Water Level Data Analysis - Secondary Data Analysis

CHAPTER 5: DATA SYNTHESIS AND RESULTS................................................... 26-67

5.2 Secondary Data Analysis

5.2.3 Water Level Data Analysis

For each of these stations, the water level data of several specific cyclonic events where data for all of the nine stations are available were selected (Table 5.14) based on the BMD cyclone list (Table 2.2). Water level data (both high tide and low tide) of five days (including the day of landfall, one day prior, and three days after) was taken for comparison of the Water Level stations (Table 5.13).

Table 5.13: Selected Cyclone Events for Secondary Analysis of Water Level Data (Source: BMD) Landfall

Date Landfall Area Maximum Wind

Speed (km/hr)

Tidal Surge Height (m)

24.05.85 Chattogram 154 4.57

29.11.88 Khulna 160 0.60 - 4.42

18.12.90 Cox's Bazar Coast 115 1.52 - 2.13

29.04.91 Chittagong 225 3.66 - 6.71

02.05.94 Cox's Bazar-Teknaf Coast 220 1.52 - 1.83

19.05.97 Sitakundu 232 4.57

20.05.98 Sitakundu 173 0.91

The cyclonic events had been chosen from a period such that the effect and contrast of dense mangrove buffer and the exposed coastal area can be easily discernable. For each cyclonic event, the water levels at different levels for the five days (including the day of landfall, one day prior, and three days after) are given in Tables 5.14 to 5.20. Figures 5.23 to 5.36 show these tidal water levels graphically. The stations have been plotted on the horizontal axis, with the left to right showing the southern to eastern stations. On the vertical axis, the maximum water levels at both high and low tides for each of the five days have been plotted. This shows the overall status of the water level along the coast for the whole of the coastal zone of Bangladesh and brings a unique perspective of assessing the overall water levels including the storm surges and tidal effects. It is to be noted that the station SW222 at Noakhali Khal exhibited closer levels for the High Tide and Low Tide data, indicating a lower tidal response.

Table 5.14: Water Levels at Different Stations for Cyclone in ’85 (Source: BWDB) Cyclonic

Event Water

Level Station ID

Water Level (m)

23.05.85 24.05.85 25.05.85 26.05.85 27.05.85 High

Tide Low

Tide High Tide Low

Tide Severe

Cyclonic Storm (24.05.85)

Landfall:

Chattogram

SW26 2.32 -1.98 2.07 -1.68 2.04 -1.70 1.76 -1.58 1.62 -1.59 SW29 2.98 -0.29 3.10 -0.29 3.00 -0.09 2.95 -0.09 2.73 -0.08 SW244 2.00 -0.30 2.10 -0.40 1.90 -0.40 1.85 -0.30 1.95 -0.10 SW39 2.40 -0.90 2.70 -0.40 2.10 -0.60 2.00 -0.50 2.00 -0.02 SW278 2.50 -0.25 3.80 -0.10 3.70 -0.15 2.51 -0.15 2.31 -0.17 SW222 3.10 3.02 3.55 3.33 4.40 4.00 4.55 4.00 4.60 4.02 SW121 2.95 -0.60 3.40 -0.45 3.85 0.90 3.20 0.80 2.80 0.48 SW250 2.01 0.10 3.30 0.30 2.50 0.30 1.95 0.11 1.86 0.06 SW200 2.30 -0.80 3.85 -0.50 2.60 -0.70 2.10 -0.90 2.00 -1.00

Figure 5.23: Maximum Water Levels during High Tide (HT) and Low Tide (LT) at Different Stations for Cyclone in 1985

Figure 5.24: Maximum Water Levels during High Tide at Different Stations for Cyclone in 1985

SW26 SW29 SW244 SW39 SW278 SW222 SW121 SW250 SW200

Stations around mangrove cover:

Consistently lower water levels

Stations virtually exposed to the coast: Consistently higher water levels

Stations around mangrove cover:

Consistently lower water levels at SW26 and SW 244, with moderately high water level at SW29

Table 5.15: Water Levels at Different Stations for Cyclone in ’88 (Source: BWDB) Cyclonic

Event Water

Level Station ID

Water Level (m)

28.11.88 29.11.88 30.11.88 01.12.88 02.12.88 High

Tide Low

Tide High Tide Low

Tide Severe

Cyclonic Storm (29.11.88)

Landfall:

Khulna

SW26 2.28 -0.29 2.74 -0.02 2.11 -0.02 1.95 -0.53 2.29 -0.16 SW29 2.59 0.04 2.94 0.58 2.82 0.07 2.64 0.28 2.48 0.18 SW244 1.74 -0.56 2.14 0.64 1.94 -0.06 1.64 -0.26 1.74 -0.36 SW39 1.92 -0.20 2.30 0.50 1.30 0.00 1.20 -0.03 1.17 -0.10 SW278 2.40 -0.05 2.65 0.50 2.90 0.18 2.01 0.12 1.49 0.08 SW222 3.95 3.91 4.02 3.70 4.15 4.10 4.00 3.95 3.99 3.95 SW121 2.65 -0.40 2.70 -0.50 2.90 -0.10 2.70 -0.10 2.40 -0.30 SW250 2.35 -0.35 2.55 -0.35 2.35 -0.30 2.25 -0.25 2.20 -0.20 SW200 2.20 -0.90 2.40 -0.60 2.60 -0.50 2.20 -0.80 1.90 -0.80

Figure 5.25: Maximum Water Levels during High Tide (HT) and Low Tide (LT) at Different Stations for Cyclone in 1988

Stations around mangrove cover:

Comparatively lower water levels

Stations virtually exposed to the coast:

Comparatively higher water levels during HT

Figure 5.26: Maximum Water Levels during High Tide at Different Stations for Cyclone in 1988

Table 5.16: Water Levels at Different Stations for Cyclone in ’90 (Source: BWDB) Cyclonic

Event Water

Level Station ID

Water Level (m)

17.12.90 18.12.90 19.12.90 20.12.90 21.12.90 High

Tide Low

Tide High Tide Low

Tide

Cyclonic Storm (18.12.90)

Landfall:

Cox’s Bazar

SW26 1.32 -1.70 1.54 -1.61 1.50 -1.64 1.36 -1.59 1.40 -1.56 SW29 2.09 -0.57 2.17 -0.81 2.28 -0.63 2.19 -0.57 2.14 -0.65 SW244 1.50 -0.80 1.52 -0.80 1.68 -0.90 1.70 -0.90 1.60 -0.85 SW39 1.71 -0.58 1.79 -0.61 1.90 -0.60 1.81 -0.50 1.80 -0.20 SW278 1.05 -0.48 1.37 -0.58 1.35 -0.52 1.42 -0.57 1.53 -0.53 SW222 3.49 3.49 3.64 3.49 3.67 3.65 3.67 3.64 3.66 3.64 SW121 1.90 -1.10 2.20 -1.00 2.30 -0.90 2.20 -1.00 2.10 -1.10 SW250 2.16 -0.24 2.38 -0.08 2.16 -0.22 2.14 -0.48 2.10 -0.49 SW200 2.86 -1.74 2.56 -1.64 2.36 -1.54 2.06 -1.44 1.76 -1.24 Stations around mangrove cover:

Comparatively lower water levels despite being near landfall location

Figure 5.27: Maximum Water Levels during High Tide (HT) and Low Tide (LT) at Different Stations for Cyclone in 1990

Figure 5.28: Maximum Water Levels during High Tide at Different Stations for Cyclone in 1990

0 0.5 1 1.5 2 2.5 3 3.5 4

SW26

SW29

SW244

SW39

SW278 SW222

SW121 SW250

SW200

17.12.90 (HT) 18.12.90 (HT) 19.12.90 (HT) 20.12.90 (HT) 21.12.90 (HT) Stations around mangrove cover:

Consistently lower water levels

Stations virtually exposed to the coast: Consistently higher water levels during HT

Stations around mangrove cover:

Consistently lower water levels

SW26 SW29 SW244 SW39 SW278 SW222 SW121 SW250 SW200

Table 5.17: Water Levels at Different Stations for Cyclone in ’91 (Source: BWDB) Cyclonic

Event Water

Level Station ID

Water Level (m)

28.04.91 29.04.91 30.04.91 01.05.91 02.05.91 High

Tide Low

Tide High Tide Low

Tide Severe

Cyclonic Storm (29.04.91)

Landfall:

Chattogram

SW26 2.06 -2.10 2.14 -2.21 2.19 -2.04 2.18 -1.85 2.16 -1.68 SW29 2.65 -0.65 2.73 -0.58 2.80 -0.54 2.80 -0.42 2.82 -0.53 SW244 2.05 -1.05 2.60 -0.45 2.55 -1.05 2.55 -1.05 2.45 -0.78 SW39 2.54 -0.70 2.70 -0.65 2.40 -0.70 2.30 -0.60 2.30 -0.10 SW278 2.33 -0.55 2.60 -0.50 2.57 -0.62 2.72 -0.35 2.86 0.05 SW222 3.13 3.13 3.27 3.17 4.64 4.29 4.77 4.69 4.32 4.11 SW121 3.30 -1.10 3.40 -0.90 3.80 0.00 3.50 0.15 3.30 0.00

SW250 -- -- -- -- -- -- -- -- -- --

SW200 -- -- -- -- -- -- 2.93 -1.67 2.63 -1.57

Figure 5.29: Maximum Water Levels during High Tide (HT) and Low Tide (LT) at Different Stations for Cyclone in 1991

Stations around mangrove cover:

Consistently lower water levels

Stations virtually exposed to the coast: Consistently higher water levels

SW26 SW29 SW244 SW39 SW278 SW222 SW121

Figure 5.30: Maximum Water Levels during High Tide at Different Stations for Cyclone in 1991

Table 5.18: Water Levels at Different Stations for Cyclone in ’94 (Source: BWDB) Cyclonic

Event Water

Level Station ID

Water Level (m)

01.05.94 02.05.94 03.05.94 04.05.94 05.05.94 High

Tide Low

Tide High Tide Low

Tide Severe

Cyclonic Storm (02.05.94)

Landfall:

Cox’s Bazar

SW26 2.11 -1.57 1.85 -1.15 1.64 -0.83 1.59 -0.87 1.80 -1.03 SW29 2.57 -0.53 2.45 -0.34 2.26 0.05 2.29 0.23 2.35 0.15 SW244 2.15 -1.05 2.10 -0.80 1.90 -0.60 1.70 -0.55 1.70 -0.60

SW39 1.58 -0.37 1.48 -0.29 1.46 0.09 1.39 0.31 1.41 0.06 SW278 2.51 0.53 2.40 0.56 2.36 0.76 2.72 0.81 2.58 0.86 SW222 4.18 4.14 4.14 4.11 4.09 4.07 4.03 4.00 3.92 3.90 SW121 2.75 -0.45 2.85 -0.45 2.85 -0.35 2.76 -0.25 2.75 -0.15 SW250 2.64 -0.48 3.43 -0.52 2.29 -0.23 2.83 0.12 2.89 -0.34 SW200 1.76 -0.64 2.26 -0.24 1.66 -0.84 1.46 -0.94 1.36 -1.04 Stations around mangrove cover:

Consistently lower water levels

Figure 5.31: Maximum Water Levels during High Tide (HT) and Low Tide (LT) at Different Stations for Cyclone in 1994

Figure 5.32: Maximum Water Levels during High Tide at Different Stations for Cyclone in 1994

