Abstract A detailed laboratory investigation of the mechanical and low-solubility gas coupling between wind and water has been undertaken using a suite of microphysical measurement techniques. Under a variety of wind conditions and in the presence and absence of mechanically generated short waves, approximately fetch-independent surface conditions have been achieved over short laboratory fetches of several metres. The mechanical coupling of the surface is found to be consistent with Banner (J. Fluid Mech. vol. 211, 1990, pp. 463–495) and Banner & Peirson (J. Fluid Mech. vol. 364, 1998, pp. 115–145). Bulk observations of re-aeration are consistent with previous laboratory studies. The surface kinematical behaviour is in accordance with the observations of Peirson & Banner (J. Fluid Mech. vol. 479, 2003, pp. 1–38). Also, their predictions of a strong enhancement of low-solubility gas flux at the onset of microscale breaking is confirmed and direct observations show a concomitant onset of very thin aqueous diffusion sublayers. It is found that the development of strong parasitic capillary waves towards the incipient breaking limit does not noticeably enhance constituent transfer. Across the broad range of conditions investigated during this study, the local instantaneous constituent transfer rate remains approximately log-normally distributed with an approximately constant standard deviation of $0.62\pm 0.15({\mathrm{log}}_e(\mathrm{m}~ {\mathrm{s}}^{-1}))$ . Although wind-forced water surfaces are shown to be punctuated by intense tangential stresses and local surface convergence, localized surface convergence does not appear to be the single critical factor determining exchange rate. Larger-scale orbital wave straining is found to be a significant constituent transfer process in contrast to Witting (J. Fluid Mech. vol. 50, 1971, pp. 321–334) findings for heat fluxes, but the measured effects are consistent with his model. By comparing transfer rates in the presence and absence of microscale breaking, low-solubility gas transfer was decomposed into its turbulent/capillary ripple, gravity-wave-related and microscale breaking contributions. It was found that an efficiency factor of approximately $17\, \%$ needs to be applied to Peirson & Banner’s model, which is extended to field conditions. Although bulk thermal effects were observed and thermal diffusion layers are presumed thicker than their mass diffusion counterparts, significant thermal influences were not observed in the results.

Abstract The input of energy by wind to water waves is compared with the observed growth of the waves using a suite of microphysical measurement techniques in the laboratory. These include measured tangential stresses in the water and air immediately adjacent to the interface with corresponding form drag measurements above wind-forced freely propagating waves. The drag data sets are consistent but the comparison has highlighted important issues in relation to the measurement of fluctuating pressures above freely propagating waves. Derived normalized wind input values show good collapse as a function of mean wave steepness and are significantly in excess of the assembly of net wave growth measurements by Peirson & Garcia (J. Fluid Mech., vol. 608, 2008, pp. 243–274) at low steepness. Sheltering coefficients in the form of Jeffreys (Proc. R. Soc. Lond. Ser. A, vol. 107, 1925, pp. 189–206) are derived that are consistent with values previously obtained by Donelan & Pierson (J. Geophys. Res., vol. 92, 1987, pp. 4971–5029), Donelan (Wind-over-Wave Couplings: Perspectives and Prospects, Clarendon, 1999, pp. 183–194) and Donelan et al. (J. Phys. Oceanogr., vol. 36, 2006, pp. 1672–1689). The sheltering coefficients exhibit substantial scatter. By carefully measuring the associated growth of the surface wave fields, systematic energy budgets for the interaction between wind and waves are obtained. For non-breaking waves, there is a significant and systematic misclose in the radiative transfer equation if wave–turbulence interactions are not included. Significantly higher levels of turbulent wave attenuation are found in comparison with the theoretical estimates by Teixeira & Belcher (J. Fluid Mech., vol. 458, 2002, pp. 229–267) and Ardhuin & Jenkins (J. Phys. Oceanogr., vol. 36, 2006, pp. 551–557). Suitable normalizations of attenuation for wind-forced wave fields exhibit consistent behaviour in the presence and absence of wave breaking. Closure of the surface energy flux budget is obtained by comparing the normalized energy loss rates due to breaking with the values previously determined by Banner & Peirson (J. Fluid Mech., vol. 585, 2007, pp. 93–115) and Drazen et al.(J. Fluid Mech., vol. 611, 2008, pp. 307–332) when expressed as a function of mean wave steepness. Their normalized energy loss rates obtained for non-wind forced breaking wave groups are remarkably consistent with the levels found during this present study when breaking waves are subject to wind forcing.

