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Nevertheless, between 2006 and 2008 several tropical storms and hurricanes had taken place close to the Progreso beaches. During the 2006 Atlantic Hurricane Season 10 tropical cyclones occurred (NHC, 2006). The tropical storm Alberto was particularly close to Yucatan beaches. During the 2007 Atlantic Hurricane season, 17 tropical cyclones occurred, with two tropical cyclones in close proximity, Olga and Erin, and Hurricane Dean crossed the Peninsula in August (NHC, 2007). In 2008 10 tropical cyclones occurred before the satellite acquisition, Hurricane Dolly crossed very close to the Progreso beaches (NHC, 2008). All these tropical cyclones events were likely to have had an impact on the wave conditions and on the Yucatan beaches.

Figure 5 Nevertheless, between 2006 and 2008 several tropical storms and hurricanes had taken place close to the Progreso beaches. During the 2006 Atlantic Hurricane Season 10 tropical cyclones occurred (NHC, 2006). The tropical storm Alberto was particularly close to Yucatan beaches. During the 2007 Atlantic Hurricane season, 17 tropical cyclones occurred, with two tropical cyclones in close proximity, Olga and Erin, and Hurricane Dean crossed the Peninsula in August (NHC, 2007). In 2008 10 tropical cyclones occurred before the satellite acquisition, Hurricane Dolly crossed very close to the Progreso beaches (NHC, 2008). All these tropical cyclones events were likely to have had an impact on the wave conditions and on the Yucatan beaches.

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Table 1. Wavelengths detected by each band from the SPOT satellite. Table 2. Tide levels relative to BMI for each SPOT image used to extract shoreline and the measured slope f rom beach profiles and their dates.
Table 1. Wavelengths detected by each band from the SPOT satellite. Table 2. Tide levels relative to BMI for each SPOT image used to extract shoreline and the measured slope f rom beach profiles and their dates.
Figure 1. Spectral profiles on cross shore sections between shallow water and land showing the spectral intensities from the three bands (Green, Red, Near Infrared) for locations where the largest offset between the ground and satellite-derived shoreline occurs (A) and where the offset is less than 0.5 m (B). = Sto =e UL oe TS ES ee Oe ee ee ee ee es ea ee ee lm el el ee The reflectance values from red, green and near infrared wavelengths show a more limited variation anc smaller spectral intensities over the water than over the land, as shown in the examples in Figure 1. The smallest values, around 50 to 100, correspond to water, and the higher values, around 150-250, belong tc land. The shoreline has been identified as the boundary between the groups, at 70 m in the cases shown ir Figure 1A-B. The sharpness of the change in intensities from sea to land means that the shoreline can be identified with an accuracy of a pixel, and this has been confirmed by tests which show that shoreline location is insensitive to the convergence threshold used in the clustering algorithm.
Figure 1. Spectral profiles on cross shore sections between shallow water and land showing the spectral intensities from the three bands (Green, Red, Near Infrared) for locations where the largest offset between the ground and satellite-derived shoreline occurs (A) and where the offset is less than 0.5 m (B). = Sto =e UL oe TS ES ee Oe ee ee ee ee es ea ee ee lm el el ee The reflectance values from red, green and near infrared wavelengths show a more limited variation anc smaller spectral intensities over the water than over the land, as shown in the examples in Figure 1. The smallest values, around 50 to 100, correspond to water, and the higher values, around 150-250, belong tc land. The shoreline has been identified as the boundary between the groups, at 70 m in the cases shown ir Figure 1A-B. The sharpness of the change in intensities from sea to land means that the shoreline can be identified with an accuracy of a pixel, and this has been confirmed by tests which show that shoreline location is insensitive to the convergence threshold used in the clustering algorithm.
Over the 8 km shoreline stretch measured, the ground shoreline and the satellite-derived shoreline differ from each other by a mean of -0.19 m and a standard deviation of 4 m. The mean and the standard variation are much smaller than the pixel size, showing that in fact the satellite-derived shoreline is close to the edge of the run up maximum. Figure 2 shows an example of the fitting of both shorelines. The agreement over most of the beach stretch is well within the pixel size. Figure 2. Ground shoreline and satellite-derived shoreline superimposed on the image from 20" of September 2008. The upper image shows close up from the lower image. The black dots represent the ground shoreline and the red line the satellite-derived shoreline.
Over the 8 km shoreline stretch measured, the ground shoreline and the satellite-derived shoreline differ from each other by a mean of -0.19 m and a standard deviation of 4 m. The mean and the standard variation are much smaller than the pixel size, showing that in fact the satellite-derived shoreline is close to the edge of the run up maximum. Figure 2 shows an example of the fitting of both shorelines. The agreement over most of the beach stretch is well within the pixel size. Figure 2. Ground shoreline and satellite-derived shoreline superimposed on the image from 20" of September 2008. The upper image shows close up from the lower image. The black dots represent the ground shoreline and the red line the satellite-derived shoreline.
