Design of Cyclone Shelters Based on Wind Tunnel Studies

Transcription

1 The Eighth Asia-Pacific Conference on Wind Engineering, December 10 14, 013, Chennai, India Design of Cyclone Shelters Based on Wind Tunnel Studies S. Selvi Rajan Chief Scientist and Head, Wind Engineering Lab., CSIR-SERC, CSIR Road, Taramani, Chennai, India, About to 3 tropical cyclones form every year along the coastal line of South Asia, bringing intense winds and severe flooding with large loss of lives due to storm surge. Therefore, construction of shelters along vulnerable coastal areas is one of the important cyclone mitigation measures. As these shelters are to be protected from storm surge, ground floor area is left open to allow storm water to pass through during the event. This however leads to increased loading on the bottom floor slab for which, currently there are no Codal provisions for computing wind loads using pressure coefficients on such kind of special structures. Hence, CSIR-SERC has carried out pressure measurement studies on 1:50 scale models of two cyclone shelters under simulated cyclonic conditions using an auxiliary device developed in-house. The pressure distribution on various faces of the cyclone shelter for 0 and 90 angles of wind incidence are evaluated. As the critical angle of wind incidence is 0, the pressure coefficients corresponding to 0 is only presented in this paper. Keywords: Cyclone, Wind tunnel, Auxiliary device, Pressure coefficient, Angles of wind incidence. Introduction The coastal line of Bay of Bengal in South Asia has a maximum length of 5000 km (approximate) and the weather conditions there are often brutal due to heavy monsoon rains, both summer and winter ( About to 3 tropical cyclones form every year, bringing intense winds and severe flooding. Owing to the funnel shape of the shallow Bay of Bengal and low-lying zigzag terrain, waters churn up due to variation in atmospheric pressure during cyclones, throwing up s of water produced directly by the strong winds, from 8 m to 10 m high (Murthy et. al., 1986). There were about 10 cyclones that hit Asia s Bay of Bengal during the period from 006 to October 013. The statistical data as given in Table 1 shows the large loss of lives (according to wikipedia), which is attributed primarily due to very severe storm surge in the region. Table 1 clearly reveals that the number of fatalities had drastically reduced lately, when compared to the year 008, due to progression in the warning system. This warrants development of infra structure to abandon the local people living in low-lying areas. Cyclone shelters form a major infrastructural facility to save vulnerable population from the onslaught of storm surges during the occurrence of cyclone disturbances. The marooned people are temporarily evacuated on receipt of warnings and they need to be looked after for 3 days by arranging shelter, food and health care facilities. Therefore, construction of shelters along vulnerable coastal areas is one of the important cyclone disaster mitigation measures (Guidelines for Design and Construction of Cyclone/Tsunami Shelters, GoI 006). In addition, these shelters are to be protected from storm surge and hence the ground floor area has to be left open to allow storm water to pass through during the event. At the request of Indian and German Red Cross and KfW, Germany, CSIR-SERC has designed a stilted cyclone shelter for use in Orissa coast with specialized foundation. The 3 cyclone shelters constructed at Orissa have saved nearly 46,000 people during the Super Cyclone which hit Orissa in 1999 and many of these structures are still in service, withstanding the recently hit Phailin cyclone in October 013. Recently, National Disaster Management Authority (NDMA, Govt. of India) is also embarking on a large project involving many cyclone shelters being built along the entire cyclone prone regions of the country. Based on the experiences of various stages in construction and maintenance of cyclone shelters, a common strategy has been evolved for sustainable use of the shelters, by Ministry of Home Affairs, Government of India (006). Copyright 013 APCWE-VIII. Published by Research Publishing Services. ISBN: :: doi: / _key-10 K-199

