Vertical Water Mixing Research Articles

Adaptation of heat and mass exchange model to the calculation of diurnal water temperature regime in a stratified reservoir

A box model algorithm for heat and mass exchange model for a valley reservoir was adapted for the diurnal calculation of hydrological characteristics. Field observation data were used to obtain an empirical dependence for the calculation of the rate of vertical water mixing of water in a water body. The verification of the new model demonstrated a good quality of calculation for short-term prediction of water temperature. Recommendations are given for the further improvement of the calculation algorithm.

Effects of stream‐aquifer disconnection on local flow patterns

Disconnected stream‐aquifer systems are becoming increasingly common because of lowering groundwater tables. This work focuses on the pathways and rates of infiltration and seepage as streams transition from fully connected to disconnected conditions. HYDRUS‐2D simulations show for a connected stream, water infiltrates vertically then moves laterally below the top of the preflooding capillary fringe height, finally causing an upward displacement of antecedent water into the vadose zone. This contradicts the commonly held assumption that stream water moves laterally for some distance into the stream bank, forming a wedge above the antecedent water. Even for shallow disconnections (<1 m), there is an increase in infiltration losses from the stream, elimination of postflooding seepage, and increased lateral and vertical mixing of stream water and antecedent pore water.

An examination of the physical variability around the Pribilof Islands in 2004

The Pribilof Islands form a unique ecosystem. An anti-cyclonic oceanic flow exists around the islands. Nutrients are introduced into this circulation from two sources: (1) the flow along the 100-m isobath with intrusions of nutrient-rich water in Pribilof Canyon, and (2) the westward transport and vertical mixing of middle-shelf water, which contains nutrient-rich bottom water. Enhanced tidal mixing around the Pribilofs introduces this deeper, nutrient-rich water into the euphotic zone and thus supports prolonged production around the islands. Further, the middle-shelf water that is advected into the region causes the upper 50 m of the water column around the islands to freshen throughout the summer. This enhances the frontal structure and strengthens the baroclinic flow along the 100-m isobath. The strengthening of the frontal structure can moderate ecosystem productivity by limiting the intrusion of slope water rich in nutrients and oceanic copepods.

Satellite Assessment of Bio-Optical Properties of Northern Gulf of Mexico Coastal Waters Following Hurricanes Katrina and Rita.

The impacts of major tropical storms events on coastal waters include sediment resuspension, intense water column mixing, and increased delivery of terrestrial materials into coastal waters. We examined satellite imagery acquired by the Moderate Resolution Imaging Spectroradiometer (MODIS) ocean color sensor aboard the Aqua spacecraft following two major hurricane events: Hurricane Katrina, which made landfall on 29 August 2005, and Hurricane Rita, which made landfall on 24 September. MODIS Aqua true color imagery revealed high turbidity levels in shelf waters immediately following the storms indicative of intense resuspension. However, imagery following the landfall of Katrina showed relatively rapid return of shelf water mass properties to pre-storm conditions. Indeed, MODIS Aqua-derived estimates of diffuse attenuation at 490 nm (K_490) and chlorophyll (chlor_a) from mid-August prior to the landfall of Hurricane Katrina were comparable to those observed in mid-September following the storm. Regions of elevated K_490 and chlor_a were evident in offshore waters and appeared to be associated with cyclonic circulation (cold-core eddies) identified on the basis of sea surface height anomaly (SSHA). Imagery acquired shortly after Hurricane Rita made landfall showed increased water column turbidity extending over a large area of the shelf off Louisiana and Texas, consistent with intense resuspension and sediment disturbance. An interannual comparison of satellite-derived estimates of K_490 for late September and early October revealed relatively lower levels in 2005, compared to the mean for the prior three years, in the vicinity of the Mississippi River birdfoot delta. In contrast, levels above the previous three year mean were observed off Texas and Louisiana 7-10 d after the passage of Rita. The lower values of K_490 near the delta could be attributed to relatively low river discharge during the preceding months of the 2005 season. The elevated levels off Texas and Louisiana were speculated to be due to the presence of fine grain sediment or dissolved materials that remained in the water column following the storm, and may also have been associated with enhanced phytoplankton biomass stimulated by the intense vertical mixing and offshore delivery of shelf water and associated nutrients. This latter view was supported by observations of high chlor_a in association with regions of cyclonic circulation.

