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The structure of a silicon avalanche photodiode (APD) has a significant impact on the probability of light creating charge carriers and on the generated noise due to the multiplication process or excess noise factor (ENF). In this paper, we will review front-illuminated and back-illuminated APD structures and their impact on ENF as a function of wavelength from 400 nm to 1000 nm for recently commercially produced silicon APDs targeting LIDAR and other applications. The experimental setup developed for characterization will be described and highlight the differences between the studied structures. APDs with different junction profiles were produced and measurement of ENF was found to match McIntyre’s theory for experimental k-factors (ratio of the hole impact ionization rate to that of electrons) ranging from approximately 0.05 to 0.008. The generated illuminated noise as a function of responsivity can be used as a guideline to select the APD achieving the best signal-to-noise ratio (SNR) for a given application. To help meeting this condition, optimizing the electrical field profile of an APD and making certain the electrons are the primary carriers initiating the avalanche is critical.

Silicon Avalanche Photodiodes (APDs) are used in NASA’s Global Ecosystem Dynamics Investigation (GEDI) that was launched in December 2018 and is currently measuring the Earth’s vegetation vertical structure from the International Space Station. The APDs were specially made for space lidar with a much lower hole-to-electron ionization coefficient ratio (k-factor ~0.008) than that of commercially available silicon APDs in order to reduce the APD excess noise. A silicon heater resistor was used under the APD chip to heat the device to 70°C to improve its quantum efficiency at the 1064-nm laser wavelength while maintaining a low dark current such that the overall signal to noise ratio is optimized. Special APD protection circuits were included to raise the overload damage threshold to prevent device damage from strong laser returns from specular surfaces, such as still water bodies, and space radiation events. The APD and a hybrid transimpedance amplifier circuit were hermetically sealed in a TO-8 type metal package with a sufficiently low leak rate to ensure a multi-year operation lifetime in space. The detector assemblies underwent a series of pre-launch tests per NASA Goddard Environmental Verification Standard for space qualification. The APDs have performed exactly as expected in space. A detailed description of the GEDI detector design, signal and test results are presented in this paper.

Torrey Pines State Beach, a site with large seasonal fluctuations in sand level, received a small shoreface beach fill (about 160,000 m3) in April 2001. The 600 m-long, flat-topped nourishment pad extended from a highway riprap revetment seaward about 60 m, terminating in a 2 m-tall vertical scarp. A 2.7 km alongshore span, centered on the nourishment region, was monitored prior to the nourishment and biweekly to monthly for the following 2 years. For the first 7 months after the nourishment, through fall 2001, significant wave heights were small, and the elevated beach fill remained in place, with little change near and above Mean Sea Level (MSL). In contrast, the shoreline accreted on nearby control beaches following a seasonal pattern common in southern California, reducing the elevation difference between the nourished and adjacent beaches. During the first winter storm (3 m significant wave height), the shoreline retreated rapidly over the entire 2.7 km survey reach, forming an alongshore-oriented sandbar in 3 to 4 m water depth [Seymour, R.J., Guza, R.T., O'Reilly, W., Elgar, S., 2004. Rapid erosion of a Southern California beach fill. Coastal Engineering 52 (2), 151–158.]. We show that the winter sandbar, most pronounced offshore of the nourishment, moved back onto the beach face during summer 2002 (following the usual seasonal pattern) and formed a wider beach above MSL at the site of the original nourishment than on the control beaches. Thus, the April 2001 shoreline nourishment was detectable until late fall 2002, persisting locally over a full seasonal cycle. In an extended 7-year time series, total sand volumes (summed between the back beach and 8 m water depth, over the entire 2.7 km reach) exhibit multi-year fluctuations of unknown origin that are twice as large as the nourishment volume.

Airborne light detecting and ranging (lidar) systems can survey hundreds of kilometers of shoreline with high spatial resolution (several elevation estimates per square meter). Sequential surveys yield spatial change maps of beach and dune sand levels. However, lidar data include elevations of the exposed, subaerial beach and, seaward of the waterline, the ocean surface. Here, a simple method is developed to find the waterline and eliminate returns from the ocean surface. A vertical elevation cutoff is used, with the waterline elevation (W) above the known tide level because of the superelevation from wave setup and runup. During each lidar pass, the elevation cutoff (W) is assumed proportional (C) to the offshore significant wave height Hs. Comparison of in situ and lidar surveys on a moderately sloped, dissipative California beach yields C ≈ 0.4, which is qualitatively consistent with existing observations of runup and setup. The calibrated method rejects ocean surface data, while retaining subaerial beach points more than 70 m seaward of the mean high waterline, which is often used as a conservative default waterline.