Optical distortions have previously prevented non-intrusive measurements of dissolved oxygen concentration profiles by Laser induced fluorescence (LIF) to within 200 μm of the air–water interface. It is shown that by careful experimental design, reliable measurements can be obtained within 28 μm of moving air–water interfaces. Consideration of previously unidentified optical distortions in LIF imagery due to non-linear effects is presented that is critical for robust LIF data processing and experimental design. Phase resolved gas flux measurements have now been accomplished along wind forced microscale waves and indicate that the highest mean gas fluxes are located in the wave troughs. The local mean oxygen fluxes as determined by LIF techniques can be reconciled to within 40% of those obtained by bulk measurement in the water. These data provide a new perspective on wind-wave enhancement of low solubility gas transfer across the air–water interface.

Over the last few years, compelling evidence has emerged that the exchange of low-solubility gases across air-water interfaces is strongly enhanced by microscale breaking (e.g. Jahne and HauBecker [12], Zappa et al. [28]). jahne and HauBecker [12] observe that low-solubility gas flux rates are enhanced by up to a factor of 5 in the presence of small scale waves. Investigations using surface infrared imagery [10, 22, 27, 28] have demonstrated a strong correlation between total flux and a proportional area of surface with a high infra-red radiation emission associated with the passage of microscale breaking waves. The mechanisms causing this significant enhancement in exchange rate remain unclear. Zappa et al. [28] proposed that thinning of the aqueous diffusion sublayer by subsurface turbulence in the vicinity of the high infra-red emission region was primarily responsible for this enhancement. Alternate to this is a relationship between the air-water surface exchange rate and the passage rate of wind-forced microscale breaking waves proposed by Peirson and Banner [21]. They have suggested that subduction of the aqueous diffusion sublayer by the microscale wave spilling regions coupled with a weak surface divergence on the upwind faces of the waves primarily determines the microscale-breaking associated flux rate. We have completed a sequence of precise oxygen re-aeration measurements with the specific objective of testing the findings of Peirson and Banner [21]. Specifically, we have compared the flux rates of wind-forced, flat water surfaces in the absence of waves with those in the presence of wind-forced, steep, unbroken waves and wind-forced, microscale breaking waves. With the introduction of steep, unbroken micro-scale waves the surface exchange rate is enhanced by a factor of approximately 2.5. The transition from incipient breaking of the waves to the microscale breaking state induces a significant increase in the associated wind stress [1]. The observed rapid increase in flux rate is approximately proportional to the increase in the wind stress. For the microscale-breaking state, the observed flux rates show good agreement with the predictions of Peirson and Banner [21].

Cooling water for a 2,600MW Power Station is sourced from a nearby coastal saline lake. Prior to the water entering the power station it passes through a series of drum screens to remove weed, fish, sand and shell material. As part of the Power Station's environmental operating license, the fish must be returned to the lake and the weed removed for disposal. The Power Station constructed a small weed-settling pond into which the weed, fish, sand and shell material flowed. The weed-settling pond was linked to the lake by a 2km culvert. The envisaged concept was that the weed would settle in the pond and the fish would swim along the culvert back to the lake. However, much of the weed either floated or was neutrally buoyant, resulting in a large proportion of the weed being returned to the lake via the culvert. Advice was sought by the Power Station to improve the weed removal efficiency of the pond. This involved developing a concept to separate the fish from the weed by non- mechanical methods and utilising the existing weed-settling pond. The proposed solution used a combination of hydraulic barriers and side crest weirs retrofitted into the existing weed-settling pond. This paper discusses the proposed solution and the hydraulic physical modelling undertaken at the University of New South Wales (Australia), Water Research Laboratory.