Iron supply to the western subarctic Pacific: Importance of iron export from the Sea of Okhotsk

Iron is an essential nutrient and plays an important role in the control of phytoplankton growth (Martin et al., 1989). Atmospheric dust has been thought to be the most important source of iron, supporting annual biological production in the western subarctic Pacific (WSP) (Duce and Tindale, 1991; Moore et al., 2002). We argue here for another source of iron to the WSP. We found extremely high concentrations of dissolved and particulate iron in the Okhotsk Sea Intermediate Water (OSIW) and the North Pacific Intermediate Water (NPIW), and water ventilation processes in this region probably control the transport of iron through the intermediate water layer from the continental shelf of the Sea of Okhotsk to wide areas of the WSP. Additionally, our time series data in the Oyashio region of the WSP indicate that the pattern of seasonal changes in dissolved iron concentrations in the surface‐mixed layer was similar to that of macronutrients, and that deep vertical water mixing resulted in higher winter concentrations of iron in the surface water of this region. The estimated dissolved iron supply from the iron‐rich intermediate waters to the surface waters in the Oyashio region was comparable to or higher than the reported atmospheric dust iron input and thus a major source of iron to these regions. Our data suggest that the consideration of this source of iron is essential in our understanding of spring biological production and biogeochemical cycles in the western subarctic Pacific and the role of the marginal sea.

Mixing and stirring in the Southern Ocean

The vertical and horizontal mixing of waters within the ocean affects the Earth's climate because it controls the poleward transport of heat and carbon within the ocean, the structure of the large‐scale currents, and the character of water that upwells and interacts with the atmosphere. Particularly in the Southern Ocean, ocean general circulation models can show different behavior depending on the exact representation of mixing. To date, however, measurements of this mixing have not been available because of the remoteness and difficult working conditions in the Southern Ocean.The Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES) seeks to understand the regional importance of the Southern Ocean to global circulation by measuring large‐ and small‐scale mixing processes. In the ocean, water moves primarily along surfaces of constant potential density (isopycnals), but it can also mix and change density between these surfaces in what is known as diapycnal mixing. This experiment, a joint effort between U.S. and U.K. scientists, will begin fieldwork in early 2009.

Elucidating the dynamics and mixing agents of a shallow fjord through age tracer modelling

The mixing agents and their role in the dynamics of a shallow fjord are elucidated through an Eulerian implementation of artificial tracers in a three-dimensional hydrodynamic model. The time scales of vertical mixing in this shallow estuary are short, and the artificial tracers are utilized in order to reveal information not detectable in the temperature or salinity fields. The fjord's response to external forcing is investigated through a series of model experiments in which we quantify vertical mixing, transport time scales of fresh water runoff and estuarine circulation in relation to external forcing. Using age tracers released at surface and bottom, we quantify the time scales of downward mixing of surface water and upward mixing of bottom water. Wind is shown to be the major agent for vertical mixing at nearly all depth levels in the fjord, whereas the tide or external sea level forcing is a minor agent and only occasionally more important just close to the bottom. The time scale of vertical mixing of surface water to the bottom or ventilation time scale of bottom water is estimated to be in the range 0.7 h to 9.0 days, with an average age of 2.7 days for the year 2004. The fjord receives fresh water from two streams entering the innermost part of the fjord, and the distribution and age of this water are studied using both ageing and conservative tracers. The salinity variations outside this fjord are large, and in contrast to the salinity, the artificial tracers provide a straight forward analysis of river water content. The ageing tracer is used to estimate transport time scales of river water (i.e. the time elapsed since the water left the river mouth). In May 2004, the typical age of river water leaving the fjord mouth is 5 days. As the major vertical mixing agent is wind, it controls the estuarine circulation and export of river water. When the wind stress is set to zero, the vertical mixing is reduced and the vertical salinity stratification is increased, and the river water can be effectively exported out of the fjord. We also analyse the river tracer fields and salinity field in relation to along estuary winds in order to detect signs of wind-induced straining of the along estuary density gradient. We find that events of down estuary winds are primarily associated with a reduced along estuary salinity gradient due to increased surface salinity in the innermost part of the fjord, and with an overall decrease in vertical stratification and river water content at the surface. Thus, our results show no apparent signs of wind-induced straining in this shallow fjord but instead they indicate increased levels of vertical mixing or upwelling during down estuary wind events.

