Kuo-Chuin Wong

On the relative importance of the remote and local wind effects to the subtidal variability in a coastal plain estuary

A set of month-long sea level and current measurements is used to examine the subtidal variability in the Delaware estuary and assess the relative importance of remote and local wind effects on the observed subtidal variability. The evidence indicates that the remote wind effect, through the impingement of coastal sea level at the mouth of the estuary, is more important than the local wind effect in producing the subtidal sea level fluctuations in the interior of the estuary. On the other hand, the local wind effect dominates over the remote wind effect in producing the observed subtidal current fluctuations at the mooring site. It appears that the remote wind effect is important to the sectionally averaged subtidal transport into or out of the estuary. However, the local wind effect may be more important than the remote wind effect in producing the subtidal currents at any given point along the estuarys cross section. The spatial structure associated with the local wind-induced circulation is very important to the long term transport and distribution of waterborne material.

Wind‐forced dynamics at the estuary‐shelf interface of a large coastal plain estuary

[1] Wind-current relationships at the mouth of a large coastal plain estuary are examined using seasonal current, wind, and sea level records. Observations indicate a small portion of subtidal current is forced by along-shelf (AL) wind-induced coastal sea level fluctuations (remote forcing). However, subtidal fluctuations on the order of 10 cm/s appear to be largely forced by along-estuary wind stress (local forcing). Local forcing dominates the wind-driven subtidal current during principal AL and along-estuary wind conditions, but along-estuary wind stress accentuates the local effect while diminishing the remote effect. A simple two-dimensional analytical model helps clarify the competition between remote and local wind forcing of current near the estuary mouth, supporting the existence of a strong bi-directional current that is locally wind-driven, regardless of principal wind forcing. Both observed and model results indicate that the relative orientation of the wind stress with respect to the estuary and the adjacent shelf are important in determining the roles of remote and local forcing. Findings from this study challenge the application of traditional coastal pumping models for estuary-shelf exchange and have implications for larval recruitment and exchange of material between estuaries and inner shelves.

The axial salinity distribution in the delaware estuary and its weak response to river discharge

We use long term salinity and river discharge data from the Delaware estuary, U.S.A. to determine the mean axial salinity distribution and the salinity response to fresh water discharge. The Delaware is a weakly stratified estuary with a typical vertical salinity variation of only 1 psu. We find that over most of the estuarine salt intrusion length the mean axial distribution of salinity is surprisingly close to a linear decrease with axial distance. Using linear regression analysis, we find that the response of salinity to river discharge is surprisingly weak. The equivalent displacement of a given isohaline for a change in discharge of one standard deviation is only about 4 km, about half the amplitude of the M 2 tidal displacement. This implies that some powerful buffering agent exists to reduce the salinity response. We suggest two possible mechanisms for this agent: the action of vertical shear flow dispersion in a tidally stirred regime and the action of lateral shear coupled to strong lateral salinity gradients.

Mechanisms of sediment flux and turbidity maintenance in the Delaware Estuary

[1]xa0An observational study was conducted to identify mechanisms of suspended sediment flux and turbidity maintenance in the Delaware River estuary. From March through October 2005, instrumented moorings were deployed to obtain continuous measurements of currents and suspended sediment concentration at sites along the estuarine channel and on flanking subtidal flats. Data time series were analyzed to determine the relative influence of nontidal advection and tidal pumping on residual fluxes of sediment. Results indicate that the estuarine channel is a strongly advective transport environment with residual sediment fluxes driven mostly by gravitational circulation. Tidal pumping is a contributing process of residual sediment flux in the channel near the estuarine null point and turbidity maximum, though the magnitude and direction of pumping vary with river flow and resident sediment inventory in the upper estuary. Sediment pumping in the channel is driven by tidal asymmetries in velocity and particle settling and perhaps by tidal variations in internal mixing in the stratified lower estuary. In contrast to the estuarine channel, residual sediment fluxes over the subtidal flats are weak and dominated by tidal pumping. Landward advective fluxes of sediment in bottom waters of the lower estuarine channel are strongest during neap tides; during large spring tides sediment is mixed high in the water column and the advective flux reverses to seaward under the residual surface outflow. Despite these transient seaward fluxes, the estuary has an enormous capacity to buffer extreme freshwater discharges and suppress export of suspended sediment to Delaware Bay.

