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The Lynher Estuary in Southwest England is a small, strongly tidal sub-estuary of the Tamar Estuary. It is a Site of Special Scientific Interest (SSSI), a Marine Conservation Zone (MCZ), a part of the Plymouth Sound and Estuaries Special Area of Conservation (SAC) and a Special Protection Area (SPA). Management of the Lynher SSSI and MCZ stipulates that good water quality and sediment quality should be maintained; as such, a good understanding of its responses to influences such as climate change and changes in agricultural practices within its catchment area is required. Observations of salinity, temperature, suspended particulate matter (SPM) concentrations, estuarine turbidity maximum (ETM) behaviour, and chlorophyll-a are presented for the Lynher over a 1-y period. The dataset provides important baseline information with which to identify future changes and guide management of the SSSI and MCZ as well as adding to our knowledge of estuarine systems. Salt intrusion is largely controlled by tides and runoff. A persistent ETM occurs that is closely associated with the freshwater-saltwater interface at high water (HW) and with a minimum in dissolved oxygen concentrations. HW depth-averaged ETM magnitudes are relatively low, less than 60 mg l−1 and typically 30 mg l−1 over the observation period. Larger tides and stronger flood-tide wind speeds lead to a stronger ETM. Tidal river HW SPM concentrations are intrinsically small (8 ± 8 mg l−1 during the observation period). Surface chlorophyll-a concentrations are low during winter (when they often peak near the ETM) and are much higher during spring and summer.

Observations are presented of estuarine infragravity oscillations in a very small estuary, the Ria de Santiuste, northern Spain, that discharges into a bay. Time-scale calculations and measurements indicate that the most likely source of amplified infragravity waves within the estuary are coastal edge waves, which appear to be resonating between the headlands of the bay, and onshore-offshore seiching of waters that may be resonating between the beach and the bay's seaward limit. Infragravity waves in the estuary have a narrow, power-dominant periodicity range of 4.3–4.8 min. Generally, only very weak waves, and at other periodicities, occur within the tidal river when it is unaffected by the tide. Although infragravity water-level amplitudes are small in the tidal river, typically ∼0.01 m during high water, the corresponding velocity amplitudes are significant, typically ∼0.1 m s −1 , despite the occurrence of high runoff conditions during these measurements. Within the estuary and close to the mouth, near-bed salinity shows considerable variability that is a consequence of salt wedge oscillations with periodicities that are within the range of 4.3–4.8 min. Model results for simulated neap to spring high-water water levels show that wave periodicities in the range 3.5–4.5 min correspond approximately to 3 λ /4 resonances. The model also indicates that an effect of low runoff is to greatly enhance resonant behaviour within the estuary, such that water level amplitudes of 0.02 m at the mouth can produce wave currents as fast as 0.4 m s −1 within the estuary, compared with ∼0.1 m s −1 during high runoff.

Observations are presented of water levels, currents, salinity, turbidity, sediment grain sizes and sediment transport in the Devonshire Avon Estuary, UK, in order to improve knowledge of freshwater, wave and tidal influences on small, strongly tidal ria estuaries. A large reduction in tidal range occurred progressing from the coastal zone to the upper estuary that was mainly a consequence of rising bed and river water levels. The spring-neap cycle also had an influence on the reduction in tidal range along the length of the estuary. Surface gravity waves were completely dissipated propagating into the estuarine channel from the coastal zone, and despite strong wave-induced resuspension, suspended sediment was not transported into the lower estuary in observable amounts during the ensuing flood tide, indicating that the wave-suspended material was too coarse to remain in suspension once transported away from the surf zone. Turbidity in the lower estuary was relatively low during low runoff summer conditions and had largest values over low water, when turbid waters from farther up-estuary had been transported there. Strong resuspension events occurred at peak currents in the upper estuary during summer, reflecting the presence of finer-grained sediment sources. Turbidity was similar but greater in the lower estuary during high runoff winter conditions and strong resuspension occurred at peak currents, indicating an easily erodible, nearby sediment source, due to down-estuary movement and relocation of finer sediment over the winter. A large shoal in the lower estuary exhibited a consistent pattern of accretion/erosion during the high runoff months of late autumn and winter to spring that also was qualitatively consistent with sediment transport modelling and implied: (a), erosion from the up-estuary limit of the shoal with (b), down-estuary bed-load and suspended-load transport that accreted the centre and down-estuary limit of the shoal until (c), a diminished supply led to erosion via continued down-estuary transport from the shoal centre.

