Troels Aagaard

MEAN CURRENTS AND SEDIMENT TRANSPORT IN A RIP CHANNEL

Measurements of hydrodynamics and suspended sediment transport were carried out along a cross-shore transect in a rip channel. A tidally induced threshold for rip flow initiation was found; this threshold depended upon the degree of wave energy dissipation and could be identified through a critical value of the ratio of significant wave height to water depth (γs) in the rip neck. With normally incident waves, rip current velocities were well correlated with velocities predicted from a simple model involving the onshore mass transport in asymmetric breaking waves crossing the bar. Hydrodynamics and sediment transport were tidally modulated with strong offshore directed rip currents and associated offshore sediment transport due to the mean flow at low tide. At high tide when the rip was inactive, the tendency was for weak onshore directed mean flows and/or oscillatory incident waves transporting smaller amounts of sediment landward. The net result was a gradual lowering of the topographic relief along the rip channel with net deposition seaward of the rip. Outside the surf zone, transport was mainly landward due to oscillatory incident waves.

Cross-shore suspended sediment transport in the surf zone: a field-based parameterization

Existing cross-shore sediment transport models for two-dimensional surf zone bathymetries almost invariably predict offshore-directed sand transports and bar migrations during storm conditions. However, onshore-directed suspended sediment fluxes and associated nearshore bar migration were observed during recent field experiments on a gently sloping beach on the Danish North Sea coast. Field measurements of suspended sediment flux obtained during three experiments on two different beaches are used to parameterize the observed fluxes. This parameterization predicts suspended sediment transport due to incident waves and undertow across bars in two-dimensional surf zones. First, a non-dimensional sediment flux index is formulated which describes the tendency towards net onshore or offshore transport and the strength of that tendency. The non-dimensional formulation circumvents the problem of measurement inconsistencies due to varying elevations of sediment concentration sensors relative to the bed. The index is found to depend upon the undertow velocity, the incident wave skewness and the cross-correlation between orbital velocity and sediment concentration. However, some of these parameters are difficult to predict, particularly in barred surf zones and therefore, the independent variables are recast in terms of a set of more easily obtainable parameters. The sediment flux index depends on a combination of the following: non-dimensional bed shear stress (the Shields parameter), relative water depth, wave orbital velocity, relative wave height and bed slope. Finally, a formulation of suspended sediment transport across bars is obtained by linking the flux index with a parameterization of the sediment concentration/distribution in the water column. These concentrations are found to depend on non-dimensional bed shear stress, relative wave height and water depth. The formulation predicts a tendency for onshore-directed sediment transport due to incident waves on gently sloping beaches and/or with large bed shear stresses. On steeply sloping beaches and/or in the inner part of the surf zone there is a tendency towards offshore sediment transports due to the undertow.

Suspended sediment transport and nearshore bar formation on a shallow intermediate-state beach

Abstract Estimates of cross-shore suspended sediment flux were obtained during a 3-day storm event on the North Sea coast of Denmark. The suspended sediment flux estimates were correlated with volumetric beach change as registered at 28 permanently deployed survey rods crossing the intertidal beach and inner surf zones. During this storm, a nearshore bar initially migrated onshore and welded to the beach face at its updrift end; subsequently a new bar formed on the seaward slope of the former. Suspended sediment transport, recorded using optical backscatter sensors and electromagnetic current meters, corresponded well qualitatively with the morphological evolution; indications are that such measurements may account for a significant fraction of the actual cross-shore sediment transport. The onshore migration of the nearshore bar was the result of onshore directed mean currents associated with a cell circulation. Renewed bar formation took place when this circulation was (locally) replaced by an offshore-directed mean current (undertow). Suspended sediment transport due to oscillatory waves at incident and infragravity frequencies was generally subordinate to the mean transport.

