Enhancing submerged coastal constructions by incorporating multifunctional purposes

Surfing has becoming more and more attractive in the past few decades, constituting nowadays an important source of revenue for many countries with extensive coastlines. For this purpose and also for environmental reasons, the conventional ways of protecting a coastline appear to entail some disadvantages. An innovative and interesting way of improving surfing capacities and contributing to protect a local coastal zone is by means of multifunctional artificial reefs. A multifunctional artificial reef (MFAR) is a submerged structure that serves several purposes; in particular, it may enhance the surfing possibilities and the environmental value of the local area. This structure has some promising new aspects, too: first, it provides an unimpaired visual amenity; second, it offers tourist and economic benefits by improving the surfing conditions; third, it can enhance the environmental value of the area where it is built, and fourth, if designed properly, the down drift erosion can be minimal. An appropriate reef design in terms of ‘surfability’, i.e. the possibility to surf a wave, for the Leirosa beach, located to the south of Figueira da Foz, midway along Portugal’s West Atlantic coast, has been investigated. In order to achieve the best design several steps were conducted. First, the performance of the Boussinesq-type COULWAVE model is tested with experimental data. Next, this numerical model is used to define the best values for three design parameters: reef angle; geometry of the reef (without or with a platform), and horizontal dimensions for the appropriate geometry. A preliminary design was achieved step by step, making use of the theory and of the state of the art of multifunctional reefs. This reef geometry is used in the numerical study. In terms of ‘surfability’ and for the conditions of the local coastline of Leirosa, the following values were found for the main parameters: a reef angle of 66°; a structure height of 3.20 m; a reef geometry composed of a delta without a platform; a reef submergence of 1.50 m, and a structure seaward slope of 1:10.     

RANS-based simulation of transverse turbulent mixing in a 2D geometry

Although transverse mixing is a significant process in river engineering when dealing with the discharge of pollutants from point sources or the mixing of tributary inflows, no theoretical basis exists for the prediction of its rate, which is indeed based upon the results of experimental works carried on in laboratory channels or in streams and rivers. The paper presents the preliminary results of a numerical study undertaken to simulate the transverse mixing of a steady-state point source of a tracer in a two-dimensional rectangular geometry, which is expected to reproduce a shallow flow. This geometry is that of Lau and Krishnappan (J Hydraul Div 13(HY10):1173–1189, 1977), who collected turbulent mixing data for a shallow flow. In the numerical study an approach based on the Reynolds Averaged Navier–Stokes (RANS) equations was applied, where the closure problem was solved by using turbulent viscosity concept. Particularly, the classical two-equations k–? model was used. Two methods were applied to the model results to evaluate the turbulent transverse mixing coefficient. The effect on transverse mixing of a grid located upstream the tracer source was also studied. Numerical results were generally higher than the experimental data. This overestimation could be explained considering the hypothesis of isotropic turbulence underlying the k–? model, which can lead to large turbulent viscosities and rate of mixing. However, RANS-based results may still be considered acceptable also providing the large uncertainties associated with literature predictive equations.     

Development of an empirical equation for the transverse dispersion coefficient in natural streams

In this study, a new empirical equation for the transverse dispersion coefficient has been developed based on the hydraulic and geometric parameters in natural streams using a regression technique. First, a total of 32 data sets in 16 streams were collected. Among those sets, 16 sets were used for deriving the new equation, and the other 16 sets were used for verifying the equation. Then, through dimensional analysis, it was found that the normalized transverse dispersion coefficient is associated with several parameters such as sinuosity, aspect ratio, and a friction term. The robust least square method was applied to estimate regression coefficients. The newly proposed equation was proven to be superior in explaining the dispersion characteristics of natural streams more precisely compared to the existing equations.     

