Static stability of submerged partial revetment structures under regular and irregular waves

Physical models of submerged partial revetment structures were built on natural beach sand with a diameter of 0.35 mm and specific gravity of 2.63. The armor units, the diameter and specific gravity of which varied in the range of 8.5–67.95 mm and 1.81–2.77 respectively, were placed only on wave breaking areas. A series of experiments has been conducted on the conditions of different armor units and different wave characteristics using regular waves and irregular waves. Based on the experimental data, the effects of wave height, wave period, diameter and specific gravity of armor units, water depth in the channel, and wave types on static damage of given structures are assessed. Some empirical formulas have been suggested through regression analysis to describe static stability and stability number of submerged partial revetment structures under pure regular waves, pure irregular waves, and regular–irregular waves. The suggested formulas compared with Van der Meer’s (1988) formulas and some differences have occurred because of differences among revetment types and test conditions; therefore, proposed formulas give reasonable results for the test conditions used.     

On the interaction of a solitary wave and a submerged dike

The unsteady, two-dimensional Navier–Stokes equations and the exact free surface boundary conditions were solved to study the interaction of a solitary wave and a submerged dike. A piston-type wavemaker was set up in the computational domain to produce the incident solitary waves. The incident wave and the associated boundary layer flow in a wave tank with a flat bed were compared with the analytical solutions to verify the accuracy of this numerical scheme. Effects of the incident wave height and the size of the dike on the wave transformation, the flow fields, and the drag forces on the dike were discussed. Our numerical results showed that even though the induced local shear stress on the top surface of the dike is large at some particular locations, the resultant pressure drag is much larger than the friction drag. The primary vortex generated at the lee side of the dike and the secondary vortex at the right toe of the dike may scour the bottom and cause a severe problem for the dike.     

Temporal changes and spatial variation of soil oxygen consumption, nitrification and denitrification rates in a tidal salt marsh of the Lagoon of Venice, Italy

The aim of the present study was to investigate seasonal and spatial patterns of soil oxygen consumption, nitrification, denitrification and fluxes of dissolved inorganic nitrogen (DIN) in a tidal salt marsh of the Lagoon of Venice, Italy. In the salt marsh, intact soil cores including overlying water were collected monthly at high tide from April to October in salt marsh creeks and in areas covered by the dominant vegetation, Limonium serotinum. In May, cores were also collected in areas with vegetation dominated by Juncus maritimus and Halimione portulacoides. In laboratory incubations at in situ temperature in the dark, flux rates of oxygen and DIN were monitored in the overlying water of the intact cores. ¹⁵N-nitrate was added to the overlying water and nitrification and denitrification were measured using isotope-dilution and -pairing techniques. The results show that highest soil oxygen consumption coincided with the highest water temperature in June and July. The highest denitrification rates were recorded in spring and autumn coinciding with the highest nitrate concentrations. Soil oxygen consumption and nitrification rates differed between sampling sites, but denitrification rates were similar among the different vegetation types. The highest rates were recorded in areas covered with L. serotinum. Burrowing soil macrofauna enhanced oxygen consumption, nitrification and denitrification in April and May. The data presented in this study indicate high temporal as well as spatial variations in the flux of oxygen and DIN, and nitrogen transformations in the tidal salt marshes of the Venice lagoon during the growth season. The results identify the salt marshes of the Venice lagoon as being metabolically very active ecosystems with a high capacity to process nitrogen.     

Hydrodynamic studies on vertical seawall defenced by low-crested breakwater

This paper presents results obtained from a series of experiments conducted in wave flume to assess the influence of the offshore low-crested breakwater as a defence structure in reducing the wave forces on vertical seawall. The main aim of the tests was to know the effect of crest elevation of the offshore low-crested breakwater as a rehabilitation structure for the existing damaged shore protection structures. In this study five relative breakwater heights are used and associated flow evolution was analyzed. With the sections proposed in this study, it is possible to achieve considerable reduction of wave force on the seawall. Modification factor is proposed to estimate the shoreward force on the seawall defenced by low-crested breakwater.     

Numerical Investigation on Submerged Horizontal Plate

Hydrodynamic characters on a horizontal, thin, rigid plate located beneath the free surface are numerically investigated. Assuming a linear, time-harmonic potential flow and utilizing Green identity, the governing Laplace equation can be simplified into Fredholm integral equation ofthe second kind. Supposing linear-order discontinuous elements along intersecting vertical boundaries, and by use of the boundary element method, numerical solution about source strength distribution on the plate can be changed into a series of algebraic equations. The 3D Green function is introduced to set up the integral equations, and the GMRES solver is performed for solving the large dense linear system of equations. The added-mass, damping force and exciting force are evaluated directly from the equations. It is found that the added-mass coefficient becomes negative for a range of frequencies when the plate is sufficiently close to the free surface.     

