天然气水合物体系动态演化研究(Ⅱ):海底滑坡

天然气水合物被认为是大陆边缘沉积物强度变弱的一个因子，从而能解释大陆边缘海底滑坡的一些观测现象。天然气水合物的形成使沉积物强度增加，而其分解则使沉积物强度变弱。虽然无法直接观测沉积物中天然气水合物的活动过程与相应的海底滑坡，大量的背景资料表明，天然气水合物崩解常常有助于触发海底沉积物块体的运动。此外，大型滑塌可以释放大量的固态天然气水合物，水合物在水柱中上浮。大块天然气水合物可以在分解前到达海洋的上部层，一些甲烷可以直接进入大气中。本文综述与天然气水合物体系演化有关的海底滑坡的研究现状。     

Evaluation of sediment contamination in the Red Sea coastal area combining multiple pollution indices and multivariate statistical techniques

In the recent years,the Red Sea coast of Yemen has been severely affected by intensive anthropogenic activities.The current study constitutes a thorough inquiry to evaluate the extent of heavy metals pollution in Yemen's Red Sea coast sediment and identifies the possible sources of pollution.The concentrations of five metals(copper(Cu),zinc(Zn),cadmium(Cd),lead(Pb),and nickel(Ni))collected from nine sites along the Red Sea coast of Yemen were assessed using an atomic absorption spectrophotometer(ASS).Sediment quality indices,such as the sediment quality guidelines(SQGs),potential ecological risk(RI),contamination factor(CF),pollution load index(PLI),geoaccumulation index(Igeo),and modified degree of contamination(mCd)were computed.In addition,multivariate statistical techniques(principal component analysis(PCA),hierarchical cluster analysis,and Pearson's correlation analysis)were applied to identify the potential sources of metals.The mean concentrations of Cu,Zn,Cd,Pb,and Ni were 51.3,61.9,4.02,9.9,and 33.4 mg/kg dry wt,respectively.The spatial distribution revealed that the metals concentrations were high at the middle zone and low southward of Hodeida city.According to the SQGs,the adverse biological effects of metals were occasionally associated with Cu and Cd,frequently associated with Ni,and not expected to occur with Zn and Pb.The RI indicated that the sediment of the studied sites pose low(RI<50)to considerable(100≤RI<200)ecological risk.The mean effect range-median quotient(M-ERM-Q)indicated that the combination of the studied metals had the toxicity probability of 21%at all studied sites.Igeo and CF indicated that the metals concentrations were in the descending order of:Zn>Ni>Pb>Cd>Cu,whereas the PLI and mCd indicated that Ras Isa(Site 5)and Urj village(Site 6)were the most polluted sites.PCA,cluster analysis,and correlation analysis found that Cd,Pb,and Ni mostly originated from anthropogenic sources while Cu and Zn were mainly derived from natural sources.Thus,it is evident that the intensive anthropogenic activities had negative influence on metals accumulation in the sediment of the Red Sea coast of Yemen leading to detrimental effects to the whole ecosystem.These comprehensive findings provide valuable information and data for future monitoring studies regarding heavy metals pollution and sediment quality at the Red Sea coast of Yemen.     

Climatic and anthropogenic impacts on the decrease of sediment discharge to the Mediterranean coast from the largest river of Maghreb

The current study focuses on the issue of the decrease in sediment discharge to the Mediterranean Sea by the largest river in Algeria,the Wadi Cheliff(i.e.Cheliff River).This study clarifies the effect of climatic and anthropogenic factors on the changes in the sedimentary dynamics of the Cheliff River discharging to the sea.The data used(rainfall,water discharge,and sediment discharge)concern the Sidi Bel Attar gaging station on the Cheliff River,only 18 km from discharge to the Mediterranean Sea.A power-type statistical regression model was used to fill the 74.2% gap in Suspended Solids(SS)measurements in the establishment of a SS database for the period 1951e2012.The study results show that the transport of suspended sediment discharged to the sea is about 487 t/km^2/yr.Statistical tests of breaks highlight that rainfall decreased by 26% and that water and sediment input to the sea declined from 74%to 63% for the period of 1981e2012.The correlation analysis shows a decrease in the contribution of rainfall on sediment and water input at the outlet equal to 77.9% and 77.8%,respectively,during the period of 1980 e2012,compared to the period of 1968e1980.However,the double mass method reflects the contribution of factors other than rainfall to the decrease of sediment input to the sea.These other factors mainly include large dams,which intercepted about 71%of the total volume of sediment discharged to the sea during the period of 1968e2010.In addition the contribution of large dams to the reduction of sedimentary input to the sea is more important than that of the decrease in rainfall.The management of large dams also contributes to the increase in the sediment deficit to the sea through the prioritization of interception of sediment at the expense of releases,for socio-economic purposes,68.4%of the Cheliff River discharge is diverted for human use.This has led to an increase in the mean water bed level at bankfull downstream,where the Cheliff River gave up 51%of its width to the floodplain between 1996 and 2009.In the light of the scarcity of sediment transport data in North Africa and in many other areas,the current study provides a reference framework for other studies:providing useful information for the study of the transfer of sediment from land to sea,and the links with the socio-economic needs.     

Mud deposit formation on the open coast of the larger Patos Lagoon–Cassino Beach system

This paper proposes an explanation of the mud deposits on the inner Shelf of Cassino Beach, South Brazil, by using computational modeling. These mud deposits are mainly formed by sediments delivered from Patos Lagoon, a coastal lagoon connected to the Shelf, next to Cassino Beach. The deposits are characterized by (soft) mud layers of about 1 m thick and are found between the −5 and −20 isobaths.     