The tidal and subtidal variations in the transverse salinity and current distributions across a coastal plain estuary

The transverse structure in the current and salinity distributions across the mouth of Delaware Bay are examined over the tidal and subtidal time scales. Results show that the across-estuary variation in bathymetry, in the form of a channel-shoal configuration, has a very significant impact on the characteristics of transverse variability. The mean current over the channel is characterized by a strong outflow of low salinity water in the upper layer and a strong inflow of high salinity water in the lower layer, consistent with the density-induced gravitational circulation. The mean flow pattern in the shallow waters over the shoals is marked by transverse rather than vertical variation. The subtidal current and salinity fluctuations are primarily driven by the effect of local atmospheric forcing. The subtidal current fluctuations in the upper layer of the channel are frictionally coupled to the local wind, resulting in downwind currents. The subtidal current fluctuations in the lower layer of the channel, however, flow in the direction of local setup and against the wind. With a wind blowing down the estuary, the wind-induced current tends to reinforce the two-layer structure of the gravitational circulation and substantially enhance the vertical shear and surface to bottom salinity difference. The reverse occurs with a wind in the up-bay direction. The subtidal currents in the shallow areas to the right of the channel exhibit largely depth-independent response to the effect of local wind, with downwind currents at both the surface and the bottom. At tidal frequencies the currents show only a modest variation across the bay mouth. Tidal currents are highly deterministic, but the characteristics of the tidal variability in salinity exhibit significant changes over long time scales. These long-term changes in the intratidal salinity variability are caused by the nonlinear interactions between the tidal and subtidal motions. The residual salt flux through the bay mouth shows significant subtidal fluctuations. The leading factor responsible for producing such subtidal fluctuations is the advection of salt by the wind-induced subtidal currents, but the effect of tidal pumping also contributes significantly to the overall residual salt flux into the estuary.

Sea-level fluctuations in a coastal lagoon

Abstract Sea-level observations made during December, 1979, at six stations in Great South Bay (which is a coastal lagoon on the south shore of Long Island, New York) reveal that there were significant subtidal fluctuations in addition to the tidal oscillations. Harmonic analysis of the tidal oscillations of sea level indicates that M 2 is the dominant tidal constituent. The M 2 amplitude, however, suffered a more than 50% reduction in the interior of the Bay due largely to the narrow inlet. The subtidal sea level fluctuations within the Bay were forced primarily by the low-frequency fluctuations of the adjacent shelf water. The active subtidal exchange induced by this Bay-shelf coupling appeared to have suffered only minor attenuation within the Bay. As a consequence, the variance associated with subtidal sea level fluctuations was greater than that associated with the tidal oscillations over most of Great South Bay.

The effect of coastal sea level forcing on Indian River Bay and Rehoboth Bay, Delaware

Abstract Previous studies have suggested that sea level and current variability in Indian River Bay and Rehoboth Bay, Delaware are primarily forced by coastal sea level fluctuations. A linearized frequency-dependent pumping mode model is developed to (1) examine the response characteristics of the Indian River Bay-Rehoboth Bay system, and (2) assess the relative importance of the coastal forcing from the Indian River Inlet and the Lewes-Rehoboth Canal. The results indicate that the pumping mode model can adequately address the first-order response of the two bays. The results further indicate that sea level variabilities in the two bays are almost entirely forced by coastal forcing from the Indian River Inlet at both tidal and subtidal frequencies. The coastal forcing from the inlet also dominates the volume flux through the system at all frequencies. but the coastal forcing conveyed through the Lewes and Rehoboth Canal can generate up to 20% of the total volume transport at very low frequencies. The lowfrequency volume flux through the canal, however, generates a flow through the entire system and produces minimal sea level response. The overall response of the two bays to coastal forcing depends strongly on the degree to which the two bays are coupled. The ditch connecting the two bays acts as an effective low-pass filter to preferentially damp out high-frequency tidal motions in Rehoboth Bay. Because of the coupled nature of the response, the ditch also exerts substantial influence on the response characteristics of Indian River Bay.

Water Level and Velocity Characteristics of a Salt Marsh Channel in the Murderkill Estuary, Delaware

ABSTRACT Dzwonkowski, B.; Wong, K.-C., and Ullman, W.J., 2014. Water level and velocity characteristics of a salt marsh channel in the Murderkill estuary, Delaware. High-frequency observations of water level and velocity over one year in Murderkill estuary, a tributary estuary of Delaware Bay, are used to examine changes in tidal and subtidal flow characteristics as water propagates from the mouth of the estuary (Bowers site) into a minor channel that connects to a contiguous salt marsh (channel site), a setting where long-term continuous data sets are uncommon. These data provide insight into the flow behavior in marsh channels, the driving mechanisms of water exchange, and the potential for particle and solute exchange between tidal marshes and their adjacent estuaries. At both sites, tidal forcing is normally the dominant driving mechanism in water level and velocity signals with remote wind forcing having a limited contribution. As the tidal signal propagates from the Bowers site into the salt marsh, however, the progressive-type mixed wave is transformed into a standing wave with minor velocity distortions resulting from a reduction in the M2 tidal constituent as well as an amplification of the M4 overtide. The variability at both sites is typically well explained as a linear superposition of high-frequency tidal constituents and remotely forced wind-driven subtidal fluctuations. During storm events at intermediate to spring tides, however, both tidal and subtidal velocity characteristics in the channel are more nonlinear compared to both typical channel conditions and those at the Bowers site. During these events, in response to remote winds, tidal flow in the channel becomes progressive in form and flood dominant, while subtidal velocities are ebbward and nearly an order of magnitude stronger than typical conditions.