One-dimensional modelling of sediment deposition in reservoirs with sinking streams

It is common in karst regions that rivers are occasionally cut by mountains, resulting in the alternate appearances of open channel flow and pressurized flow. With more and more reservoirs being built in this region, the complicated sediment transport processes of such mixture flow are urgently needed to study. In this paper a one-dimensional numerical model with free surface and pressurized flows coupled together is presented. The simulated results are analyzed to explore deposition process in reservoirs with sinking streams; impacts of different hydraulic conditions on the sedimentation are also studied. To verify the computed results, a flume experiment is also conducted. The results show that deposition of sediments mainly occurs in open channel reaches where the longitudinal profile is similar to that of conventional reservoirs, i.e. typical delta has formed, demonstrating characteristics of deltaic deposition morphology in that the crest of delta moves to the downstream direction over time. The model provided by this paper is not only proved to simulate the characteristics of deposition in karst reservoirs successfully, but also reveal the impacts of hydraulic conditions in such circumstances.     

两种常见沉水植物与藻的相互作用

选用太湖常见的沉水植物苦草(VallisneriaspiralisL.)和轮叶黑藻(HydrillaverticillataRoyle),构建微型生态系统。通过对沉水植物作用下水体中藻类叶绿素a及试验前后沉水植物生物量变化的观测,分析沉水植物与藻类的相互作用。结果表明,(1)相同条件下,苦草和轮叶黑藻对藻类变化的影响没有显著差异。(2)在苦草生物量相同的情况下,藻类叶绿素a初始值越大,其变化速率也越快。叶绿素a初始值差别越大,其变化速率差异越显著。轮叶黑藻也有相似的结果。(3)从试验前后苦草和轮叶黑藻生物量的变化可知,苦草受藻类影响较大,轮叶黑藻几乎没有受到藻的影响。     

Analytical model for vertical velocity profiles in flows with submerged shrub-like vegetation

An analytical solution for the vertical profiles of the horizontal velocity of channel flow with submerged shrub-like vegetation is investigated in this paper. At first, a shape function is proposed to fit the diameter change of different types of shrub-like vegetation. Using the momentum theorem and the mixing-length turbulence model, an analytical solution for the vertical profile of the horizontal velocity within the vegetation is obtained. The velocity distribution of the whole column is determined in tandem with the logarithmic velocity profile above the vegetation. The solution is compared with experimental data in excellent agreement. The results show that the flow above the vegetation has a logarithmic velocity profile while the flow within the vegetation is impacted greatly by the shape and density of vegetation. The flows within shrub-like vegetations are non-uniform and vary inversely with the shrub diameter.     

Decomposition of diverse litter mixtures in streams

In view of growing interest in understanding how biodiversity affects ecosystem functioning, we investigated effects of riparian plant diversity on litter decomposition in forest streams. Leaf litter from 10 deciduous tree species was collected during natural leaf fall at two locations (Massif Central in France and Carpathians in Romania) and exposed in the field in litter bags. There were 35 species combinations, with species richness ranging 1-10. Nonadditive effects on the decomposition of mixed-species litter were minor, although a small synergistic effect was observed in the Massif Central stream where observed litter mass remaining was significantly lower overall than expected from data on single-species litter. In addition, variability in litter mass remaining decreased with litter diversity at both locations. Mean nitrogen concentration of single- and mixed-species litters (0.68-4.47% of litter ash-free dry mass) accounted for a large part of the variation in litter mass loss across species combinations. For a given species or mixture, litter mass loss was also consistently faster in the Massif Central than in the Carpathians, and the similarity in general stream characteristics, other than temperature, suggests that this effect was largely due to differences in thermal regimes. These results support the notion that decomposition of litter mixtures is primarily driven by litter quality and environmental factors, rather than by species richness per se. However, the observed consistent decrease in variability of decomposition rate with increasing plant species richness indicates that conservation of riparian tree diversity is important even when decomposition rates are not greatly influenced by litter mixing.     

