Modeling waves and longshore transport potential in Half Moon Bay, Grays Harbor, Washington, USA

A local-scale phase-resolving wave transformation model with CGWAVE is established in connection with a regional-scale coupled STWAVE-ADCIRC wave-current model for its application in the Half Moon Bay, Grays Harbor. Wave transformation from offshore to the harbor entrance is simulated by the STWAVE model which includes wave-current interaction. The STWAVE results provide incident wave conditions for the local-scale CGWAVE model at its outer boundary. A simple method is developed to take into ac- count the lateral variation of wave height in constructing the model's wave boundary conditions. The model was validated for three wave condition cases which yielded good agreement with field data. The validated model was applied to predicting nearshore waves in the Half Moon Bay and longshore transport parameters along the wave breaking line for the existing condition and three engi- neering alternatives. A comparative analysis indicated that storm waves that have a combination of long period and large height are the most destructive to the crenulate shoreline in the Half Moon Bay; both 152 m jetty extension (Alt. 2) and diffraction mound enlargement ( Alt. 3) would significantly reduce breaking wave height and longshore transport potential in the southwest comer of Half Moon Bay.     

Terrestrial-marine transition in the Cambrian rocks of Kangaroo Island, South Australia

The upper part of the Lower Cambrian succession in northeast Kangaroo Island comprises three interbedded facies associations. The fine-grained association is composed of siltstone, mudstone and minor sandstone. It contains flat lamination and abundant ripple cross-lamination which shows bipolar palaeocurrents, and occurs in combinations of flaser bedding, lenticular bedding and wavy lamination. Although body fossils are relatively rare, trilobite traces and desiccation cracks are common, and the association is interpreted as a predominantly subtidal to intertidal deposit. The conglomerate facies association contains horizontally bedded cobble to boulder conglomerate, with subordinate trough cross-stratified coarse sandstone to granule/pebble conglomerate. Fabrics and structures in the coarse conglomerates are consistent with alluvial transport (stream and debris flow), but not beach deposition. The conglomerate association is attributed to tectonic uplift and erosion of a Precambrian-Lower Cambrian succession developed adjacent to the present north coast of Kangaroo Island. Southward progradation of an alluvial fan complex occurred across east-west oriented tidal flats on which limited wave activity reworked sand and fine gravel, but not coarser material. The sandstone facies association mainly comprises trough cross-stratified and plane-laminated sandstone, the latter with current lineation predominantly sub-parallel to the east-west shoreline. Trough cross-stratification is ascribed to onshore waves and longshore currents, and current lineation to predominantly shore-parallel tidal currents, augmented by longshore drift and storm surge. Tectonic movements gave rise to cycles of transgression and regression as tidal and alluvial processes dominated alternately.     

Petrologic Evolution of Anorthoclase Phonolite Lavas at Mount Erebus, Ross Island, Antarctica

Mount Erebus, Ross Island, Antarctica, is an active, intraplate,alkaline volcano. The strongly undersaturated sodic lavas rangefrom basanite to anorthoclase phonolite, and are termed theErebus lineage (EL). The lavas are porphyritic with olivine(Fo₈₈–51), clinopyroxene (Wo₄₅–53En₃₆–41Fs₈–30),opaque oxides (Usp₃₁–76), feldspar (An₇₂–11), andapatite. Rare earth element (REE) contents increase only slightlywith increasing differentiation compared with other incompatibleelements. The light REE are enriched (La_N/Yb_N= 14–20)and there are no significant Eu anomalies. ⁸⁷Sr/⁸⁶Sr is uniformand low ({small tilde} 0.7030) throughout the EL, suggestingderivation of the basanites from a depleted asthenospheric mantlesource, and lack of significant crustal contamination duringfractionation of the basanite. Regular geochemical trends indicatethat the EL evolved from the basanites by fractional crystallization.Major element mass balance calculations and trace element modelsshow that fractionation of 16% olivine, 52% clinopyroxene, 14%Fe-Ti oxides, 11% feldspar, 3% nepheline, and 3% apatite froma basanite parent leaves 23.5% anorthoclase phonolite. Minor volumes of less undersaturated, more iron-rich benmoreite,phonolite, and trachyte are termed the enriched iron series(EFS). The trachytes have ⁸⁷Sr/⁸⁶Sr of 0.704, higher than otherEFS and EL rocks, and they probably evolved by a combined assimilation-fractionalcrystallization process. The large volume of phonolite at Mt. Erebus requires significantbasanite production. This occurs by low degrees of partial meltingin a mantle plume (here termed the Erebus plume) rising at arate of about 6 cm/yr.     

