ABSTRACT Observations of foreshore morphologic change and swash flow velocities were made at Duck... more ABSTRACT Observations of foreshore morphologic change and swash flow velocities were made at Duck, NC in an effort to estimate cross-shore sediment flux magnitudes. The three-dimensional foreshore surface over an approximately 10 x 20m study area was determined repeatedly to roughly centimeter accuracy using a stereometric video method. Sediment flux magnitudes derived from the temporal gradient of these data showed erosion rates of over 25 cm/hr. Near-bed, cross-shore swash velocities were measured at multiple cross-shore locations using a separate video technique. Swash velocities estimated using this method were found to be consistent with current measurements obtained using acoustic Doppler and ducted impeller current meters. The swash zone profile observations and velocity estimates were used to test an energetics-based total load sediment transport model. Although the trends of both the model and the observations were qualitatively consistent, the magnitudes and positioning of observed sediment fluxes did not match the transport model predictions. This discrepancy implies that other factors, such as water depth variations, infiltration, or sediment advection, may be important.
Previous investigations of the response of Plio-Pleistocene climatic records to long-term, orbita... more Previous investigations of the response of Plio-Pleistocene climatic records to long-term, orbitally induced changes in radiation have considered a linear response of climate. While the second-order statistics of power spectra and cross spectra provide necessary information on linear processes, insight into the nonlinear characteristics of Pliocene and Pleistocene climate is not provided by these statistical quantities. Second-order statistics do not contain the phase information necessary to investigate nonlinear, phase-coupled processes. Such information is provided by higher-order statistical quantities. In particular, bispectral analysis indicates that nonlinear couplings are present in the climatic (radiative) forcing at the Milankovitch frequencies. Through a linear transfer, this forcing produces similar nonlinear couplings in deep-sea sedimentary oxygen isotope records (ODP site 677 and DSDP site 607) from 1.0 to 0 Ma during the late Neogene. This analysis suggests that during the late Pleistocene, the dominance of the 100,000 year cycle in the climate record is consistent with a linear, resonant response to eccentricity forcing. In the period from 2.6 to 1.0 Ma, a change in the nature of the climatic response to orbital forcing is indicated, as phase couplings present in the isotopic time series are not similar to the phase couplings present in the insolation forcing. Third-order moments (skewness and asymmetry) are used to quantify the shape of the climatic response. From 2.6 Ma to present, an increase in the asymmetry (sawtoothness) of the oxygen isotopic records is accompanied by a corresponding decrease in the skewness (peakedness) of the records. This indicates an evolution in the nature of the phase coupling within the climate system. These results may provide important constraints useful in development of models of paleoclimate.
... REPORT NUMBER 9. SPONSORING / MONITORING AGENCY NAHES(S) AND ADDRESSES) Office of Naval Resea... more ... REPORT NUMBER 9. SPONSORING / MONITORING AGENCY NAHES(S) AND ADDRESSES) Office of Naval Research 800 North Quincy Street Arlington ... Herbers, THC, Steve Elgar, RT Guza, and WC O'Reilly, 1995 Infragravity-frequency (0.005-0.05 Hz) motions on the shelf ...
Bispectral analysis is used to compare and contrast ordered and chaotic vortex shedding from a cy... more Bispectral analysis is used to compare and contrast ordered and chaotic vortex shedding from a cylinder in low-Reynolds-number flow. Auto-bicoherence spectra of the wake velocity field for ordered shedding are characterized by sharp peaks, indicating coupling between fluid motions at the vortex shedding frequency, its side bands, and its harmonics. Wake velocities for chaotic shedding have broad power spectral peaks and substantial low-frequency energy. The corresponding auto-bicoherences show coupling between motions with a relatively wide range of frequencies, especially low frequencies.
Near-bottom currents observed on a tidal flat are compared with those observed 50 m away inside a... more Near-bottom currents observed on a tidal flat are compared with those observed 50 m away inside a shallow (0.25 to 0.40 m deep) channel. For water depths between 0.5 and 2.5 m (when both current meters are submerged), current speeds 0.13 m above the bed on the flat are about 30% greater than those observed 0.13 m above the bed in the channel, and are approximately equal to those observed 0.58 m above the channel bed (0.26 m above the flat elevation). Flow directions on the flat are similar to those in the channel. For flows parallel to the channel axis, the ratio of speeds 0.13 m above the bed on the flat to those 0.13 m above the bed in the channel decreases from about 1.4 to about 1.1 with increasing water depth, consistent with conservation of mass. For flows directed across the channel axis, the ratio of speeds increases from about 1.3 to about 2.2 with increasing water depth. The corresponding ratio of the vertical velocity variances (a proxy for turbulence) decreases from abou...
LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surfa... more LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surface-gravity waves, allowing more skillful predictions of wave fields on continental shelves and enabling the estimation of characteristics of the seafloor from wave observations. OBJECTIVES The objective of the Wave Propagation Across Muddy Seafloors project is to develop, test, and improve models for mud-induced dissipation of waves in shallow water. Specific goals are to: -Observe waves along a cross-shore transect spanning several km of the Louisiana inner shelf between about 5-and 1-m water depth, -Extend existing wave models to account for damping by mud, -Use the observations and models to test hypotheses for mud-induced damping, and -Calibrate, test, and improve the models by comparing their predictions with the observations. Additional objectives in FY07 included analysis of waves, currents, and morphological change onshore of complex shallow-water bathymetry dominated by two subm...
LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surfa... more LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surface-gravity waves, circulation, sediment transport, and the subsequent morphological response in shallow waters on sandy and muddy coasts and on macrotidal flats. OBJECTIVES The objective of our studies in FY08 is to develop, test, and improve models for mud-induced dissipation of waves in shallow water, and for circulation, sediment transport, and morphological change on macrotidal flats. Specific goals relating to waves over muddy seafloors are to: -Observe waves along a cross-shore transect spanning several km of the Louisiana inner shelf between about 5-and 1-m water depth, -Extend existing wave models to account for damping by mud, -Use the observations and models to test hypotheses for mud-induced damping, and -Calibrate, test, and improve the models by comparing their predictions with the observations. Specific goals relating to macrotidal flats are to: -Investigate the relative im...
Predictions of sediment transport models driven with observed inner-surf and swash zone velocitie... more Predictions of sediment transport models driven with observed inner-surf and swash zone velocities will be compared with each other. Classical energetics-based models (e.g., Bagnold, 1966; Bailard, 1981; Bowen, 1980) for cross-shore sediment transport predict the offshore migration of sandbars observed in the surf zone during storms. Recent studies have suggested that onshore transport and bar migration observed when mean flows are weak can be predicted by accounting for skewed orbital velocity accelerations under asymmetric (pitched-forward) waves. Flow accelerations also are expected to be important in the swash zone. Velocities were measured for 140 hours at 7 cross-shore locations between the shoreline and about 3-m water depth on the fine-grained, low-sloped Scripps Beach, CA. Bed levels were measured hourly at each sensor location, and swash zone beach profiles were surveyed daily. Offshore significant wave heights ranged from about 55 to 80 cm. In the swash zone, mean flows w...
ABSTRACT Observations of foreshore morphologic change and swash flow velocities were made at Duck... more ABSTRACT Observations of foreshore morphologic change and swash flow velocities were made at Duck, NC in an effort to estimate cross-shore sediment flux magnitudes. The three-dimensional foreshore surface over an approximately 10 x 20m study area was determined repeatedly to roughly centimeter accuracy using a stereometric video method. Sediment flux magnitudes derived from the temporal gradient of these data showed erosion rates of over 25 cm/hr. Near-bed, cross-shore swash velocities were measured at multiple cross-shore locations using a separate video technique. Swash velocities estimated using this method were found to be consistent with current measurements obtained using acoustic Doppler and ducted impeller current meters. The swash zone profile observations and velocity estimates were used to test an energetics-based total load sediment transport model. Although the trends of both the model and the observations were qualitatively consistent, the magnitudes and positioning of observed sediment fluxes did not match the transport model predictions. This discrepancy implies that other factors, such as water depth variations, infiltration, or sediment advection, may be important.
Previous investigations of the response of Plio-Pleistocene climatic records to long-term, orbita... more Previous investigations of the response of Plio-Pleistocene climatic records to long-term, orbitally induced changes in radiation have considered a linear response of climate. While the second-order statistics of power spectra and cross spectra provide necessary information on linear processes, insight into the nonlinear characteristics of Pliocene and Pleistocene climate is not provided by these statistical quantities. Second-order statistics do not contain the phase information necessary to investigate nonlinear, phase-coupled processes. Such information is provided by higher-order statistical quantities. In particular, bispectral analysis indicates that nonlinear couplings are present in the climatic (radiative) forcing at the Milankovitch frequencies. Through a linear transfer, this forcing produces similar nonlinear couplings in deep-sea sedimentary oxygen isotope records (ODP site 677 and DSDP site 607) from 1.0 to 0 Ma during the late Neogene. This analysis suggests that during the late Pleistocene, the dominance of the 100,000 year cycle in the climate record is consistent with a linear, resonant response to eccentricity forcing. In the period from 2.6 to 1.0 Ma, a change in the nature of the climatic response to orbital forcing is indicated, as phase couplings present in the isotopic time series are not similar to the phase couplings present in the insolation forcing. Third-order moments (skewness and asymmetry) are used to quantify the shape of the climatic response. From 2.6 Ma to present, an increase in the asymmetry (sawtoothness) of the oxygen isotopic records is accompanied by a corresponding decrease in the skewness (peakedness) of the records. This indicates an evolution in the nature of the phase coupling within the climate system. These results may provide important constraints useful in development of models of paleoclimate.
