High Water Slack Research Articles

Tidal current asymmetry in shallow estuaries and tidal creeks

We examine tidal asymmetry in a 100-km long coastal plain estuary, a 15-km long tidal creek closed at the end, and a small side channel of a coastal plain estuary closed at low water. Current meter and water depth data were harmonically analyzed for M2 and the M4 and M6 overtide. The results are displayed in a plot of tidal current versus water level and the effects of the M2, M2+M4, and M2+M4+M6 are compared in order to show how overtides affect tidal asymmetry in the different systems.The long estuary showed a gradual increase in tidal current asymmetry. The current takes on a more progressive nature as it goes into the estuary, as indicated by increasing lags between high or low water and the corresponding slack water. On the other hand, the tidal creek has its reflection point at the head, a relatively short distance from the observations, which reflects the incident tidal wave more efficiently.The tidal creek has a high salt marsh which is covered only an hour or so before high water. Since its intertidal areas flood and drain near the time of high water, maximum flood and ebb currents occur a short time before and after high water slack throughout the tidal creek. Most of the tidal energy is dissipated at the end of the creek where M6/M2 of the tidal current almost doubles.The small side channel has tidal flats that become covered soon after the time of low water. The relative time of maximum flood and ebb currents occurs much closer to the time of low-water-slack, which contrasts significantly to the timing in the tidal creek channel surrounded by high salt marsh.

Remotely Sensed Observations of the Turbidity Maximum in the Highly Turbid Humber Estuary, UK

The Humber Estuary, UK, divides into the Ouse and Trent estuaries at the so-called Apex within its upper reaches. Remotely sensed Compact Airborne Spectrographic Imager (CASI) images and boat measurements were used to observe a strong turbidity maximum in the upper Humber and Ouse during a spring tide in November 1995. Surface suspended particulate matter (SPM) concentrations during the late ebb, as estimated from the CASI data, increased from approximately 6 to 13 g I−1 moving up-estuary into the Ouse. Greater SPM concentrations (∼10 g I−1) were evident in the deeper channels of the Ouse, compared with shallower areas, possibly due to faster ebb currents there and differential down-estuary advection of the turbidity maximum. Ribbons, or streaks, of lower SPM and slightly cooler waters were observed. It appears that slightly cooler and lower turbidity waters from the confluent Trent estuary remained fairly distinct for distances of approximately 2 km down-stream of its confluence with the upper Humber and Ouse. These waters eventually broke into ribbon-like or streak-like structures within the higher SPM-laden and slightly warmer waters of the Humber. They were discernible for more than 5 km down-estuary of the confluence of the Humber, Ouse, and Trent. Boat measurements showed that the turbidity maximum occurred over a fairly restricted region of the upper Humber, between about 20 to 50 km from the tidal limit at high water. The turbidity maximum’s sediment load was largely suspended in the water column during stronger currents. SPM rapidly settled close to the bed during high water and low water slack periods. At these times, SPM concentrations in a thin, near-bed layer were >60 g I−1 in the turbidity maximum region of the Ouse and >30 g 1−1 in the upper Humber (where channel volumes were much greater). SPM within the turbidity maximum comprised very fine-grained material and its low organic content demonstrated that the SPM was essentially mineral, clastic sediment derived originally from erosion and decay of crustal rocks.

A simple analytical expression to describe tidal damping or amplification

Can a single line describe tidal damping or amplification? Tidal damping and amplification in alluvial estuaries are constrained by an implicit feedback mechanism. The time lag ϵ between the occurrence of high water and high water slack is crucial in this regard. An analytical solution of the St. Venant equations appears to result in a surprisingly simple explicit relation for the tidal range, consisting of an exponential and a linear term. In alluvial estuaries, the linear term is dominant, particularly in the case of tidal amplification. In the case of tidal damping the exponential term only becomes important in the upper reaches of the estuary (preventing the expression for the tidal range from becoming negative). In tidal amplification, the exponential term is suppressed by the newly defined tidal Froude Number which (as it contains sin ϵ) tends to zero when the tidal wave gets a predominantly standing wave character. This negative feedback prevents the development of an exponentially increasing tidal range. Finally, the expression obtained is a very useful explicit equation to determine estuary parameters that are difficult to determine from direct observations, such as the roughness and the mean water depth.

