David G. Aubrey

Non-linear tidal distortion in shallow well-mixed estuaries: a synthesis☆

Abstract The importance of asymmetric tidal cycles in the transport and accumulation of sediment in shallow well-mixed estuaries is well established. Along the U.S. Atlantic Coast, tidal amplitude, bottom friction, and system geometry determine tidal distortion as documented at 54 tide gauges in 26 tidally dominated estuaries of varying geometry having negligible freshwater inflow. Analyses of sea-surface heights are compared to the results of one-dimensional numerical modelling to clarify the physics of tidal response in well-mixed estuaries. Concise measurements of estuarine geometry and ocean tidal range are used to predict consistently the nature of tidal sea-surface distortion. Numerical modelling then is utilized to extend theoretical and observational relationships between geometry and sea-height to predict trends in velocity distortion and near-bed sediment transport. Non-linear tidal distortion is a composite of two principal effects: (1) frictional interaction between the tide and channel bottoms (reflected in a h = tidal amplitude channel depth ) causes relatively shorter floods; (2) intertidal storage (measured by V s V c = volume of intertidal storage volume of channels at mean sea level) causes relatively shorter ebbs. Variations in V s V c and a h trigger consistent and predictable changes in tidal distortion as measured through the first harmonic of the principal tidal constitutent.

Transport of groundwater-borne nutrients from watersheds and their effects on coastal waters

Anthropogenic activities on coastal watersheds increase nutrient concentrations of groundwater. As groundwater travels downslope it transports these nutrients toward the adjoining coastal water. The resulting nutrient loading rates can be significant because nutrient concentrations in coastal groundwaters may be several orders of magnitude greater than those of receiving coastal waters. Groundwater-borne nutrients are most subject to active biogeochemical transformations as they course through the upper 1 m or so of bottom sediments. There conditions favor anaerobic processes such as denitrification, as well as other mechanisms that either sequester or release nutrients. The relative importance of advective vs. regenerative pathways of nutrient supply may result in widely different rates of release of nutrients from sediments. The relative activity of denitrifiers also may alter the ratio of N to P released to overlying waters, and hence affect which nutrient limits growth of producers. The consequences of nutrient (particularly nitrate) loading include somewhat elevated nutrient concentrations in the watercolumn, increased growth of macroalgae and phytoplankton, reduction of seagrass beds, and reductions of the associated fauna. The decline in animals occurs because of habitat changes and because of the increased frequency of anoxic events prompted by the characteristically high respiration rates found in enriched waters.

A study of non-linear tidal propagation in shallow inlet/estuarine systems Part II: Theory☆

Abstract The generation of tidal asymmetries is clarified via numerical integration of the one-dimensional equations for channel geometries characteristic of shallow estuaries. Channels without tidal flats develop a time asymmetry characterized by a longer falling than rising tide. This behavior is enhanced by strong friction and large channel cross-sectional area variability over a tidal cycle. Resulting tidal currents have a shorter, intense flood and a longer, weak ebb (flood-dominant). Addition of tidal flats to the channels can produce a longer rising tide and stronger ebb currents (ebb-dominant), if the area of tidal flats is large enough to overcome the effects of time-variable channel geometry. Weaker friction with flats can also produce this asymmetry. Despite the physical complexity of these systems, essential features of estuarine tidal response can be recovered from one-dimensional models. Shallow estuaries are shown to have a system response leading to stable, uniform senses of tidal asymmetry (either flood- or ebb-dominated, due to phase-locking of forced tidal constituents), with down-channel development in magnitude of asymmetry. These concepts are illustrated by modeling idealized representations of tidal channels at Nauset Inlet, MA, and Wachapreague Inlet, VA, which have flood- and ebb-dominance, respectively.