2 K-00 S. Selvi Rajan Table 1. Statistical data of tropical cyclones that hit Asia s Bay of Bengal during the period from 006 to 013. Sl.No. Cyclones Storm surge height m (ft.) Economic loss USD Fatalities 1 Phailin (.4 ) 696 million 45 total Nilam (5 ) 37 million 75 total 3 Thane to 1.0 (1.5 to 3 ) 35 million 48 total 4 Jal to 4.5 (5 to 15 ) 39 million at least Laila (5.8 ) million 65 total 6 Giri (1 ) 359 million 157 direct 7 Nisha to 1.5 (4 to 5 ) 800 million 04 8 Nargis to 4.5 (5 ) 10 billion 138,366 total 9 Sidr (9.8 ) 1.7 billion ~10,000 total 10 Mala (3 ) 6.7 million direct Cyclonic Wind Characteristics Schroeder and Smith (003) while analyzing the Hurricane Bonnie wind characteristics, it was found that additional energy at low frequency in the longitudinal power spectral density (PSD) was observed in comparison to the power spectra of normal winds. The intensities of turbulence for the WEMITE anemometer at a height of 10.7 m varied between 0.1 to 0.35 during normal winds and 0.17 to 0.41 during cyclones. This suggests an increase in turbulence intensities by 15 to 30% during cyclones. Using the data of Florida Coastal Monitoring Programme (FCMP), during hurricanes, Yu and Chowdhury (008) also opined that there was increased energy at low frequencies in cyclonic wind spectra. They compared the non-dimensionalised spectrum obtained from FCMP experiments with different models used for normal wind climate and came out with an expression for power spectral curve for hurricane winds, as a function of RMS velocity and a few constant values derived at 10 m height. A close look at the longitudinal wind speed PSD of the 10.7 m WEMITE data shows that there is a shift in reduced frequency (nl/u, where, n is the frequency, L is the length scale and U is the mean wind velocity) from 0.03 to 0.01, which also tallies roughly with the hurricane wind power spectra given by Yu and Chowdhury (008). Also, based on the R&D studies conducted on cyclonic wind characteristics using full scale experiments at CSIR-SERC (Harikrishna et al., 010, Ramesh Babu et al., 01), increased turbulence intensities by 15 to 30% during cyclones was observed. This is in consistent with the observation of increased energy at low frequencies in wind spectra during hurricanes observed in Florida Coastal Monitoring Programme (FCMP). Simulation of tornado like vortices had been reported by Zifeng Yang et al., 011. Simulation of In the present study, an attempt has been tried to simulate wind spectrum having high energy content at the low frequency range. Loading The recommended value of the imposed loads as per IS: 875 (Part ) 1987, for assembly buildings, without fixed seats is equal to 500 kg/m. An imposed load of 500 kg/m can be considered (Guidelines for Design and Construction of Cyclone/Tsunami Shelters, 006). An additional imposed load of 50 kg/m has to be included taking into consideration of loading due to overcrowding. Hence, a total imposed load of 750 kg/m may be considered for multi-purpose cyclone shelters. As per IS: 875 (Part 3) 1987, the maximum basic wind speed for the cyclone prone coastal regions of India is 50 m/s. As per IS: (004), a basic wind speed of 65 m/s is to be considered for cyclonic condition. However, based on the risk analysis carried out at CSIR-SERC, Chennai, it has been shown that cyclonic wind speeds averaged over a duration of one minute can be as high as 70 m/s. This would correspond to a 3-second averaged wind speed of 84 m/s. Hence, for the design of the cyclone shelter carried out by CSIR-SERC, a basic wind speed of 84 m/s has been considered. The design wind speed can further be computed as per IS: 875 (Part 3) 1987 using probability factor (k 1 ) of 1.08 (considering 100 year life of the cyclone shelter), A load factor of 1.0