Reprint of “Processes influencing dissolved iron distributions below the surface at the Atlantic Ocean–Celtic Sea shelf edge”

Shelf break systems are highly dynamic environments. However little is known about the influence that benthic interactions and water mass mixing may have on vertical distributions of iron in these systems. Dissolved Fe (< 0.4 μm) concentrations were measured in samples from nine vertical profiles across the upper slope (150–2950 m water depth) at the Atlantic Ocean–Celtic Sea shelf break. Dissolved iron concentrations varied between less than 0.2 and 5.4 nM, and the resulting detailed section showed evidence of a range of processes influencing the Fe distributions. The near sea floor data were interpreted in terms of release and removal processes. The concentrations of dissolved Fe present in near seabed waters were consistent with release of Fe from in situ remineralisation of particulate organic matter at two upper slope stations, and possibly release from pore water upon resuspension on shelf. Lateral transport of dissolved iron was evident from elevated Fe concentrations in an intermediate nepheloid layer and its advection along isopycnals. Surface waters at the shelf break also showed evidence of vertical mixing of deeper iron-rich waters. These waters contained macronutrients that sustained primary productivity in these otherwise nutrient-depleted surface waters. The data also suggest some degree of stabilisation of relatively high concentrations of iron, presumably through ligand association or as colloids. This study supports the view that lateral export of dissolved iron to the interior of the ocean from shelf and coastal zones and may have important implications for the global budget of oceanic iron.

Salinity changes in the western tropical South Atlantic during the last 30 kyr

The hydrographic changes in the western tropical South Atlantic during the last 30 kyr were reconstructed based in the faunal and isotopic analyses of planktonic foraminifera of three cores taken along the Brazilian Continental Margin between 14°S and 25°S. The application of the SIMMAX–MAT method on faunal counts data provided the sea surface temperature estimates. Sea surface salinity estimates were based on the oxygen isotope composition of Globigerinoides ruber (white). Additionally, the abundance record of the planktonic foraminifera Globorotalia truncatulinoides (right) was used as a proxy for vertical mixing of surface waters. Sea surface temperature estimates suggest a relative stability of the area during the last 30 kyr. However, significant changes in the isotopic composition of G. ruber (white) suggest that the isotopic signal is dominated by the influence of sea surface salinity changes. The observed salinity changes are related to both the local hydrological balance and global circulation. Orbital forcing and sea surface salinity changes were responsible for considerable changes in the stability of the upper water column and consequently in the depth of the mixed layer, as indicated by the abundance record of G. truncatulinoides (right).

Processes influencing dissolved iron distributions below the surface at the Atlantic Ocean–Celtic Sea shelf edge

Shelf break systems are highly dynamic environments. However little is known about the influence that benthic interactions and water mass mixing may have on vertical distributions of iron in these systems. Dissolved Fe (< 0.4 μm) concentrations were measured in samples from nine vertical profiles across the upper slope (150–2950 m water depth) at the Atlantic Ocean–Celtic Sea shelf break. Dissolved iron concentrations varied between less than 0.2 and 5.4 nM, and the resulting detailed section showed evidence of a range of processes influencing the Fe distributions. The near sea floor data were interpreted in terms of release and removal processes. The concentrations of dissolved Fe present in near seabed waters were consistent with release of Fe from in situ remineralisation of particulate organic matter at two upper slope stations, and possibly release from pore water upon resuspension on shelf. Lateral transport of dissolved iron was evident from elevated Fe concentrations in an intermediate nepheloid layer and its advection along isopycnals. Surface waters at the shelf break also showed evidence of vertical mixing of deeper iron-rich waters. These waters contained macronutrients that sustained primary productivity in these otherwise nutrient-depleted surface waters. The data also suggest some degree of stabilisation of relatively high concentrations of iron, presumably through ligand association or as colloids. This study supports the view that lateral export of dissolved iron to the interior of the ocean from shelf and coastal zones and may have important implications for the global budget of oceanic iron.