Benthic-pelagic exchange of microalgae at a tidal flat. 2. Taxonomic analysis

Benthic-pelagic exchange of diatoms over a tidal cycle has been examined using a combination of field measurements and annular flume experiments. The relative contribution of benthic and planktonic species to water-column abundance and biomass has been estimated using taxonomic counts, and cell volume to carbon conversions. In June, peaks in chlorophyll a (chl a) biomass occurred either side of the high-water slack period, which corresponded with increased abundance of benthic diatoms, as well as the large centric diatom Coscinodiscus spp. In numerical terms, benthic diatoms accounted for, on average, 15 to 42% of total diatom abundance over the flood-ebb period, while Coscinodiscus spp. accounted for 12 to 17% in June and 3% in late August. Because of the large size of planktonic forms (e.g., Coscinodiscus spp., Rhizosolenia spp., Biddulphia spp.) compared with the small benthic Nitzschia and Navicula species, the contribution of benthic diatoms was significantly lower in terms of biomass. The average contribution of benthic diatoms to total diatom carbon was <1% in June, but up to 25% in late August. In contrast, the relative contribution of Coscinodiscus spp. was ~90% in June and 73 to 80% in late August. The flume experiments confirmed that, in addition to increased benthic abundance in response to stepwise increases in current velocity, the large centric diatom Coscinodiscus spp. was also resuspended from sediments at high current velocities. At low current velocities, these cells rapidly sank to the seabed, the rate probably enhanced by deposition of sand grains out of the water column. Ecological implications related to water-column biomass and production, macro-infauna diet, and composition of benthic chl a are discussed.

Observations of currents, salinity, turbidity and intertidal mudflat characteristics and properties in the Tavy Estuary, UK

Results are presented from a study of the velocity, salinity, temperature and turbidity behaviour on intertidal mudflats bounding a 400 m wide cross-section in the central reaches of a small, macrotidal, ria estuary (the Tavy Estuary, Southwest England, UK). Measurements were made during low freshwater inflow, summer spring tides. The mudflats comprised a muddy mixture of predominantly silt and clay, with bulk densities typically in the range 1.2–1.4 g ml −1. Bed sediment in the main channel, and on the upper shores of both banks, comprised a mixture of predominantly coarse, non-cohesive sediment, with very small fractions of silt and clay. Two near-bed instrument packages were continuously deployed during the observational period, one on each of the intertidal mudflats located on opposite sides of the estuarine main channel. The instruments recorded water level, velocity, temperature, salinity and turbidity at 0.25 m above the bed. Tidal-cycle measurements of these variables were additionally made throughout the water column in the cross-section's main channel. Longitudinal and vertical surveys of temperature, salinity and turbidity were made throughout the estuary in order to aid interpretation. Maximum bed shear stresses were flood dominant in the main channel and strongly ebb dominant on the upper mudflats. Currents over the intertidal mudflats had much slower peak speeds than those in the main channel. The data presented here indicate that vertically mixed, relatively high-salinity, high-turbidity waters flooded onto the upper mudflats during spring tides, and that the suspended particulate matter largely settled to the bed there, both during the flood and over high-water slack. With one exception, there was little evidence of any subsequent strong sediment resuspension during the salinity-stratified ebb (at least when depths >0.25 m). These results are consistent with the long-term depositional environment associated with rapidly rising sea levels of the early Holocene, and with the present, relatively slower rate of sea-level rise.

Transport and Retention of Suspended Particulate Matter and Bacteria in the Humber-Ouse Estuary, United Kingdom, and Their Relationship to Hypoxia and Anoxia