Hydrodynamics and Sediment Fluxes across an Onshore Migrating Intertidal Bar

Abstract Detailed hydrodynamic and morphological data are presented from a field deployment spanning 2 days (four tide cycles). The data include bed-elevation changes measured at each low tide and continuous records of water-surface elevation, cross-shore and long-shore current velocities, and suspended sediment concentrations all measured within 20 cm of the bed. During the deployment, an intertidal bar migrated onshore and infilled a runnel on its landward side. The depth of this runnel was initially 0.6 m. During the migration of the bar, the significant wave height in deep water was ca. 2 m and wave period was 7 seconds. The significant wave height over the intertidal bar crest was about 0.25 m. Suspended sediment fluxes were estimated (product of current velocity and suspended sediment concentration profile) and partitioned between mean and oscillatory components with the latter further partitioned between short and long wave contributions. When the bar was migrating shoreward and infilling the runnel, estimated suspended sediment flux for all components was directed landward on the bar crest. Once the migrating bar had infilled the runnel, however, the suspended sediment fluxes for the mean component were directed seaward, whereas the short wave-driven flux was still directed landward. These results represent a clear example of morphodynamic interactions—(a) as waves cross the intertidal bar the onshore mean and oscillatory components transport sediment shoreward, (b) the presence of the runnel reduces the offshore component of oscillatory transport by channeling the flow alongshore, (c) the runnel rapidly infills due to the strong transport asymmetry, (d) once the runnel has infilled, the mean cross-shore current and mean sediment flux reverse direction. When the runnel is present, the general intertidal circulation is a horizontal cell circulation with rip currents, whereas it becomes a vertical undertow circulation when the runnel has infilled.

Suspended sediment transport and morphological response on a dissipative beach

Abstract This paper reports on some results from a field experiment conducted under highly dissipative surf zone conditions, occurring during a storm in Lake Huron, Ontario, Canada. Measurements of sediment resuspension and sediment flux were conducted at a number of stations across a nearshore bar. This bar moved approximately 25 m offshore during the storm, at a rate of ≈2.5 m h−1. The maximum erosion depth over the former bar crest was 0.92 m, with simultaneous accretion on the lakeward slope of the bar reaching 0.63 m. During the early hours of the storm, the accretion rate over the lakeward slope was approximately 0.05 m h−1. Mean sediment concentrations were up to 6.4 g 1−1, time series of sediment concentration were characterized by low-frequency fluctuations and the sediment resuspension was strongly constrained by infragravity waves. The major mechanism responsible for the offshore sediment flux was an offshore directed mean current, which reached a velocity of −0.34 m s−1. However, the reason for the erosion and the offshore migration of the bar was the strong spatial gradient in the cross-shore suspended sediment flux. This gradient was induced mainly by the infragravity waves, which transported sediment onshore at the former bar crest where the oscillatory flux balanced the mean flux, and offshore over the lakeward slope, reinforcing the mean flux at this location.

Infragravity waves and nearshore bars in protected, storm-dominated coastal environments

Abstract Field measurements of run-up oscillations were conducted on fetch-restricted, storm-wave dominated beaches in Denmark. Experiments took place under conditions with breaker heights of 1.5–3 m and wave periods of 5–7 s. Statistically significant spectral peaks were identified in the infragravity regime. The frequencies of these peaks were compared to frequencies predicted by the edge wave cut-off model, originally proposed by Huntley (J. Geophys. Res., 81, 1976), and it was found that the waves were probably cut-off mode edge waves with the mode number corresponding to the number of nearshore bars at the locality in question. The waves were usually standing alongshore, but during an experiment in which a very high energy level prevailed, the edge waves were probably progressive. The selection of only one or two cut-off modes may be the result of an interrelationship between the topographic cut-off and offshore wave groups. The nearshore region was surveyed following storms. It was found that there was generally a good correlation between bar positions and rhythmic dimensions and the theoretical cross-shore and longshore structure of the recorded edge waves. It is concluded that infragravity waves may be important in the formation, migration and morphology of nearshore bars, with the number of bars depending on the edge-wave mode number.