3 Design of Cyclone Shelters Based on Wind Tunnel Studies K-01 is recommended for wind loads, whereas a load factor of 1.5 is recommended for both dead and imposed loads. As these cyclone shelters are to be protected from storm surge, ground floor area is left open to allow storm water to pass through during the event. This however leads to additional wind loading on the bottom floor slab. IS:875 (Part 3) 1987 does not give recommendations for this type of stilted structures for computing suction wind pressure acting on the bottom slab of the building wherein wind flow can occur below the floor slab. Hence, CSIR-SERC has carried out pressure measurement studies on two selected models of cyclone shelters to evaluate pressure distribution based on wind tunnel studies under simulated cyclonic wind characteristics in comparison to the pressure distribution under normal simulated boundary layer wind conditions. Structural Configuration of Cyclone Shelter The detailed structural configuration of the cyclone shelter models 1 and have been presented in this section. Majority of the previous cyclone shelters in India are circular in plan. The force coefficients for circular cylinders are lower compared to other structural forms, and hence the above shape may have been chosen earlier. However, the effective floor area utilization is significantly lower with a circular plan. Hence, the selected cyclone shelters have been designed with rectangular plan shape even though it attracts more wind loads. However, in case of model, the efficiency of the structural layout has been improved by providing rounded corners for better aerodynamic effect. Model 1 The cyclone shelter of Model 1 is a reinforced concrete framed structure with rectangular plan shape with chamfered corners and two projected head rooms. The overall plan dimensions at the roof level are 8.3 m x 3.0 m (excluding the projecting head rooms). The columns of the frame are circular in shape. The ground floor area is left open excepting for the columns supporting the super structure to allow storm water to pass through in case of storm surge. The stilt height is about 3.5 m. The entire structure is on raised platform of height 1.05 m to reduce effects of storm surge. A view of the prototype cyclone shelter in Orissa is shown in Fig. 1(a). Model The cyclone shelter Model has rectangular plan shape with aerodynamically rounded at the four corners to reduce wind load effects. The structure essentially consists of ten reinforced concrete frames. The overall length of the building between the end frames is 1.0 m. The radius of the Fig. 1(a). View of prototype cyclone shelter (Model 1).

4 K-0 S. Selvi Rajan Fig. 1(b). View of prototype cyclone shelter (Model ). Flexible ceiling Roughening Elements Trip Board Tunnel floor BL depth roughening elements Test section length = 18 m trip board Wind Fig.. Vortex generators used for simulation of normal wind. circular curve at the inner edge has been chosen to be 1.0 m. To reduce the effects of storm surge, the ground level in the area is to be raised by 1.05 m. The building has a corridor of width 1.95 m at first floor level (+4.65 m). A stair case of width m is provided to reach the corridor. The overall dimensions at the roof level are 7.65 m x 3.0 m, which includes an overhang of 1.95 m in front and 0.9 m at the rear of the building. The columns of the frame are circular in shape. The stilt height is about 3.5 m. Since the ground floor is left open, there tends to be additional wind loading on the bottom floor slab, which has been duly considered in the design. A view of the full-scale structure of the cyclone shelter located in Orissa is shown in Fig. 1(b). Wind Tunnel Studies Based on the reported literature on cyclonic wind characteristics, wind tunnel investigations have been carried out to attain the following objectives: to realise high energy content at low frequency region in the turbulence spectra to obtain an increased intensity of turbulence by about 15 % Existing vortex generators, namely, trip board and roughening elements in BLWT facility of CSIR- SERC, as shown in Fig. have consistently reproduced the normal boundary layer wind profiles and spectra of turbulence corresponding to different scales and terrain categories. Having identified the above objectives, it is felt that cyclonic wind characteristics may not be realised simply by

5 Design of Cyclone Shelters Based on Wind Tunnel Studies K-03 changing the sizes of trip board and roughening elements. Hence, a physical mechanism has been developed to reproduce the high energy content at low frequency region. An auxiliary device to simulate the cyclonic wind spectra has been designed to increase the turbulence intensity by about 15%. The details of the auxiliary device are explained elsewhere (Abraham, et al., 014). Brief Details About Auxiliary Device The device primarily consists of a circular plate (diameter 1000 mm) made of perspex sheet, roughening elements and two square plates (size of 100 mm x 100 mm) kept along two radial lines separated at right angles to each other at the edges. The device has been located at the upstream side of the turn table and supported by caster wheels at the edges so that the device is intended to rotate on the tunnel floor about its vertical axis i.e., in yaw direction, manually. A small gap of 5 mm has been provided between the device and tunnel floor. A steel rod has been connected to the device and taken through the bottom of the tunnel floor for operation purposes. Since the diameter of the device has been 1000 mm, there is a need of channelising the approach flow at the upstream side. Hence, two plates (height of 165 mm and length of 1750 mm) have been fixed on the tunnel floor and separated by a distance of 160 mm (dimension relatively larger than diameter of circular plate). The above setup has been located symmetrically about the centre line of the wind tunnel. The device has been separated by a distance of 1575 mm from the centre of the turn table. Fig. 3 shows the dimensions and location of auxiliary device. When the device has been under stationary position (when not rotated), it reproduces the wind characteristics corresponding to normal terrain condition i.e., the frequency obtained based on several spectra reveals that the peak amplitude for longitudinal fluctuation occurs around 1 Hz. By rotating the device about its vertical axis i.e., in yaw direction, the peak spectral amplitude is realised below 1 Hz i.e., around 0.3 Hz. It is clearly to be noted that the device has been kept at the upstream side of turn table so as to retain the modified flow characteristics over a short fetch length up to location of model. Turn table Paddlers Pitot tube Anti-clockwise Clockwise Auxiliary device 1.6 m dia =1 m 0 0 Roughening elements 0 Circular plate Position of plates used for channelsing the flow Direction of rotation of auxiliary device Fig. 3. Details of auxiliary device. Pressure Coefficient A number of experiment have been carried out to obtain cyclonic wind characteristics, viz. increased level of turbulence intensity and high energy content at low frequency region. The

