Individual Tidal Cycles Research Articles

Application of a Novel Automatic Erosion and Deposition Monitoring System at a Channel Bank Site on the Tidal River Trent, U.K.

There is a well-defined need to improve understanding of the dynamics of sediment erosion and deposition on intertidal channel banks, given their importance to channel stability, sediment budgets, depth maintenance, pollutant and nutrient transport, and ecological processes in estuarine systems. Conventional, manual methods for field monitoring of erosion and deposition, however, normally deliver information of low temporal resolution conditioned by infrequent field resurveys. To address this problem, this paper discusses a recently developed and improved automatic erosion and deposition monitoring technique, the Photo-Electronic Erosion Pin (PEEP) system, and its application to a tidal channel bank site at Burringham on the River Trent, U.K. The PEEP system allows the magnitude, frequency and timing of individual erosion and deposition events to be monitored much more precisely than with conventional manual methods. PEEP sensors also monitor light intensity and sediment temperature, variables which can influence bank stabilizing and destabilizing processes. Example results at both the event and spring-neap timescales are presented from a short PEEP system deployment between March and May 1997 at Burringham. These establish that discrete erosion events of >60mm and 100mm can occur in response to individual tidal cycles, events which are readily monitored automatically and quasicontinuously by the PEEP system. The capability of the PEEP approach to enhance temporal resolution of monitoring is demonstrated by the determination of the timing of the 100-mm bank erosion incident to an ‘event window’ of 2·75h: this converts to mean bank erosion rate of 36mm h−1over the period of inundation. In addition, the PEEP system defines the magnitude and date of two example deposition events of 47 and 92mm on the lower bank during a sequence of rising spring tides. These represent mean deposition rates of 4·5 and 8·4mm h−1respectively over the periods of inundation. The Burringham site is shown to be highly active, with regular and dynamic erosion and deposition cycles. Upper bank surface elevation oscillations, driven by this sediment cycling, were characterized by a strong 14-day cycle which clearly reflected the spring-neap cycling of tidal range. Sediment was deposited on the bank relatively quickly, but removed by erosion rather slowly, giving an asymmetric sediment cycling profile. Higher bank elevations were strongly correlated with high tidal ranges, and especially to water level peaks 2 days previously. Incorporation of a simple Wind Stress Index further improved the statistical explanation of tidal bank elevation, and suggested that high on-shore wind speeds were associated with increased bank erosion. Such vigorous sediment cycling means that many erosion-deposition sequences on tidal banks can be self-concealing and therefore may not be recorded by infrequent manual resurveys which will inevitably underestimate total activity. This reinforces the need for an automated method such as the PEEP system to determine these typically cyclic sequences of self-cancelling accretion and removal activity if site dynamism is to be correctly quantified. The ability of the PEEP approach to generate such detailed and high-resolution information on the temporal distribution of erosion and deposition events in tidal environments should significantly enhance future process and applied studies, especially of entrainment thresholds, sediment recycling, estuarine sediment supply and sediment fluxes, the operation of biomediation mechanisms, bank erodibility changes, storage and residence times of contaminants and site management options.

Patterns of flow and suspended sediment concentration in a macrotidal saltmarsh creek, Bay of Fundy, Canada.

Abstract Measurements of velocity and suspended sediment concentration were carried out in a saltmarsh tidal creek network in the Cumberland Basin, Bay of Fundy, Canada. The study area was located on the NW shore of the basin in part of an undyked marsh that is about 200 m wide with a simple reticulate creek network. The area is macrotidal with spring tides greater than 12 m and suspended sediment concentrations in the basin characteristically range from 150–300 mg l −1 . The purpose of the study was to determine vertical and along channel variations in these two parameters over individual tidal cycles and to use these data to assess the role of the tidal creeks in the import and export of water and sediment from the marsh surface. Measurements using a vertical array of co-located electromagnetic current meters and OBS probes for measuring suspended sediment concentration were carried out over four spring tides at a cross section in the lower part of Middle Creek. Six sets of measurements were carried out at four locations along the length of the creek, a distance of about 200 m over six tides ranging from spring to neap. Maximum mean velocities measured over sampling times of eight minutes did not exceed 0.1 m sec −1 in Middle Creek and 0.15 m sec −1 in Main Creek. Transient high velocities associated with the overbank flows were weakly developed as a result of the absence of significant levees or embankments on the marsh surface. Suspended sediment concentrations in the creek generally decreased steadily over the period of inundation. Flow across the marsh margin occurred simultaneously with the achievement of bankful conditions and the creeks themselves appear to play a relatively minor role in the movement of water and sediment onto and out of the marsh. Despite the fact that the marsh surface is still low in the tidal frame and active sedimentation is still occurring, the low flow velocities and observations in the field suggest that the tidal creek network is unable to flush itself and that it is contracting.

Controls on suspended sediment deposition over single tidal cycles in a macrotidal saltmarsh, Bay of Fundy, Canada

Abstract A field study was conducted on a section of Allen Creek marsh in the Bay of Fundy to examine changes in suspended sediment circulation and deposition over single tidal cycles. Net flow velocity, suspended sediment concentration and sediment deposition were measured over 13 individual tidal cycles during the summer of 1998. A vertical array was deployed in the low marsh region, consisting of three pairs of electromagnetic current meters, OBS tm probes and one pressure transducer. Sediment deposition was measured using full-cycle sediment traps. The temporal distribution of sediment deposition was monitored using sequential sediment traps exposed at different tidal stages. The data suggest that sediment deposition on the marsh surface is primarily controlled by the interaction of water flow, marsh morphology and vegetation. The highest amounts of sediment are deposited during conditions of high suspended sediment concentration and low wave activity, particularly when the relative roughness of the vegetation is the highest. Loss of suspended sediment from the water column was shown to be correlated with the sediment trap data; however, predictions of sediment deposition based on the variation in suspended sediment concentration were found to be valid only for conditions with less than 0.15 m high waves. For higher wave conditions, the use of suspended sediment loss calculations should be used primarily for estimating the relative rather than absolute values of deposition on the marsh surface.