Tidal propagation in strongly convergent channels

Simple first- and second-order analytic solutions, which diverge markedly from classical views of cooscillating tides, are derived for tidal propagation in strongly convergent channels. Theoretical predictions compare well with observations from typical examples of shallow, “funnel-shaped” tidal estuaries. A scaling of the governing equations appropriate to these channels indicates that at first order, gradients in cross-sectional area dominate velocity gradients in the continuity equation and the friction term dominates acceleration in the momentum equation. Finite amplitude effects, velocity gradients due to wave propagation, and local acceleration enter the equations at second order. Applying this scaling, the first-order governing equation becomes a first-order wave equation, which is inconsistent with the presence of a reflected wave. The solution is of constant amplitude and has a phase speed near the frictionless wave speed, like a classical progressive wave, yet velocity leads elevation by 90°, like a classical standing wave. The second-order solution at the dominant frequency is also a unidirectional wave; however, its amplitude is exponentially modulated. If inertia is finite and convergence is strong, amplitude increases along channel, whereas if inertia is weak and convergence is limited, amplitude decays. Compact solutions for second-order tidal harmonics quantify the partially canceling effects of (1) time variations in channel depth, which slow the propagation of low water, and (2) time variations in channel width, which slow the propagation of high water. Finally, it is suggested that phase speed, along-channel amplitude growth, and tidal harmonics in strongly convergent channels are all linked by morphodynamic feedback.

A study of non-linear tidal propagation in shallow inlet/estuarine systems Part I: Observations☆

The offshore tide becomes strongly distorted as it propagates into shallow estuarine systems. Observations of sea surface elevation and horizontal currents over periods ranging from three days to one year, at nine stations within Nauset inlet/estuary, document the non-linear interaction of the off-shore equilibrium tidal constituents. Despite strong frictional attenuation within the estuary, the overtides and compound tides of M2, S2 and N2, in particular, reach significant amplitude, resulting in strong tidal distortion. High frequency forced constituents in sea surface are phase-locked, consistently leading the forcing tides by 60–70°, resulting in a persistent distortion where falling tide is longer than rising tide. Forced constituents in currents are more nearly in phase with equilibrium constituents, producing flood currents which are shorter but more intense than ebb currents. A compound fortnightly tide, MSf, modulates the mean water level such that lowest tides occur during neap phase instead of spring phase. This fortnightly tide can be contaminated by storm surge, changing the phase characteristics of this constituent. Implications of the overtides, compound tides, and lower frequency tides on near-bed, suspended and dissolved material transport are profound.

Coastal neo-tectonics of the Mediterranean from tide-gauge records

Abstract Records from tide gauges in Israel and Egypt supplement the many geological and archeological investigations that have contributed information about relative sea-level changes in the Mediterranean region. Seven such records reveal changes during the past few decades that accord with prior inferences about land movements in this region (emergence along the coast of Israel and at Alexandria and subsidence at the Nile Delta and the head of the Gulf of Suez). Twenty-four other tide-gauge records for the rest of the Mediterranean region indicate more uniformity (submergence of land or rise of sea level) in the west, but with greater movements of the land attributed to probable plate underthrusting in Turkey and Greece, to volcanism near Mount Etna, to deltaic compaction at Izmir, and to deltaic compaction coupled with water pumping at the Po Delta.

Contamination by organochlorine compounds in sturgeons from Caspian Sea during 2001 and 2002.

Organochlorine compounds (OCs) were determined in the five species of sturgeons collected from coastal waters of Caspian Sea in Kazakhstan, Azerbaijan, Turkmenistan and Iran during 2001 and 2002 to understand their status of contamination and accumulation features. Among OCs examined, concentrations of DDTs (DDT and its metabolites) were predominant in all the sturgeon samples with concentrations ranging from 73 to 31,000 ng/g on lipid weight basis, followed by PCBs, CHLs, HCHs, HCB, dieldrin, TCPMOH, and heptachlor epoxide in order. The concentrations of OCs in beluga (Huso huso) were the highest among all the five species. When comparing residue levels among same species, OC residues were highest in sturgeons from Azerbaijan and Kazakhstan and lowest in Turkmenistan. However, the concentrations of HCHs, dieldrin, and heptachlor epoxide in sturgeons showed less geographical variability.

