y will put such sediment directly back on the beach if they arrive normal to the beach around the peryphery of the bay. If no swell is available the offshore bar, or a good proportion of it, remains in place to be built up again when higher waves arrive. The shape and indentation of these crenulate-shaped bays when in equilibrium, or when no further sand supply is available, are predictable, and can therefore be used for design in a stabilization programme (Silvester, 1976). This has been done successfully on a large reclamation scheme in Singapore (Silvester and Ho, 1972). It could also be employed in defending the island on which the airport is to be located (Silvester, 1982). REFERENCES Silvester, R., 1976. Headland defence of coasts. Proc. 15th Conf. Coastal Eng., 2: 1394--1406. 1Coastal Engineering, 5 : 5 1 - - 8 1 (1981).
0378-3839/83/$03.00
© 1983 Elsevier Science Publishers B.V.
176 Silvester, R., 1982. Planning for artificial islands. Symp. Engineering in Marine Environments, Brugge, 2: 115--122. Silvester, R. and Ho, S.K., 1972. Use of crenulate shaped bays to stabilize coasts. Proc. 13th Conf. Coastal Eng., 2: 1347--1365. Silvester, R., Tsuchiya, Y. and Shibano, Y., 1980. Zeta bays, pocket beaches and headland control. Proc. 17th Conf. Coastal Eng. Toyoshima, O., 1974. Design of a detached breakwater system. Proc. 14th Conf. Coastal Eng., 2: 1419--1431. Toyoshima, Y., 1976. Changes of sea bed due to detached breakwaters. Proc. 15th Conf. Coastal Eng., 2: 1572--1589.
175
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
Discussions LABORATORY STUDY ON THE SHORELINE CHANGES BEHIND A COASTAL STRUCTURE by K. Mizumura and N. Shirashi 1 -- DISCUSSION RICHARD SILVESTER
Department of Civil Engineering, University of Western Australia, Nedlands, W.A. 6009 (Australia) (Received September 15, 1982)
The siltation and associated erosion t h a t is predicted from the offshore airport in Osaka Bay introduced groynes and closely spaced offshore breakwaters as means of stabilizing the coast. The present writer has discussed the inefficiency and uneconomic nature of these two types of structures (Silvester et al., 1980). Certainly if offshore breakwaters are employed, the groynes appear unnecessary. The offshore breakwater concept has been p r o m o t e d by Toyoshima (1974) who concludes that the main problem to be solved is their undermining by waves (Toyoshima, 1976). There is the additional mechanism of scouring during storms when the water forced into the leeward side of the structures must pass through the gaps at high velocity. This removes a good proportion of the sand accreted in the form of spits or even tombolos. Experience has shown that seawalls are still necessary on the centre line of the openings. The alternative of spacing headlands at greater distances apart permits zeta shaped bays to form between them. These shorelines provide the material required by storm waves to form an offshore bar. When more modest waves in the form of swell arrive, t h e y will put such sediment directly back on the beach if they arrive normal to the beach around the peryphery of the bay. If no swell is available the offshore bar, or a good proportion of it, remains in place to be built up again when higher waves arrive. The shape and indentation of these crenulate-shaped bays when in equilibrium, or when no further sand supply is available, are predictable, and can therefore be used for design in a stabilization programme (Silvester, 1976). This has been done successfully on a large reclamation scheme in Singapore (Silvester and Ho, 1972). It could also be employed in defending the island on which the airport is to be located (Silvester, 1982). REFERENCES Silvester, R., 1976. Headland defence of coasts. Proc. 15th Conf. Coastal Eng., 2: 1394--1406. 1Coastal Engineering, 5 : 5 1 - - 8 1 (1981).
0378-3839/83/$03.00
© 1983 Elsevier Science Publishers B.V.
176 Silvester, R., 1982. Planning for artificial islands. Symp. Engineering in Marine Environments, Brugge, 2: 115--122. Silvester, R. and Ho, S.K., 1972. Use of crenulate shaped bays to stabilize coasts. Proc. 13th Conf. Coastal Eng., 2: 1347--1365. Silvester, R., Tsuchiya, Y. and Shibano, Y., 1980. Zeta bays, pocket beaches and headland control. Proc. 17th Conf. Coastal Eng. Toyoshima, O., 1974. Design of a detached breakwater system. Proc. 14th Conf. Coastal Eng., 2: 1419--1431. Toyoshima, Y., 1976. Changes of sea bed due to detached breakwaters. Proc. 15th Conf. Coastal Eng., 2: 1572--1589.