Data are presented on dissolved oxygen (DO) concentrations and their relationship to salinity, suspended particulate matter (SPM), concentrations, and the turbidity maximum in the Humber-Ouse Estuary, United Kingdom, during summer 1995. Measurements in the upper Humber during March 1995 showed DO in the range 82% to 87% of saturation. Suspended particulate matter concentrations were 60,000 mg l−1) occurred within 1 m of the bed in the turbidity maximum region. A spring-neap record showed a dramatic and tidally controlled decrease in DO at very low salinities as the tides progressed from neaps to springs. An anchor station located down-channel of the turbidity maximum showed that about 95% of the variance in DO, which varied from 28% at low-water slack to 67% at high-water slack, could be explained in terms of salinity variation. At the up-channel margins of the turbidity maximum, DO increased from zero (anoxia) near high water to 60% near low water slack, in contrast to the behavior down-channel of the turbidity maximum. About 82% of the variance in DO could be explained in terms of salinity variations alone. Only 43% of the DO variance could be explained in terms of SPM alone. Up-channel of the turbidity maximum, SPM concentrations were relatively low (<3000 mg l−1) and DO levels varied from 48% of saturation near high water to 83% near low water slack. About 76% of the variance in DO could be explained in terms of salinity variations alone. Within the turbidity maximum region, DO varied from <2% saturation on the early flood and late ebb and maximized around 7% at high water slack. About 63% of the variance in DO could be explained in terms of salinity variation alone. This increased to 70% when suspended particulate matter was taken into account. Only 29% of the DO variance could be explained in terms of suspended particulate matter alone. Because bacteria were likely to have been the cause of the observed reduction in DO, the numbers of bacteria, both free-living and attached to particles, were measured in the turbidity maximum region. Numbers of free-living bacteria were low and most of the bacteria were attached to sediment particles. There was a linear correlation between total bacterial number and suspended particulate matter concentration, suggesting that the strong DO demand was exerted locally as a result of bacterial activity associated with increased suspended particulate matter concentrations. An order of magnitude analysis of DO consumption within the Ouse’s turbidity maximum, based on the premise that DO depletion was directly related to suspended particulate matter concentrations and that DO addition was due to reaeration, indicates that complete deoxygenation could have occurred with an oxygen depletion rate of ∼0.01 mg DO h−1/g suspended particulate matter during the residence time of waters within the turbidity maximum (∼7 d). This rate was sufficiently fast that anoxic to aerobic conditions were able to develop a spring-neap periodicity within the turbidity maximum, but too slow to generate substantial intratidal fluctuations in DO. This is in accordance with the observations, which show that relatively little of the intratidal variance in DO could be explained in terms of suspended particulate matter fluctuations, whereas most of the variance could be explained in terms of salinity, which behaved as a surrogate measure for the proximity of the turbidity maximum.

Unsteady Density Current in a Shallow Water Estuary

Simultaneous measurements of three components of instantaneous velocity, salinities and water temperature have been performed during a spring tide in the Ota River estuary which demonstrates a large tidal range and density gradients induced by salinity. Data are collected at various distances from the bed. Stability influence of stratification changes widely during the tidal cycle. Around low water slack, the estuary is well mixed. On the other hand, the estuary strongly stratifies near the water surface after high water slack in the ebb tide. The longitudinal density gradient, which drives an upstream bottom flow, is essential throughout all tidal phases. Owing to the longitudinal density gradient, the ebb flows in lower layers are small and bed-generated turbulence is weak.

Meetings

Suspended-material concentrations in a salt-marsh tidal channel on Kiawah Island, South Carolina, ranged from 6 to 50 mg/l in March, 1977, and from 10 to 200 mg/l in July–August, 1977. Higher summer than winter concentrations are related to more biologic activity and increased thundershower frequency. Maximum suspended-material concentrations for both periods occurred during spring tides when tidal prisms were greater and, consequently, current velocities were stronger, or during high winds or heavy rains.Monitoring of the flux of suspended material during fifteen tidal cycles in March, 1977, indicated there was an export of combustible (organic) matter and a balance of noncombustible (inorganic) matter for that period. During eight tidal cycles in July–August, 1977, there was a net export of both combustible and noncombustible matter. Export of suspended material is attributed to the time—velocity asymmetry of the tidal currents. Maximum current velocity for both flood and ebb tide occur near high slack water (time asymmetry). Peak ebb velocity is, on the average, 20–30% stronger than peak flood velocity (velocity asymmetry). Consequently, higher concentrations of suspended material are transported for longer periods of time on the ebb tide, resulting in a seaward movement of material.