Biological and chemical characteristics of high-chlorophyll, low-temperature water observed near the Sulu Archipelago

Abstract According to ocean-color remote sensing data, a low phytoplankton biomass exists in the central parts of the Sulu and Celebes seas, whereas higher concentrations of chlorophyll a are observed in the waters near the Sulu Archipelago, which separates these water bodies. These high-chlorophyll a waters have lower temperatures than those of surrounding waters, indicating vertical mixing of nutrient-rich subsurface water that is attributable to the topographical features of the Sulu Ridge. Differences in photosynthetic parameters, primary productivity and the phytoplankton community structure between the high-chlorophyll area near the Sulu Archipelago and the oligotrophic basin area in the Philippine, Sulu and Celebes seas were investigated in November 2002 during the R.V. Hakuho-Maru KH-02-4 cruise. Entrainment of subsurface water and vertical mixing by intense tidal flow from the Sulu Archipelago to the Celebes Sea created a low-temperature, high-nutrient condition at the southeastern edge of the Sulu Ridge. Chlorophyll a concentrations greater than 2 μg l−1 were observed at stations 5–15 km offshore at the edge of the Sulu Ridge. This high-chlorophyll tongue in the Celebes Sea showed high primary productivity: it was 1.6–18 times higher than those measured in oligotrophic central basin regions of the Philippine, Sulu, and Celebes seas. Microscopic observation of these high-chlorophyll waters showed the dominance of chain-forming large centric diatoms. Extension of high-chlorophyll waters was not observed at the Sulu Sea side, where the surface water salinity was lower than in the Celebes Sea and consequently prevented intrusion of the Celebes Sea water into the surface layer. Occasional extensions of productive, chlorophyll-rich surface water from the archipelagic zone to the surrounding basin area seems to be an important factor causing spatiotemporal variability in the phytoplankton biomass in the southern Sulu Sea and the western Celebes Sea.

Ammonium transport and reaction in contaminated groundwater: Application of isotope tracers and isotope fractionation studies

Ammonium (NH4+) is a major constituent of many contaminated groundwaters, but its movement through aquifers is complex and poorly documented. In this study, processes affecting NH4+ movement in a treated wastewater plume were studied by a combination of techniques including large‐scale monitoring of NH4+ distribution; isotopic analyses of coexisting aqueous NH4+, NO3−, N2, and sorbed NH4+; and in situ natural gradient 15NH4+ tracer tests with numerical simulations of 15NH4+, 15NO3−, and 15N2 breakthrough data. Combined results indicate that the main mass of NH4+ was moving downgradient at a rate about 0.25 times the groundwater velocity. Retardation factors and groundwater ages indicate that much of the NH4+ in the plume was recharged early in the history of the wastewater disposal. NO3− and excess N2 gas, which were related to each other by denitrification near the plume source, were moving downgradient more rapidly and were largely unrelated to coexisting NH4+. The δ15N data indicate areas of the plume affected by nitrification (substantial isotope fractionation) and sorption (no isotope fractionation). There was no conclusive evidence for NH4+‐consuming reactions (nitrification or anammox) in the anoxic core of the plume. Nitrification occurred along the upper boundary of the plume but was limited by a low rate of transverse dispersive mixing of wastewater NH4+ and O2 from overlying uncontaminated groundwater. Without induced vertical mixing or displacement of plume water with oxic groundwater from upgradient sources, the main mass of NH4+ could reach a discharge area without substantial reaction long after the more mobile wastewater constituents are gone. Multiple approaches including in situ isotopic tracers and fractionation studies provided critical information about processes affecting NH4+ movement and N speciation.

Bulk Recycling Models with Incomplete Vertical Mixing. Part I: Conceptual Framework and Models

Abstract A conceptual framework, within which the bulk recycling models could incorporate the effects related to an incomplete vertical mixing of water in the tropospheric column, is developed. As an example, the bulk model with the modified well-mixed atmosphere condition, incorporating the “fast recycling” process, is constructed. The model contains a tunable, spatially varying parameter K that characterizes the level of the incomplete vertical mixing effects. The simplified analytical models based on the modified well-mixed atmosphere condition are developed. The analysis of the simplified model results shows that the regional recycling ratio may depend significantly on the average of K, but it depends only slightly on the rate of variation of K, which indicates that models with a constant K can be used instead of the variable K model.

Bulk Recycling Models with Incomplete Vertical Mixing. Part II: Precipitation Recycling in the Amazon Basin

