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6.7 6.2.4. Methods of Construction The specifications and guidelines for construction as they are formulated by the Irrigation Department are adequate. However there is a large discrepancy between the embankments as they are designed and as they are actually constructed. In order to achieve proper embankments quality and quality control needs to be improved. It is beyond the scope and the competence of this paper to suggest institutional measures to improve the supervision and the quality and quantity control but when the ultimate goal is to construct proper embankment these measures are bound to be taken. As no contractor likes to work at a loss the costs for the construction of the embankment most likely will increase when proper supervision and control will be implemented as the present unit prices are presumably too low to expect that the contractors are able to work according to the specifications and still make a reasonable profit. 6.2.5 Protection of the Outer Slope The Irrigation Department has protected the outer slope of the embankments over many kilometers by dry brick pitching. The construction runs from the crest of the embankment till about 0.5m below low water level. As already stated earlier the construction is extremely expensive in comparison with the construction of an unprotected embankment. The cost of protected embankments are estimated by the Department at Rs. 3100/m, while the cost of the unprotected embankment is estimated at Rs. 200/m. According to Kanjilal (2006), there is no need to provide brick pitching at such a costly rate to the river side slope of the embankment, but possibly reconstruct the embankments once it is breached at a much lesser cost. However, it is felt here that probably the brick pitching becomes unstable due to wave dash because each brick by itself is too weak to resist the force of the wave dash. On the other hand, it is suggested that a cluster of bricks may be made by available local materials to increase the total weight and possible resist the force of the wave dash better. 6.2.6. Retiring of embankments In certain critical locations, as for example at the shores of sea facing islands or at the concave bends of the tidal creeks, it is inferred that the embankments would not be stable even after strengthening. The best option in this case would be to retire or retreat the embankment by a few tens of meters and provide mangrove plantation, if possible. According to Kanjilal (2006), the second
6.8 embankment shall have to be built with the proper sail as is technically necessary, or rather the best option would be to excavate out the existing embankment, since the material has withstood the test of time. The distance of retreat has to be according to a possible stable slope, say at least a minimum of 3 Horizontal : 1 Vertical, as shown in Figure 6-1. Figure 6 -1. Recommended minimum distance between the riverbank and embankment 6.2.7. Sluice gates The sluices are constructed with a sliding gate and a flap gate. The sliding gate is necessary to prevent outflow of water which is useful for the community of the island. For this, the sliding gate can be and should be in a closed position except at moments excess water needs to be drained. If the sliding gate is properly constructed and properly operated, it also can prevent the inflow of saline water during high tide. The flap gate prevents inflow of water when the sliding gate is not closed on time or is left open. The sliding gate is a necessity; the flap is a safety precaution which reduces the discharge capacity significantly. In theory there are two options to increase the discharge capacity of the sluices without enlarging the diameter of the culverts, which means the construction of a new structure: • To remove the flap gate and to rely completely on a proper operation of the sliding gates. In many countries this option has been chosen in cases that the storage capacity was relatively large. In periods of heavy rains when spilling is required during series of low tides and consequently the sliding gate needs to be opened and closed with every tide an extra gate keeper needs to be called on duty. • To reduce the negative effect of the flap gate by reducing the weight that pushes on the jet of water leaving the culvert and which diminishes the discharge capacity of the sluice. This can be achieved by re-construction of the flap gate structure is such a way that a counter weight
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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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Page 126 and 127: 4.26 maximum surge for this event w
Page 128 and 129: 4.28 U-Component of Velocity (Surfa
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Page 132 and 133: 4.32 V-component of Velocity (Surfa
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7.4 5. No adverse effect in drainin
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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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