Size and settling velocity distributions of flocs in the Tamar estuary during a tidal cycle

Data extracted from video recordings of individual estuarine flocs near the estuary bed during the advance and retreat of the salt intrusion show changes in size and settling velocity distributions. The recordings were taken using INSSEV —IN Situ SEttling Velocity instrument. Size coupled with effective density variations due to both changes in floc structure and ambient salinity result in changes in the settling velocity during the tidal cycle. In particular, just after high water slack, the appearance of high settling velocity medium size flocs and individual particles suggest that the lower density flocs have been broken up by the intense vertical shear in the currents caused by the salt wedge intrusion. Current shear is shown to have a significant influence on floc effective density.

In situ aggregate size and settling velocity in the Oosterschelde tidal basin (The Netherlands)

Suspended aggregate size and settling velocity at sediment concentrations < 10 mg·dm −3 were determined in situ from aggregates sampled by a diver and from observations by a submersible video camera in September and November 1991. Aggregate size and settling velocity were up to 1500 μm and 1 cm·s −1, respectively. Aggregate size, settling velocity and density were highest during flood and high-water slack, which points to flocculation and settling during flood and high-water slack, and no resuspension by the ebb current. This agrees with the observed landward transport of fine-grained sediment inside the basin.

Settling velocities of mud aggregates in the oosterschelde tidal basin (The Netherlands), determined by a submersible video system

A submersible video system was used for direct measurements of the form, size and settling velocity of mud aggregates in the Oosterschelde tidal basin. Continuous in situ measurements were obtained during one tide. They showed that even at relatively low concentrations (10 mg l −1 ) large aggregates are present. The largest aggregates and highest settling velocities were determined during high-water slack (for aggregates ≥ 50 μm: median aggregate diameter = 300 μm, median settling velocity = 1·74 mm s −1 ). The settling velocity of mud aggregates ( W ) depends on their diameter ( d a ) according to W ∼ d a ″. For the Oosterschelde an n -value of 0·92 was found for aggregates >50 μm. This value is within the range of 0·4-1 which is usually determined for mud aggregates. Likewise, the reduction of aggregate density as they grow can be related to the diameter of these aggregates (de) according to Δϱ ∼ d a m . The value of m for the Oosterschelde aggregates >50 gm ranged from − 0·69 to − 1·84.

Changes in suspended-matter floc size during the tidal cycle in the dollard estuary

Measurements of in situ particle size of suspended matter in a tidal channel in the Dollard (Dutch Wadden Sea) indicated systematic variations in floc size during the tidal cycle that can be explained by assuming settling during slack tide, resuspension during the early ebb and early flood, flocculation of fine particles into large ones during most of the tide, and deflocculation of large flocs into smaller particles during or after settling to the bottom. There was a characteristic difference in floc-size variation during ebb and during flood. During the ebb maximum floc size coincided with maximum suspended-matter concentration (maximum collision frequency); during the flood maximum floc size continued to increase towards high-water slack tide. The adjacent tidal flats had a marked influence on floc size: where the flats were small, the highest percentages of large flocs (> F128 μm) occurred around slack tide when current velocities are low. Where the adjacent flats were broad and extensive, high percentages also occured during intermediate periods. Flocs of maximum size are probably not in equilibrium with the bulk of the suspended matter: they are mainly formed during periods of high suspended-matter concentrations of short duration.

The Freshwater-Saltwater Interface and Its Relationship to the Turbidity Maximum in the Tamar Estuary, United Kingdom

Data are presented from several experiments in the freshwater-saltwater interface (FSI) region of the Tamar Estuary. Longitudinal surveys of salinity and suspended particulate matter (SPM) at high water showed that the location of the FSI could be predicted in terms of a power-law regression with freshwater runoff. Longitudinal transects also were surveyed over periods of several hours. The FSI was observed to advect into the region on the flood with strong vertical mixing. After high water, stratification became intense as fresher water ebbed in the surface layers. The near-bed water in the stratified region began to ebb between 2 h and 3 h before low water. A model of the vertical structure of longitudinal currents showed that the enhanced stratification on the ebb, coupled with the longitudinal density gradient, partly produced this long period of slack, near-bed currents following high water. A strong turbidity maximum (TM) occurred during spring tides and was located slightly up-estuary of the FSI at high water. Longitudinal transects during a period of low freshwater runoff and large neap tide showed that at the start of the flood the TM was associated with the FSI region. As the FSI advected up-estuary on the flood there was considerable resuspension of sediment at the FSI. Some of this SPM moved with the FSI and reached the limit of saline intrusion, where it formed a slowly-eroding TM as particles settled during the long, high-water slack period. As the near-bed currents increased on the ebb and the FSI moved down-estuary, strong vertical mixing and resuspension of recently deposited sediment occurred in the unstratified water behind the FSI and the associated TM advected down-estuary. Additional effects were present with stronger tides and increased runoff.

