Nidzieko
Nicholas J.
Nidzieko
Nicholas J.
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ArticleTidal asymmetry in estuaries with mixed semidiurnal/diurnal tides(American Geophysical Union, 2010-08-05) Nidzieko, Nicholas J.Tidal asymmetry in estuaries with mixed, mainly semidiurnal tides arises from both the interaction of principal tides and the higher harmonics generated by distortions within the estuary. The duration asymmetry in rise and fall of water level caused by principal tides on the west coast of the continental United States is ebb-dominant, and so the tide entering estuaries is also ebb-dominant, prior to any internal distortions within the estuary. The interaction of higher harmonics with principal constituents either augments or cancels the duration asymmetry in the principal tides. In estuaries where tidal elevation and velocity phase are near quadrature (90° out of phase), the duration asymmetry in tidal elevation leads to asymmetries in tidal current magnitude. Asymmetry can be conveniently quantified in terms of the sample skewness, γ1, the normalized third sample moment about the mean. An analytic approximation to the skewness shows that traditional metrics of asymmetry, namely the ratio of constituent amplitudes and the relative constituent phase difference, arise from calculating the third sample moment. Observations from three California estuaries of different morphologies are presented as an illustration of how skewness can be used to quantify asymmetry in real systems. As in semidiurnal systems, morphology is a good predictor of whether higher harmonics engender ebb-dominance or flood-dominance, however asymmetry imposed by principal tides at the mouth must first be overcome and so there is a spatial evolution in the total asymmetry. Quantifying observations via skewness should be considered in addition to traditional metrics in estuaries with mixed tides.
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ArticleILTER - the International Long-Term Ecological Research Network as a platform for global coastal and ocean observation(Frontiers Media, 2019-08-28) Muelbert, Jose H. ; Nidzieko, Nicholas J. ; Acosta, Alicia T. R. ; Beaulieu, Stace E. ; Bernardino, Angelo F. ; Boikova, Elmira ; Bornman, Thomas G. ; Cataletto, Bruno ; Deneudt, Klaas ; Eliason, Erika ; Kraberg, Alexandra ; Nakaoka, Masahiro ; Pugnetti, Alessandra ; Ragueneau, Olivier ; Scharfe, Mirco ; Soltwedel, Thomas ; Sosik, Heidi M. ; Stanisci, Angela ; Stefanova, Kremena ; Stéphan, Pierre ; Stier, Adrian ; Wikner, Johan ; Zingone, AdrianaUnderstanding the threats to global biodiversity and ecosystem services posed by human impacts on coastal and marine environments requires the establishment and maintenance of ecological observatories that integrate the biological, physical, geological, and biogeochemical aspects of ecosystems. This is crucial to provide scientists and stakeholders with the support and knowledge necessary to quantify environmental change and its impact on the sustainable use of the seas and coasts. In this paper, we explore the potential for the coastal and marine components of the International Long-Term Ecological Research Network (ILTER) to fill this need for integrated global observation, and highlight how ecological observations are necessary to address the challenges posed by climate change and evolving human needs and stressors within the coastal zone. The ILTER is a global network encompassing 44 countries and 700 research sites in a variety of ecosystems across the planet, more than 100 of which are located in coastal and marine environments (ILTER-CMS). While most of the ILTER-CMS were established after the year 2000, in some cases they date back to the early 1900s. At ILTER sites, a broad variety of abiotic and biotic variables are measured, which may feed into other global initiatives. The ILTER community has produced tools to harmonize and compare measurements and methods, allowing for data integration workflows and analyses between and within individual ILTER sites. After a brief historical overview of ILTER, with emphasis on the marine component, we analyze the potential contribution of the ILTER-CMS to global coastal and ocean observation, adopting the “Strength, Weakness, Opportunity and Threats (SWOT)” approach. We also identify ways in which the in situ parameters collected at ILTER sites currently fit within the Essential Ocean Variables framework (as proposed by the Framework for Ocean Observation recommendations) and provide insights on the use of new technology in long-term studies. Final recommendations point at the need to further develop observational activities at LTER sites and improve coordination among them and with external related initiatives in order to maximize their exploitation and address present and future challenges in ocean observations.