... REPORT NUMBER 9. SPONSORING / MONITORING AGENCY NAHES(S) AND ADDRESSES) Office of Naval Resea... more ... REPORT NUMBER 9. SPONSORING / MONITORING AGENCY NAHES(S) AND ADDRESSES) Office of Naval Research 800 North Quincy Street Arlington ... Herbers, THC, Steve Elgar, RT Guza, and WC O'Reilly, 1995 Infragravity-frequency (0.005-0.05 Hz) motions on the shelf ...
Bispectral analysis is used to compare and contrast ordered and chaotic vortex shedding from a cy... more Bispectral analysis is used to compare and contrast ordered and chaotic vortex shedding from a cylinder in low-Reynolds-number flow. Auto-bicoherence spectra of the wake velocity field for ordered shedding are characterized by sharp peaks, indicating coupling between fluid motions at the vortex shedding frequency, its side bands, and its harmonics. Wake velocities for chaotic shedding have broad power spectral peaks and substantial low-frequency energy. The corresponding auto-bicoherences show coupling between motions with a relatively wide range of frequencies, especially low frequencies.
Near-bottom currents observed on a tidal flat are compared with those observed 50 m away inside a... more Near-bottom currents observed on a tidal flat are compared with those observed 50 m away inside a shallow (0.25 to 0.40 m deep) channel. For water depths between 0.5 and 2.5 m (when both current meters are submerged), current speeds 0.13 m above the bed on the flat are about 30% greater than those observed 0.13 m above the bed in the channel, and are approximately equal to those observed 0.58 m above the channel bed (0.26 m above the flat elevation). Flow directions on the flat are similar to those in the channel. For flows parallel to the channel axis, the ratio of speeds 0.13 m above the bed on the flat to those 0.13 m above the bed in the channel decreases from about 1.4 to about 1.1 with increasing water depth, consistent with conservation of mass. For flows directed across the channel axis, the ratio of speeds increases from about 1.3 to about 2.2 with increasing water depth. The corresponding ratio of the vertical velocity variances (a proxy for turbulence) decreases from abou...
LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surfa... more LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surface-gravity waves, allowing more skillful predictions of wave fields on continental shelves and enabling the estimation of characteristics of the seafloor from wave observations. OBJECTIVES The objective of the Wave Propagation Across Muddy Seafloors project is to develop, test, and improve models for mud-induced dissipation of waves in shallow water. Specific goals are to: -Observe waves along a cross-shore transect spanning several km of the Louisiana inner shelf between about 5-and 1-m water depth, -Extend existing wave models to account for damping by mud, -Use the observations and models to test hypotheses for mud-induced damping, and -Calibrate, test, and improve the models by comparing their predictions with the observations. Additional objectives in FY07 included analysis of waves, currents, and morphological change onshore of complex shallow-water bathymetry dominated by two subm...
LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surfa... more LONG-TERM GOALS The long-term goal is to develop field-verified models for the evolution of surface-gravity waves, circulation, sediment transport, and the subsequent morphological response in shallow waters on sandy and muddy coasts and on macrotidal flats. OBJECTIVES The objective of our studies in FY08 is to develop, test, and improve models for mud-induced dissipation of waves in shallow water, and for circulation, sediment transport, and morphological change on macrotidal flats. Specific goals relating to waves over muddy seafloors are to: -Observe waves along a cross-shore transect spanning several km of the Louisiana inner shelf between about 5-and 1-m water depth, -Extend existing wave models to account for damping by mud, -Use the observations and models to test hypotheses for mud-induced damping, and -Calibrate, test, and improve the models by comparing their predictions with the observations. Specific goals relating to macrotidal flats are to: -Investigate the relative im...
Predictions of sediment transport models driven with observed inner-surf and swash zone velocitie... more Predictions of sediment transport models driven with observed inner-surf and swash zone velocities will be compared with each other. Classical energetics-based models (e.g., Bagnold, 1966; Bailard, 1981; Bowen, 1980) for cross-shore sediment transport predict the offshore migration of sandbars observed in the surf zone during storms. Recent studies have suggested that onshore transport and bar migration observed when mean flows are weak can be predicted by accounting for skewed orbital velocity accelerations under asymmetric (pitched-forward) waves. Flow accelerations also are expected to be important in the swash zone. Velocities were measured for 140 hours at 7 cross-shore locations between the shoreline and about 3-m water depth on the fine-grained, low-sloped Scripps Beach, CA. Bed levels were measured hourly at each sensor location, and swash zone beach profiles were surveyed daily. Offshore significant wave heights ranged from about 55 to 80 cm. In the swash zone, mean flows w...
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