Sub-estuaries, i.e. tidal creeks and also larger estuaries that branch off the stem of their main estuary, are commonplace in many estuarine systems. Their physical behaviour is affected not only by tributary inflows, winds and tides, but also by the properties and behaviour of their main estuary. Measurements extending over more than an annual cycle are presented for the Tavy Estuary, a sub-estuary of the Tamar Estuary, UK. Generally, waves are small in the Tavy because of the short wind fetch. A several-hour period of up-estuary winds, blowing at speeds of between 7 and 10 m s −1 , generates waves with significant wave heights of 0.25 m and a wave periodicity of 1.7 s that are capable of eroding the bed over the shallow, ca. 1.5 m-deep mudflats. Waves also influence sedimentation within and near salt marsh areas. An estuarine turbidity maximum (ETM) occurs in the Tavy's main channel, close to the limit of salt intrusion at HW. Suspended particulate matter (SPM) concentrations typically are less than 40 mg l −1 at HW, although concentrations can exceed 80 mg l −1 when tides and winds are strong. Flood-tide SPM inputs to the Tavy from the Tamar are greater during high runoff events in the River Tamar and also at spring tides, when the Tamar has a high-concentration ETM. Higher SPM concentrations are experienced on the mudflats following initial inundation. Without wave resuspension, this is followed by a rapid decrease in SPM for most of the tide, indicating that the mudflats are depositional at those times. SPM concentrations on the mudflats again increase sharply prior to uncovering. Peak ebb tidal speeds at 0.15 m above the mudflat bed can exceed 0.26 m s −1 at spring tides and 0.4 m s −1 following high runoff events, which are sufficient to cause resuspension. Time-series measurements of sediment bed levels show strong seasonal variability. Higher and lower freshwater flows are associated with estimated, monthly-mean sediment transport that is directed out of, or into, the upper sub-estuary, respectively. Seasonal sediment transfers between the estuary and its sub-estuary are discussed.

The macrotidal Humber system, comprising estuaries of the Humber, Trent and Ouse, and their various tributaries, is one of the largest and most turbid in the British Isles. This paper presents detailed spatial and temporal data on the estuarine turbidity maximum (ETM) within the Ouse Estuary of the Humber system during quiescent summer conditions, when freshwater runoff was very low and approximately steady. Under these conditions, an extremely turbid ETM existed in the low salinity reaches of the upper Humber, within the Trent and Ouse Estuaries. Longitudinal surveys of salinity, temperature and turbidity were obtained at approximately local high water (HW) or low water (LW) between the tidal limit of the Ouse and the upper Humber. Tidal-cycle stations were worked between the upper Ouse and the coastal zone. In situ median floc sizes were measured at some stations. Tidal water levels were very asymmetric and currents were flood dominant in the upper estuary, especially at spring tides. Frictional drag on the currents was approximately balanced by water-level slope forcing, which led to a large reduction in tidal amplitude as the tide propagated into the estuary. A tidal bore, 0.1–0.2 m high, formed at spring tides in the upper estuary, but did not cause suspension of fine sediment at locations up-estuary of the ETM. Generally, salinity was fairly well mixed vertically, despite strong SPM stratification in the ETM region. However, large salinity inversions did occur in the presence of underlying, stationary sediment suspensions (∼90 g l−1). The ETM core region, in which near-bed SPM concentrations exceeded 16 g l−1, extended over a longitudinal distance of 35 km at HW, both at spring and at neap tides. It was separated by nose and tail regions from much lower turbidity waters. The nose was much sharper than the tail and was located 15 km into the tidal river at spring tides, where salinity was less than 1. Except at very small neap tides, when fluid mud layers and stationary suspensions formed in the tail region of the ETM, maximum near-bed SPM concentrations (∼50 g l−1) occurred close to the nose in the upper core region. The ETM was displaced down-estuary by ca. 12 km during the transition from spring to neap tides. It also was displaced down-estuary between HW and LW. Floc settling led to pronounced SPM stratification over the HW, HW-slack and early ebb period. Estimates of settling velocity, corrected for hindered settling, ranged from 1.2 to 2.1 mm s−1. At HW slack, hindered settling prevented appreciable deposition of flocs to the bed in the main channel of the nose and upper core regions and at LW of spring tides there was little time for deposition. In the water column, in situ floc sizes maximised around mid-depth at HW slack. Therefore, slack water and low current shears led to increased flocculation, greater sizes and enhanced settling. These maximum median sizes typically were 300–500 μm, whereas sizes during other states of the tide were in the range 70–300 μm. A strong, negative correlation existed between depth-averaged median floc size and bulk vertical current shear in the water column for flocs that were greater than a station-dependent size. The largest median floc sizes occurred within the near-bed stationary suspensions at neap tides, where floc sizes could exceed 1 mm. Entrainment of these flocs led to floc breakage, which reduced their median sizes to less than 200 μm.