Changing coastal levels of South America and the Caribbean region from tide-gauge records

Abstract Tide-gauge records from southern Mexico, the Caribbean Islands, and Central and South America that span the interval 1940–1970 reveal long-term changes of relative sea level according to regression analysis and eigenanalysis. The results indicate such large variations in both direction and rate of secular movement as to rule out changes in volume of ocean water as being more than a subordinate factor. The only satisfactory explanation is that the land level beneath the tide gauges is rising in some places and sinking in others. Complex spatial patterns of relative seal-level change in southern Mexico and the Caribbean mirror the tectonic regime of these regions, exhibiting both submergence and emergence of the land. Central American tide-gauge records similarly show considerable complexity, responding to relative movement along plate boundaries. The Pacific coast of South America appears to correlate with the depth of the Benioff zone; subduction of aseismic ridges produces local highs in the Benioff zone, flanked by troughs at either side. Near the Benioff highs, relative land level is rising; between these ridges relative land level is falling. Sea-level trends in southern and Atlantic coasts of South America are closely linked with continental crustal rifting and subsidence. Data do not allow unambiguous separation of changes in ocean level from changes in land level, and no simple eustatic ocean level change can be estimated accurately from these data.

RAPID FORMATION AND DEGRADATION OF BARRIER SPITS IN AREAS WITH LOW RATES OF LITTORAL DRIFT

Abstract A small barrier beach exposed to low-energy waves and a small tidal range (0.7 m) along Nantucket Sound, Mass., has experienced a remarkable growth phase followed by rapid attrition during the past century. In a region of low longshore-transport rates, the barrier spit elongated approximately 1.5 km from 1844 to 1954, developing beyond the baymouth, parallel to the adjacent Nantucket Sound coast. Degradation of the barrier spit was initiated by a succession of hurricanes in 1954 (Carol, Edna and Hazel). A breach opened and stabilized near the bay end of the one kilometer long inlet channel, providing direct access for exchange of baywater with Nantucket Sound, and separating the barrier beach into two nearly equal limbs. The disconnected northeast limb migrated shorewards, beginning near the 1954 inlet and progressing northeastward, filling the relict inlet channel behind it. At present, about ten percent of the northeast limb is subaerial: the rest of the limb has completely filled the former channel and disappeared. The southwest limb of the barrier beach has migrated shoreward, but otherwise has not changed significantly since the breach. A new mechanism is proposed for spit elongation when the inlet thalweg parallels the beach axis, in which material scoured from the lengthening inlet is the dominant source for spit accretion (perhaps initially deposited as a linear channel-margin bar which later becomes subaerial). The lengthening spit causes the parallel inlet to elongate, which in turn further lengthens the spit, in a self-generating fashion. This mechanism provides both a source of sediment for elongating the barrier spit, and a sink for material scoured from the lengthening inlet. The proposed mechanism for spit growth may be applicable to other locations with low wave energy, small tidal prisms and low longshore sand transport rates, suggesting that estimates of directions and rates of longshore sand transport based on spit geomorphology and development be scrutinized on a case-by-case basis.

Relative sea levels of Japan from tide-gauge records

We attempted to separate tectonic, ocean-ographic, and eustatic components of change in relative sea levels derived from the many tide-gauge records of Japan acquired during the past 50 yr. A modified eigenanalysis permitted use of station data of unequal lengths. Regression analysis and eigenanalysis revealed systematic spatial differences in relative sea-level rise of ∼25 mm/yr. Although the difference is too great to be solely eustatic in origin, data are insufficient to permit accurate separation of tectonic and eustatic effects on relative sea levels. The low-frequency patterns (periods longer than 50 yr) may have been derived from subduction of the Pacific and Philippine plates beneath Japan. Higher frequencies (12-, 6-, and 2-yr periods) are correlated with oceanographic factors such as shifts in the position of the Kuroshio and the El Nino/Southern Oscillation. The contours of relative change in sea levels provide convincing evidence that neither a single nor several Japanese tide gauges can be selected to authentically demote present or past eustatic sea-level rise.