CHAPTER 5: DATA SYNTHESIS AND RESULTS................................................... 26-67
5.2 Secondary Data Analysis
5.2.1 NDVI Analysis
NDVI analysis of 1990, 1995, 2000, 2005, 2010, 2015, and 2020 for all of the 19 coastal districts of Bangladesh have been carried out (Annex B) to find out the chronological change in the coastal vegetation in different years. The NDVI value classification has been kept the same for all the analyses following the discussion presented in Chapter 4, with the class +1 to 0.4 denoting denser vegetation, and 0.4 to 0.15 denoting less dense vegetation. Figure 5.1, and 5.2 shows the analysis results for 1990, and 2020.
Figure 5.1: NDVI (1990) of all Coastal Districts Lack of dense vegetation buffer in the Central Coastal Zone
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Figure 5.2: NDVI (2020) of all Coastal Districts
From these analyses, it is obvious that the central part of coastal Bangladesh has usually had less vegetation cover. Whereas the area around Sundarbans (western coastal zone) always has had dense to medium dense vegetation cover due to the presence of the mangrove forest. The area just north of the Sundarbans is showing less dense vegetation cover for it being low lands with numerous small water bodies. On the eastern side, the coastline is showing a similar trend of medium to less dense vegetation cover.
Table 5.1 shows the land area falling under the different ranges of NDVI classifications carried out for this study, along with the statistical properties of the data. From this table, it can be observed that there appears to be a high degree of deviation and skewness in the data, which indicates sharp variability. Furthermore, it can be observed that there is a sharp increase in the +1 to 0.4 class area for the year 2000 and a sharp decrease in the same class area for the year 2010 which is not coherent with the analysis data from the adjacent years as can be observed from the yearly NDVI analysis (Annex C). A possible reason for this anomaly is the presence of cloud cover. Discarding the sharply varied data of these two years reduces the deviation and the skewness of the +1 to 0.4 NDVI value class significantly (Table 5.2). In any case, these two tables indicate that the presence of dense vegetation in the coastal area is rare, and the coastal vegetation is generally less dense.
Increase in denser vegetation buffer in the Central and Eastern Coastal Zone
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Table 5.1: Area for Different NDVI Ranges and Statistical Properties
NDVI Range +1 to 0.4 0.4 to 0.15 0.15 to -0.15 -0.15 to -1
Area (km2) within the NDVI Range in Different
Years
1990 10679.42 18031.98 8983.81 1855.21 1995 6230.91 20157.09 11893.34 1333.29 2000 16470.25 15344.36 6157.30 1621.27 2005 3503.46 25429.36 9373.80 1331.06 2010 477.38 12251.44 25140.36 1768.73
2015 1644.92 31774.14 6181.07 32.99
2020 8442.66 25615.03 5483.12 84.00
Statistical Properties of the Data
Median 6230.91 20157.09 8983.81 1333.29 Standard
Deviation 5612.49 6766.04 6861.95 769.48
Skewness 0.73 0.28 2.08 -0.96
Table 5.2: Area for Different NDVI Ranges and Statistical Properties, Discarding Anomalous Data NDVI Range +1 to 0.4 0.4 to 0.15 0.15 to -0.15 -0.15 to -1 Area (km2) within the
NDVI Range in Different Years
1990 10679.42 18031.98 8983.81 1855.21 1995 6230.905 20157.09 11893.34 1333.294 2005 3503.457 25429.36 9373.796 1331.056 2015 1644.915 31774.14 6181.074 32.985 2020 8442.662 25615.03 5483.123 84.0024 Statistical Properties of the
Data Median 6230.905 25429.36 8983.81 1331.056
Standard
Deviation 3642.323 5368.211 2594.363 821.5559 Skewness 0.025191 0.371511 0.223853 -0.28807
Figure 5.3 and 5.4 graphically represents Tables 5.1 and 5.2. The figures indicate that the comparative extent of dense vegetation is much lower than less dense vegetation. However, there is an increasing trend for the dense vegetation extent since 2010, which seems to have accelerated after the year 2015.
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Figure 5.3: Area for Different NDVI Ranges for all 19 Coastal Districts
Figure 5.4: Area for Different NDVI Ranges for all 19 Coastal Districts, Discarding Anomalous Data Based on this assessment of the NDVI for all 19 coastal districts of Bangladesh, the NDVI analysis for four particular coastal districts (Barguna, Patuakhali, Bhola, and Noakhali) has been carried out for the yearly analysis from 1990 to 2020 (Annex C). Figures 5.5 to 5.8 show some selected years of NDVI analysis of the abovementioned four central coastal districts.
Increasing trend in area with denser vegetation since 2010
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Figure 5.5: NDVI analysis of 4 Central Coastal Districts of Bangladesh (1991)
Figure 5.6: NDVI analysis of 4 Central Coastal Districts of Bangladesh (1994)
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Figure 5.7: NDVI analysis of 4 Central Coastal Districts of Bangladesh (2019)
Figure 5.8: NDVI analysis of 4 Central Coastal Districts of Bangladesh (2020)
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Figure 5.9 graphically represents the total area falling under the +1 to 0.4 and 0.4 to 0.15 NDVI value classes for the four central coastal districts for every year from 1990 to 2020, following Table 5.3. From these, there appears to be a significant lack of densely vegetated area extents in these four districts. This lack of dense vegetation enhances the vulnerability to strong storm surges and also exposes this area to strong winds during cyclonic events. The dense vegetation seems to have been particularly decreased between 2013 and 2017. However, the less dense vegetation extent seems to have increased since around 2013. The graphical representation of 5 yearly average data of areas falling under different NDVI classes also seems to confirm this notion and indicates a decrement in the dense vegetation while exhibiting an increase in the less dense vegetation extent (Figure 5.10). It is also apparent from this figure that these central coastal districts have historically had a lack of significant dense vegetation.