Results are presented of turbidity measurements made in the Tweed Estuary during 14–29 September 1993. The observations covered a spring-neap period of very strong and relatively weak tides, which included strong wave activity at the mouth of the estuary, the aftermath of a strong freshwater inflow event and a minor inflow event which coincided with neap tides. Turbidity levels between the mouth and the limit of saline intrusion during this period were observed to lie in the range 2–30 ppm. Temperature–salinity relationships, based on rapid sampling throughout the estuary, often indicated conservative mixing between riverine and coastal waters. Turbidity–salinity relationships throughout the estuary were approximately linear for most of the lower salinity range (

The Tamar (England) and Weser (Germany) Estuaries exhibit strong turbidity maxima in their low-salinity reaches. Whereas their morphologies and freshwater inflows are different, the estuaries have similar mean tidal ranges and mean tidal current speeds. Despite the differences, the turbidity maxima show similar intratidal, spring–neap and seasonal behaviour for the two estuaries. On an intratidal time scale, strong suspended particulate matter (SPM) variability occurs in both turbidity maxima regions due to deposition over slack-water periods, subsequent re-suspension and depletion of easily erodible bed-source sediment during the following flood or ebb, and transport by tidal currents. Over a spring–neap cycle, the spring-tide SPM levels are greater than those during neap tides owing to enhanced re-suspension in the stronger spring-tide currents. Both estuaries exhibit a hysteresis in SPM during the falling and rising spring–neap cycle. On a seasonal basis, the occurrences of river spates in both the Tamar and the Weser move both the limits of saline intrusion and the turbidity maxima further down-estuary. Following the return to normal freshwater inflows, the saline intrusion moves rapidly up-estuary, whereas the turbidity maximum exhibits a time lag.