Abstract An efficient method for precipitation recycling calculations based on the general bulk recycling model is developed and applied to the Amazon basin region. The results are compared with the estimates provided by simplified recycling models, and with the estimations obtained in other studies. Then the numerical procedure is adjusted to the modified recycling model incorporating some effects of incomplete vertical mixing of water in the tropospheric column as described in Part I of this paper. The modified numerical model is applied to the Amazon basin region and the results are compared with those from the unmodified model. It is seen from the results that the effects related to an incomplete vertical mixing produce significant changes in the level and distribution of precipitation recycling over the Amazon basin. The general changes in the recycling patterns occurring as a result of modifications include a significant increase in the contribution of recycled water to precipitation in the eastern part of the region and weakening continental recycling gradients. The regional recycling ratio values for the Amazon basin estimated by the modified model are significantly higher than the values provided by the unmodified model. The results of application of the models to several other regions on the globe are described in short. In particular, a comparison of the results with the estimates by the modified Budyko model is made for different regions, and it is found that the modified Budyko's model estimates are, in general, closer to the modified model numerical results than it was for the unmodified models. The results for precipitation recycling over the central United States are presented in more detail. For this region, the information on the degree of incomplete vertical mixing available from the GCM tracer transport modeling results has been used in the modified model calculations. It has been found that, as distinct from the Amazon basin, the effects related to an incomplete vertical mixing do not produce significant changes in the level and distribution of precipitation recycling over the central United States.

A PERSPECTIVE ON BOTTOM WATER TEMPERATURE ANOMALIES IN LONG ISLAND SOUND DURING THE 1999 LOBSTER MORTALITY EVENT

Abstract Analyses of time series data for bottom or near bottom temperatures for 50 stations distributed throughout Long Island Sound reveal distinctive features of the bottom water temperature history during the lobster mortality event of 1999. These include: temperatures that exceeded 23.5°C in shallow, well-mixed areas; markedly higher temperatures, in general, in those areas with water column depth <20 m; basin-averaged bottom temperatures that were the highest for the decade during the months of July and August; and a rapid increase in bottom temperatures in late August caused by the vertical mixing of warm surface waters during a strong wind event. Results indicate that anomalies in the local surface heat flux made an important contribution to bottom temperature anomalies.

Interannual variation in Neocalanus biomass in the Oyashio waters of the western North Pacific

AbstractWe examined the interannual variation in Neocalanus copepod biomass in the Oyashio waters in spring and summer from 1972 to 1999. In the mid‐1970s, mesozooplankton biomass in spring was high; however, it decreased significantly in the late 1970s. The timing of the decrease in mesozooplankton biomass corresponded to the 1976/77 climatic regime shift. The biomass of N. flemingeri, which dominated the Neocalanus community, was roughly constant from 1980 to 1999. Although species‐level estimates of Neocalanus biomass were not available for the 1970s, a previous study reported that Neocalanus copepods were the predominant mesozooplankton in the Oyashio waters in spring during the 1970s. Neocalanus copepods dominated the mesozooplankton community throughout the 1970s, and their biomass decreased in the late 1970s. Springtime net community production, an index of new production, also decreased in the late 1970s. We suggest that the reduction in new production negatively affected Neocalanus food availability, resulting decreased copepod biomass. New production may have been limited by a combination of subsurface iron supplies, increased vertical density gradient, and reduced vertical water mixing in winter, which resulted in diminished iron entrainment in winter. In summer, mesozooplankton biomass significantly decreased and increased synchronously with the 1976/77 and 1988/89 climatic regime shifts. The biomass of N. plumchrus, which dominated the Neocalanus community, was low in the 1980s and increased in the early 1990s. The biomass of the second‐most dominant copepod, N. cristatus, also increased in the early 1990s. Neocalanus copepods were reported to be a dominant component of the mesozooplankton community in the 1970s; Neocalanus biomass was high in the mid‐1970s and decreased in the late 1970s. Japanese sardine (Sardinops melanostictus), an important predator of Neocalanus copepods, exhibited interannual variation in standing stock that was inversely related to mesozooplankton biomass. At their peak in 1984, sardines consumed 32–138% of the daily Neocalanus production during summer. Therefore, predation pressure on Neocalanus by Japanese sardine is likely to affect interannual variation in mesozooplankton biomass during the summer.

