Cyclone and Storm Surge - Iczmpwb.org
Cyclone and Storm Surge - Iczmpwb.org Cyclone and Storm Surge - Iczmpwb.org
2.50 embankments. Crops and properties over Rs.900 million have been destroyed and around 500 people have lost their lives. These numbers are likely to increase manifold in future. The study also notes that the Sagar Islands is the most populated and vulnerable island of the Hoogly estuary. Over the last few decades the island has been seen to register a land loss of 12 Km2 with marginal accretion on the portion of the southern and eastern fringe. Within the last few decades, nearly 3800 persons have been rendered homeless from Sagar Island itself due to sea level rise, coastal erosion and flooding due to cyclones and consequent storm surges. In addition, four refugee colonies for the displaced persons from other islands (Lohachara, Ghoramara etc) have been set up at Sagar. A mathematical model coupled with GIS database developed by the School of School of Oceanographic Studies, Jadavpur University; demonstrate that with the current rate of sea level rise, the Sagar island will loose around 15% of its existing land area by the 2020. An estimate considering the population growth rate and future population density of Sagar indicates that the number of displaced persons due to sea level rise and associated storm surges/coastal flooding will be around 30,000 by 2020. 2.7.3. Inundation As mentioned before, the problem of inundation due to high rainfall within the circuit embankments of the inhabited islands is inevitable at the time of a high tide, coupled with a storm surge. As such, proper drainage is required along with right kind of sluices and vents. At the time of a high sea level, the island dwellers are also at a constant threat of further inundation likely to be caused by possible embankment failure. Since the dwellers here have settled before the full maturity of development of the islands, they continue to suffer from the gradual rise of the surrounding riverbeds compared to their lands. Due to premature reclamation, the average ground level of some of the populated islands remains between 1.55 m to 0.75mts below the mean high water level of the tides and in the coastal area, the ground level remains about 2.60m below. This accentuates further the risk of embankment failure and washout. These embankments have now become the lifeline of the Sundarbans islands, as because, the safety of the lives and properties as well as agriculture and subsistence are dependant on the stability of the embankments. Excess rain water is drained from the fields into a system of canals and ponds. Some of the canals and ponds are excavated by the farmers for this purpose. Others are the result of the construction of embankments and roads. On many islands the canal system partly consists of small natural creeks in which dams has been constructed at their downstream end. The internal system of canals and ponds
2.51 is used to store fresh water for domes-tic use and for drinking water of the animals. This fresh water is also used to water the garden crops which are grown in the dry season. Culvert type sluices, mostly provided with flap gates and with sliding gates enable spilling of excess water to the tidal rivers during periods when the water levels of these rivers are lower than the water levels in the internal drainage systems of the islands. When the water level of the rivers are higher than in the internal system, drainage is not possible. All water that flows into the system during these periods needs to be stored in the canals and ponds. Consequently there are two reasons water needs to be stored in the internal system a socio-economic one and one which is of a more technical nature. When the sliding gates of the sluices are open water will be drained to the river as soon as the water level in the river is lower than the water level in the internal system. The flap gates will open automatically; at first only slightly but when the difference in water level inside and outside increases the gates will open wider. When during outgoing tide the water level of the river becomes lower than the water level in the internal system and the sliding gates are open, the culvert might be in a submerged condition but soon the river level will be lower than the top of the culvert bringing the sluice in the intermediate condition. For other culverts in the Sundarban discharge starts under these intermediate conditions. Near the end of the outgoing tide the sluice might enter the free flow condition but as most of the sluices are not built close to the river often the water level in the level in the canal section connecting the sluice with the river will remain higher than the bottom of the culvert. Due to all this the discharges of the sluices cannot be computed but can only be determined by measurements. The presence of the flap reduces the discharge capacity of the sluices significantly. By its weight the flap is pushed downwards and this hampers the outflow of water. Some of the flaps in the Sundarban are constructed of wood, others are of cast iron. Although the latter are better from a maintenance point of view, they are heavier and reduce the outflow more than the wooden flaps. As the formula indicates, the discharge of a given sluice gate is determined by the water head. When the gate is spilling water the system of canals and ponds needs to convey water to this gate. For a proper drainage it is important that this transport of water is realized with a minimum loss of head. In theory there are two methods to reduce the loss of head in the canals: 1. To reduce the slope in the canals required to transport the volume of water that needs to be spilled by the sluice. This can be realized by wide and smooth canals. This means that the canals to the sluices need to be cleaned and as much as possible free of any obstacles.