SW26 SW29 SW244 SW39 SW278 SW222 SW121 SW250 SW200

Stations around mangrove cover:

Consistently lower water levels

Stations virtually exposed to the coast: Consistently higher water levels

Stations around mangrove cover:

Consistently lower water levels

Table 5.19: Water Levels at Different Stations for Cyclone in ’97 (Source: BWDB) Cyclonic

Event Water

Level Station ID

Water Level (m)

18.05.97 19.05.97 20.05.97 21.05.97 22.05.97 High

Tide Low

Tide High Tide Low

Tide Severe

Cyclonic Storm (19.05.97)

Landfall:

Sitakundu

SW26 1.83 -1.19 2.44 -1.31 2.55 -1.39 2.71 -1.54 2.80 -1.68 SW29 2.37 -0.33 2.73 -0.33 2.90 -0.36 3.00 -0.42 3.05 -0.70 SW244 1.92 -0.08 1.75 -0.10 2.65 -0.20 2.82 -0.23 2.82 -0.18 SW39 1.67 -0.17 2.03 -0.32 2.52 -0.18 2.53 -0.17 2.43 -0.17 SW278 1.93 0.00 2.28 0.08 3.45 -0.07 3.30 0.08 3.29 0.08 SW222 2.48 2.46 2.46 2.45 2.81 2.79 2.78 2.72 3.24 2.97 SW121 2.10 1.20 1.95 1.20 2.00 1.00 1.80 0.90 1.90 1.00 SW250 3.19 0.76 3.10 -0.52 3.12 -1.02 2.79 -1.07 2.66 -0.92

SW200 -- -- -- -- -- -- -- -- -- --

Figure 5.33: Maximum Water Levels during High Tide (HT) and Low Tide (LT) at Different Stations for Cyclone in 1997

SW26 SW29 SW244 SW39 SW278 SW222 SW121 SW250

Stations around mangrove cover:

Consistently lower water levels

Stations virtually exposed to the coast: Consistently higher water levels

Figure 5.34: Maximum Water Levels during High Tide at Different Stations for Cyclone in 1997

Table 5.20: Water Levels at Different Stations for Cyclone in ’98 (Source: BWDB) Cyclonic

Event Water

Level Station ID

Water Level (m)

19.05.98 20.05.98 21.05.98 22.05.98 23.05.98 High

Tide Low

Tide High Tide Low

Tide Severe

Cyclonic Storm (20.05.98)

Landfall:

Sitakundu

SW26 1.22 -1.73 1.17 -1.79 1.53 -1.98 1.37 -2.17 1.51 -2.40 SW29 2.92 0.10 2.80 0.20 2.97 0.20 2.95 -0.10 3.25 -0.26 SW244 2.00 -0.60 2.20 -0.65 2.30 -0.80 2.40 -0.90 2.65 -1.00 SW39 1.93 -0.07 1.93 0.03 2.07 0.18 2.48 0.06 2.48 -0.02 SW278 2.22 0.18 2.59 0.21 2.67 0.28 2.92 0.35 3.19 0.36 SW222 3.07 3.02 3.18 3.10 3.24 3.19 3.16 3.10 3.01 2.98 SW121 4.00 2.00 3.90 2.10 3.50 1.90 3.00 1.80 3.20 1.80 SW250 2.90 -0.22 2.72 0.27 2.77 -0.07 3.09 0.00 3.23 0.36

SW200 -- -- -- -- -- -- -- -- -- --

Stations around mangrove cover:

Comparatively lower water levels on the day of cyclone (19.05.97) that increases in the next days

Figure 5.35: Maximum Water Levels during High Tide (HT) and Low Tide (LT) at Different Stations for Cyclone in 1998

Figure 5.36: Maximum Water Levels during High Tide at Different Stations for Cyclone in 1998

From the above tables and graphs, it is discernable that the water levels were consistently lower in the gauge stations located near the mangrove cover; while the water levels were recorded to be higher in the coasts exposed to the Bay of Bengal. This trend is especially prominent in the cases of moderately strong cyclones, with diminishing effects on the stronger cyclones. The

