Decrease Of Tidal Amplitude Research Articles

Holocene development of Brabrand Fjord, eastern Jylland, Denmark.

The Brabrand Sø area west of Aarhus in eastern Jylland, Denmark, was deglaciated about 18 000 to 17 000 years ago. Coring in the present-day lake area revealed Early Holocene stream deposits overlain by marine deposits. The area was transgressed by the sea at c. 8500 cal. years BP and a 12 km long, narrow fjord was formed. In the beginning, the fjord housed a species-poor marine or brackish-water fauna with molluscs Hydrobia sp. (mudsnail), Littorina littorea (winkle), Mytilus edulis (blue mussel) and Cerastoderma sp. (cockle). This phase was followed by a phase during which the fjord housed a species-rich fauna that included Ostrea edulis (European flat oyster) and Ruditapes decussatus (palourde clam). During this phase the salinity and summer water temperatures were higher than in present day Aarhus Bugt and we also see evidence for strong bottom currents. This phase was probably characterised by a fairly large tidal amplitude. Two radiocarbon ages of O. edulis shells of c. 6250 and 6700 cal. years BP indicate that such conditions peaked during the period of the Ertebølle culture. The high-salinity phase was followed by a phase with a more species-poor fauna, this phase was also characterised by a high sedimentation rate – a feature seen in other fjords in the region. We suggest that the shift could be due to a decrease in tidal amplitude. Brabrand Fjord was eventually transformed into a lake due to land uplift and closure of the connection to Aarhus Bugt due to longshore sediment transport but the timing of the transition from fjord to lake is still unknown.

Salt Marsh and Tidal Flat Area Distributions Along Three Estuaries

Estuarine landscapes form through interactions between fluvio-coastal processes and ecological processes within the boundaries imposed by hard substrate layers and man-made dikes and dams. As estuaries are ecologically valuable areas, monitoring and quantification of trends in habitats is needed for objective comparison and management. However, datasets of tidal flat and saltmarsh habitats along entire estuaries are scarce. The objective was to compare trends of biogeomorphological areas and habitat transitions along three estuaries in the Netherlands and assess whether these are generally comparable or mainly determined by system-specific histories. We present data for these estuaries obtained by automated classification of false-color aerial imagery. The automated method allows objective mapping of entire estuaries at unprecedented resolution. The estuaries are dominated by subtidal areas and tidal flats. The tidal flats have similar area along the estuaries while saltmarsh area decreases. Collective lengths of ecologically important transitions between saltmarsh, low-energy tidal flats and water differ more between the estuaries. These variations are due to presence of mid-channel bars and shore-connected embayments. Saltmarsh area is mainly determined by the different formation and embankment histories of the estuaries, embayments and side-branches. Much of the past saltmarsh flanking the estuaries was lost due to past land reclamation. In one system, ecologically important low-energy tidal flats are reduced by a sudden decrease of tidal amplitude, causing increase of subtidal area at the cost of intertidal area. Large areas of high-energy tidal flats in one estuary remain unexplained. The automated method can be applied in other estuaries, provided that high-quality areal imagery is available. Extensions of the data to other estuaries would allow for system-scale trend comparison between estuaries of ecologically relevant biogeomorphological characteristics.

Spatial and temporal variability of the velocity and hydrographic structure in a weakly stratified system, Broad Sound, Casco Bay, Maine

AbstractThe velocity and hydrographic structure across Broad Sound, a north‐south orientated subsystem of Casco Bay, ME that lacks continuous coastal boundaries, were characterized using velocity observations from two moorings in late summer/fall of 2013 and velocity and density observations from a repeat‐transect ship survey conducted over a tidal cycle during the same period. At tidal time scales, the system is dominated by a barotropic semidiurnal standing wave with a west to east decrease in tidal amplitude and relatively minimal phase change across the majority of the transect. The stratification (vertical differences of 0.5–1.0 kg m−3) was generally laterally uniform and stronger during the flood phase which is hypothesized to result from stronger offshore stratification. The mean circulation had strong lateral shear with inflow over the deepest point in the bathymetric cross section and eastern slope and outflow over the western slope. There was also vertical shearing of the horizontal velocities with stronger northward (or northward trending) velocities at depth. The depth‐averaged subtidal fluctuations were relatively small (∼2–3 cm s−1) and uncorrelated between mooring sites suggesting the vertically uniform current response associated with remote wind forcing is of limited importance. On the other hand, the depth‐dependent velocity fluctuations at the subtidal time scale were, in large part (∼36–72%), driven by wind forcing. The net flux ratio, a means of quantifying the relative importance of the vertical and lateral shear in the flow field, was typically ∼0.44 indicating the structure of the local wind response favored vertically sheared flow.

Influence of varying tidal prism on hydrodynamics and sedimentary processes in a hypertidal salt marsh creek

ABSTRACTRecent initiatives directing tidal power development in the Bay of Fundy have raised questions about far‐field environmental impacts related to energy extraction. It is understood that commercial scale tidal power installations in the Minas Passage will result in an overall decrease in tidal amplitude in the Minas Basin. Corresponding changes in sedimentation patterns may or may not be within the natural range of variability, and it is hypothesized that intertidal sedimentation rates will demonstrate a non‐linear response to modification of the tidal energy regime. This research considers acoustic Doppler velocimeter (ADV) and optical backscatter sensor (OBS) data from a sheltered tidal creek in the Minas Basin, for analysis of tidal characteristics in a hypertidal creek environment over spring and neap tidal cycles. Sediment deposition in the creek was also measured. Results show a first‐order control of topography on flow magnitude in the tidal creek, which impacts net sediment deposition through resuspension and removal of newly introduced material. This study demonstrates that tides which peak around the bankfull level show reduced early ebb stage turbulence and flow velocity and encourage an extended depositional period. The dynamics of marshfull tides may be responsible for the maximum sediment deposition in tidal creeks, providing large amounts of material that is eventually distributed to and deposited on marsh surfaces. Copyright © 2012 John Wiley & Sons, Ltd.

Increase in water exchange ratio due to a decrease in tidal amplitude in Ariake Bay, Japan

The year-to-year variations in the water exchange ratio and the tidal amplitude in Ariake Bay, Japan, were investigated using observed river discharge, salinity and tide gauge data. The water exchange ratio between Ariake Bay and the open ocean has increased from the early to the late 1990s due to the intensification of the estuarine circulation in the bay. The intensification of the estuarine circulation in Ariake Bay has resulted from a decrease in the tidal amplitude there.

埋め立てに伴う潮汐・潮流の減少とそれが物質分布に及ぼす影響

In Bays of Tokyo and Ise it is clearly shown that the emplitude and the phase lag of tide have gradually decreased for several decades. This is attributed to the remarkable reclamation of long standing in the respective bays, because the period of proper oscillation in the bay becomes smaller on account of the reduction of bay area and the response of bay water is weakened to the incidental tidal wave from the open sea. In Bay of Osaka, however, the rate of decrease of tide is rather small, because the semi-diurnal tide is low to be comparable to the diurnal tide and the tide is also influenced by the Seto Inland Sea. As for the tidal components, the decrease of amplitude is conspicuous in case of the semi-diurnal tide more than in case of the diurnal tide, corresponding to a difference between the period of them. The decrease of tidal amplitude follows the decrease of tidl current naturally, and the lowering of water exchange is inevitably anticipated between the bay and the outer sea. Then we discussed the effect of reclamation on the distribution of material based on simple theoretical considerations.