The beneficial role of rubble mound coastal structures on seawater oxygenation

Abstract

The bene®cial role of rubble mound coastal structures on oxygenation under the eect of waves is discussed, based on analytical considerations and experimental data from laboratory experiments with permeable and impermeable structures. Signi®cant oxygenation of the wave-protected area was observed as a result of horizontal transport through the permeable structure. A two-cell model describing the transport of dissolved oxygen (DO) near a rubble mound breakwater structure was developed and used for the determination of the oxygen transfer coecients from the experimental data. Oxygen transfer through the airwater interface is considered a source term in the transport equation and the oxygen¯ux through the structure is taken into account. The mass transport equations for both sides of the structure are solved analytically in terms of time evolution of DO concentration. The behaviour of the solution is illustrated for three dierent characteristic cases of initial conditions. The oxygen transfer through the air±water interface in the wave-in¯uenced area increases the DO content in the area; the resulting oxygen¯ux through the structure is discussed. The analytical results depend on the initial conditions, the oxygen transfer coecient and the exchange¯ow rate through the structure. Experiments with impermeable structures show that air water oxygen transfer in the harbour area is negligible in the absence of waves. In addition the ratio of the horizontal DO¯ux to the vertical¯ux into the seaward side tends towards a constant value, independent of the initial conditions.