SW26 SW29 SW244 SW39 SW278 SW222 SW121 SW250

Stations around mangrove cover:

Consistently lower water levels

Stations virtually exposed to the coast: Consistently higher water levels

Stations around mangrove cover:

Comparatively lower water levels

retarding effect on water levels is naturally greater if the landfall location of the cyclones is at a distance from the protected coasts, and it is also pertinent in the cases of landfall locations on mangrove covers compared to the exposed coastal locations. This is further reinforced by the analysis of the storm surge levels as discussed in Chapter 4. The maximum storm surge levels for the southern, central, and eastern coastal zone for each of the cyclonic events have been shown in Table 5.21, and graphically represented in Figures 5.37 to 5.43.

Table 5.21: Maximum Surge Levels at Selected Stations for Different Cyclonic Events (Source: after BWDB water level data)

Cyclone

Year Classification

(collected from BMD) Landfall Location Surge Levels (m) Southern

Coastal Zone

Central Coastal Zone

Eastern Coastal Zone 1985 Severe Cyclonic Storm Chattogram

(Eastern Coastal Zone) 0.95 1.82 1.85 1988 Severe Cyclonic Storm with

a core of hurricane wind Khulna

(Western Coastal Zone) 1.54 1.24 0.50 1990 Cyclonic Storm (crossed as

a depression) Cox’s Bazaar

(Eastern Coastal Zone) 0.20 0.10 0.54 1991 Severe Cyclonic Storm with

a core of hurricane wind Chattogram

(Eastern Coastal Zone) 1.42 1.44 1.20 1994 Severe Cyclonic Storm with

a core of hurricane wind Cox’s Bazaar

(Eastern Coastal Zone) 0.45 0.62 0.90 1997 Severe Cyclonic Storm with

a core of hurricane wind Sitakundu

(Eastern Coastal Zone) 1.10 1.60 0.44 1998 Severe Cyclonic Storm with

a core of hurricane wind Sitakundu

(Eastern Coastal Zone) 0.75 1.15 1.50

Figure 5.37: Maximum Surge Levels for Cyclone in 1985

Figure 5.38: Maximum Surge Levels for Cyclone in 1988

Figure 5.39: Maximum Surge Levels for Cyclone in 1990

Figure 5.40: Maximum Surge Levels for Cyclone in 1991

Figure 5.41: Maximum Surge Levels for Cyclone in 1994

Figure 5.42: Maximum Surge Levels for Cyclone in 1997

Figure 5.43: Maximum Surge Levels for Cyclone in 1998

It is to be noted that for the cyclone in 1991, data was unavailable for two of the selected eastern coastal zones (SW250 and SW200), while for the cyclones in 1997 and 1998, data was unavailable for SW 200. In the case of the central coastal zone, data was unavailable only for one of the stations (SW222) for the cyclone in 1995. However, from the figures 5.37 to 5.43, it is evident that the western coastal zone experiences generally lower values of storm surges when the landfall locations of cyclones are not there. The cyclone of 1988 remains an outlier when the landfall location was in Khulna, and the cyclone was a severe cyclonic storm with a core of hurricane wind. The maximum storm surge in the western coastal zone for the 1988 cyclone was found to be 19.48% higher than the maximum storm surge in the central coastal zone. This also indicates that while the protection against storm surges resulting from severe cyclones may be diminished to a degree, it still offers significant protection when there is a coastal buffer impeding the surge. On the other hand, for the 1990 cyclone which crossed as a depression through the eastern coastal zone, the western coastal zone recorded an 80% less storm surge compared to the surge in the central coastal zone. This indicates that the coastal buffers are highly significant in reducing the storm surges resulting from lower-strength cyclones.

Dalam dokumen assessment of the potential of bioengineering intervention (Halaman 57-72)