A winter storm eroded a small (160,000 m3) beach fill at Torrey Pines State Beach in southern California. The fill, constructed in April 2001, was a 600-m long flat-topped berm, extending from a highway revetment seaward about 80 m, terminating in a 2-m tall, near-vertical scarp. The size distributions of the preexisting and fill beach sand were similar (median ∼0.2 mm). A total of 56 cross-shore transects were surveyed between the revetment and 8 m water depth biweekly along 2.7 km of the beach centered on the fill area. During summer and fall, the incident significant wave heights measured 1 km offshore of the fill usually were below 1 m, the scarp was not overtopped, and the fill did not change greatly. The beach face alongshore of the fill accreted, consistent with the usual seasonal cycle in southern California. During a storm (3 m significant wave height) in late November, erosion began when wave uprushes overtopped the scarp and reached the relatively flat elevated fill, where the overwash flowed alongshore to initially small depressions that channeled the flow seawards. The offshore flow rapidly deepened and widened the channels, which maintained steep vertical faces and eroded by slumping. Thirty hours after the storm began, the shoreward end of the eroded channels had retreated to the highway revetment, leaving uneroded sand peninsulas protruding seawards ∼50 m from the revetment and elevated ∼1.75 m above the surrounding beach. Erosion of the beach adjacent to the fill was much less variable alongshore than within the fill region. During the next few days, the peninsulas eroded almost completely.

Records of deepwater sea surface elevation alone, even when taken at high sampling rates, do not allow determination of those waves in the record that are breaking. Although visual observations of steep random waves clearly show even the beginning of a breaking event, the white capping and collapse of the front face of a breaking wave do not appear in any meaningful way in the recorded output of an elevation gauge. Considerable research has been devoted to finding a method of detecting breaking events in a wave elevation record. Some criteria have been advanced, but most of the models show little skill. The present study evaluates the power of the Phase-Time Method based on the Hilbert transform, in detecting breaking waves in large-scale laboratory experiments under a variety of deep water wave conditions. A limited validation of the method in the field was performed. A useful model of deepwater breaking wave detection is developed, and its limits are stated. The behavior of the model suggests that these results may be useful in the understanding of the breaking mechanism in deep water.

Several transient wave trains containing an isolated plunging or spilling breaker at a prescribed location were generated in a two-dimensional wave flume using an energy focusing technique. Surface elevation measurements of each transient wave train were made at locations before and after breaking. Applying a nonlinear deterministic decomposition approach to the measured elevation, the free-wave components of the transient wave train were derived by excluding the contribution from bound-wave components. The comparison of the amplitude or energy spectra of free-wave components before and after a breaker can accurately reveal the energy dissipation as a function of frequency. It is found that the energy loss is almost exclusively from wave components at frequencies higher than the spectral peak frequency. Although the energy density of the wave components of frequencies near the peak frequency is the largest, they do not significantly gain or lose energy after the breaking. It is also observed that wave components of frequencies significantly below or near the peak frequency gain a small portion (about 12%) of energy lost by the high-frequency waves. These findings are quite different from the empirical formulas presently used for determining wave dissipation due to wave breaking. Hence, they have important implications to the ocean wave energy budget, specially to the energy transfer at frequencies below and near the spectral peak frequency.

A method is developed for the deterministic decomposition of nonlinear short-crested irregular ocean waves accurate up to second order in wave steepness. Initially, an alternative maximum likelihood method is used to determine the directional spreading of wave energy. Then a least squares fitting scheme is used to calculate the initial phases of directional free-wave components. The effects of nonlinear wave-wave interactions among these free-wave components are calculated using a conventional second-order wave-wave interaction model and subtracted from the measurements. The final results are obtained through an iterative process of computing the free-wave components and their nonlinear interactions. Given an ocean wave field defined by multiple feed-point measurements, the method is capable of decomposing the wave field into a set of directional free-wave components. On the basis of the derived free-wave components, the wave characteristics of the wave field can be predicted in the vicinity of the measurements. The method has been applied to two sets of ocean wave pressure measurements which were collected from an array of sensors mounted on an offshore production platform near the California coast. Satisfactory agreement between the predictions based on the decomposed free-wave components and related measurements indicates the proposed method is reliable. It is expected that this method will have a variety of applications to ocean science and engineering.

The performance of the Datawell Directional Waverider and the National Data Buoy Center (NDBC) 3-m discus buoy, widely used to measure the directional properties of surface gravity waves, are evaluated through comparisons to an array of six pressure transducers mounted 14 m below the sea surface on a platform in 200-m depth. Each buoy was deployed for several months within a few kilometers of the platform. The accuracy of the platform ground-truth array was verified by close agreement of wavenumber estimates with the theoretical linear dispersion relation for surface gravity waves. Buoy and array estimates of wave energy and directional parameters, based on integration of the directional moments across the frequency band of energetic swell (0.06–0.14 Hz), are compared for a wide range of wave conditions. Wave energy and mean propagation direction estimates from both buoys agree well with the platform results. However, the Datawell buoy provides significantly better estimates of directional spread...