The mechanism of retention of pelagic tomcod, Microgadus tomcod, larvae and juveniles in the well-mixed part of the St. Lawrence Estuary

We tested the hypothesis that the mechanism of retention of tomcod, Microgadus tomcod, larvae and juveniles in the well-mixed part of the St. Lawrence Estuary is similar to that of sympatric smelt, Osmerus mordax, larvae who actively migrate to the surface during flood tides and to the bottom during ebb tides so as to minimize net downstream displacement. The vertical distribution of tomcod larvae and juveniles was documented during two 98-h sampling series at 2 anchor stations in June and July, 1986. An hourly index of the center of mass of fish in the water column calculated to take into account daytime net avoidance in surface waters suggested that tomcod remained deep in the water column and that their accumulation at the head of the estuary was the result of passive upstream transport by net residual circulation rather than active tidal migrations. For both series, depth of fish was inversely related to density of the water suggesting that the buoyancy of fish influenced their vertical distribution. Tomcod larvae and juveniles were advected by tidal currents. In June, larger larvae were found at low slack water indicating that they were located upstream of smaller larvae. In July, larger juveniles were located downstream of smaller juveniles, the difference in mean length between low and high slack water attaining 20 mm. Ontogenetic buoyancy changes may be responsible for these differences in the vertical distribution of tomcod. Comparisons of the early life-history stages of tomcod and smelt retained in the same area under the same hydrodynamical conditions indicate that more than one mechanism permits retention in a well-mixed estuary and that the observed species-specific patterns of vertical distribution are not simply interpretable as adaptations to retention.

Salt intrusion model for high-water slack, low-water slack, and mean tide on spread sheet

A steady-state model is presented which can calculate the low-water slack and mean tidal salt intrusion from the calibrated high-water slack salt intrusion. A relation is presented between the dispersion coefficients at high-water slack, at low-water slack, and at mean tide. The model has been adapted for use on spread sheet, which makes the model accessible for anyone who has basic knowledge of computers.

Hydraulic control of flow over the sill in Observatory Inlet

Observations of density, velocity, and acoustic backscatter obtained over the sill in Observatory Inlet, British Columbia, during the ebb tide illustrate several features of internal hydraulic control in a tidally forced stratified flow. Following high water slack, the flow field was initially determined by critical control of the second internal mode at the sill crest. As the flow speed increased, a transition occurred, and control changed to an “approach control” of the second internal mode located upstream of the sill crest. Whereas the critical control represents a minimum energy condition at the crest, the approach control corresponds to a maximum energy condition upstream of the sill crest. Later, when the ebb currents were just past their maximum values, the control condition changed to critical control of the first internal mode. The concept of approach control of flow over topography has not previously been discussed in the literature. The data described here are believed to represent the first field observations of this phenomenon. The changing hydraulic conditions are analyzed in terms of two‐ and three‐layer hydraulic models. Model results are consistent with the evolution of the flow patterns, although the model cannot reproduce the apparent hysteresis in the response after maximum ebb. The concept of an approach control is demonstrated with a simple two‐layer laboratory model.

Characteristics of Suspended Cohesive Sediment of the Severn Estuary, U.K.

Characteristics of suspended cohesive sediment from the benthic boundary layer of the Bristol Channel/Severn Estuary were examined at one location (51°27.66′N. 2°53.20′W). Samples were collected in vertical profiles over the bottom 2 m of the water column at times near to, and at low and high slack water periods on spring and neap tides. Primary particle size distributions obtained using a Coulter Counter (range 1.59–50 μm) and electron microscopy (range 0.1–20 μm) reveal bimodal (volume) frequency distributions with a lower mode fixed at 1.59 μm and a higher mode variable between 6 and 30 μm. When these data are converted to number frequency distributions, the higher mode effectively vanishes, while the lower is retained. Suspended sediment mineralogy, of dispersed samples, was determined by individual and multiple particle examination using analytical electron microscopy. Average suspended sediment composition was found to be illite 44%, quartz 23%, chlorite 19%, calcium carbonate 5%, kaolinite 4%, and organic carbon 3% (by weight). Floc diameters estimated from hindered settling experiments were ≈ 140 μm possessing a sedimentation velocity in the range 1.8–2.0 mm∙s−1, in good agreement with field observations of temporal changes in the vertical distribution of turbidity.Key words: suspended cohesive sediment, flocculation, Bristol Channel.

