Structural control considering soil–structure interaction effects

A discussion of the effects of Soil-Structure Interaction (SSI) on the formulation of active structural control algorithms is presented. Two approaches for incorporating SSI effects in linear optimal control theory are developed: one which performs the control analysis on a structure with a fixed base and then considers the effects of SSI on the controlled structural response; and another which performs the control analysis using a structural equation of motion reformulated to include SSI effects. The two control formulations are studied and compared using a single-degree-of-freedom structure supported through a rigid foundation resting on a linear, elastic half-space. Results show that control effectiveness is affected by the approach used in formulating the equations of motion of the interacting system.     

Am improved source mechanism for the 1935 Timiskaming,Quebec earthquake from regional waveforms

The Timiskaming earthquake, which occurred near the Quebec-Ontario border at the northwest end of the Western Quebec seismic zone in 1935, is one of the five largest instrumentally recorded southeastern Canadian earthquakes. Previous studies of this earthquake concentrated on modeling teismograms recorded at regional distances, a better constrained focal mechanism is obtained. The waveforms indicate thrust faulting on a moderately dipping northwest striking plane at a depth of 10 km. TheM _w of 6.1 determined in this study is in good agreement with previous magnitude estimates (m _b 6.1,M _s 6.0, andm _bLg 6.2–6.3). The focal mechanism is similar to those of many recent small to moderate earthquakes in the region, and the inferred (from theP axis) acting stress of northeast compression is consistent with the overall eastern North American stress field. The Lake Timiskaming Rift Valley in which the earthquake occurred, comprises several northwest striking faults consistent with the strike of the 1935 event. Thus, the 1935 earthquake appears to be a result of faulting on the reactivated Timiskaming graben.     

Large-Scale Differences in Community Structure and Ecosystem Services of Eelgrass (<Emphasis Type="Italic">Zostera marina</Emphasis>) Beds Across Three Regions in Eastern Canada

Eelgrass (Zostera marina) forms extensive beds in temperate coastal and estuarine environments worldwide and provides important ecosystem services, including habitat for a wide range of species as well as nutrient cycling and carbon storage. However, little is known about how eelgrass ecosystem structure and services differ naturally among regions. Using large-scale field surveys, we examined differences in eelgrass bed structure, carbon and nitrogen storage, community composition, and habitat services across three distinct regions in Eastern Canada. We focused on eelgrass beds with low anthropogenic impacts to compare natural differences. In addition, we analyzed the relationships of eelgrass bed structure with environmental conditions, and species composition with bed structure and environmental conditions, to elucidate potential drivers of observed differences. Our results indicate that regional differences in eelgrass bed structure were weakly correlated with water column properties, whereas differences in carbon and nitrogen storage were mainly driven by differences in eelgrass biomass. There were distinct regional differences in species composition and diversity, which were particularly linked to temperature, as well as eelgrass bed structure indicating differences in habitat provision. Our results highlight natural regional differences in ecosystem structure and services which could inform spatial management and conservation strategies for eelgrass beds.     

Subtidal flow and its variability at the entrance to a subtropical lagoon

Spatial and temporal variability of the subtidal exchange flow at West Pass, an inlet at the entrance to a subtropical lagoon (St. Andrew Bay, Florida), was studied using moored and towed current velocity profiles and hydrographic data. Towed and hydrographic measurements were captured over one diurnal tidal cycle to determine intratidal and spatial changes in flow. Hydrographic profiles over the tidal cycle showed that tidal straining modified density stratification asymmetrically, thus setting up the observed mean flow within the inlet. During the towed survey, the inlet's mean flow had a two-layer exchange structure that was moderately frictional and weakly influenced by Coriolis accelerations. Moored current profiles revealed the additional contribution to the dynamics from centrifugal accelerations. Along channel residual flows changed between unidirectional and exchange flow, depending on the forcing from the along-estuary wind stress and, to a lesser extent, the spring–neap tidal cycle. Increases in vertical shear in the along channel subtidal flow coincided with neap tides and rain pulses. Lateral subtidal flows showed the influence on the dynamics of centrifugal accelerations through a well-developed two-layer structure modulated in magnitude by the spring–neap tidal cycle.     

Sources of organic matter in surface sediments of the Louisiana Continental margin: Effects of major depositional/transport pathways and Hurricane Ivan

Lignin and pigment biomarkers were analyzed in surface sediments of the Louisiana Continental margin (LCM) to distinguish differences in the degradative state of sedimentary organic matter along and between two major depositional pathways (along shore and offshore to the Mississippi Canyon) from Southwest (SW) Pass in July 2003. Barataria Bay, an inter-distributary estuary, was also assessed as a potential source of terrestrial organic matter to the LCM. Sediment signatures taken along the same pathways after Hurricane Ivan (October 2004) were compared with the pre-Ivan signature to elucidate carbon dynamics after major hurricane events. Density fractions were investigated at key stages across the LCM. Mississippi Canyon sediments are a depocenter for labile and refractory organic matter derived from river and previously deposited shelf sediments. Barataria Bay material may be a contributing source of sedimentary organic matter in shallow shelf areas bordering the bay and is thus potentially important in carbon cycling in sediments of these shallow areas; however, our results show that organic matter inputs from the bay were likely rapidly decomposed and/or diluted. Hurricane Ivan mobilized sedimentary organic carbon (SOC) offshore and homogenized terrestrial sediment parameters and gradients. As observed through pigment concentrations sediments tended to equilibrate to a more steady-state condition within months of the disturbance. Insights from density fractions show that selective degradation and aggregation/flocculation processes were also very important processes during cross-shelf transport. Zooplankton grazing, largely on diatoms and other algae, was a shelf wide phenomenon, however, grazing products dominated the marine-derived SOC in margin sediments west of the birdsfoot delta indicated by the abundance of steryl chlorin esters (SCEs).     