Estimating the Net Flux of Nutrients between a Salt Marsh and a Tidal Creek

Statistical aspects of estimating net fluxes of nutrients between a salt marsh and a tidal creek on a tidal cycle basis and an annual basis are explored. For individual tidal cycles, the instantaneous flux of a nutrient is written in a constrained linear model as a function of time. The model is rewritten as an unconstrained model, and net flux is shown to be a linear combination of the parameters of the model. Standard linear models techniques can be used to make inferences about net fluxes on a tidal cycle basis. Considering a year as a finite population of tidal cycles, annual net flux can be estimated using a regression estimator. In the case of the flux of dissolved nitrite plus nitrate, the marsh was found to be a statistically significant sink for nitrogen (in this form) from adjacent tidal creeks.

A sand wave field in the entrance to Bahia Blanca Estuary, Argentina

Sand waves in the mouth of the Bahía Bllanca Estuary show differences in wavelength, crestlength and crest shape characteristics between two sectors of the field. These differences may be explained by assuming that the residual ebboriented currents are stronger in the southern sector. Crest bifurcations on the centerline are suggestive of intermediate energy conditions and the development of secondary flows oblique to the main current direction. The large lee slopes and the presence of megaripple fans are indicative of flow separation, a condition seldom mentioned for subtidal sand waves. Boundaries of the sand wave field may be controlled by the thickness of the surficial sand layer, rapid reduction of the water depth as produced by a large marginal shoal, and variations in grain size. An average migration rate of 33 m yr −1 was calculated from repeated bathymetric surveys. Assuming that sand waves preserve shape and size, the established net sediment flux over the crests is 0.025 cm 3 cm −1 s −1. Current measurements made during four individual tidal cycles yielded a net bedload sediment transport of 0.048 cm 3 cm −1 s −1.

Nutrient flux in the Rhode River: Tidal transport of microorganisms in brackish marshes

Concentrations of bacteria, chlorophyll a, and several dissolved organic compounds were determined during 11 tidal cycles throughout the year in a high and a low elevation marsh of a brackish tidal estuary. Mean bacterial concentrations were slightly higher in flooding (7·1 × 10 6 cells ml −1) than in ebbing waters (6·5 × 10 6 cells ml −1), and there were no differences between marshes. Mean chlorophyll a concentrations were 36·7 μg l −1 in the low marsh and 20·4 μg l −1 in the high marsh. Flux calculations, based on tidal records and measured concentrations, suggested a small net import of bacterial and algal biomass into both marshes. Over the course of individual tidal cycles, concentrations of all parameters were variable and not related to tidal stage. Heterotrophic activity measured by the uptake of 3H-thymidine, was found predominantly in the smallest particle size fractions (< 1·0 μm). Thymidine uptake was correlated with temperature ( r = 0·48, P < 0·01), and bacterial productivity was estimated to be 7 to 42 μg Cl −1 day −1.

Tidal variations in the movement of organic carbon in New Jersey salt marshes

The movement of organic carbon was assessed by statistical and simulation modeling analyses in two marsh types in New Jersey;each marsh contained three water-drainage systems in which three tidal cycles were sampled in May and June 1973. Hourly water samples were obtained and filtered through a gelman Type A glass filter for separation into dissolved (DOC) and particulate (POC) organic carbon components of the total organic carbon (TOC). Simulation data showed that individual creeks and marshes functioned differently on the sampled tidal cycles in regard to net movement of water and organic carbon components. Organic carbon components exhibited similar tidal variations, with significantly lower concentrations at flood slack than at ebb slack. Mid-ebb concentrations were significantly higher than mid-flood concentrations for TOC and POC. Individual marshes showed significantly different concentrations in the latter segments of the tidal cycle for TOC's and POC's. Results indicate that individual creeks, marshes and tidal cycles are not representative of the total movement of organic carbon in estuaries.

Malleable bronze

Bedform measurements were obtained under shoaling and breaking wave conditions using a pencil-beam sonar in the intertidal zone at Skallingen, Denmark. Measurements were made during four individual tidal cycles starting from flat bed conditions at low tide and as the tide rose and the instruments entered the surf zone, megaripples appeared when the mobility number exceeded 240, which equates to the classic flat-bed criterion proposed by Dingler and Inman (1976). The megaripples were oriented cross-shore and their wavelength scaled with the wave orbital diameter. Wave ripples were observed when the mobility number was smaller which occurred mainly under shoaling waves. Since all bedforms were erased as the seabed became dry at low tide and perpendicular longshore currents were occasionally strong, the megaripples were of small amplitude but their dimensions were consistent with predictions from the literature. Their migratory behaviour in the first few hours of existence appeared more strongly related to self-organisation rather than to ambient hydrodynamics. A clear gap in bedform spacing at λ = 120 cm was identified between wave ripples and megaripples.