Daytime deposition and nighttime dissolution of calcium carbonate controlled by submerged plants in a karst spring-fed pool: insights from high time-resolution monitoring of physico-chemistry of water

Water temperature, dissolved oxygen (DO), pH, and specific conductivity (spc) were measured in a time interval of 15 min in a karst spring and the spring-fed pool with flourishing submerged plants in Guilin, SW China under dry weather for periods of 2 days. Measurements allowed calculation of calcium and bicarbonate concentrations ([Ca²⁺] and [HCO₃ ⁻]), and thus CO₂ partial pressure ( ) and saturation index of calcite (SIc). Results show that there were not any diurnal variations in the physico-chemical parameters of the water for the spring. However, during daytime periods, pool water decreased to far less than the spring water in a few hours, pH and SIc increased to greater than the spring, and [Ca²⁺] and [HCO₃ ⁻] decreased to less than the spring. During nighttime periods, pool water returned to or even increased to greater than the spring, pH and SIc decreased to less than the spring, and [Ca²⁺] and [HCO₃ ⁻] increased to greater than the spring. The decrease in [Ca²⁺] and [HCO₃ ⁻] to less than the spring during daytime periods implies daytime deposition of calcium carbonate, while the increase in [Ca²⁺] and [HCO₃ ⁻] to greater than the spring during nighttime periods implies nighttime dissolution of calcium carbonate. The direction of the observed changes depended essentially on the illumination, indicating that daytime photosynthetic and nighttime respiratory activities in the pool aquatic plant ecosystem, which were further evidenced by the increase and decrease in DO during daytime and nighttime periods respectively, were the main processes involved. The large variations of the components of the carbonate system imply considerable changes of the capacities of CO₂ and O₂ in water. The finding has implications for water sampling strategy in slow-flowing karst streams and other similar environments with stagnant water bodies such as estuaries, lakes, reservoirs, and wetlands, where aquatic plant ecosystem may flourish.     

Effects of hydraulic structures on river morphological processes

Study of hydraulic structures such as groins and bandal-like structures can provide valuable information on their influences on morphological processes in natural rivers.These structures usually used for bank protection and formation of deep navigation channel can locally create complex flow patterns,reduce flow velocities and also increase the flood levels.Most of the previous studies are focused on structures like groins under non-submerged flow condition.However,the recent demand of nature friendly low cost and sustainable methods for river bank protection and channel formation leads to the necessity of study different type of structures like bandal-like structures.In this context, this study investigates the flow characteristics and sediment transport process influenced by bandal-like structures through laboratory experiments.The experiments were carried out under live-bed scour condition with sediment supplied from the inlet for two submergence(non-submerged and submerged)conditions.The experimental measurements contribute to better understand the mechanism of deposition/erosion process around different type of hydraulic structures.The performance of bandal-like structures considering the erosion around the structures,the deposition near the bank and the formation of deep main channel show promising results compared with conventional structures such as groins(impermeable and permeable ones).     

The mechanism of energy loss and transition in a flow with submerged vegetation

The mechanism of energy balance in an open-channel flow with submerged vegetation was investigated. The energy borrowed from the local flow, energy spending caused by vegetation drag and flow resistance, and energy transition along the water depth were calculated on the basis of the computational results of velocity and Reynolds stress. Further analysis showed that the energy spending in a cross-section was a maximum around the top of the vegetation, and its value decreased progressively until reaching zero at the flume bed or water surface. The energy borrowed from the local flow in the vegetated region could not provide for spending; therefore, surplus borrowed energy in the non-vegetated region was transmitted to the vegetated region. In addition, the total energy transition in the cross-section was zero; therefore, the total energy borrowed from the flow balanced the energy loss in the whole cross-section. At the same time, we found that there were three effects of vegetation on the flow: turbulence restriction due to vegetation, turbulence source due to vegetation and energy transference due to vegetation, where the second effect was the strongest one.     

Submerged hydraulic jump with sediment-laden flow

In this experimental study,the effect of suspended sediment concentration on the characteristics of a submerged hydraulic jump in a rectangular channel has been investigated,based on the sediment conce...     

Nutrients and organic matter in the surface sediment of a submerged macrophyte zone in a eutrophic lake:Implications for lake management

Little is known about the distribution and risk levels of nutrients and organic matter(OM) in the surface sediment of shallow submerged macrophyte-dominated lakes. In the current study, sixty surface sediment samples were collected from Xukou Bay, a typical submerged macrophyte-dominated zone in Lake Taihu, China. A 60-day degradation experiment of Potamogeton malaianus, a dominant species in the bay,was done in the laboratory. The results demonstrated that the ranges of total nitrogen(TN) and t...