6 K-04 S. Selvi Rajan analysis of pressure data have been mainly focused on pressure coefficients and their spectral variations. The pressure coefficients are defined, as given below: Ê ˆ Á p- pso Cp = Á 1 V Ë Á r (1) where C p = mean pressure coefficient p, p so = mean, static pressures ρ = mass density of air, and V = mean wind velocity È 1 T C p = Î Í 1/ pt ()-p dt Ê ˆ Á p- pso ; Cˆ ˆ p = Ê 1 ˆ Á 1 ; V V Ë Á r Á r Ë Ú ( ) Cp Ê ˆ Á p- pso = Á 1 Ë Á rv () where C p, Ĉ p and C p = r.m.s., maximum and minimum pressure coefficients p(t) = instantaneous pressure at time t, and pˆ, p = maximum, minimum pressure Pressure Measurement Studies Detailed experimental investigations have been carried out in the Boundary Layer Wind Tunnel of CSIR-SERC to evolve the design pressure distributions applicable to the geometry of the cyclone shelter Model 1 and Model. The schematic diagrams showing the dimensions corresponding to full-scale of Model 1 and Model are given in Figs. 4 and 5, respectively. Pressure measurement studies have been conducted on 1:50 scaled models of these cyclone shelters. The models have been made of acrylic material of 3 mm thickness. The Model 1 has an overall height of around 170 mm and plan dimensions of 463 mm x 04 mm. The schematic diagrams of the model 1 and model are shown in Figs. 4 and 5, respectively. The model has an overall height of around 154 mm and plan dimensions of 463 mm x 153 mm. Both the models have a stilt height of around 70 mm to account for storm surge. A total of 60 and 50 pressure taps have been 10. m Section- Section-1 3 m 8.3 m Section- Section-1 3 m 7.65 m 3.5 m 3.5 m θ = 0 θ = 0 Fig. 4. Schematic diagram of Model 1. Fig. 5. Schematic diagram of Model.

7 Design of Cyclone Shelters Based on Wind Tunnel Studies K-05 provided for Models 1 and, respectively, in one half of the building envelope, considering the symmetric dimensions. The pressure measurement studies on the cyclone shelter models have been carried out under simulated cyclone conditions and normal wind conditions. The wind tunnel experiments were conducted with a novel experimental technique to introduce low frequency component in the longitudinal wind spectra and to increase the intensity of turbulence by at least 15% as observed during cyclone. Experiments have been conducted by rotating the auxiliary device in clockwise (clock), anti-clockwise (anticlock) directions, in both directions simultaneously for half circular rotation (half) and without any rotation (norot). No rotation case corresponds to normal simulated wind condition. It has been noted that this device was not able to retain the modified flow characteristics over a long fetch. Hence, the models of the cyclone shelters have been placed at a distance of 15 cm (case A) and then 5 cm (case B) from the centre of the auxiliary device, as shown in Fig. 6. The pitot tube has been placed at the level of height of the cyclone shelter model. Two pressure scanners have been used to acquire the data simultaneously with a sampling rate of about 650 Hz for duration of 15 seconds, for each test case. More details on the experimental studies are included in Keerthana et al. (014). (a) (b) Fig. 6. Models of the cyclone shelters in wind tunnel (a) Model 1 and (b) Model. Results and Discussions The acquired pressure data are analysed for mean and RMS pressure coefficients with reference to the height of the cyclone shelter model. Figs. 7 and 8 show the distribution of mean pressure coefficient along Section-1 and Section- (as referred in Fig. 4) of the building envelope for cyclone shelter model 1 for case A and case B respectively. Figs. 9 and 10 show the distribution of mean pressure coefficient along Section-1 and Section- (as shown in Fig. 5) of the building envelope for cyclone shelter Model for case A and case B respectively. Table gives the maximum values of pressure coefficients along Sections-1 and of cyclone shelter Models 1 and. For Model 1, from the measured mean pressure coefficients, a maximum value of 1.45 has been observed on the windward. Maximum values of 1 and 1.19 have been observed for the mean suction pressure coefficients on roof slab and bottom slab, respectively. For the leeward, maximum suction pressure coefficient of 0.46 has been observed. For model, a maximum positive pressure of 1.05 has been observed in the windward. Maximum suction pressure coefficients of 0.73 and 0.49 have been observed for the roof slab and bottom slab, respectively. For leeward, maximum suction pressure coefficient of 0.5 has been