- Page 10 and 11: 1.4 5. Proposals for location of cy
- Page 12 and 13: 2.2 region is that which struck the
- Page 14 and 15: 2.4 Figure 2-2. Track of the 1970 B
- Page 16 and 17: 2.6 The most destructive element as
- Page 18 and 19: 2.8 the two regions is rather diffe
- Page 20 and 21: 2.10 2.4. The eastern coastal zone
- Page 22 and 23: 2.12 velocity is first checked. As
- Page 24 and 25: 2.14 Strong winds known as cyclones
- Page 26 and 27: 2.16 2.4.6. Soil characteristics Th
- Page 28 and 29: 2.18 All rivers have in general sou
- Page 30 and 31: 2.20 Another interesting feature of
- Page 32 and 33: 2.22 formed does not pose so much o
- Page 34 and 35: 2.24 2.4.11. Forest composition The
- Page 36 and 37: 2.26 Canning (Bidyadhari Outfall in
- Page 38 and 39: 2.28 Administrative information Pop
- Page 40 and 41: 2.30 Mangrove plantation beyond the
- Page 42 and 43: 2.32 A retired embankment of river
- Page 44 and 45: 2.34 The beaches are flattened by s
- Page 46 and 47: 2.36 of a doiminant tidalbasin of t
- Page 48 and 49: 2.38 primary mode is centered on th
- Page 50 and 51: 2.40 Aquaculture ponds located betw
- Page 52 and 53: 2.42 All the islands are situated w
- Page 54 and 55: 2.44 • Transport the water to the
- Page 56 and 57: 2.46 The other type of failure may
- Page 58 and 59: 2.48 channel floor, some of the riv
- Page 62 and 63: 2.52 2. To reduce the average dista
- Page 64 and 65: 3.2 a storm surge caused by a tropi
- Page 66 and 67: 3.4 these merchants may have been t
- Page 68 and 69: 3.6 Figure 3-1. Map of India during
- Page 70 and 71: 3.8 larger volume of sewage and dra
- Page 72 and 73: 3.10 receive, at least during the r
- Page 74 and 75: 3.12 3.9. Drainage scheme of Dr. B.
- Page 76 and 77: 3.14 3.10. CMPO and KMDA (originall
- Page 78 and 79: 3.16 Flows are diverted to the loca
- Page 80 and 81: 3.18 the Bhangar region through the
- Page 82 and 83: 3.20 the ocean level as a result of
- Page 84 and 85: 3.22 Table 3-6. Major pumping stati
- Page 86 and 87: 3.24 Table 3-9. Drainage basin area
- Page 88 and 89: 3.26 will lead to significant impro
- Page 90 and 91: 3.28 locks, one at Chitpur and anot
- Page 92 and 93: 3.30 Respectively. The discharge of
- Page 94 and 95: 3.32 Figure 3-6. The downstream of
- Page 96 and 97: 3.34 Figure 3-10 shows the tail end
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- Page 102 and 103: 4.2 In US, there is a steady decrea
- Page 104 and 105: 4.4 This shows that Orissa (with 10
- Page 106 and 107: 4.6 30 25 20 15 10 Severe Cyclone C
- Page 108 and 109: 4.8 Table 3: Return periods of cycl
2.51<br />
is used to store fresh water for domes-tic use <strong>and</strong> for drinking water of the animals. This fresh water<br />
is also used to water the garden crops which are grown in the dry season.<br />
Culvert type sluices, mostly provided with flap gates <strong>and</strong> with sliding gates enable spilling of excess<br />
water to the tidal rivers during periods when the water levels of these rivers are lower than the water<br />
levels in the internal drainage systems of the isl<strong>and</strong>s. When the water level of the rivers are higher<br />
than in the internal system, drainage is not possible. All water that flows into the system during these<br />
periods needs to be stored in the canals <strong>and</strong> ponds. Consequently there are two reasons water<br />
needs to be stored in the internal system a socio-economic one <strong>and</strong> one which is of a more technical<br />
nature.<br />
When the sliding gates of the sluices are open water will be drained to the river as soon as the water<br />
level in the river is lower than the water level in the internal system. The flap gates will open<br />
automatically; at first only slightly but when the difference in water level inside <strong>and</strong> outside increases<br />
the gates will open wider.<br />
When during outgoing tide the water level of the river becomes lower than the water level in the<br />
internal system <strong>and</strong> the sliding gates are open, the culvert might be in a submerged condition but<br />
soon the river level will be lower than the top of the culvert bringing the sluice in the intermediate<br />
condition. For other culverts in the Sundarban discharge starts under these intermediate conditions.<br />
Near the end of the outgoing tide the sluice might enter the free flow condition but as most of the<br />
sluices are not built close to the river often the water level in the level in the canal section connecting<br />
the sluice with the river will remain higher than the bottom of the culvert. Due to all this the<br />
discharges of the sluices cannot be computed but can only be determined by measurements.<br />
The presence of the flap reduces the discharge capacity of the sluices significantly. By its weight the<br />
flap is pushed downwards <strong>and</strong> this hampers the outflow of water. Some of the flaps in the Sundarban<br />
are constructed of wood, others are of cast iron. Although the latter are better from a maintenance<br />
point of view, they are heavier <strong>and</strong> reduce the outflow more than the wooden flaps.<br />
As the formula indicates, the discharge of a given sluice gate is determined by the water head. When<br />
the gate is spilling water the system of canals <strong>and</strong> ponds needs to convey water to this gate. For a<br />
proper drainage it is important that this transport of water is realized with a minimum loss of head.<br />
In theory there are two methods to reduce the loss of head in the canals:<br />
1. To reduce the slope in the canals required to transport the volume of water that needs to be<br />
spilled by the sluice. This can be realized by wide <strong>and</strong> smooth canals. This means that the<br />
canals to the sluices need to be cleaned <strong>and</strong> as much as possible free of any obstacles.
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