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ArticleThe impact of wind forcing on the thermal wind shear of a river plume(American Geophysical Union, 2019-10-31) Mazzini, Piero L. F. ; Chant, Robert J. ; Scully, Malcolm E. ; Wilkin, John L. ; Hunter, Elias J. ; Nidzieko, Nicholas J.A 38-day long time series obtained using a combination of moored Wirewalkers equipped with conductivity-temperature-depth profilers and bottom-mounted and subsurface acoustic Doppler current profilers provided detailed high-resolution observations that resolved near-surface velocity and vertical and cross-shelf density gradients of the Chesapeake Bay plume far field. This unprecedented data set allowed for a detailed investigation of the impact of wind forcing on the thermal wind shear of a river plume. Our results showed that thermal wind balance was a valid approximation for the cross-shelf momentum balance over the entire water column during weak winds (|𝜏w 𝑦 | < 0.075 Pa), and it was also valid within the interior during moderate downwelling (−0.125< 𝜏w 𝑦 < −0.075 Pa). Stronger wind conditions, however, resulted in the breakdown of the thermal wind balance in the Chesapeake Bay plume, with thermal wind shear overestimating the observed shear during downwelling and underestimating during upwelling conditions. A momentum budget analysis suggests that viscous stresses from wind-generated turbulence are mainly responsible for the generation of ageostrophic shear.
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ArticleInferring tidal wetland stability from channel sediment fluxes : observations and a conceptual model(John Wiley & Sons, 2013-10-07) Ganju, Neil K. ; Nidzieko, Nicholas J. ; Kirwan, Matthew L.Anthropogenic and climatic forces have modified the geomorphology of tidal wetlands over a range of timescales. Changes in land use, sediment supply, river flow, storminess, and sea level alter the layout of tidal channels, intertidal flats, and marsh plains; these elements define wetland complexes. Diagnostically, measurements of net sediment fluxes through tidal channels are high-temporal resolution, spatially integrated quantities that indicate (1) whether a complex is stable over seasonal timescales and (2) what mechanisms are leading to that state. We estimated sediment fluxes through tidal channels draining wetland complexes on the Blackwater and Transquaking Rivers, Maryland, USA. While the Blackwater complex has experienced decades of degradation and been largely converted to open water, the Transquaking complex has persisted as an expansive, vegetated marsh. The measured net export at the Blackwater complex (1.0 kg/s or 0.56 kg/m2/yr over the landward marsh area) was caused by northwesterly winds, which exported water and sediment on the subtidal timescale; tidally forced net fluxes were weak and precluded landward transport of suspended sediment from potential seaward sources. Though wind forcing also exported sediment at the Transquaking complex, strong tidal forcing and proximity to a turbidity maximum led to an import of sediment (0.031 kg/s or 0.70 kg/m2/yr). This resulted in a spatially averaged accretion of 3.9 mm/yr, equaling the regional relative sea level rise. Our results suggest that in areas where seaward sediment supply is dominant, seaward wetlands may be more capable of withstanding sea level rise over the short term than landward wetlands. We propose a conceptual model to determine a complex's tendency toward stability or instability based on sediment source, wetland channel location, and transport mechanisms. Wetlands with a reliable portfolio of sources and transport mechanisms appear better suited to offset natural and anthropogenic loss.