Suspended Sediment in the Swash Zone: Heuristic Analysis of Spatial and Temporal Variations in Concentration

Abstract Swash zone sediment transport is generally measured in an Eulerian reference frame, but this approach tends to give insufficient information on spatial transport characteristics. This paper utilizes a novel technique to map measured suspended sediment concentrations (SSC) onto the x–t plane, thus providing a visualization of SSC at all locations throughout a swash cycle. The widely adopted energetics or bed shear stress approaches to modeling suspended sediment transport in the swash predict that SSC varies directly with the horizontal velocity, u, to the power of either 2 or 3. A calibrated ballistic swash model was used to predict SSC(x,t) ∝ u2 and SSC(x,t) ∝ u3. The qualitative comparison between field measurements and transport model predictions in x–t space provides the opportunity to examine the transport model performance throughout the swash zone. The agreement between observed and modeled SSC patterns was generally poor. Similar to previous observations, SSC was larger during the uprush than the backwash. During the uprush, however, the models predict a maximum in SSC at the moving shoreline and a rapid drop-off behind the leading swash edge, whereas observed SSC was maintained at high levels for most of the uprush phase. During backwash, there was very little correspondence at all between the patterns of predicted and observed SSC; in particular, observed SSC was unexpectedly small in the mid- and late stages of the backwash. A general consensus is emerging that existing swash zone sediment transport models that assume SSC is in equilibrium with the horizontal flow velocity are inadequate. This study suggests that this is probably due to presuspended sediment being advected into the swash zone, the effects of bore-generated turbulence being advected into the swash zone, and both bed shear stress and Reynolds stresses being out of phase with the horizontal flow velocity.

Nearshore wave height variation in unsaturated surf

The nearshore evolution of wave height is presented from field observations during unsaturated surf conditions from 10 different beaches characterized by microtidal conditions and predominantly swell-dominated wave climates. Wave evolution is presented in terms of wave height to water depth ratio (gamma) for comparison with previous data from saturated surf. Both conventional time-averaged (gamma(rms)) and a new wave-by-wave analysis (gamma(w)) are performed. Values of gamma increase with increasing offshore wave height, indicating unsaturated surf. The observations show a variation in gamma values from near constant values in the mid surf zone to rapidly and asymptotically increasing gamma values in the inner surf zone. In contrast to previous data from saturated surf, gamma shows no dependence on either the absolute beach slope or the relative beach slope beta/k (h) over bar. The skewness of the distributions of gamma(w) is consistent with waves that are not depth limited. The inner surf zone wave heights are approximately equally dependent on the water depth and offshore wave height. The previous observations of gamma from saturated surf are shown to be consistent with a terminal bore height at the shoreline which is in excellent agreement with a previously derived value for the Miche parameter. In contrast, for the present unsaturated surf conditions, the terminal bore height at the shoreline can be approximated by H-b approximate to 0.12H(o), which is consistent with recent laboratory data sets.

Sediment supply to beaches: Cross-shore sand transport on the lower shoreface

Many beaches have been built by an onshore supply of sand from the shoreface, and future long-term coastal evolution critically depends on cross-shore sediment exchange between the upper and the lower shorefaces. Even so, cross-shore sediment supply remains poorly known in quantitative terms and this reduces confidence in predictions of long-term shoreline change. In this paper, field measurements of suspended sediment load and cross-shore transport on the lower shoreface are used to derive a model for sediment supply from the lower to the upper shoreface at large spatial and temporal scales. Data collection took place at five different field sites that exhibit a wide range of wave conditions and sediment characteristics. Data analysis shows that both suspended sediment load and cross-shore sediment transport scale with the grain-related mobility number which ranged up to ψ ≈ 1000 in the measurements while the effect of orbital velocity skewness is more limited. A 1 year long simulation of sediment transfers between the lower and the upper shorefaces on a natural beach compares well with transport rates estimated from long-term bar migration patterns and aeolian accretion on the same beach.

Cyclic sand bar migration on a spit-platform in the Danish Wadden sea-spit-platform morphology related to variations in water level

Abstract Cyclic morphology is related to water levels on a shallow spit-platform in the Danish Wadden Sea. Water levels were used as a dynamic parameter because low water levels, water levels at moderate wind set ups and high water levels occur at different wind situations and they result in different morphology. Low water levels result in a flat sand bar with one deep and narrow ebb channel. Water levels at moderate wind set ups result in several wide and shallow ebb channels. After a storm the spit-platform is flat and low with no dissecting ebb channels. Sand bars on the spit-platform migrate up to 35 m/month. A model proposing the different morphology at the three different water level stages is presented.