Results are presented from a 2-week field programme in the Tweed Estuary, U.K. Maximum values of the empirically based Estuarine Richardson Number, RiE, occurred during neap tides, and minimum values occurred during spring tides. Estimated values of RiEvaried between 0·3 and 2·3, suggesting the occurrence of partially mixed to stratified conditions, depending on tidal state and freshwater inflow. These relatively large values of RiEwere consistent with both observed strong salinity stratification and large salt fluxes due to vertical shear transport. Low values ( 0·5) values in the halocline. A velocity maximum occurred within the halocline during the early flood. Wave-like spatial oscillations of the halocline occurred on the ebb. The oscillation troughs were situated above deep holes located just down-estuary of the rail and old road bridges. There was an indication that the constricted flow between the bridges’ arches resulted in enhanced mixing of near-surface waters and a thickening of the halocline. It is also possible that these wave-like structures were stationary, near-critical internal lee waves, triggered by the deep holes. Trapping of high-salinity waters occurred on the ebb. Saline pools were isolated within a deep hole or deeper section of bed by the falling halocline. When the salt wedge moved further down-estuary, the ‘ trapped ’ waters were subjected to strongly ebbing, overlying freshwater, and were subsequently entrained and flushed. The salinity intrusion was a strong function of spring–neap tidal state and a weaker function of freshwater inflow. The estimated salinity intrusion varied from about 4·7 to 7·6 km during the fieldwork period. The strong dependence on tidal range followed from the comparable lengths of the tidal excursion and salinity intrusion. Long excursion lengths were also partly responsible for the short residence (or flushing) times and their strong dependence on spring–neap tidal state. For typical summer freshwater inflows of 40 m3s−1, the estimated residence times varied from one tide at large spring tides (removal on one ebb) to two tides (removal over two ebbs) at neap tides. Computed residence times were shorter for typical winter inflows.

Measurements describing the intratidal, spring-neap and seasonal variations of fine, cohesive, suspended particulate matter (SPM) concentrations at two sites (CK and HQ) within the upper reaches of the Tamar Estuary, U.K. are presented. The data were obtained using two, near-bed instrument packages. Correlations between daily-averaged SPM concentrations and runoff and tidal range during the separate deployments often showed a significant dependence of SPM on these variables. Where statistically significant, increasing tidal range led to enhanced SPM levels because of resuspension of bed sediments. There was an asymmetry between the daily-averaged SPM concentrations on the rising and falling 15 day spring-neap cycle. On the rising cycle, the SPM levels were significantly lower than on the falling cycle for the same tidal range. This asymmetry was a feature of all deployments. The daily-averaged SPM during 3·8 m tidal-range rising spring tides at stations CK and HQ was approximately 0·3 kg m -3 in July and November 1988 and then fell to 0·01 kg m -3 during December 1988. This reduction in SPM was a manifestation of the winter period of bed-sediment depletion. Sediment was scoured from the upper estuary by strong ebb currents, which were enhanced by high and increasing runoff. SPM levels remained low during March 1989 (0·08 kg m -3 ) at the end of the high runoff, winter period. The daily-averaged SPM at HQ during April was much higher (0·77 kg m -3 ) than that further up-estuary at CK (0·11 kg m -3 ), which was comparable with the March levels. This indicates that the bed-sediment supply had moved up-estuary to the vicinity of HQ, so that local resuspension was very important there at that time, but had not reached the vicinity of CK. Fine sediment continued to move into the upper reaches during the spring to summer period. By May 1989 the daily-averaged SPM concentrations at CK were much higher than those further down-estuary at HQ(0·98 as opposed to 0·22 kg m -3 ). For the June and (both) July 1989 deployments, SPM levels at CK exceeded those at HQ, although at both stations the levels were lower in July. As the limited amount of fine sediment moved further up-estuary, through CK and closer to the head, the amount of bed-source sediment available for local erosion at CK and HQ was reduced.

Two synoptic, spring-tide, multi-station experiments in the 31 km-long Tamar Estuary were analysed for suspended particulate matter (SPM) concentrations and transport. SPM concentrations were very low in the seaward reaches and were largely associated with the flushing of fresh water from the estuary. Depth-averaged SPM concentrations in the upper 8·5 km of estuary were of the order of 103 higher than those near the mouth under similar, low-runoff, spring-tide, summer conditions. Local resuspension and advection of locally resuspended sediment from nearby bed-sources in the upper 8·5 km of estuary dominated the SPM loads during the flood. Highest SPM concentrations occurred during peak flood currents in the fresh water at 4·5 km from the head. At high-water, this location was in the vicinity of the freshwater-saltwater interface (FSI). During the ebb, SPM maxima occurred in fresh water throughout the upper 8·5 km of estuary. Near-bed measurements and longitudinal-transect data in the upper estuary showed that the FSI had an important influence on intratidal SPM variations.