Modelling of periodic wave transformation in the inner surf zone

In this paper, we analyse the ability of the nonlinear shallow-water (NSW) equations to predict wave distortion and energy dissipation of periodic broken waves in the inner surf zone. This analysis is based on the weak-solution theory for conservative equations. We derive a new one-way model, which applies to the transformation of non-reflective periodic broken waves on gently sloping beaches. This model can be useful to develop breaking-wave parameterizations (in particular broken-wave celerity expression) in both time-averaged wave models and time-dependent Boussinesq-type models. We also derive a new wave set-up equation which provides a simple and explicit relation between wave set-up and energy dissipation. Finally, we compare numerical simulations of both, the NSW model and the simplified one-way model, with spilling wave breaking experiments and we find a good agreement.     

Budgets of Mn,Cd and Cu in the macrotidal Gironde estuary (SW France)

The behavior and budget of Mn, Cd and Cu in the Gironde estuary were investigated through data from both the water column (WC) and sediment depth profiles. In the estuarine freshwater reaches, Mn and Cd removal from and Cu addition to the dissolved phase occurs with a magnitude equivalent to 10%, 30% and 25% of their respective annual fluvial gross dissolved input, respectively. In the saline estuary, diffusive benthic outflow is the main source of dissolved Mn (74% of the total gross dissolved input within the estuary) to the WC. In contrast, Cd (96%) and Cu (89%) are mainly released into the dissolved phase of the WC from fluvial, estuarine and dredging-related particles through complexation (Cd) and organic carbon mineralization (Cu). Anthropogenic activities (sediment dredging) induce pore water inputs, particulate sulfide oxidation and sediment resuspension, significantly contributing to the metal budget of the WC. The related amounts of metals released could be equivalent to 20–50% (Cd) and up to 70% (Cu) of their respective net dissolved addition. Mass balances suggest that a large part of the metals previously released into the dissolved phase from processes within the estuary are removed by suspended particles due to (co-)precipitation of Fe/Mn (oxy)hydroxides and scavenging on autochthonous organic matter. On an annual basis, the Gironde estuary acts as a net sink of dissolved Mn, removing 60% of the dissolved fluvial inputs, and as a net source of dissolved Cd and Cu, contributing ∼ 85% and 20–45% to the dissolved Cd and Cu fluxes to the ocean.     

Influence of Wind on Dynamics and Flushing of Shallow Estuaries

Observations of two small estuaries in Cape Cod, U.S.A. indicate large variations in salinity structure that are forced by variations in along-estuary wind stress. During onshore winds, the estuarine circulation is reduced, and the along-estuary salinity gradient increases as freshwater accumulates. During offshore winds, the surface outflow is enhanced, freshwater is flushed out of the estuary, and the along-estuary salinity gradient becomes weak. Constrictions block the wind-induced flushing, resulting in strong salinity fronts across the constrictions. The residence time of one of the estuaries varies by more than a factor of three in response to variations in wind-induced flushing. The other estuary has little variation of flushing associated with winds, due to a constriction at the mouth that inhibits the wind-induced exchange. The strong influence of winds on the flushing of these estuaries is due in part to their shallow depths, which accentuates the influence of wind stress relative to the effects of the horizontal density gradient. In addition, the residence times of the estuaries are comparable to the time scale of wind forcing, allowing large changes in water properties during wind events.     

随机海浪的列率谱分析研究

首次将列率谱分析的方法引入到随机海浪的研究。根据不同环境条件下实验室资料，对列率谱与频率谱进行了比较。结果表明，列率频对频率谱高频部分有明显改善，特别对二倍频含能段的分辨具有明显的优越性。另外，列率谱具有计算速度快、简单易行、分辨率高等优点。     

Distribution of nonlinear wave crests

The distribution of nonlinear wave crests is examined on the basis of a theoretical probability density previously given elsewhere (J. Eng. Mech. 120 (1994) 1009). Certain errors contained in the original theoretical density are corrected, and the corresponding exceedance distribution is derived. The resulting theoretical forms of the probability density and exceedance distribution are then slightly simplified and compared with nonlinear wave data gathered under hurricane conditions. The results indicate that the proposed theoretical forms describe the observed distributions of large wave crests better than the Rayleigh law. However, the quantitative accuracy of the predictions is somewhat poor, as is typical of approximate theories based on Gram–Charlier-type expansions.     

Spatio-Temporal Distribution of Algal and Animal Communities in a Posidonia oceanica Meadow

Abstract. Investigations were carried out in a Posidonia oceanica meadow at Ischia (Gulf of Naples) along a depth gradient from 1 to 32 m in November 1979 and May 1981.
In these two months, different discontinuities were found at the sampling stations (1, 5, 10, 20, 30 m) at 10 m for leaf features, between 1 and 5 m for the algal community and the vagile fauna. The leaf features (length, surface, and biomass) along the transects show a delay in leaf production toward the deeper stations, below the thermocline.
The algal community shows in both seasons and in all stations a persistence of an encrusting layer, mostly represented by Corallinaceae and the brown alga Myrionema orbiculare , while at the shallowest station (1 m) the community is characterized by a more developed upright layer.
The vagile fauna of the leaf stratum, mostly represented by Polychaeta, Mollusca , and Crustacea (Peracarida and Decapoda) , shows a consistent zonation in both seasons along the transect. A superficial community at I m, characterized by a low number of species and individuals and specialized for high environmental energy levels, and a deeper community, persistent in time and more strictly related to the Posidonia oceanica meadows, are identified.
The influence of environmental factors and the importance of meadow structure for the zonation of the algal and animal communities are discussed.     

Statistics of nonlinear wave crests and groups

Groups of large nonlinear waves with sharper higher crests can pose hazards to ships, induce harbor resonance and cause wave-overtopping of fixed and floating structures. Past interest in wave groups has mostly been focused on the statistics and modeling of linear wave groups. Studies on nonlinear wave groups are surprisingly few, and address deep water waves only. Here, statistics of nonlinear wave crests and wave-crest groups in deep and transitional water depths are considered, using an appropriate second-order representation for crest heights and the continuous wave-envelope approach. In particular, theoretical expressions describing the statistics of nonlinear wave crests and their groups are posed in the form of a simple second-order transformation of well-known results on linear waves. Predictions from the transformation so posed compare well with nonlinear wave data gathered in the North Sea, and demonstrate that nonlinearities do affect the statistics of large wave crests and their groups significantly.     

Robust and adaptive path following for underactuated autonomous underwater vehicles

This paper proposes a nonlinear robust adaptive control strategy to force a six degrees of freedom underactuated underwater vehicle with only four actuators to follow a predefined path at a desired speed despite of the presence of environmental disturbances and vehicle’s unknown physical parameters. The proposed controller is designed using Lyapunov’s direct method, the popular backstepping and parameter projection techniques. The closed loop path following errors can be made arbitrarily small. Interestingly, it is shown that our developed control strategy is easily extendible to situations of practical importance such as parking and point-to-point navigation. Numerical simulations are provided to illustrate the effectiveness of the proposed methodology.     

泥石流平面形态的统计分析

泥石流扇的平面形态可用规划的几何图形来模拟。在研究了中国西南地区小江流域和意大利东北部阿尔卑斯山区的５２个泥石流扇的基础上，本文提出了两种泥石流扇的平面形态：中国西南地区的“等腰三角形＋半圆形”泥石流扇和意大利东北部的“几何扇”。用统计分析建立了两组泥石流扇和泥石流流域之间的形态度量关系。通过两个研究区域的对比，两个流域参数：流域面积和主沟比降确定为影响泥石流扇平面形态和面积的最重要的因素。