The platinum-group metals, ie, platinum, palladium, rhodium, ruthenium, osmium, and iridium, all have high melting points and exceptional corrosion resistance. Platinum, rhodium, and iridium are particularly stable to oxidation at high temperatures. These metals, especially platinum and palladium, show exceptional catalytic properties. In nature, platinum-group metals frequently occur in conjunction with nickel, copper, and iron sulfides. The principal producing region is South Africa, but other commercially important deposits exist in Canada and CIS. In the refining of these metals, solvent extraction technology is increasingly replacing conventional hydrometallurgical processes. Benefits include increased recovery, improved efficiency, and significantly shortened process time. Resources and recovery processes are discussed. The platinum-group metals are extensively used as catalysts. Applications include pollution control catalysts for gasoline and diesel vehicles, control of emissions of organics and nitrous oxides from industrial installations, fuel cells and process catalysts in the chemical industry, including nitric acid and chlorine production, and petroleum organic synthesis refining. Platinum-group metals are also used as jewelry, in the glass industry, in electronic components, in spark plugs, on turbine blades, and in medical devices. Modern cardiovascular surgery and anticancer chemotherapy both employ these metals.

The effects of seawater exposure on the mechanics of Mode I fracture and crack growth in unidirectional AS-4/3501 graphite/epoxy were investigated utilizing the previously developed compliant load-frame (CLF) technique. Mode I double-cantilever- beam (DCB) specimens were exposed to fresh flowing seawater under both static and fa tigue loading conditions. Long-term fatigue loading in air resulted in extensive bridging of reinforcing fibers across the crack tip, increasing the apparent fracture resistance of the material. The primary effect of seawater was to weaken the fiber-to-matrix bond in the composite, causing an increase in the number of bridged fibers. Fiber bridging increased the fracture resistance of the DCB test specimens and reduced the rate of crack growth to near zero. An increase in the fracture resistance of specimens soaked in seawater but not loaded was also attributed to a weakened fiber-to-matrix bond. The compliant load-frame (CLF) technique was found to be a simple and cost-effective method for studying the mechanics of crack growth under environmental exposure conditions. However, linear- elastic fracture mechanics (LEFM) relationships derived previously for the CLF become invalid when the fracture resistance of the material increases during crack growth, e.g., during fiber bridging.

INTRODUCTION 261 A PARTIAL HISTORY OF MAJOR SPILLS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 PREDICTING THE TRAJECTORIES OF SPILLS . . . . ....... . ... . . . . . . . . . . . . 263 WEATHERING , BIODEGRADATION. AND CHEMICAL MODIFICATION . . ... . 266 SHORT-TERM ENVIRONMENTAL EFFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 LONG-TERM ENVIRONMENTAL EFFECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 THE ROLE OF BIOREMEDIATION, DISPERSANTS, AND OTHER INTERVENTION METHODS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 277 DISCUSSION AND CONCLUSIONS 280

Model predictions of bound (i.e., nonlinearly forced by and coupled to wave groups) infragravity wave energy are compared with about 2 years of observations in 8- to 13-m depths at Imperial Beach, California, and Barbers Point, Hawaii. Frequency-directional spectra of free waves at sea and swell frequencies, estimated with a small array of four pressure sensors, are used to predict the bound wave spectra below 0.04 Hz. The predicted total bound wave energy is always less than the observed infragravity energy, and the underprediction increases with increasing water depth and especially with decreasing swell energy. At most half, and usually much less, of the observed infragravity energy is bound. Bound wave spectra are also predicted with data from a single wave gage in 183-m depth at Point Conception, California, and the assumption of unidirectional sea and swell. Even with energetic swell, less than 10% of the total observed infragravity energy in 183-m depth is bound. Free waves, either leaky or edge waves, are more energetic than bound waves at both the shallow and deep sites. The low level of infragravity energy observed in 183-m depth compared with 8- to 13-m depths, with similarly moderate sea and swell energy, suggests that leaky (and very high-mode edge) waves contribute less than 10% of the infragravity energy in 8–13 m. Most of the free infragravity energy in shallow water is refractively trapped and does not reach deep water.

A simple technique is presented for characterizing crack growth in compos ite materials subjected to a hostile environment. The method, based on linear-elastic frac ture mechanics (LEFM), retains the simplicity of constant-strain testing while greatly in creasing the observed crack growth. Load is applied using a rigid frame in series with an extension spring. The enhancement of crack propagation leads to a more accurate assess ment of long-term environmental cracking susceptibility. Crack growth rates were measured in fiberglass/epoxy double-cantilever beam (DCB) specimens to verify the theory for stress-corrosion-cracking (SCC). No SCC threshold was observed for the fiberglass/epoxy in distilled water. Applicability to corrosion-fatigue (CF) testing is dis cussed.

Renewable ocean energy sources are typically highly variable and uncontrolled, resulting in the production of low value electricity. Storing energy in the form of compressed air is a mature technology on land. Utilizing hydrostatic pressure at depth in the ocean to maintain constant pressure in the air supply chamber offers large recovery efficiency advantages. If salt dome caverns are not available, the design challenge is the development of a low cost bottom-founded air storage chamber.