Flow Velocities From Intra Wave Video Observations example essay topic

Introduction Alongshore currents drive changes of coastal morphology and also embody a potential threat for swimmer safety. This explains the interest of coastal managers and scientists in cost-efficient techniques to quantify alongshore flow patterns. Traditional in-situ methods like electro magnetic flowmeters are expensive, time-consuming, dangerous to deploy during rough wave conditions and lack synoptic coverage. Recently a new technique was developed which quantifies flow velocities from intra-wave video observations of the nearshore (Chickadee et al., 2003).

This Optical Current Meter (OCM) was successfully tested against extensive field experiments at a swell-dominated, intermediate to reflective beach at Duck, NC (USA). In this paper, we investigate the applicability of the OCM at a dissipative beach at Noordwijk (the Netherlands), characterized by shorter waves and a mild beach slope. The Optical Current Meter (OCM) The OCM was developed at Oregon State University as part of the Argus video program (Holman et al., 1993). It is based on the analysis of short time series of image intensities, sampled from an alongshore array of pixels (cf. Fig. 3). This yields alongshore time stack images (Fig. 1), which typically reveal the bright horizontal bands of passing breaking waves and the oblique traces of foam patches drifting with the prevailing alongshore current.

The velocity of the foam patches gives a measure of the alongshore surface current velocity. To quantify the surface foam drift, intensity time stacks are first Fourier transformed to a frequency-wavenumber spectrum, and finally to a velocity distribution. A model of the velocity distribution is fit to the observed distribution to estimate the foam drift velocity. Field test comparisons against an in-situ bidirectional electro magnetic flowmeter, involving one month of video data sampled during the 1997 SandyDuck experiment, showed a rms error of 0.10 m / 's. OCM application in Egmond and Noordwijk (The Netherlands) To verify the generic applicability of the OCM, the model was applied to quantify alongshore flow velocities at two field sites along the Dutch coast. The first test, performed at Egmond, involved a comparison of video-derived flow velocities and ground-truth flowmeter measurements, both collected at 500 m north of the video station.

At this distance, alongshore pixel resolutions are in the order of 2 m. This resolution turned out to be insufficient to resolve foam patches, which explains poor OCM performance at Egmond. Fig. 1: Alongshore intensity time-stack, collected during SandyDuck (1997) The second test involved the validation of OCM against a two-week dataset of hourly measured alongshore flow velocities, sampled directly in front of the Noordwijk video station. With a rms off-set of about 30 cm, the technique shows improved performance (Fig. 2). For positive (i.e. southward-directed) velocities, video-derived velocities consistently overestimate the corresponding ground-truth measure-ments. This can be explained from the observation that the OCM estimates surface velocities at variable tidal levels, whereas the electro magnetic flowmeter measures a velocity in the water column, at about 25 cm above the sea bed.

We are now in the process of scrutinizing the data to explain the outliers in our data set. To illustrate the potential of the OCM, the model was used to quantify flow-velocities along 5 alongshore arrays spacing 10 m cross-shore. The result (Fig. 3) shows a realistic distribution of flow velocities across the surf zone, with maximum velocities up to 0.65 m /'s at about 40 m off the shoreline. Fig. 3: Video-derived cross-shore distribution of alongshore flow velocities at Noordwijk In summary This study demonstrates the generic applicability of a method to quantify alongshore flow velocities from video, over a range of wave climates.

Fig. 2: OCM validation against ground-truth flowmeter measurements at Noordwijk. -0.4 0.6 0 0.4 0.2 0.8 -0.2 vEM F (m /'s ) -0.4 0.2 -0.2 0.6 0.8 0.4 Vrms = 0.31 Vme an = 0.21 'o V = 0.22 0.7 0.2 0 0.5 0.3 0.4 0.1 0.6 Alongshore flow velocity (m /'s ) 125 140 135 130 150 155 145 160 Noordwijk (the Netherlands) The background of the model deviations found at Noordwijk needs further consideration, even though the video-derived alongshore flow field looks highly realistic. Providing enhanced opportunities for the cost-efficient, synoptic mapping of nearshore flow patterns, we conclude that this innovative technique is expected to contribute significantly to our understanding of nearshore processes and the assessment of coastal management issues such as swimmer safety. video (m) Cross-shore location (m).