Steep wave,turbulence, and sediment concentration statistics beneath a breaking wave field and their implications for sediment transport

A new methodology based on wavelet analysis is used to estimate steep wave statistics under depth-limited conditions and the corresponding high concentration sediment statistics. Steep waves here are defined as wave crests within the wavelet transform exceeding a root mean square derived acceleration threshold. The method is applied to laboratory data obtained in a large-scale wave-flume experiment conducted in 2005 at Oregon State University's O. H. Hinsdale Wave Research Laboratory from an acoustic Doppler velocimeter and a fiber optic backscatter sensor array above a mobile sand bed. The steep wave and high concentration statistical results for the erosive condition suggest that sand suspensions are intermittent when a wave-breaking timescale (the ratio of breaking wave height and rms wave velocity) is used to detect the concurrence among steep wave, high velocity turbulent fluctuations, and sand concentration events near the bed. More importantly, at 1 cm above the bed, though the accretive case has more steep wave events, the erosive case has more steep waves and concurrent high concentration events, suggesting a more intense breaking wave process near the sensors. The use of a longer time window, based on the dominant wave period in the detection process of steep wave and high concentration events at 1 cm above the bed, does not change the resulting statistics for the erosive condition. However, increased percentages of high concentration events correlated with steep wave and high velocity turbulence events for the accretive condition are obtained. These increased percentages are conjectured to be due to advection of non-local turbulent events and sediment concentration peaks from upstream.     

A two-phase numerical model for suspended-sediment transport in estuaries

This paper presents a full 2-D X/Z numerical model for sediment transport in open channels and estuaries using a two-phase (fluid–solid particle) approach. The physical concept and the mathematical background of the model are given and test-cases have been carried out to validate the proposed model. In order to illustrate its feasibility for a real estuary, the model has been applied to simulate the suspended-sediment transport and the formation of turbidity maximum in the Seine estuary. The numerical results show that the main characteristics of estuarine hydro-sediment dynamics in the Seine estuary are in fact reproduced by the proposed model. A qualitative agreement between the numerical results and the actual observations has been obtained and is presented in this paper.     

NUMERICAL SIMULATION OF SEDIMENT RELEASE FROM RESERVOIRS

1 INTRODUCTION Reservoirs sedimentation is a serious problem in many countries, including the I. R. of Iran. Accumulation of sediment deposits decreases worldwide reservoir storage capacity by one percent per year (Mahmood, 1987). The loss of reservoir st…     

LIVE-BED SCOUR IN LONG CONTRACTIONS

1 INTRODUCTION Scour within a channel contraction occurs where the waterway is restricted by the foundations or road approach embankments of a bridge. Alternately, contraction scour takes place when a bridge is located at a natural contraction in the rive…     

Field accumulative behavior of organochlorine pesticides. The role of crabs and sediment characteristics in coastal environments

The influence of intertidal crab beds on the concentrations of organochlorine (OC) pesticides in sediment was studied in two different coastal environments in Argentina. Samples of male burrowing crabs (Chasmagnathus granulatus) were collected for this study. Our field data showed lower bioaccumulation of OC pesticides in crabs from sediments with a higher total organic carbon (TOC) and higher clay content. Thus, concentrations in crabs depend on the physico-chemical characteristics of the sediment where they live more than on the OC pesticide concentrations in the environment. The distribution patterns in sediment from inside and outside crab burrows were similar for both coastal areas being HCHs  γ-chlordane > p,p′-DDE for San Antonio Bay (SAO), and HCHs > p,p′-DDE  γ-chlordane for Mar Chiquita (MCh) coastal lagoon. OC pesticide concentrations in sediment were significantly lower inside than outside crab burrows, irrespective of the sediment physico-chemical characteristics due to the bioturbation activity of C. granulatus.     

Stochastic nature of three dimensional bursting events and sediment entrainment in vortex chamber

In this study, three dimensional quadrant analysis of bursting process was used to recognize the susceptible regions for sediment entrainment and deposition at the bed of a vortex chamber. From the analysis, it was found that two dimensional quadrant analysis in unable to find the turbulent coherent structure of flow near the bed of the vortex chamber. Therefore, a new method based on three dimensional bursting process is introduced in this study to define the turbulent flow structure. Based on the new methodology in this study, the bursting event is divided into eight different cube zones according to three dimensional velocity fluctuations. It was realized that, four cube zones interactions are toward the central orifice of the vortex chamber and four cube zones interactions are toward the wall of the chamber and they are categorized as classes A and B, respectively. The results from the experiments showed that in class A, the internal sweep events (class IV-A) moves the settled sediment particles toward the central orifice of the chamber, whereas in class B the external sweep events (class IV-B) moves the settled sediment particles toward the external region of the chamber. Also the transition probabilities of the bursting events in 64 particular movements were determined. The result showed that stable organizations of each class of the events had highest transition probabilities whereas cross organizations had lowest transition probabilities. Additionally, an effort was made to find the average inclination angle of the three dimensional bursting events in each cube zone. The results showed that near the bed of the vortex chamber by increasing the tangential velocity toward the center of the chamber, the average inclination angle of the events in the cube zones decreased. Also, at the region where the sediment particles were deposited, the inclination angles had higher values.