Skill assessment of multiple hypoxia models in Chesapeake Bay and implications for management decisions

The Chesapeake Bay Program (CBP) has used their coupled watershed-water quality modeling system to develop a set of Total Maximum Daily Loads (TMDLs) for nutrients and sediment in an effort to reduce eutrophication impacts which include decreasing the seasonal occurrence of hypoxia within the Bay. The CBP is now considering the use of a multiple model approach to enhance the confidence in their model projections and to better define uncertainty. This study statistically compares the CBP regulatory model with multiple implementations of the Regional Ocean Modeling System (ROMS) in terms of skill in reproducing monthly profiles of hydrodynamics, nutrients, chlorophyll and dissolved oxygen at ~30 stations throughout the Bay. Preliminary results show that although all the models substantially underestimate stratification throughout the Bay, they all have significant skill in reproducing the mean and seasonal variability of bottom dissolved oxygen. This study demonstrates that multiple community models can be used together to provide independent confidence bounds for management decisions based on CBP model results

Evaluating Confidence in the Impact of Regulatory Nutrient Reduction on Chesapeake Bay Water Quality

Excess nutrients derived from anthropogenic activity have resulted in the degradation of coastal water quality and an increase in low-oxygen and hypoxic events worldwide. In an effort to curb these impacts and restore water quality in the Chesapeake Bay, a maximum load of nutrients has been established based on a framework of regulatory standards and models. This research aims to evaluate the projected changes in water quality resulting from the implementation of these nutrient reductions by applying the regulatory methodology to two different models that have been previously shown to have similar model skill. Results demonstrate that although the two models differ structurally and produce a different degree of absolute change, they project a similar relative improvement in water quality along the main stem of the Chesapeake Bay and the lower reaches of the tributaries. Furthermore, the models largely agree on the attainment of regulatory water quality standards as a result of nutrient reduction, while also establishing that meeting water quality standards is relatively independent of hydrologic (wet/dry) conditions. By developing a Similarity Index that compares model results across habitat, time, and methodology, this research identifies the locations and causes of greatest uncertainty in modeled projections of water quality. Although there are specific locations and times where the models disagree, overall this research lends support and increased confidence to the appropriateness of the nutrient reduction levels and in the general impact of nutrient reduction on Chesapeake Bay water quality under current environmental conditions

Associated dataset: The competing impacts of climate change and nutrient reductions on dissolved oxygen in Chesapeake Bay

This research uses an estuarine-watershed hydrodynamic–biogeochemical modeling system along with projected mid-21st-century changes in temperature, freshwater flow, and sea level rise to explore the impact climate change may have on future Chesapeake Bay dissolved-oxygen (DO) concentrations and the potential success of nutrient reductions in attaining mandated estuarine water quality improvements

Introduction

https://scholarworks.wm.edu/vimsbooks/1035/thumbnail.jp

Binary black hole spacetimes with a helical Killing vector

Binary black hole spacetimes with a helical Killing vector, which are discussed as an approximation for the early stage of a binary system, are studied in a projection formalism. In this setting the four dimensional Einstein equations are equivalent to a three dimensional gravitational theory with a $SL(2,\mathbb{C})/SO(1,1)$ sigma model as the material source. The sigma model is determined by a complex Ernst equation. 2+1 decompositions of the 3-metric are used to establish the field equations on the orbit space of the Killing vector. The two Killing horizons of spherical topology which characterize the black holes, the cylinder of light where the Killing vector changes from timelike to spacelike, and infinity are singular points of the equations. The horizon and the light cylinder are shown to be regular singularities, i.e. the metric functions can be expanded in a formal power series in the vicinity. The behavior of the metric at spatial infinity is studied in terms of formal series solutions to the linearized Einstein equations. It is shown that the spacetime is not asymptotically flat in the strong sense to have a smooth null infinity under the assumption that the metric tends asymptotically to the Minkowski metric. In this case the metric functions have an oscillatory behavior in the radial coordinate in a non-axisymmetric setting, the asymptotic multipoles are not defined. The asymptotic behavior of the Weyl tensor near infinity shows that there is no smooth null infinity.Comment: to be published in Phys. Rev. D, minor correction

Cross-shoreface Suspended Sediment Transport : A Response to the Interaction of Nearshore and Shelf Processes, Fall 1994 Duck, NC Field Experiment

Deployment : The tripods were assembled, tested and secured onboard the RIV Sea Diver, which left the Little Creek Amphibious Base in Norfolk, Virginia early on 26 September. While underway to the deployment site, the continuous surface water conductivity and temperature survey was run and several CTD casts were made. The tripods were deployed on 26 September and secured to the sea floor with sand anchors by VIMS divers. The R/V Sea Diver then began the series of on/off shore transects at the tripod deployment site for approximately 12 hours. The vessel returned to port on 27 September . Recovery: The tripod recovery cruise began late on 21 October and repeated the underway data collection scheme of the September cruise. On October 22 divers removed the sand anchors and the tripods were recovered without incident. The on/off shore transects were repeated and transit to port occurred on 23 October

Numerical modeling of gravity-driven sediment transport and deposition on an energetic continental shelf: Eel River, northern California

A two-dimensional numerical model was applied to predict large-scale deposition by wave-supported sediment gravity flows on the Eel River continental shelf for four consecutive flood seasons using measured bathymetry, waves and river forcing. The model assumes that sediment-induced stratification maintains the near-bed Richardson number at its critical value, which determines the sediment carrying capacity of the wave boundary layer. Deposition is predicted when the gravity-driven flux of sediment exceeds the carrying capacity. The model predicted 26% of fine sediment discharged by the Eel River to be deposited on the midshelf with a magnitude and distribution largely consistent with field observations. Greatest deposition on the midshelf was predicted well north of the river mouth despite greater sediment input nearest the river mouth. Model results indicate that when the river delivers sufficient sediment to critically stratify the wave boundary layer, wave intensity and the bathymetry of the Eel shelf are the dominant factors controlling the observed pattern of deposition. Large wave energy caused the majority of fine sediment (65%) to escape the shelf as gravity-driven flows. The greatest amount of sediment was predicted to leave the shelf from the region off-shelf of the river mouth (including 11% into the Eel Canyon) where inshore sediment input was high and the concave downward bathymetry associated with the Eel River subaqueous delta prevents significant midshelf gravity-driven deposition