Figure 3. Predicted tide in meters in the horizontal axis and corrected height to a standard atmospheric pressure along the beach surveyed in Progreso, Mexico on September 9" 2008. ea <—)e eeee eeeeeeeeeeeeeeeee e eeeee The DGPS measurements of the shoreline began at the eastern side of the pier and then moved to the we stern side and during this period the tide level was falling, with a predicted drop of around 19 cm between t he start and end of the measurement period. To see whether this drop is detectable we have computed separ ate means and standard deviations of the differences between the measured and satellite-derived shorelines for the eastern and the western sides of the pier. For the eastern side, the resulting mean is +2.3 m with a sta ndard deviation of 2.1 m whilst to the west the mean is -4.2 m with a standard deviation of 2.8 m. A positiv e mean indicates that the DGPS shoreline is landward of the satellite shoreline, so the drop in the mean fro m east to west is qualitatively consistent with a drop in sea level over the measurement period. However th e magnitude of the difference is much greater than expected on the basis of the predicted drop in tide level; a 19 cm drop on a beach slope between 0.05 and 0.3 gives a difference of only 0.6 — 3.8 m, whilst the obser ved difference from east to west is 6.5 m. Interestingly however the measured vertical elevations of the DG PS survey also provide some independent evidence that the actual drop in mean water level was considerab ly larger than the tide prediction. Figure 3 shows a plot of the DGPS vertical elevation, corrected for atmos pheric pressure variations, versus the predicted tide level. The rapid variations may represent fluctuations i n the swash line but there is a longer term slope which fits a linear trend with a high correlation (r°=0.8). H owever the slope of this trend is about three times greater than expected, suggesting a drop of sea level of a round 40 cm; with atmospheric pressure effects included this increases to around 50 cm. The equivalent hor izontal shoreline excursion would be between 1.6 and 10 m, thus spanning the observed difference between the means to the east and west of the pier.
Figure 3. Predicted tide in meters in the horizontal axis and corrected height to a standard atmospheric pressure along the beach surveyed in Progreso, Mexico on September 9" 2008. ea <—)e eeee eeeeeeeeeeeeeeeee e eeeee The DGPS measurements of the shoreline began at the eastern side of the pier and then moved to the we stern side and during this period the tide level was falling, with a predicted drop of around 19 cm between t he start and end of the measurement period. To see whether this drop is detectable we have computed separ ate means and standard deviations of the differences between the measured and satellite-derived shorelines for the eastern and the western sides of the pier. For the eastern side, the resulting mean is +2.3 m with a sta ndard deviation of 2.1 m whilst to the west the mean is -4.2 m with a standard deviation of 2.8 m. A positiv e mean indicates that the DGPS shoreline is landward of the satellite shoreline, so the drop in the mean fro m east to west is qualitatively consistent with a drop in sea level over the measurement period. However th e magnitude of the difference is much greater than expected on the basis of the predicted drop in tide level; a 19 cm drop on a beach slope between 0.05 and 0.3 gives a difference of only 0.6 — 3.8 m, whilst the obser ved difference from east to west is 6.5 m. Interestingly however the measured vertical elevations of the DG PS survey also provide some independent evidence that the actual drop in mean water level was considerab ly larger than the tide prediction. Figure 3 shows a plot of the DGPS vertical elevation, corrected for atmos pheric pressure variations, versus the predicted tide level. The rapid variations may represent fluctuations i n the swash line but there is a longer term slope which fits a linear trend with a high correlation (r°=0.8). H owever the slope of this trend is about three times greater than expected, suggesting a drop of sea level of a round 40 cm; with atmospheric pressure effects included this increases to around 50 cm. The equivalent hor izontal shoreline excursion would be between 1.6 and 10 m, thus spanning the observed difference between the means to the east and west of the pier.
Figure 4A-B shows the three different satellite-derived shorelines overlaid on the 2008 satellite image. There are differences of more than 40 m between 2004 and 2008 shoreline. It is possible to appreciate that along the whole beach the oldest shoreline is consistently seaward of the most recent one and the 2006 shoreline shows that the landward movement has been progressive over this period. Figure 4. The upper images show a close up within the A and B squares shown in the lower image, showing three satellite-derived shorelines from 2004, 2006 and 2008.
Figure 4A-B shows the three different satellite-derived shorelines overlaid on the 2008 satellite image. There are differences of more than 40 m between 2004 and 2008 shoreline. It is possible to appreciate that along the whole beach the oldest shoreline is consistently seaward of the most recent one and the 2006 shoreline shows that the landward movement has been progressive over this period. Figure 4. The upper images show a close up within the A and B squares shown in the lower image, showing three satellite-derived shorelines from 2004, 2006 and 2008.
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