Table 5.3: Area in Different NDVI Ranges for Four Central Coastal Districts
NDVI Range +1 to 0.4 0.4 to 0.15
Area (km2) within the NDVI Range for the Years 1990 to 2020 in Four Central
Coastal Districts
1990 800.44 4119.10
1991 670.59 5346.10
1992 1270.75 5137.69
1993 2405.51 4461.54
1994 685.89 4949.94
1995 428.38 5163.57
1996 202.86 3232.30
1997 404.85 4035.91
1998 799.60 5657.46
1999 657.46 5380.78
2000 4852.77 2483.72
2001 339.40 4489.29
2002 21.50 1342.99
2003 132.83 3450.79
2004 138.40 4114.27
2005 370.90 5389.39
2006 1059.16 5089.62
2007 1055.42 5406.37
2008 3043.58 4401.68
2009 1602.25 5136.36
2010 42.82 1055.14
2011 2064.30 4568.12
2012 3908.67 3245.01
2013 756.13 6723.00
2014 416.40 6958.62
2015 226.02 7088.48
2016 458.59 6630.63
2017 219.10 6939.19
2018 938.57 6215.98
2019 599.16 6741.83
2020 1028.51 6414.70
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Figure 5.9: Area in Different NDVI Ranges for Four Central Coastal Districts from 1990 to 2020 Table 5.4 shows the average area in the different NDVI ranges for different stretches of years from 1990 to 2020. Figure 5.10 represents the data graphically. This reveals a lack of increase in areal extent on average for the dense vegetation in the four selected central coastal districts.
Table 5.4: Area in Different NDVI Ranges for Four Central Coastal Districts from 1990 to 2020
NDVI Range +1 to 0.4 0.4 to 0.15 0.15 to -0.15
Average Area (km2) within the NDVI Ranges
for Different Year Ranges
1990-1994 1166.64 4802.87 2002.22
1995-1999 498.63 4694.01 2948.14
2000-2004 1096.98 3176.21 3543.12
2005-2009 1426.26 5084.68 1514.18
2010-2014 1437.67 4510.00 2094.18
2015-2020 578.32 6671.80 1003.66
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Figure 5.10: Average Area in Different NDVI Ranges for Four Central Coastal Districts
Figures 5.11 and 5.12 are focused on the recent years for the four central coastal districts following Tables 5.5 and 5.6. There is an obvious gap between the area extent of +1 to 0.4 and 0.4 to 0.15 NDVI classes. From 2010 to 2014 there seemed to be a decline in the denser vegetation area (Figure 5.11) as well as some randomness in the short range of data. However, it seems to have improved slightly and there is a slight upward trend in this denser vegetation area from 2015 to 2020 (Figure 5.12).
Table 5.5: Area in Different NDVI Ranges for Four Central Coastal Districts from 2010 to 2014
NDVI Range +1 to 0.4 0.4 to 0.15 0.15 to -0.15
Area (km2) in different NDVI Ranges for Four Central Coastal Districts
from 2010 to 2014
2010 42.82 1055.14 6816.95
2011 2064.30 4568.12 1310.87
2012 3908.67 3245.01 667.84
2013 756.13 6723.00 780.71
2014 416.40 6958.62 894.51
1990-1994 1995-1999 2000-2004 2005-2009 2010-2014 2015-2020
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Table 5.6: Area in Different NDVI Ranges for Four Central Coastal Districts from 2015 to 2020
NDVI Range +1 to 0.4 0.4 to 0.15 0.15 to -0.15
Area (km2) in different NDVI Ranges for Four Central Coastal Districts
from 2015 to 2020
2015 226.0179 7088.478 943.605
2016 458.5887 6630.629 1159.166
2017 219.1005 6939.187 1106.396
2018 938.5704 6215.976 1118.75
2019 599.1597 6741.833 880.0938
2020 1028.506 6414.695 813.9609
Figure 5.11: Area in Different NDVI Ranges for Four Central Coastal Districts (2010-2014)
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Figure 5.12: Area in Different NDVI Ranges for Four Central Coastal Districts (2015-2020) Figure 5.13 and Figure 5.14 isolate the area for the +1 to 0.4 NDVI range. Figure 5.13 shows the decline of dense vegetation in the years 2013 and 2014. However, since that decline, the upward trend in recent years (2015 to 2020) is apparent in Figure 5.14.
Figure 5.13: Area under +1 to 0.4 NDVI Range for Four Central Coastal Districts (2010-2014)
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Figure 5.14: Area under +1 to 0.4 NDVI Range for Four Central Coastal Districts (2015-2020) This interpretation indicates the impact of the afforestation programs taken in the coastal zone of Bangladesh in the past few decades. As the afforested areas mature, the less dense vegetation extents increase. Further on, as the vegetation keeps maturing, the less dense extent shrinks and contributes to increasing the more dense vegetation extents in the more recent years. In the coming years, the denser vegetation may be expected to increase further and can provide a conducive environment for adopting bioengineering interventions.
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