Cyclone and Storm Surge - Iczmpwb.org

4.25
⎛ 2Rr
⎞
V = Vm
⎜
2 2
⎟
⎝ R + r ⎠
(Jelesnianski, 1972) (36)
With the surface winds estimated one can proceed to the computation of the stress at the sea surface.
The surface stress is expressed by conventional quadratic law as:
2
F S ρ c u ( u + v
2
=
a D a a a
)
1
2
1
2
= ) 2
a D a a a
2
G S ρ c v ( u + v
where, ua and va are the x and y components of surface wind, cD is the drag coefficient. Observational
studies suggest that the drag coefficient may be related to the wind speed by:
(37)
cD = (1.00 + 0.07 v10 ) x 10-3 (38)
in which v10 is the wind speed at 10m from the mean sea level. This expression is generally valid for
wind speed less than 14 m sec-1. At wind speeds between 10 and 30 m sec-1 , cD varies from 2 x 10-
3 to 3 x10-3 with no significant dependence on wind speed. Most of the workers use a uniform value of
drag coefficient (Cd=2.8 x 10-3).
4.9. Case Studies of Storm Surges in coastal belt of West Bengal
Storm surges associated with tropical cyclones having their landfall in the coastal belt of West Bengal
distributed in block-wise for the period 1891-2005, is depicted in Table-2. The block-wise frequency of
these cyclones has been grouped based on their severity based upon the central pressure drop (∆P)
while approaching the littoral belt of West Bengal state. As noticed from Table-2, the frequency of severe
cyclones was highest in the Gosaba block with six severe extreme events in a total of 20 cases. Based
on severe cyclones, the other blocks following Gosaba are Namkhana and Ramnagar which
experienced two severe cyclones. Accordingly, four severe cyclones were selected viz; 1998 Gosaba
cyclone, 1988 Gosaba cyclone, 1981 Namkhana cyclone and 202 Kultali cyclone. Among these four
extreme events, the 1988 Gosaba cyclone was considered the most severe.
Case -1: 1998 Gosaba Cyclone
The track and computed surge for the 1998 Gosaba cyclone is depicted in Figure-2(a). The simulation
run was performed for 16 hours prior to the cyclone making its landfall off Gosaba. The computed