不同特性底质对沉水植物恢复生长的影响

通过室内模拟实验,研究了3种土壤基质（沙土、粘土和腐殖土）分别与河岸边土壤按5个质量分数（5%、10%、30%、50%和80%）掺混形成的不同成分底质分别对菹草（Potamogeton crispus）、轮叶黑藻（Hydrilla verticillata）和微齿眼子菜（Potamogeton maackianus）顶枝断枝恢复生长的影响。结果表明：（1）在3种基质与岸边土壤掺混的底质上,实验结束时菹草的平均最高植株分别出现在5%SA（沙土与岸边土掺混）底质、50%NA（粘土与岸边土掺混）底质和10%FA（腐殖土与岸边土掺混）底质,株高分别达到30、48和45 cm;轮叶黑藻的平均最高植株分别出现在10%SA底质、50%NA底质和30%FA底质,株高分别达到47、82和59 cm;微齿眼子菜在3种基质掺混的底质上植株株高几乎没有增高;（2）生长于同种底质的植株,单位高度上菹草的生物量大于轮叶黑藻,微齿眼子菜的生物量几乎没有增加。因此,适宜菹草和轮叶黑藻恢复生长的基质顺序为粘土〉腐殖土〉沙土,微齿眼子菜在3种基质掺混的底质上几乎没有生长;植株对于特性底质适应性强弱的顺序依次为轮叶黑藻〉菹草〉微齿眼子菜。     

Quantifying vertical mixing in estuaries

Estuarine turbulence is notable in that both the dissipation rate and the buoyancy frequency extend to much higher values than in other natural environments. The high dissipation rates lead to a distinct inertial subrange in the velocity and scalar spectra, which can be exploited for quantifying the turbulence quantities. However, high buoyancy frequencies lead to small Ozmidov scales, which require high sampling rates and small spatial aperture to resolve the turbulent fluxes. A set of observations in a highly stratified estuary demonstrate the effectiveness of a vessel-mounted turbulence array for resolving turbulent processes, and for relating the turbulence to the forcing by the Reynolds-averaged flow. The observations focus on the ebb, when most of the buoyancy flux occurs. Three stages of mixing are observed: (1) intermittent and localized but intense shear instability during the early ebb; (2) continuous and relatively homogeneous shear-induced mixing during the mid-ebb, and weakly stratified, boundary-layer mixing during the late ebb. The mixing efficiency as quantified by the flux Richardson number Rf was frequently observed to be higher than the canonical value of 0.15 from Osborn (J Phys Oceanogr 10:83–89, 1980). The high efficiency may be linked to the temporal–spatial evolution of shear instabilities.     

Chromium accumulation in submerged aquatic plants treated with tannery effluent at Kanpur, India

Aquatic macrophytes have been widely studied because of their capability of absorbing contaminants from water and their subsequent use in biomonitoring. This study presents a comparison of Cr accumulating potential of submerged aquatic plants viz Vallisneria spiralis and Hydrilla verticillata. These plants were treated with various concentrations of treated tannery effluent collected from UASB, Jajmau, Kanpur under repeated exposure in controlled laboratory conditions in order to assess their maximum bioaccumulation potential. The maximum accumulation of 385.6 and 201.6 microg g(-1) dry weight was found in roots of V. spiralis and the whole plants of H. verticillata, respectively at 100% concentration after 9th day of effluent exposure. The chlorophyll and protein content of both species decreased with increase in effluent concentration and duration. At highest concentration and duration a maximum reduction of 67.4 and 62.66% in total chlorophyll content, 9.97 and 4.66% in carotenoid content and 62.66 and 59.36% in protein content was found in V. spiralis and H. verticillata respectively. Anatomical studies in both V. spiralis and H. verticillata was carried out to assess the effects of metal accumulation within the plants. Changes in the anatomical structures of both plants exhibits the capacity of these species to act as indicator of effluent toxicity. The high accumulation potential of Cr by both plants revealed their capability to remove pollutants from effluent.     

Enhancing Avifauna in Commercial Plantations

Abstract:  The occurrence of fauna in commercial plantations is often associated with structural complexity. Through a meta-analysis, we tested whether the structural complexity of plantations could enhance bird species assemblages and whether bird assemblages respond differently depending on taxonomic affiliation, body size, and diet. Our analyses included 167 case studies in 31 countries in which bird assemblages in forests and plantations were compared and 42 case studies in 14 countries in which bird assemblages in plantations of different structural complexity were compared. Species richness, but not abundance, was higher in forests than in plantations. Both species richness and abundance were significantly higher in complex than in structurally simple plantations. Taxonomic representation and body size did not differ between forest and plantations, except that there were fewer insectivorous birds in plantations than in forests. In the comparison of simple versus complex plantations, abundance of all taxonomic and dietary groups was higher in complex plantations. Body size did not affect bird species richness or abundance. Independent of the type of plantation, bird richness and abundance were greater in structurally complex plantations. Enhancing the structural complexity of plantations may mitigate their impact and offer habitat for some native species.     

