Marsdiep Inlet Research Articles

Deployment of a floating tidal energy plant in the Marsdiep inlet: resource assessment, environmental characterization and power output

In this work, BlueTEC, a tidal energy floating platform deployed in the Marsdiep inlet, is introduced. The patterns of circulation of the Marsdiep inlet and an energy resource assessment followed by the respective site selection for the platform’s deployment are reported. This manuscript analyses different datasets sampled during the project, including current speeds, wave parameters, turbine power output, platform excursions, and other environmental parameters such as temperature and salinity. Most of the datasets were measured by instruments installed on the platform, with the exception of the wave data, which was sampled by a nearby oceanographic mooring, and except the environmental parameters, which were estimated via satellite and numerical modelling. The effect of the turbine on the water velocity and preliminary results on the power output are discussed. Finally, discussions on the marine growth, potential environmental impacts and future perspectives are also addressed.

Long-term ferry-based observations of the suspended sediment fluxes through the Marsdiep inlet using acoustic Doppler current profilers

Long-term measurements with a hull mounted acoustic Doppler current profiler (ADCP) under the ferry, crossing the Marsdiep inlet between the mainland and the island of Texel (the Netherlands), were used to determine the volume flux and the flux of suspended particulate matter (SPM) through this inlet for the period 2003–2005. Profiles of the SPM concentration were estimated from profiles of the acoustic backscatter intensity in which the shift between the low and the high turbulent regime is taken into account. Calibration constants and tuning parameters were estimated by using data collected during 7 different 13hour anchor stations. The residual (water) volume flux through the inlet appears to vary strongly on a variety of time scales from daily to inter-annual. A regression analysis indicates that the daily residual volume transport correlates well with the daily mean wind component from the south; the latter likely drives the residual flow along the coast of Holland. The observed residual SPM transport of 7 to 11Mton/yr is dominated by the correlation between tidal velocity and SPM concentration variations. This leads to an import as currents and SPM concentrations during flood were higher than those during ebb, a process generally known as tidal asymmetry. Our analysis has shown that regular observations with a ferry mounted ADCP is an effective method to monitor the volume and SPM transport processes in an estuary.

Variability of secondary currents in a weakly stratified tidal inlet with low curvature

Long-term ferry acoustic Doppler current profiler (ADCP) and surface density data collected in the period 2000–2002, and ADCP data and vertical profiles of density collected during several 13-h surveys in the period 2003–2006 provide insight in the variability of instantaneous secondary (transverse) currents in the 4 km wide and 27 m deep Marsdiep tidal inlet, the Netherlands. The application of the data to the transverse momentum balance indicates that centrifugal and Coriolis accelerations and baroclinic pressure gradients govern the observed secondary circulations. In well-mixed conditions, local curvature ( | R s | ≈ 10 km ) dominates over Coriolis forcing in driving secondary currents. In stratified conditions transverse density gradients become more important than centrifugal and Coriolis accelerations. The data suggests that the well-mixed Marsdiep inlet falls in the regime where stratification still enhances secondary currents. In the inlet the vertical stratification of maximally O ( 1 kg m - 3 ) is in part set up due to transverse density gradients. The stratification dampens turbulent mixing, which allows for surface secondary currents up to 0.4 m s - 1 ( ∼ 30 % of the streamwise currents).

Long-term evolution of sand waves in the Marsdiep inlet. I: High-resolution observations

A seven-year long data set from 1998 to 2005 of sand waves in the Marsdiep tidal inlet in the Netherlands is presented. The sand waves are visible on digital terrain models (DTMs) that were compiled from water depths obtained with a ferry-mounted acoustic Doppler current profiler. This multi-year data set allows to study the spatial and temporal evolution of sand-wave migration, height, length, asymmetry, and orientation. Horizontal migration rates were successfully obtained with a simple cross-correlation technique similar to techniques used in particle tracking. The study area can be divided in roughly two halves. In the southern half of the inlet, the sand waves are of the progressive type with area-mean heights of about 3 m and lengths of about 190 m. In the northern half, the sand waves are asymmetrical-trochoidal with area-mean heights of about 2 m and lengths of about 165 m. Across the inlet, the sand waves migrate in the flood direction with rates between 0 and 90 m y - 1 . Only the progressive waves in the southern half migrate in directions perpendicular to their crests. A striking observation is the seasonal variability in height and migration of the sand waves in the northern half of the inlet. The sand-wave heights are about 0.5 m higher in fall than in spring and the migration rates are about 30 m y - 1 higher in winter than in summer. This paper only presents observations and analysis techniques. In a subsequent paper, the ADCP-current data will be applied to explain the spatial and seasonal variability of the sand waves.

