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EARSeL: 2nd Workshop on Remote Sensing of the Coastal Zone Porto, Portugal, 9-11 June 2005 |
SESSION COASTAL ZONE MANAGEMENT |
Susanne Quartel, Elisabeth A. Addink
Department of Physical Geography, Utrecht University
P.O.Box 80.115, 3508 TC Utrecht, The Netherlands
s.quartel@geog.uu.nl, e.addink@geog.uu.nl
The intertidal zone of the beach extends from the high to the low water line. Morphological features such as bars, troughs and rip channels can be found on the intertidal beach. A bar is a shore-parallel or oblique elongated body of sand with a landward-located depression, which is the trough, and may be intersected every few hundreds of metres by cross-shore rip channels. The presence of the bar protects the shoreward located dunes against erosion by forcing waves to break, whereas the currents in the rip channels are potentially dangerous to swimmers. In this contribution the focus is on the semi-automated detection of bars, troughs and rip channels from routinely collected video images (i.e. ARGUS system). A time series of 12 consecutive low-tide images recorded at Noordwijk beach (The Netherlands) was selected and studied in detail. The three morphological phenomena can be distinguished by classifying the image into three classes: dry sand, wet sand and water. The extraction of this information from the images is done by an object-oriented approach. This resembles the human cognition system much closer than the conventional per-pixel approach, since it provides spectral, shape and context information for image interpretation. Quadratic discriminant analysis (QDA) was used to create a classification model for each day.
Visual inspection shows a high similarity of the shapes in digitized maps of the area and the classification results. Accuracy values of the individual classifications range from 85 to 92%. The sequence of the classifications clearly shows the dynamics of the intertidal bars and the associated troughs and rips over the selected 12 days. In conclusion, object-oriented image analysis combined with QDA offers a valuable method to semi-automatically extract intertidal morphological phenomena from video imagery. This opens up possibilities to quantify and study intertidal beach morphodynamics over long time scales (weeks to years) and large spatial scales (kilometres alongshore).
Last Update: 2005-03-17