Sea Ice-Upper Ocean Ecosystems and Global Changes in the Arctic

In the Arctic, a clear temperature trend toward warming has been observed during the last two decades. The warming has led to a reduction in the area and thickness of the Arctic sea ice cover because of increased melting. Melt water is accumulated within the 0- to 30-m surface layer, markedly changing its hydrological and hydrochemical characteristics: this water is warmer and fresher, with lower nutrient concentrations. A stable thermocline at depths of 30–35 m weakens vertical mixing of water and hampers active exchange between nutrient-enriched water lying below the thermocline and nutrient-poor water under ice. This affects the qualitative and quantitative composition of upper-ocean phyto- and zooplankton. The changes in the climate and the ice environment cause marked alterations in the composition and structure of the biological communities of sea ice and under-ice surface water. Comparative analysis of materials collected in the anticyclonic Arctic Gyre at the ice stations Severnyi Polyus-22 (1975– 1981) and SHEBA (1997–1998) shows that over the last two decades the number of species of ice diatoms markedly decreased and the role of freshwater algae increased. The number of nematodes, copepods, amphipods, and turbellarians, which were the dominant groups in perennial ice, also declined sharply. The results suggest that the sea ice-upper ocean ecosystem is changing from typically marine to brackish-water. The main reason for these changes is probably global warming in the Arctic.

Oceanic Suess effect of δ13C in subpolar region: The North Pacific

We propose a simple approach for estimating the recent oceanic Suess effect of δ13C in the subpolar region of the North Pacific, an area which experiences a large seasonal variability of carbonate species, due to the biological activity and vertical water mixing. This approach is based simply on time‐series data of δ13C from July 1997 to July 2001 and an equation of the Fourier sine expansion. Applying our approach to the fixed sampling location in the western North Pacific, station ‘KNOT’, we estimated values of −0.012‰‐δ13C yr−1 for the Suess effect of δ13C with an amplitude of 1.3‰‐δ13C and 1.0 μmol‐C kg−1 yr−1 for the increase rate of dissolved inorganic carbon with an amplitude of 134 μmol‐C kg−1 in surface water. The Suess effect of δ13C in this region was a similar value to that estimated in the Southern Ocean producing the deep water, while it was half as much as values estimated in the subtropical regions which experience a small seasonal variability. The ratio of Suess effect to the increase rate of dissolved inorganic carbon was −0.012‰ (μmol kg−1)−1, which was different from the global mean values calculated by recent modeling study. Therefore, we conclude that the Suess effect of δ13C in the polar and subpolar regions is generally smaller than found in other regions, suggesting that it is necessary to reevaluate the uptake rate of anthropogenic carbon which has been based on the assumption that the ratio is uniform over the entire ocean.

Bacterial abundance and production during the unique spring phytoplankton bloom in the central Yellow Sea

To elucidate the factors responsible for the resulting bacterial properties during the spring phytoplankton bloom in the central Yellow Sea, we investigated bacterial abundance and pro- duction together with physico-chemical and biological parameters. In spring, the central Yellow Sea is characterized by the horizontal intrusion of high-temperature and high-salinity Yellow Sea Warm Current (YSWC), which forms a thermohaline front between the central Yellow Sea and cold coastal waters, and by the vertical mixing of high-nutrient Yellow Sea Cold Water (YSCW) over the previous winter. The combination of high nutrient conditions with weak vertical density gradients because of increased irradiance, which increases the residence time of phytoplankton within the euphotic layer, seems to trigger the spring phytoplankton bloom. Enhanced bacterial biomass, production, and turnover rates were observed in the bloom area. Bacterial variables were significantly correlated with chl a concentration, but not with temperature. These results indicated that resource supply from phytoplankton primarily stimulated bacterial growth. Despite the enhanced bacterial growth in the bloom area, abundant heterotrophic nanoflagellates and their grazing on bacteria were responsible for the relatively smaller increase in bacterial biomass in the bloom area. Higher bacterial growth but smaller increases in bacterial biomass indicated that bacterial growth and biomass are independently controlled during the spring bloom in the central Yellow Sea, in which bacterial growth is primarily stimulated by organic material produced from the phytoplankton bloom, but the enhanced biomass is more tightly controlled by grazing.

INFLUENCE OF A PERMEABLE BREAKWATER ON THE FLOW FIELD IN A HARBOR

The water pollution in a harbor is one of the most crucial topics today.In order to improve the sea water quality in a harbor, it may be estimated that water exchange ability through harbor breakwaters is important. Such water exchange may be realized by adopting permeable breakwaters, including the new type breakwater with special overtopping works in it.In this study, we examined the mass flux through the harbor entrance and the permeable breakwater to understand the resultant flow field in the model harbor basin. We also carried out the experiment on the comparative case, in which impermeable breakwaters were used instead. It was observed that the direction of mass flux through the permeable breakwater is toward inside the harbor. On the contrary, that is toward outside the harbor through the harbor entrance. In the case of permeable breakwater, the vertical mixing of water appears under the condition that the strong vortex flows are generated around the permeable breakwater.