Development of site-specific design ground motions in Western and Eastern North America

This paper presents results recently obtained for generating site-specific ground motions needed for design of critical facilities. The general approach followed in developing these ground motions using either deterministic or probabilistic criteria is specification of motions for rock outcrop or very firm soil conditions followed by adjustments for site-specific conditions. Central issues in this process include development of appropriate attenuation relations and their uncertainties, differences in expected motions between Western and Eastern North America, and incorporation of site-specific adjustments that maintain the same hazard level as the control motions, while incorporating uncertainties in local dynamic material properties. For tectonically active regions, such as the Western United States (WUS), sufficient strong motion data exist to constrain empirical attenuation relations for M up to about 7 and for distances greater than about 10–15 km. Motions for larger magnitudes and closer distances are largely driven by extrapolations of empirical relations and uncertainties need to be substantially increased for these cases.

For the Eastern United States (CEUS), due to the paucity of strong motion data for cratonic regions worldwide, estimation of strong ground motions for engineering design is based entirely on calibrated models. The models are usually calibrated and validated in the WUS where sufficient strong motion data are available and then recalibrated for applications to the CEUS. Recalibration generally entails revising parameters based on available CEUS ground motion data as well as indirect inferences through intensity observations. Known differences in model parameters such as crustal structure between WUS and CEUS are generally accommodated as well. These procedures are examined and discussed.     

Aircraft Measurements of Surface Albedo in Relation to Climatic Changes in Southern Israel

Summary Recent studies imply that significant climatic changes over the central and southern coastal plain in Israel may be due to changes in land usage, which have taken place since the National Water Carrier operation in the early 1960’s. Such changes are reflected in the spatial distribution of the surface albedo pattern, obviously resulting in changes in the surface radiation balance and, subsequently, modifying the surface heat fluxes and the stability conditions of the Planetary Boundary Layer (PBL). An Eppley PSP Pyranometer facing downward was mounted on a small Cessna aircraft and flown along the coastal plain from Tel Aviv to the northern Negev south of Beer Sheva, at an altitude of approximately 500 feet, measuring surface reflection. The incoming solar radiation was measured simultaneously, at several surface radiation stations of the Israel Meteorological Service, along the flight path. The results show large differences in surface reflection distributions, between the cultivated areas in southern Israel (as low as 0.15), and the adjacent arid regions (with values of up to 0.35). Historical albedo maps were reconstructed according to land utilization maps of the 1930’s and the 1960’s. A comparison between recent albedo map and the reconstructed maps, indicates temporal changes in the surface albedo pattern during the last decades. Received January 15, 1998 Revised June 23, 1998     

Flume Experiments on Turbulent Flows Across Gaps of Permeable and Impermeable Boundaries

Laser Doppler anemometery and laser-induced fluorescence techniques were used to explore the spatial structure of the flow within and above finite cavities created within porous and solid media. The cavities within these two configurations were identical in size and were intended to mimic flow disturbances created by finite gaps and forest clearing. Because flows over permeable boundaries differ from their solid counterparts, the study here addresses how these differences in boundary conditions produce differences in, (i) bulk flow properties including the mean vorticity within and adjacent to the gaps, (ii) second-order statistics such as the standard deviations and turbulent stresses, (iii) the relative importance of advective to turbulent stress terms across various regions within and above the gaps, and (iv) the local imbalance between ejections and sweeps and momentum transport efficiencies of updrafts and downdrafts. Both configurations exhibited a primary recirculation zone of comparable dimensions inside the gap. The mean vorticity spawned at the upstream corner of the gap was more intense for the solid configuration when compared to its porous counterpart. The free-shear layer spawned from the upstream corner-edge deeper into the gap for the porous configuration. The momentum flux at the interface within and above the gap was enhanced by a factor of 1.5–2.0 over its upstream value, and this enhancement zone was much broader in size for the porous configuration. For the turbulent transport terms in the longitudinal and vertical mean momentum balances, these transport terms were significant inside the gap for both boundary configurations when compared to their upstream counterpart. The effectiveness of using incomplete cumulant expansion methods to describe the momentum transport efficiencies, and the relative contributions of ejections and sweeps to turbulent stresses, especially in this zone, were also demonstrated. The flatness factor for both velocity components, often used as a measure of intermittency, was highest in the vicinity of the upstream corner in both configurations. However, immediately following the downstream corner, the flatness factor remained large for the porous configuration, in contrast to its solid configuration counterpart.     