LONG-TERM FATE OF DISPERSED AND UNDISPERSED CRUDE OIL IN TWO NEARSHORE TEST SPILLS

ABSTRACT The fate and effects of two nearshore discharges of Murban crude oil at Long Cove, Searsport, Maine in August 1981 were studied following a one-year, pre-spill baseline study of the test areas. An upper and a lower intertidal sampling area within a 60 × 100 meter test plot were exposed to dispersed oil in water resulting from the discharge of 250 gallons of oil pre-mixed with 25 gallons of Corexit 9527 dispersant. Release of treated oil was around high-water slack tide on the surface of the water, with added mixing energy provided by mixing gates deployed by small boats. The maximum water depth over the test areas was 3.5 meters. Untreated crude oil (250 gallons) was released on an ebbing tide within a separate, boomed-off 60 × 100 meter test plot. A third test plot served as an oil-free reference plot. Water samples taken near the surface and near the bottom during and after discharge showed that chemically dispersed oil loses lower boiling hydrocarbons in both the aliphatic and aromatic fractions below n-C17as the droplets diffuse downward. Data are given for sediment samples taken from the test plots 11 months pre-spill and 10 months post-spill. Hydrocarbon analyses of the sediment samples show little incorporation of dispersed oil into the sediments of the treated oil plot relative to the sediments exposed to undispersed oil.

Suspended-material transport in marsh tidal channels, Kiawah Island, South Carolina

Suspended-material concentrations in a salt-marsh tidal channel on Kiawah Island, South Carolina, ranged from 6 to 50 mg/l in March, 1977, and from 10 to 200 mg/l in July–August, 1977. Higher summer than winter concentrations are related to more biologic activity and increased thundershower frequency. Maximum suspended-material concentrations for both periods occurred during spring tides when tidal prisms were greater and, consequently, current velocities were stronger, or during high winds or heavy rains. Monitoring of the flux of suspended material during fifteen tidal cycles in March, 1977, indicated there was an export of combustible (organic) matter and a balance of noncombustible (inorganic) matter for that period. During eight tidal cycles in July–August, 1977, there was a net export of both combustible and noncombustible matter. Export of suspended material is attributed to the time—velocity asymmetry of the tidal currents. Maximum current velocity for both flood and ebb tide occur near high slack water (time asymmetry). Peak ebb velocity is, on the average, 20–30% stronger than peak flood velocity (velocity asymmetry). Consequently, higher concentrations of suspended material are transported for longer periods of time on the ebb tide, resulting in a seaward movement of material.

HYDRODYNAMICS AND SEDIMENT TRANSPORT IN A SALT MARSH TIDAL CHANNEL

Processes and sediment transport were investigated in a salt marsh drainage system at Kiawah Island, South Carolina. A general survey of the tidal current was done in the major tidal channel (Bass Creek) for a 10 tidal cycle period in August, 1977. Detailed determinations of current velocity, discharge, and suspended load were conducted during 15 tidal cycles in March, 1977 and again during 8 tidal cycles in July- August, 1977. For each of these periods, mass budget for the total suspended load were computed. The tidal currents have a pronounced time velocity asymmetry with the maximum current velocity occurring nearer high slack water and the peak ebb velocity being 20 - 30% stronger than the flood. Suspended load transport is significantly affected by the time velocity asymmetry. Peak current occurring nearer high slack water causes a net displacement of suspended material in an ebb or seaward direction under normal conditions. This process is enhanced by the stronger ebb currents. Mass budgets reflect the ebb dominance of the system showing a net export of combustible (organic) material during the March sampling period and a net export of both noncombustible (inorganic) and combustible material during the July-August period. Also important to suspended load transport in marsh systems are stressed meteorological conditions. High winds or heavy rains increase suspended load concentration and can cause significant import or export of fine-grained material.