3.2. Plume extent and duration
Fig. 10 combines the MODIS and Landsat data to show the estimated areal extent of the plumes as a function of time. For this representation, delineations made using RGB-only images, as well as instances where high Rrs(667) would be expected during normal (non-dredging) conditions (‘environmental flag’), are separately indicated. A clear increase in the size and frequency of plumes is visible after the start of dredging activities. Prior to the start of dredging, sediment plumes were identified on 10 (of 44) images. For two of those delineations, the threshold tests indicated that none of the pixels was anomalous, while another 6 delineations were on environmentally flagged dates. Indeed, a two-tailed Wilcoxon rank sum test for pre-dredging data indicated that plume area on environmentally flagged dates was significantly different (larger) than that on dates where environmental conditions did not favor high Rrs(667) (Z-statistic = 2.4, p = 0.016).