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ArticleSimilarity scaling of turbulence spectra and cospectra in a shallow tidal flow(American Geophysical Union, 2011-10-15) Walter, Ryan K. ; Nidzieko, Nicholas J. ; Monismith, Stephen G.Measured turbulence power spectra, cospectra, and ogive curves from a shallow tidal flow were scaled using Monin-Obukhov similarity theory to test the applicability to a generic tidal flow of universal curves found from a uniform, neutrally stable atmospheric boundary layer (ABL). While curves from individual 10 min data bursts deviate significantly from similarity theory, averages over large numbers of sufficiently energetic bursts follow the general shape. However, there are several differences: (1) Variance in the measured curves was shifted toward higher frequencies, (2) at low frequencies, velocity spectra were significantly more energetic than theory while cospectra were weaker, and (3) spectral ratios of momentum flux normalized by turbulent kinetic energy (TKE) indicate decreased fluxes and/or elevated TKE levels. Several features of the turbulence structure may explain these differences. First, turbulent dissipation exceeded production, indicating nonequilibrium turbulence, possibly from advection of TKE. Indeed, using the production rate rather than dissipation markedly improves agreement in the inertial subrange. Second, spectral lag of the largest eddies due to inhomogeneous boundary conditions and decaying turbulence could explain spectral deviations from theory at low frequencies. Finally, since the largest eddies dominate momentum transfer, the consequence of the cospectra difference is that calculated ogive curves produced smaller total momentum fluxes compared to theory, partly because of countergradient fluxes. While ABL similarity scaling applied to marine bottom boundary layers (MBBLs) will produce curves with the general shape of the universal curves, care should be taken in determining details of turbulent energy and stress estimates, particularly in shallow and inhomogeneous MBBLs.
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ArticleTurbulent mixing in a far‐field plume during the transition to upwelling conditions : microstructure observations from an AUV(John Wiley & Sons, 2018-09-23) Fisher, Alexander W. ; Nidzieko, Nicholas J. ; Scully, Malcolm E. ; Chant, Robert J. ; Hunter, Elias J. ; Mazzini, Piero L. F.A REMUS 600 autonomous underwater vehicle was used to measure turbulent mixing within the far‐field Chesapeake Bay plume during the transition to upwelling. Prior to the onset of upwelling, the plume was mixed by a combination of energetic downwelling winds and bottom‐generated shear resulting in a two‐layer plume structure. Estimates of turbulent dissipation and buoyancy flux from a nose‐mounted microstructure system indicate that scalar exchange within the plume was patchy and transient, with direct wind mixing constrained to the near surface by stratification within the plume. Changing wind and tide conditions contributed to temporal variability. Following the separation of the upper plume from the coast, alongshore shear became a significant driver of mixing on the shoreward edge of the plume.
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PreprintEffects of estuarine and fluvial processes on sediment transport over deltaic tidal flats( 2012-01-25) Ralston, David K. ; Geyer, W. Rockwell ; Traykovski, Peter A. ; Nidzieko, Nicholas J.Tidal flats at a river mouth feature estuarine and fluvial processes that distinguish them from tidal flats without river discharge. We combine field observations and a numerical model to investigate hydrodynamics and sediment transport on deltaic tidal flats at the mouth of the Skagit River, in Puget Sound, WA during the spring freshet. River discharge over tidal flats supplies a mean volume flux, freshwater buoyancy, and suspended sediment. Despite the shallow water depths, strong horizontal density fronts and stratification develop, resulting in a baroclinic pressure gradient and tidal variability in stratification that favor flood-directed bottom stresses. In addition to these estuarine processes, the river discharge during periods of low tide drains through a network of distributary channels on the exposed tidal flats, with strongly ebb-directed stresses. The net sediment transport depends on the balance between estuarine and fluvial processes, and is modulated on a spring-neap time scale by the tides of Puget Sound. We find that the baroclinic pressure gradient and periodic stratification enhance trapping of sediment delivered by the river on the tidal flats, particularly during neap tides, and that sediment trapping also depends on settling and scour lags, particularly for finer particles. The primary means of moving sediment off of the tidal flats are the high velocities and stresses in the distributary channels during late stages of ebbs and around low tides, with sediment export predominantly occurring during spring low tides that expose a greater portion of the flats. The 3-d finite volume numerical model was evaluated against observations and had good skill overall, particularly for velocity and salinity. The model performed poorly at simulating the shallow flows around low tides as the flats drained and river discharge was confined to distributary channels, due in part to limitations in grid resolution, seabed sediment and bathymetric data, and the wetting-and-drying scheme. Consequently, the model predicted greater sediment retention on the flats than was observed.