Cyclone and Storm Surge - Iczmpwb.org

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6.9 is attached to the structure. The flap is as strong as it was before the reconstruction, but the resulting weights much less. Flap gates with counter weights are operational with success in many countries. While reconstructing the flap gates the distance between the center of the culvert and the hinge of the flap gate structure should be enlarged. With the information presently available it is not possible to estimate whether improvement of the discharge capacity of the existing sluices is still required when the other measures have been taken. It is also difficult to recommend which of the options should be chosen when some improvement is required. The first option requires fewer investments but requires an alert organization in charge of operation and maintenance of the water control system. 6.2.8. Recommendations regarding construction of embankments In the Sunderban region, it is observed that almost all the islands that have been reclaimed for habitation over the years (as explained in Chapter 2) are bounded by circuit embankments in order to protect the people and property settled on the islands. In fact, they protected lands represent more like the polders of Netherlands. The original embankments (dykes) were constructed without any proper engineering guidelines and thus the slopes on the riverside and on the landside, embankment crest width and its elevation were all decided upon repeated human experiences. However, rising waters of the rivers cause failures of these embankments due to overtopping, sloughing, or slip failure due to erosion of the base due to river current at its base if it is too close to the river. Specific recommendation is, therefore, needed based upon certain design considerations. Basic engineering provides guidelines for the crest width, bank slopes, and material that need to be used for construction of these embankments. However, the crest width should be decided upon a chosen maximum rise of the river (or ocean) water that is likely to overtop the embankments. Thus, it is necessary to know the expected rise of water levels based on previously recorded data. This requires information about the recurrence interval or return period of the occurrence of sea level rise. Naturally, this depends upon the repetition of sea level rise due to cyclonic storms. In this regard, the Bureau of Meteorology Research Centre, Australia, in its Global Guide to Tropical Cyclone Forecasting, (Chapter 7: Warning Strategies, Section 7.5 Hazard, Vulnerability And Risk Assessment) mentions about a study of return period of tropical cyclone based on a study of 95 year data by Jayanthi and Sen Sharma in 1988. The data for West Bengal coastline from the study is given as under:
6.10 Recurrence interval (in years) 10 25 50 100 200 Maximum wind speed (knots) 90 105 116 125 135 Storm surge height (m) 4.5 6.3 7.8 9.2 10.9 A reasonable recurrence interval for assessing the crest of embankments facing the sea may be considered as 50 years. Thus, adding a freeboard, one may say that an embankment crest elevation of 8m above Mean Sea Level may be thought of as safely withstanding the overtopping of surges that is likely to occur with a recurrence of 50 years. As for the embankments along the rivers, channels and creeks, the rise of the water levels as a consequence of a rise of the sea water level due to cyclonic storm surges has to be decided upon the river hydrodynamics. Considering a rise of 7.8m of the sea (with a recurrence interval of 50 years) causes rise of water levels that is much less as the dynamic nature of the surge is dampened as it enters the channels and creeks. From the note of Superintending Engineer, eastern Circle, I&W Department, Government of West Bengal that was prepared in December 2005, titled “Scheme for the revetment works of the Embankments of Sundarban Areas in the districts of North & South 24-Parganas”, it is necessary to provide immediate revetment protection to about 235 kms critical length of embankments in the Sundarbans out of a total of about 3500 kms. The regions that require immediate attention include: (a) Sea facing dyke – 17 km, (b) Embankments facing major rivers – 139 km, and (c) Embankments facing medium rivers – 79 km. It is estimated that the total cost estimate is around Rs. 467.82 Crores. Further, 91 km length out of 225 km length of embankments already protected with revetment under the scheme “Urgent Development Works of Sundarban” need a thorough maintenance, which requires a sum of Rs.72.18 Crore. Thus, a total sum of Rs.540.00 Crore is required in the first phase for protection of the most critical of the Sundarban embankments. The technical and financial details of the proposal are presented below. Embankment design (a) Major rivers to armour the riverside slopes (3:1 to 4:1) of the embankments by providing 32.5 cm thick dry brick pitching / 25 cm thick brick block pitching over a layer of 12.5 cm to 15 cm thick jhama khoa filter with provision of a brick sausage toe wall.
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HAZARD ASSESSMENT AND DISASTER MITI
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Project Team Though the outcome of
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Preamble The State of West Bengal,
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1.2 IMD, in its website (http://www
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1.4 5. Proposals for location of cy
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2.2 region is that which struck the
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2.4 Figure 2-2. Track of the 1970 B
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2.6 The most destructive element as
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2.8 the two regions is rather diffe
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2.10 2.4. The eastern coastal zone
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2.12 velocity is first checked. As
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2.14 Strong winds known as cyclones
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2.16 2.4.6. Soil characteristics Th
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2.18 All rivers have in general sou
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2.20 Another interesting feature of
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2.22 formed does not pose so much o
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2.24 2.4.11. Forest composition The
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2.26 Canning (Bidyadhari Outfall in
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2.28 Administrative information Pop
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2.30 Mangrove plantation beyond the
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2.32 A retired embankment of river
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2.34 The beaches are flattened by s
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2.36 of a doiminant tidalbasin of t
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2.38 primary mode is centered on th
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2.40 Aquaculture ponds located betw
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2.42 All the islands are situated w
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2.44 • Transport the water to the
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2.46 The other type of failure may
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2.48 channel floor, some of the riv
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2.50 embankments. Crops and propert
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2.52 2. To reduce the average dista
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3.2 a storm surge caused by a tropi
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3.4 these merchants may have been t
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3.6 Figure 3-1. Map of India during
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3.8 larger volume of sewage and dra
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3.10 receive, at least during the r
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3.12 3.9. Drainage scheme of Dr. B.
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3.14 3.10. CMPO and KMDA (originall
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3.16 Flows are diverted to the loca
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3.18 the Bhangar region through the
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3.20 the ocean level as a result of
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3.22 Table 3-6. Major pumping stati
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3.24 Table 3-9. Drainage basin area
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3.26 will lead to significant impro
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3.28 locks, one at Chitpur and anot
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3.30 Respectively. The discharge of
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3.32 Figure 3-6. The downstream of
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3.34 Figure 3-10 shows the tail end
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3.36
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3.38
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4.2 In US, there is a steady decrea
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4.4 This shows that Orissa (with 10
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4.6 30 25 20 15 10 Severe Cyclone C
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4.8 Table 3: Return periods of cycl
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4.10 Table 5: The number and return
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4.12 work station (the Automated Tr
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4.14 through this trough into the d
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4.16 Mean Low water 1.51 1.39 1.34
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7.6 15 LT Panel L.S. 2,200,000 16 I
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7.8 Bagjola, Kestopur and SWF chann
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Executive Cost Summary of Recommend
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References Kanjilal (2005) “Sunde
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“……………….Designingadis
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