Long-term evolution of sand waves in the Marsdiep inlet. II: Relation to hydrodynamics

A discussion is presented about the mechanisms that govern the spatial and seasonal variability in sand-wave height and migration speed in the 4 km wide Marsdiep tidal inlet, the Netherlands. Since 1998, current velocities and water depths have been recorded with an ADCP that is mounted under the ferry ‘Schulpengat’. In this paper, the current measurements were used to explain the sand-wave observations presented in Buijsman and Ridderinkhof [this issue. Long-term evolution of sand waves in the Marsdiep inlet. I: high-resolution observations. Continental Shelf Research, doi: 10.1016/j.csr.2007.10.011]. Across nearly the entire inlet, the sand waves migrate in the flood direction. In the flood-dominated southern part of the inlet, the ‘measured’ (i.e. based on sand-wave shape and migration speed) and predicted bedload transport agree in direction, magnitude, and trends, whereas in the ebb-dominated northern part the predicted bedload and suspended load transport is opposite to the sand-wave migration. In the southern part, 55% of the bedload transport is due to tidal asymmetries and 45% due to residual currents. In addition to the well-known tidal asymmetries, asymmetries that arise from the interaction of M 2 and its overtides with S 2 and its compound tides are also important. It is hypothesised that in the northern part of the inlet the advection of suspended sand and lag effects govern the sand-wave migration. The relative importance of suspended load transport also explains why the sand waves have smaller lee-slope angles, are smaller, more rounded, and more three-dimensional in the northern half of the inlet. The sand waves in this part of the inlet feature the largest seasonal variability in height and migration speed. This seasonal variability may be attributed to the tides or a seasonal fluctuation in fall velocity. In both cases sediment transport is enhanced in winter, increasing sand-wave migration and decreasing sand-wave height. The influence of storms and estuarine circulation on the sand-wave variability is negligible.

Water transport at subtidal frequencies in the Marsdiep inlet

Long-term time series of subtidal water transport in the 4-km wide Marsdiep tidal inlet in the western Dutch Wadden Sea have been analysed. Velocity data were obtained between 1998 and the end of 2002 with an acoustic Doppler current profiler that was mounted under the hull of the ferry ‘Schulpengat’. Velocities were integrated over the cross-section and low-pass filtered to yield subtidal water transport. A simple analytical model of the connected Marsdiep and Vlie tidal basins was extended to include wind stress and water-level and density gradients and applied to the time series of subtidal water transport. In accordance with the observations, the model calculates a mean throughflow from the Vlie to the Marsdiep basin. The mean water transport through the Marsdiep inlet consists of an export due to tidal stresses and freshwater discharge and an import due to southwesterly winds. In contrast, the variability in the subtidal water transport is mainly governed by wind stress. In particular, southwesterly winds that blow along the main axis of the Marsdiep basin force a throughflow from the Marsdiep to the Vlie basin, whereas northwesterly winds that blow along the main axis of the Vlie basin force a smaller mean water transport in the opposite direction. The contribution of remote sea-level change to the water transport, or coastal sea-level pumping, has been found to be much smaller than the contribution of local wind stress.

Long-term ferry-ADCP observations of tidal currents in the Marsdiep inlet

A unique, five-year long data set of ferry-mounted ADCP measurements in the Marsdiep inlet, the Netherlands, obtained between 1998 and 2003, is presented. A least-squares harmonic analysis was applied to the water transport, (depth-averaged) currents, and water level to study the contribution of the tides. With 144 tidal constituents, maximally 98% of the variance in the water transport and streamwise currents is explained by the tides, whereas for the stream-normal currents this is maximally 50%. The most important constituent is the semi-diurnal M 2 constituent, which is modulated by the second-largest S 2 constituent (about 27% of M 2). Compound and overtides, such as 2MS 2, 2MN 2, M 4, and M 6, are important in the inlet. Due to interaction of M 2 with its quarter-diurnal overtide M 4, the tidal asymmetry in the southern two thirds of the inlet is flood dominant. The amplitudes of all non-astronomic constituents are largest during spring tides, strongly distorting the water level and velocity curves. The M 2 water transport is 40° ahead in phase compared to the M 2 water level, reflecting the progressive character of the tidal wave in the inlet. The currents are strongly rectilinear and they are sheared vertically and horizontally, with the highest currents at the surface above the deepest part of the inlet. During spring tides, near-surface currents can be as large as 1.8 m s − 1 . Due to the relative importance of inertia compared to friction, the M 2 currents near the centre (surface) lag maximal 20° (3°) in phase with the currents near the sides (bottom). The tidal-mean currents are directed into the basin in the shallower channel to the south and out of the basin in the deeper channel to the north.