Strontium in coral aragonite: 3. Sr coordination and geochemistry in relation to skeletal architecture

Use of the coral Sr palaeothermometer assumes that the Sr in coral skeletons is substituted randomly for Ca in the aragonite structure. The presence of Sr in additional phases e.g., strontianite, or the non random distribution of Sr across metal sites in aragonite, would complicate the Sr/Ca-sea surface temperature relationship. We have used Sr K-edge microEXAFS (extended X-ray absorption fine structure) to determine the structural state of Sr across selected microvolumes of four coral skeletons (Porites lobata, Acropora palmata, Pavona clavus, and Montastrea annularis). We used a 5 × 3 μm beam to analyse specific areas of the coral skeletal architecture, i.e., centres of calcification, fasciculi, and dissepiments. All EXAFS analyses refine, within error, to an ideally substituted Sr in aragonite, and we found no evidence of strontianite or partly ordered structural states. Anisotropy in the first shell responses results from the fact that the analysed microvolumes are not necessarily averaged for the responses of all crystal orientations in the aragonite. Although secondary ion mass spectrometry confirmed that Sr/Ca composition can vary substantially between skeletal components, we find no evidence for any contrast in Sr structural state. Sr heterogeneity may result from kinetic effects, reflecting complex disequilibrium processes during crystal precipitation, or biological effects, resulting from variations in the composition of the calcifying fluid which are biologically mediated.     

Observing Convective Aggregation

Convective self-aggregation, the spontaneous organization of initially scattered convection into isolated convective clusters despite spatially homogeneous boundary conditions and forcing, was first recognized and studied in idealized numerical simulations. While there is a rich history of observational work on convective clustering and organization, there have been only a few studies that have analyzed observations to look specifically for processes related to self-aggregation in models. Here we review observational work in both of these categories and motivate the need for more of this work. We acknowledge that self-aggregation may appear to be far-removed from observed convective organization in terms of time scales, initial conditions, initiation processes, and mean state extremes, but we argue that these differences vary greatly across the diverse range of model simulations in the literature and that these comparisons are already offering important insights into real tropical phenomena. Some preliminary new findings are presented, including results showing that a self-aggregation simulation with square geometry has too broad distribution of humidity and is too dry in the driest regions when compared with radiosonde records from Nauru, while an elongated channel simulation has realistic representations of atmospheric humidity and its variability. We discuss recent work increasing our understanding of how organized convection and climate change may interact, and how model discrepancies related to this question are prompting interest in observational comparisons. We also propose possible future directions for observational work related to convective aggregation, including novel satellite approaches and a ground-based observational network.     

Sand settling through bedform-generated turbulence in rivers

Fluvial bedforms generate a turbulent wake that can impact suspended-sediment settling in the passing flow. This impact has implications for local suspended-sediment transport, bedform stability, and channel evolution; however, it is typically not well-considered in geomorphologic models. Our study uses a three-dimensional OpenFOAM hydrodynamic and particle-tracking model to investigate how turbulence generated from bedforms and the channel bed influences medium sand-sized particle settling, in terms of the distribution of suspended particles within the flow field and particle-settling velocities. The model resolved the effect of an engineered bedform, which altered the flow field in a manner similar to a natural dune. The modelling scenarios alternated bed morphology and the simulation of turbulence, using detached eddy simulation (DES), to differentiate the influence of bedform-generated turbulence relative to that of turbulence generated from the channel bed. The bedform generated a turbulent wake that was composed of eddies with significant anisotropic properties. The eddies and, to a lesser degree, turbulence arising from velocity shear at the bed substantially reduced settling velocities relative to the settling velocities predicted in the absence of turbulence. The eddies tended to advect sediment particles in their primary direction, diffuse particles throughout the flow column, and reduced settling likely due to production of a positively skewed vertical-velocity fluctuation distribution. Study results suggest that the bedform wake has a significant impact on particle-settling behaviour (up to a 50% reduction in settling velocity) at a scale capable of modulating local suspended transport rates and bedform dynamics. © 2020 John Wiley & Sons, Ltd.     

Vibration control of cable-stayed bridges—part 1: modeling issues

The objective of the research presented here is to increase the understanding of how the complexities associated with modeling cable-stayed bridges, such as non-linear behaviour and the participation of highly coupled, high-order vibration modes in the overall dynamic response, affect the overall effectiveness of active control schemes. The 316-degree-of-freedom analytical model studied here is based on the Jindo Bridge located in South Korea. Computational considerations associated with control analyses require the size of the model to be significantly reduced, without loss of the important vibration characteristics and complexities. Three separate reduced-order modelling techniques for creating effective control models are studied here: the IRS method, the internal balancing method, and a modal reduction method. These methods are studied and compared on their ability to capture the complex dynamic response of cable-stayed bridges subjected to multiple-support excitation and their ability to create viable and computationally sound state-space models for control analyses. Results show that the modal reduction technique, because of the ability to select only those modes causing the largest force and displacement response, is most effective for control applications. © 1998 John Wiley & Sons, Ltd.