Hydrodynamics and bed stability of open channel flows with submerged foliaged plants

In this work we investigate experimentally and numerically the flow structure around foliaged plants deployed in a channel with gravels on the bed under submerged and barely submerged conditions. Velocity and Reynolds stress were measured by using a NORTEK Vectrino profiler. Visual observation shows that the initial motion of gravels is easier to be triggered under the condition of flow with barely submerged vegetation. This is confirmed by the measured velocity, Reynolds stress and total kinetic energy (TKE) profiles. The velocity exhibits a speed up in the near-bed region, and the associated Reynolds stress and TKE increase there. A 3D numerical model is then verified against the experiments and used to investigate systematically the effect of degree of submergence of foliaged plants on the channel bed shear stress. The results show that the maximum bed shear stress occurs when the foliage is situated slightly below the water surface, which can enhance channel bed instability.     

Scalar mixing in an urban canyon

The scalar dynamics within a unit-aspect-ratio street canyon are studied using large-eddy simulation. The key processes of ventilation and mixing are analysed with the canyon-averaged concentration, mean tracer age and variance. The results are sensitive to the source location and can be classified according to the streamline geometry. The canyon-averaged concentrations for the corner vortices, vortex sea and central vortex do not converge to the same value at large times, though the mean decay rates do. The variance measured with respect to the canyon average shows two distinct decay regimes: the early regime reflects large-scale straining and enhanced diffusion across streamlines, while the late regime is associated with escape from the canyon, i.e., ventilation. Analytical predictions for the variance-decay or mixing time scales are verified for the early regime. It is argued that the presence of an open boundary at the roof level suppresses rapid mixing of the scalar field and is responsible for differences with respect to scalar dynamics within closed domains.

     

Microprofiles of oxygen in epiphyte communities on submerged macrophytes

Mussels (Mytilus edulis) transferred in net bags from clean to chronically mercury polluted water readily accumulated mercury during an exposure period of three months. Growth of the transplanted mussels had a diluting effect on the mercury concentration, but the absolute weight of mercury uptake increased throughout the entire period, though there was a tendency for decreased efficiency of the removal of mercury per liter of water filtered by the mussels. Mussels were also translocated from polluted to clean (laboratory) water to depurate mercury. The biological half-lives of mercury was 293 d for M. edulis from the chronically polluted area in contrast to only 53 d for mussels from a temporary massive mercury polluted area near a chemical deposit. In both cases about 75% of the total mercury in the mussels was inorganic, and it is suggested that both inorganic and organic mercury species were immobilized in mussels from the long-term mercury polluted area, whereas the immobilization capacity was exceeded in the short-term mercury exposed mussels near the chemical deposit. Very slow elimination of mercury was observed in the deposit-feeding bivalve Macoma balthica from the chronically polluted area, and about 6% of the total mercury was methyl-+phenyl-mercury. This is more than three times lower than found in M. edulis from the same collecting site. A pronounced difference in the mercury speciation (i.e., total mercury, total organic mercury, methyl-mercury and phenyl-mercury) in M. edulis from the two mercury polluted areas is thought to reflect the different character of the mercury pollution in the two areas.     

太湖典型入湖河道中氨氮去除研究

入湖河流是湖泊营养物质的一个重要来源,因此入湖河道水质的改善是控制湖泊富营养化的关键。文章采用自行研制的新型仿生填料直接布设在河道内的生物接触氧化法,在不影响河流生态结构和原有使用功能的前提下,对河流微污染水体进行氨氮的去除研究。研究结果显示,氨氮的净去除率在5.35%~39.91%,其中最高为7月的39.91%,最低月为12月的5.35%。试验期间生物膜的平均厚度为0.8~1.1mm(雨后略厚),生物膜内的微生物主要是一些贫营养微生物,数量少,因此形成的生物膜较薄,生物膜由表及里几乎全是好气层。同时,根据氨氮去除率与硝化菌的数量关系也可以看出试验河段内氨氮的去除主要是靠硝化菌的硝化作用来完成的。     