Estimating suspended sediment concentration using backscatterance from an acoustic Doppler profiling current meter at a site with strong tidal currents

Since 1998, ferry observations have been carried out in the Marsdiep tidal inlet (Dutch Wadden Sea), using amongst other instruments a vessel-mounted acoustic Doppler current profiler (ADCP). Besides 32 cross-sections a day of current velocity data, the instrument also records the echo intensity, which has been interpreted in terms of suspended sediment concentration (SSC) before (Thorne and Hanes, Cont Shelf Res 22:603–632, 2002). However, we show herein that the random phase scattering model as outlined by Thorne and Hanes (Cont Shelf Res 22:603–632, 2002), predicts unrealistically high values of SSC if the depth-averaged current velocity exceeds approximately 0.7 m/s. Therefore, we extended the random phase scattering model by including scattering by turbulence-induced variability in SSC. The important mechanism is that when SSC fluctuations are present at length, scales of the order of the acoustic wavelength, the phase of the returned signal is no longer random and causes stronger backscatter. Such SSC fluctuations occur as a result of turbulent eddies in combination with an SSC gradient. The extended model was compared with data of two field surveys carried out in the Marsdiep inlet. The extended model, when compared with the classical random phase model, showed a large improvement of accuracy of the estimated SSC, which allows us to apply the model to the ferry data set to analyse suspended sediment transports through the Marsdiep tidal inlet.

Year-class strength of plaice Pleuronectes platessa in the Southern Bight of the North Sea:a validation and analysis of the inverse relationship with winter seawater temperature

The previously (1974-1982) observed inverse relationship between the February seawater temperature at the spawning grounds in the Southern Bight of the North Sea and subsequent year-class strength of 0-group plaice on the Balgzand tidal flats (western Dutch Wadden Sea) was revalidated for the period 1991-1998. However, seawater temperatures determined from NOAA satellite images showed that water temperatures during egg and larval drift in the open sea were more constant than the February temperatures at the spawning grounds suggested. In all years, and under all conditions, there was a persistent tongue of English Channel water entering through the Dover Strait into the Southern Bight of the North Sea, in which plaice eggs and larvae developed and were transported. The intensity of this plume varied between years, but even during the coldest period it penetrated as far as offshore Texel and the Marsdiep inlet The absence of interruptions in the pattern of larval immigrations at the nursery implies a link between variations in water temperatures and in the period of drift. Between and within years, the number of settling larvae at Balgzand was inversely related to the median temperature during drift. Overall, also year-class strength estimates were still inversely related to water temperature during drift. However, the relationship broke down for the very strong 1996 year-class, suggesting that other factors were acting too. Cold winters (reflected in low temperatures in the coastal zone are associated with persistent winds from the east which induce an atypical water circulation pattern that affects the strength and direction of the residual current. Easterly winds might also generate upwelling near the coast stimulating larval immigration.

Assessment of growth rate limiting nutrients in the north sea by the use of nutrient-uptake kinetics

In eutrophication studies it is important to estimate the impact of various macro-nutrients. Nutrient-uptake kinetics as a tool for identification of nutrient-limited growth was applied on field samples from the North Sea and the western-most inlet of the Wadden Sea (Marsdiep). It was demonstrated that low concentrations of dissolved nutrient do not automatically imply that they are limiting the growth rate of natural phytoplankton. Subsurface samples (5 m) taken on the North Sea in July 1988 showed nutrient limitation at all stations. At greater depths nutrient limitation was less severe. N-limitation dominated. At some stations more than one nutrient was limiting the multi-species population. During spring 1989 the diatom bloom in the Marsdiep inlet (The Netherlands) became silicate-limited. At the end of the Phaeocystis bloom, N-limitation was severe whereas during the decay of the bloom the N-limitation disappeared. During this transient period high numbers of ciliates (up to 54·10 3 cells·dm −3) and tintinnids (17·10 3 cells·dm −3) were present.