When monitoring coastal erosion or accretion by remote sensing data, a common difficulty is the determination of the coastline. Where is it located on the coastal profile in the dominated – tide and dominated - wave coasts? Can you share your experiences?
An interesting question! It seems simply in tidal domination coast. For wave domination coast, I don’t know how to remove wave high impact on satellite images. For Ex.: an image has taken about 3p.m but we don’t know wave condition in that time, and how to remove wave high from the image to evaluate coast condition?
This question is addressed in most coastline identification methodologies. I don't recall at the moment any references, but surely there are plenty available in the internet.
Anyway, it is common to consider one of the following as the "coastline", in order to estimate erosion/accretion rates:
- the line of separating wet/dry beach, which is essentially the line of high tide plus wave run-up (and wave set-up):
- the line of initiation of vegetation in natural beach-dune systems with natural vegetation;
- the line of initiation of the dune position (when identifiable through a sudden change of slope in the beach profile, or a change of light/colour in an aerial photography).
If one uses the instantaneous line of beach/water interface, one should theoretically correct for the above mentioned effects: tidal height, and wave set-up+run-up. Any of those require local instrumentation, the simplest to correct being the tidal level, by knowing a DEM of the beach (or a proxy of it) and the tidal level from a nearby tidal station. The correction for the instantaneous wave run-up position is generally impossible as one doesn't record the instantaneous position and synchronize with the time of the photo!
Hope it helps.
Actually we did a coast line evolution and use tidecalc program to calculate tide level at time of satellite data acquisation and did the correction based on DEM.
It proved reasonably good.
Adding on Saif Uddin said, I believe the results can be good on low energetic coasts, and as long as your tidal calculation is also good (it may fail if local phenomena affect those calculations or the bathymetry/geometry is badly represented). High energy coasts are associated with strongly variable beach morphologies, and thus the DEM may not hold at all for the time of the remote sensed data.
Cheers.
This question has been a subject of intense discussion in various circles. Determining the appropriate proxy for the shoreline position depends on several factors. The dry/wetland boundary remains the popular proxy.
Francisco is true. it probably worked well in our case because mostly it is a low energy coast. And we used teh tide calc which was supported by lots of tidal measurements. The DEM developed was also pretty accurate. I have developed a DEM using ASTER images, with pretty good resolution for this sort of work.
Thank you so much all, Vu Duy Vinh, Francisco Sancho, Erk Borg, Saif Uddin, Kwasi Appeaning Addo. Your warming suggestions are very precious.
In general set the coastline using remote sensing techs is quite tricky. According to my experience the accuracy of any methods depends on the amount of field data you have (number of images, local tide prediction, wave climate, etc). That's why we had been using Video cameras in conjunction with tide measurements and topographic surveys to set the coastline position. Using such a system you will be able to build a huge database of coastlines for each wave-tide condition. There are many algorithm to estimate the coastline position from video images (i.e: Plant et al., 2007 "The Performance of Shoreline Detection Models Applied to Video Imagery" and Almar et al., 2012 "Video-Based Detection of Shorelines at Complex Meso–Macro Tidal Beaches"), but none of them work for all conditions. Changes in light, bright, solar radiation, clouds, sand reflectance, etc may affect the detection process and also the accuracy of the method. However, those methods may work with satellite images as well.
For a more general view on RS techniques applied for Shoreline detection see Moore (2010) "Shoreline Mapping Techniques".
Hope it will be helpful
You might wish to look at a paper just published online in Geo-Marine Letters which describes a good way of defining the shoreline along a wave-dominated coast. The reference is: Shtienberg, G., Zviely, D., Sivan, D., Lazar, M. (2014). Two centuries of coastal change at Caesarea, Israel: natural processes vs. human intervention. Geo-Marine Letters (online).
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