Wetlands as an alternative stable state in desert streams

Historically, desert drainages of the American southwest supported productive riverine wetlands (ciénegas). Region-wide erosion of ciénegas during the late 19th and early 20th century dramatically reduced the abundance of these ecosystems, but recent reestablishment of wetlands in Sycamore Creek, Arizona, USA, provides an opportunity to evaluate the mechanisms underlying wetland development. A simple model demonstrates that density-dependent stabilization of channel substrate by vegetation results in the existence of alternative stable states in desert streams. A two-year (October 2004-September 2006) field survey of herbaceous cover and biomass at 26 sites located along Sycamore Creek is used to test the underlying assumption of this model that vegetation cover loss during floods is density dependent, as well as the prediction that the distribution of vegetation abundance should shift toward bimodality in response to floods. Observations of nonlinear, negative relationships between herbaceous biomass prior to flood events and the proportion of persistent vegetation cover were consistent with the alternative stable state model. In further support of the alternative-state hypothesis, vegetation cover diverged from an approximately normal distribution toward a distinctly bimodal distribution during the monsoon flood season of 2006. These results represent the first empirically supported example of alternative-state behavior in stream ecosystems. Identification of alternative stable states in desert streams supports recent hypotheses concerning the importance of strong abiotic-disturbance regimes and biogeomorphic mechanisms in multiple-state ecosystems.     

Carbon flux to seabirds in waters with different mixing regimes in the southeastern Bering Sea

The southeastern Bering Sea is characterized by three mixing regimes, separated by fronts associated with the 50, 100, and 200 m isobaths. Phytoplankton to zooplankton transfer-rates are high in waters over the outer shelf and slope (seaward of the 100 m front) relative to transfer in waters over the middle shelf (between the 50 and 100 m fronts). To see whether this difference is reflected at a higher trophic level, we computed carbon flux to the 11 commonest seabird species. Bird-density data (for the period 1975 through 1979) were combined with daily caloric requirement, which is an allometric function of body size in this endothermic group. Minimum transfer to seabirds over a 153 d period (April–August) was 30 mg C m^-2 for the middle shelf and 48 mg C m^-2 for outer shelf and slope waters. Trophic transfer to subsurface-feeding birds (shearwaters, murres and auklets) differed little between regions. In contrast, trophic transfer to surface-feeding birds (fulmars, petrels, and kittiwakes) in the outer shelf and slope waters was 3 times greater than in the waters of the middle shelf. Thus, for seabirds as a whole, pathways of energy transfer differed more between regions than did total carbon flux.     

Racial mixing in South African honeybees: the effects of genotype mixing on reproductive traits of workers

To test the hypothesis that the honeybee hybrid zone in South Africa is a tension zone due to increased reproductive conflict in colonies that contain both Apis mellifera capensis and Apis mellifera scutellata worker genotypes, we constructed mixed subspecies and hybrid colonies via a combination of artificial and natural matings. We measured emergence weight, ovary activation, and the presence/absence of a spermatheca on workers of different genotypes. We show that the measured characteristics were all affected by genotype with some traits also affected by the social environment in which the worker was reared. Workers with both an A. m. capensis mother and father had the highest emergence weight. When workers had an A. m. capensis mother, paternity affected emergence weight with A. m. capensis fathers producing heavier workers. When the queen was A. m. scutellata, paternity had less effect on weight. Presence of spermatheca was highest in mixed colonies irrespective of maternity and colonies containing pure A. m. capensis workers only. Paternity had a significant effect on the presence of a spermatheca within mixed colonies, with workers that had an A. m. capensis father being more likely to possess a spermatheca. Rates of ovary activation were highest in colonies with an A. m. scutellata queen mated to drones of both genotypes, suggesting that mixed subspecies colonies likely suffer increased reproductive strife among workers. Our results provide support for the hypothesis that the South African honeybee hybrid zone is a tension zone arising from reduced fitness of genetically mixed colonies.     

Porous media approach for RANS simulation of compound open-channel flows with submerged vegetated floodplains

Environmental Fluid Mechanics - The main goal of this study is the 3D numerical simulation of river flows with submerged vegetated floodplains. Since, vegetation layers are usually dense and...