A 20th-century record of climatologically modulated sediment accumulation rates in a Canadian Fjord

The geochronology of a gravity core collected from an anoxic, high sedimentation rate (>2.5 cm/yr) setting near the head of the Saguenay Fjord in Quebec has been established for the period 1900–1979 to a resolution of better than 1 yr using a constant flux ²¹Pb model. Interannual variations of the sedimentation rate of between 28 and 50% are caused by rapid inputs of coarse silt and fine sand during spring freshet events. A significant correlation between sand percentage and sediment accumulation rate reflects the increase in stream competence during the spring freshet; the sand percentage parameter can be used in conjunction with sediment size parameters to estimate temporal variations in river discharge intensity. Between 1914 and 1979 sediment accumulation rates were about 60% higher during the spring and summer compared to fall and winter seasons. Both the magnitude of the Saguenay River spring discharge and the quantity of precipitation stored as snow decrease through the 20th century suggesting that the freshet intensity was governed largely by natural rather than anthropogenic (e.g., dam construction) factors. A direct correlation between sand flux, peak river discharge, and a snow storage parameter provides a link between sediment texture and climate. Detailed analyses of the grain-size distributions for dated core intervals offer a method for the reconstruction of a proxy record of paleodischarge for the Saguenay River spring freshets.     

Incremental dynamic analysis and FEMA P695 seismic performance evaluation of a cold-formed steel–framed building with gravity framing and architectural sheathing

The objective of this paper is to present incremental dynamic analysis (IDA) and seismic performance evaluation results for a two-story cold-formed steel (CFS)–framed building. The archetype building was designed to current U.S. standards and then subjected to full-scale shake table tests under the U.S. National Science Foundation Network for Earthquake Engineering Simulation (NEES) program. Test results showed that the building's stiffness and capacity were considerably higher than expected and the building suffered only nonstructural damage even at excitations in excess of Maximum Considered Earthquake levels for a high seismic zone. For the archetype building, three-dimensional finite element models at different modeling fidelity levels were created using OpenSees. The models are subjected to IDA using the far-field ground motion records prescribed in Federal Emergency Management Agency (FEMA) P695. Seismic performance quantification following the FEMA P695 procedure shows that if the modeling fidelity only follows the state-of-the-practice, ie, only includes shear walls, unsafe collapse margin ratios are predicted. State-of-the-art models that account for participation from CFS gravity walls and architectural sheathing have overall performance that are consistent with testing, and IDA results indicate acceptable collapse margin ratios, predicated primarily on large system overstrength. Neglecting the lateral force resistance of the gravity system and nonstructural components, as done in current design, renders a safe design in the studied archetype, but largely divorced from actual system behavior. The modeling protocols established here provide a means to analyze a future suite of CFS-framed archetype buildings for developing further insight on the seismic response modification coefficients for CFS-framed buildings.     

Computation of a storm surge at barrow,alska

Summary The rise of the sea level during a storm surge at the coast of northern Alaska is computed by a method of numerical integration originally developed for the North Sea. Based on the most likely synoptic analysis of the storm cyclone over the Arctic Ocean, pressure gradients and wind stresses, as well as their variations with time, are obtained by grid points. The respective values at the open-sea boundary of the grid are also taken as time variatables. The numerical integration (by IBM 709) yields various curves for the changes of water level at the coast which are compared with actual observations. The computed fields of water transport are critically examined in view of the general character of the cyclone and the given distribution of water depth. The agreement between observed and computed heights of the storm surge is satisfactory.

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Zusammenfassung Die Wasserstandsschwankungen während einer Sturmflut an der Nordküste Alaskas werden unter Anwendung eines ursprünglich für die Nordsee entwickelten Integrationsverfahrens berechnet. Auf Grund einer möglichst genauen synoptischen Analyse des Sturmtiefs über dem Arktischen Ozean werden Druckgradient und Windschub nach Gitterpunkten und unter Berücksichtigung der zeitlichen Änderungen ermittelt. Auch die Randbedingungen an der Peripherie des Gitters werden als Zeitveränderliche behandelt. Die numerische Integration (IBM 709) liefert verschiedene Wasserstandskurven an der Küste, die mit den beobachteten Wasserständen verglichen werden. Die berechnete Verteilung des Wassertransports wird unter Bezugnahme auf die synoptischen Eigenschaften des Sturmtiefs und die Verteilung der Meerestiefe kritisch betrachtet. Die Übereinstimmung der beobachteten und berechneten Wasserstandsschwankungen ist im allgemeinen befriedigend.