New interpretations of Great Salt Lake ooids and of ancient non-skeletal carbonate mineralogy

Earlier interpretations of textural alteration affecting Great Salt Lake ooids have greatly influenced concepts of ooid diagenesis. Scanning electron microscope study shows, however, that the coarse radial aragonite rays are depositional, that no recrystallization of pellet cores has occurred, and that Great Salt Lake ooids have not suffered noticeable diagenesis. As suggested by Kahle (1974), radial texture in ancient calcitic ooids is probably mainly original, not diagenetic. Retention of such fine textures has been attributed to organic matter (since found to be equivalent in modern skeletal and non-skeletal grains) or to paramorphic replacement (proposed for non-skeletal grains whose original aragonite mineralogy was only inferred from modern analogs). Pleistocene ooids known to have been aragonite alter like aragonite shells to coarse neomorphic calcite, often with aragonite relics. The striking uniformity of that coarse texture in neomorphic calcite replacing known skeletal aragonites throughout the geologic record has been noted for over 100 years. In contrast, Mississippian ooids retain fine texture as do calcite layers of coexisting gastropods, but unlike the strongly altered aragonite layers of these same gastropods. Therefore, inferences of original aragonitic mineralogy of ancient non-skeletal carbonate grains (including muds) which are now calcite but retain fine texture appear unwarranted, as do assumptions of differential diagenetic behaviour of ancient aragonitic skeletal and non-skeletal grains. Accordingly, modern depositional environments of marine ooids and carbonate muds must be rejected as chemically unrepresentative of comparable ancient environments. It is inferred that ancient non-skeletal carbonates were originally predominantly or exclusively calcite because of an earlier lower oceanic Mg/Ca ratio (<2/1) which altered progressively to values favouring aragonite (modern Mg/Ca value = 5/1). Major influencing factors are: selective removal of calcium by planktonic foraminifers and coccolithophorids since Jurassic-Cretaceous time and by abundant younger, Mg-poor aragonite skeletons and an erratic trend toward decreasing dolomite formation (decreasing removal of oceanic Mg). The change to aragonite dominance over calcite for non-skeletal carbonates was probably during early to middle Cenozoic time.     

Risk assessment and drought management in the Thames basin

Abstract

A risk assessment procedure is described for use in managing a system of pumped-storage reservoirs in the Thames basin during a drought. Historical daily rainfall sequences are used as equi-probable scenarios of future rainfall. These are transformed to flow, reservoir level, and demand restriction sequences through the use of rainfall-runoff and water resource system models. The risk assessment information required is then obtained through a statistical analysis of these sequences. A novel technique is presented for incorporating monthly rainfall forecasts, presented as probabilities of rainfall being above average, average, or below average, into the risk assessment scheme. Information on current hydrological conditions is incorporated in the procedure through the use of recently observed natural flows to adjust the internal state variables of a conceptual rainfall-runoff model to achieve agreement between observed and model flow. The overall procedure is accommodated within a decision support system for drought management which is implemented on a microcomputer and makes extensive use of interactive menus, forms and colour graphic displays. A key feature of the system is the maintenance of an up-to-date archive of hydrometric data which is achieved through a real-time communication link with a second computer dedicated to real-time data acquisition via telemetry. Monitoring the reliability of the water resource system during droughts is made a quick and easy task, and the effect of a change in the operating policy on system reliability can be readily assessed. The information obtained provides valuable support for tactical decision-making within the overall long-term operating strategy.