Long-term change in October rainfall patterns in southern Israel

Summary A comprehensive study of long-term changes in October rainfall patterns in southern Israel, a semi-arid fringe zone between the Mediterranean coastal plain and the Negev Desert, was carried out. It was earlier postulated that the observed positive trend in October rainfall amounts may result from land-use changes in the area following the installation of the National Water Carrier (NWC) in the early 1960s. The purpose of the present study is to delineate the anticipated local effects, as well as their spatial extent, in order to investigate the possibility of climatic change over a larger synoptic scale.In the study region, a total number of 51 rainfall stations with long-term records up to 55 years, were analyzed, along with six control stations in other parts of the country. All stations were analyzed for trends in early seasonal rainfall, spatial variability during the two reference periods before and after the 60s, and for the ratio between rain per day (RPD) and number of rain days (NRD) during the two periods.The results of the analyses point to a well defined localized climatic change in October rainfall patterns within the study area, that is traceable to intensive and extensive land-use changes following the installation of the NWC.With 5 Figures     

Spatial and Temporal Changes in Rainfall Frequency Distribution Patterns in Israel

Summary Annual and monthly rainfall totals in Israel were analyzed to reveal any long term changes in their temporal and spatial distribution patterns, since the 1930s. The data consists of 60 rainfall stations, spread all over Israel from the far North to the Negev desert in the South, with long-term records of rainfall covering two normal periods. A gamma distribution function was fitted to the annual rainfall at each station for the two normal periods, and the shape and scale parameters of the distribution, as well as their percentage changes during the last normal period with respect to the first one, were analyzed. The analysis of the annual distribution function parameters reveals some appreciable changes, that are statistically significant, in the spatial rainfall distribution patterns in the southern, northern and central parts of the country. The most striking feature is revealed in the South, where a more than 60 percent increase in the shape parameter occurs, and a similar rate of re-scaling, i.e. a decrease of about 40 percent in the scale parameter. Analysis of the monthly distributions revealed considerable changes in October and November, at the beginning of the rainfall season, and an appreciable change in March, at the end of the rainfall season. Received December 10, 1997 Revised June 24, 1998     

Foundation design considerations for a pile supported critical facility

A large reinforced concrete structure supported on piles extending to bedrock is evaluated using a seismic soil–structure interaction analysis. The physical structure, the supporting pile system, analysis model and analytical methodology used are described. Important considerations in the design of the foundation pile system using the seismic analysis loadings and deformations are discussed.     

Spatio-Temporal Surface Shear-Stress Variability in Live Plant Canopies and Cube Arrays

This study presents spatiotemporally-resolved measurements of surface shear-stress τ _s in live plant canopies and rigid wooden cube arrays to identify the sheltering capability against sediment erosion of these different roughness elements. Live plants have highly irregular structures that can be extremely flexible and porous resulting in considerable changes to the drag and flow regimes relative to rigid imitations mainly used in other wind-tunnel studies. Mean velocity and kinematic Reynolds stress profiles show that well-developed natural boundary layers were generated above the 8 m long wind-tunnel test section covered with the roughness elements at four different roughness densities (λ = 0, 0.017, 0.08, 0.18). Speed-up around the cubes caused higher peak surface shear stress than in experiments with plants at all roughness densities, demonstrating the more effective sheltering ability of the plants. The sheltered areas in the lee of the plants are significantly narrower with higher surface shear stress than those found in the lee of the cubes, and are dependent on the wind speed due to the plants ability to streamline with the flow. This streamlining behaviour results in a decreasing sheltering effect at increasing wind speeds and in lower net turbulence production than in experiments with cubes. Turbulence intensity distributions suggest a suppression of horseshoe vortices in the plant case. Comparison of the surface shear-stress measurements with sediment erosion patterns shows that the fraction of time a threshold skin friction velocity is exceeded can be used to assess erosion of, and deposition on, that surface.     

On the significance of form-induced stress in rough wall turbulent boundary layers

This paper presents a review of recent experimental and numerical studies which deal with the analysis of form-induced stress in rough wall turbulent boundary layers. The aim of the paper is to assess the importance of this stress for various rough wall geometries and flow conditions. Analysis of the significance of form-induced stress is first performed by comparing its magnitude with the magnitude of Reynolds stress for each data set available in literature. Then, by selecting a special set of data, we analyze the comparison between the gradients of both stresses. We point out that the comparison of stress gradients gives a different perspective on the role of form-induced stress in rough wall boundary layers.