The beneficial role of rubble mound coastal structures on seawater oxygenation

Abstract

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.

Gas transfer under breaking waves: experiments and an improved vorticity-based model

“…The above solution is only valid for the case of a completely mixed fluid volume. If the source terms are not uniformly distributed, horizontal and diffusion terms are generated (Horsch, 1998;Daniil et al, 2000) and Eq. (3) is not applicable.…”

“…Their analysis concluded that the constants in the proposed equations are probably dependent on the slope of the beach and the type of the breakwater. Daniil et al (2000) proved that there is no difference in the estimation of the oxygen coefficients if the horizontal flux through the permeable breakwater is not neglected. Considering remarks from previous studies in this paper, a modified and improved vorticity-based renewal model for the prediction of gas transfer coefficient under breaking waves in the absence of wind forcing is proposed and fitted to experimental data obtained from both small-and large-scale experimental facilities.…”