Tidal Channel Research Papers - Academia.edu (original) (raw)
Strong tidal flows are observed in Cook Strait which separates the North and South Islands of New Zealand. The high velocities within the 30 km wide Strait result from a 140° phase difference in the M2 tide between the ends of the Strait.... more
Strong tidal flows are observed in Cook Strait which separates the North and South Islands of New Zealand. The high velocities within the 30 km wide Strait result from a 140° phase difference in the M2 tide between the ends of the Strait. Extraordinarily 135° of this phase difference occurs over just 40 km in the narrowest section of the Strait. Measurements from a ship mounted Acoustic Doppler Current Profiler (ADCP) over a single tidal cycle are used to determine the horizontal and vertical variation of tidal phase and amplitude in the Strait. Results show that tidal velocity amplitude ranges from 70 cm s-1 on the west of the Strait to 140 cm s-1 on the east. There was little amplitude variation over most of the water column. The eastern side of the Strait led the west by 20°. Near bottom velocity led surface velocity by approximately 10° due to the effect of bottom friction on the oscillating flow. Results from a subsequent 1 month deployment of ADCPs on the same line as the ship track are used to hindcast the semi-diurnal tide on the day of the shipboard measurements. The shipboard measured semi-diurnal tidal amplitude and phase agree extremely well with the hindcast composite of the three largest tidal constituents. Thus shipboard measurements over a single tidal cycle were able to accurately determine the horizontal and vertical variation of phase and amplitude of the semi-diurnal tide in Cook Strait
Strong tidal flows are observed in Cook Strait which separates the North and South
Recently obtained high-resolution seismic records, collected offshore the western Netherlands in a kilometer-size grid across the lower shoreface and inner shelf, suggest the presence of a dense, previously unknown network of... more
Recently obtained high-resolution seismic records, collected offshore the western Netherlands in a kilometer-size grid across the lower shoreface and inner shelf, suggest the presence of a dense, previously unknown network of tidal-channel fills in the shallow subsurface of the North Sea. Seven channel fills constituting this network were analyzed in detail to infer relationships among the fills in time and space, and to identify factors governing channel development and tidal-basin evolution. The majority of the channels are oriented perpendicular to the coast, but parts of some have a strong coast-parallel component. Shell associations and sedimentological characteristics in most channel fills are typical of back-barrier depositional environments. All but one of the channel fills are part of an Atlantic-age tidal-channel system, draining a tidal basin protected by barrier islands. One channel fill, which is correlated to an incision recognized onshore, represents a younger phase in coastal development. The presumed barrier-island chain was positioned at least 12 km seaward of the present coastline in the northern part of the study area, and at least 6 km seaward of the present coastline in the southern part. The tidal channels grew and migrated actively before partially being filled in between 7300 14C yr BP and 5500 14C yr BP, reflecting an initial tidal-prism increase followed by a substantial decrease. The tidal-prism increase was governed by an upsurge of the tidal amplitude between 8000 14C yr BP and 7000 14C yr BP, and the subsequent decrease by a gradual decrease in the rate of sea-level rise. Rapid retrogradation of the coastline put a sudden end to the existence of the tidal basin. The ensuing shoreface erosion removed all but the deepest parts of the channel fills, which do not show any evidence of systematic landward migration. This field observation corroborates recent modeling results suggesting that rapidly rising sea level may create the conditions necessary for overstepping of sandy barriers.
The dynamics of tidal flow through inlets are not fully understood; observations are scarce because of the small spatial scales over which the flow varies. This paper gives the first detailed measurements of the 2D structure of tidal... more
The dynamics of tidal flow through inlets are not fully understood; observations are scarce because of the small spatial scales over which the flow varies. This paper gives the first detailed measurements of the 2D structure of tidal currents and the dynamical terms of the momentum equation within a tidal inlet, leading to an improved understanding of the physics of tidal inlets. In the 180 cm s-1 peak flow the near-steady-state momentum balance is dominated by horizontal advection and the pressure gradient, with bottom friction playing a secondary role. At slack water, there is a balance between local acceleration and the pressure gradient. Numerical integration of the ADCP-measured terms in the momentum equation yields 60-m-resolution dynamic topography that shows a 7-cm variation at peak flood consistent with Bernoulli's equation. The surface topography because of friction forms a linear ramp with a peak irreversible head loss of 2 cm over 600 m. Tidal velocities were extracted from the ADCP measurements by extending an existing spline analysis technique. This technique is known to be sensitive to the number and location of the nodes where weights are applied to the spline. Simulations with artificial data representative of the tidally varying ADCP measurements show that, provided there are sufficient nodes to resolve the smallest spatial scale of interest, velocities predicted by the spline technique are insensitive to the number or locations of the nodes