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Résumé On calcule les variations du niveau de l'eau lors d'une marée exceptionnelle sur les côtes septentrionales de l'Alaska et cela en utilisant une méthode d'intégration développée à l'origine pour la Mer du Nord. Au moyen d'une analyse synoptique aussi exacte que possible de la position du tourbillon dépressionnaire sur l'Ocean Arctique détermine le gradient de pression et la poussée horizontale du vent en un certain nombre de points en tenant compte de leur variation dans le temps. On considère également les conditions de bord à la périphérie de la trame comme étant des variables du temps. L'intégration numérique (IBM 709) fournit diverses courbes des hauteurs de l'eau le long des côtes, hauteurs que l'on confronte aux observations maréographiques faites. La répartition calculée du déplacement de l'eau est considérée de façon critique en tenant compte des propriétés synoptiques du tourbillon dépressionnaire et la répartition des profondeurs marines. La corrélation entre les fluctuations calculées et observées de la hauteur de l'eau est en général satisfaisante.

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With 9 Figures

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Contribution No. 115, Department of Atmospheric Sciences, University of Washington, Seattle. This is an adapted version of a thesis submitted by the author in partial fulfillment of the requirements for the degree of Master of Science.     

Recent sedimentation events at the head of Saguenay Fjord,Canada

Unbioturbated marine sediments containing a record of high river discharge events occur at the head of Saguenay Fjord, near Saint Fulgence, eastern Canada. Preservation of allochthonous benthonic foraminifera, recognition of occasional rapid sedimentation events associated with the spring runoff of the Saguenay River, and the unmixed character of the sediment can be used in conjunction with several dating techniques to obtain a temporal resolution of paleoriverine events on a time scale of months to years. The geologic and geochemical record of the upper 150 cm of sediment reflects (1) the anoxic character of the benthic environment, resulting from the high flux of terrigeneous material to the bottom; (2) the transport and deposition of landslide-derived sediment into the head of the fjord over several years; (3) an indication of the nature and timing of dissolution processes that have altered or totally eliminated the allochthonous calcareous microfossil assemblage from the recycled sediment; and (4) a cyclic depositional pattern that may be useful in reconstructing pateoclimatologic trends that have influenced annual river discharge of this drainage system over the past millennium.     

When plenty is not enough: an assessment of the white stumpnose (Rhabdosargus globiceps) fishery of Saldanha Bay,South Africa

White stumpnose Rhabdosargus globiceps is the main target of the linefishery in Saldanha Bay. Increased fishing pressure over the last three decades, particularly by the recreational sector, has led to concerns regarding sustainability of the local white stumpnose stock. The fishery was exceptionally productive between 2006 and 2008, with an estimated annual catch of 141.2 tonnes (t). Only 3% of boat outings surveyed were commercial boats targeting white stumpnose, yet this sector accounted for 39.3 t (31%) of the average annual catch. The recreational boat sector accounted for most of the catch (70.0 t), and the recreational shore sector the least (31.9 t). Commercial boat catch per unit effort (CPUE; 3.7 fish angler^–1 h^–1) was more than 10 times that of recreational boats (0.3 fish angler^–1 h^–1). White stumpnose catch length-frequency differed significantly (p < 0.01) between the fishing sectors, with the commercial sector retaining larger fish (34.7 cm [SD 5.9]) than the recreational boat (33.9 cm [SD 5.9]) and shore (30.4 cm [SD 5.8]) sectors. A decline in commercial CPUE (2000–2015) of approximately 40% and a concomitant severe decline (>95%) in survey data for juvenile white stumpnose CPUE (2007–2016) indicate that the current rate of exploitation is not sustainable. Recovery of the white stumpnose stock will require a decrease in fishing mortality. Possible management regulations include sector-specific effort limitations, extending the ‘no take’ marine protected area, reducing the recreational-sector bag limit to 5 fish person^–1 day^–1, implementing a commercial-sector bag limit, and increasing the minimum size limit to 30 cm TL.