Argus & BeachWizard

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Argus: nearshore bathymetry derived from video imagery

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In the nearshore zone, extending from the beach to water depths of about 10 m, a number of functions are involved, as protection of the hinterland against flooding, nature conservation, recreational activities, settlement of coastal communities. Due to the highly dynamic nearshore morphology, research on the development of coastal environment has attracted both scientists and coastal managers.

Traditional in-situ survey methods provide excellent data but require major logistical commitments and often lack spatio-temporal resolution to resolve processes of interest. While perhaps of lower accuracy, remote sensing techniques offer the potential for cost-efficient, long term data collection with high resolution in time and space. The ARGUS system developed at Oregon State University, based on shore-based video techniques, enables the monitoring of nearshore bathymetry at spatiotemporal scales of direct management and research interest.

Image sampling scheme can be designed to collect time series of pixel intensities, typically at 2 Hz, with which wave and flow characteristics can be investigated. Collection every half an hour usually consists of three types of images (Figure 1):

• snapshot images serve as simple documentation of the ambient conditions but offers little quantitative information;
• time exposure images average out natural modulations in wave breaking to reveal a smooth pattern of bright image intensities, which are an excellent proxy for the underlying, submerged sand bar topography. Time exposures also "remove" moving objects from the camera field of view, such as ships, vehicles and people;
• variance images help identify regions which are changing in time (like the sea surface), from those which may be bright, but are unchanging (like the dry beach);

Panoramic and plan view merged images can be composed by geo-referencing the images from all the cameras of an Argus station. Plan view images enable the measurement of length scales of morphological features like breaker bars and the detection of rip currents.

Shoreline location is detected from color or luminance difference between dry and wet beach, and using actual tide and wave conditions. The Intertidal bathymetry, i.e. the three-dimensional beach surface between the low-tide and high-tide shoreline contours, is obtained by mapping a series of beach contours, sampled throughout a tidal cycle (Figure 2).

Surf zone bathymetry is obtained in cooperation with Beach Wizard, throughout the assimilation of video-observed and model-predicted patterns of wave dissipation.

Beach Wizard: up-to-date and accurate information about nearshore bathymetry

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Coastal managers need up-to-date and accurate information about the state of their coast, including the state of the nearshore bathymetry, the area that is below the tidal level but is changing everyday.

Beach Wizard can supply this information using a 'smart' translation of video images to bathymetries. Beach Wizard is developed by Deltares in close cooperation with the Naval Research Laboratories, US Geological Survey and Oregon State University, the originator of the ARGUS video system.

It is well-known that waves propagate from deeper water towards the shore and break on so-called breaker bars. Beach Wizard makes use of the unique relation between wave breaking intensity and local depth. It compares (ARGUS) video images of wave breaking intensity that are taken with shore-based camera's with a computer model output of wave breaking. If there is a difference in location of the wave breaking area, or the intensity of wave breaking, Beach Wizard gradually adjusts the depth in the computer model. It does so for many consecutive (hourly) image, so that the computer and video images of the wave breaking will coincide. If video and model match, then due to the relation between breaking and local depth, the modelled and true bathymetries will also match as has been verified by comparing with field measurements (Figure 3). Using the accurate prediction of the bathymetry, the model can predict local currents, such as rip currents.

In this way the Beach Wizard system can provide hourly information of the beach state, waves and currents with a very high resolution (Figure 4), and for a fraction of the cost of field measurements. It can provide information about the coastal state for coastal managers and about the waves and currents for the purpose of swimmer safety.