8 K-06 S. Selvi Rajan Mean pressure coefficient Windward Roof slab Leeward Port number along section - 1 Bottom slab anticlock - case A clock -case A norot - case A half - case A anticlock - case B clock -case B norot - case B half - case B Fig. 7. Distribution of C p at Section-1 for cases A and B, for θ = 0 (Model 1). Mean pressure coefficient Windward Roof slab Leeward Port number along section -1 Bottom slab anticlock - case A clock -case A norot - case A half - case A anticlock - case B clock -case B norot - case B half - case B Fig. 8. Distribution of C p at Section- for cases A and B, for θ = 0 (Model 1). Table. Maximum value of mean pressure coefficients observed for cyclone shelter Models 1 and. Model Section Case Windward Roof slab Leeward Bottom slab Model-1 Section-1 A Section-1 B Section- A Section- B Model- Section-1 A Section-1 B Section- A Section- B

9 Design of Cyclone Shelters Based on Wind Tunnel Studies K-07 Mean pressure coefficient Windward Roof slab Leeward Port number along section - Bottom slab anticlock - case A clock -case A norot - case A half - case A anticlock - case B clock -case B norot - case B half - case B Fig. 9. Distribution of C p at Section-1 for cases A and B, for θ = 0 (Model ). Mean pressure coefficient Windward Roof slab Leeward Port number along section -1 Bottom slab anticlock - case A clock -case A norot - case A half - case A anticlock - case B clock -case B norot - case B half - case B Fig. 10. Distribution of C p at Section-1 for cases A and B, for θ = 0 (Model ). observed. It is to be noted that the maximum positive and suction pressures have been observed corresponding to the rotation of the auxiliary device in anti-clockwise direction. Increase in mean pressure coefficients in the windward and the roof region for a typical low-rise building tested under simulated cyclonic characteristics has been reported earlier (Lakshmanan et al., 011). Similarly, for the stilted structure considered in the present study, increase in mean pressure coefficients in windward, roof slab and bottom slab has been observed.

10 K-08 S. Selvi Rajan The maximum positive pressure in the windward of the cyclone shelter models corresponding to case A (for which the auxiliary device is more close to the model) is higher than that of case B for both model 1 and model. Similar observation has been drawn for all other faces, viz, roof slab, bottom slab and leeward face. However, the mean pressure coefficients are similar in magnitudes as well as in trend for model 1 for case A and case B, whereas for Model, slight deviations have been observed within case A and case B. This could be attributed to the presence of open corridor in the first level of the cyclone shelter Model. It has also been observed that provision of open corridors and overhangs as in case of Model reduced the suctions pressures on the stilted slab and on the roof slab, besides reducing windward pressures for the 0 angle of wind incidence. Further from the time history of pressure coefficients, root mean square values of pressure coefficients have been derived. Figs. 11 and 1 show the comparison of rms of the pressure R M S pressure coefficient Windward Roof slab Port number along section - 1 anticlock - case A norot - case A anticlock - case B norot - case B Leeward Bottom slab clock -case A half - case A clock -case B half - case B Fig. 11. Distribution of C p along Section-1 for cases A and B for θ = 0 (Model 1). R M S pressure coefficient Windward Roof slab anticlock - case A norot - case A anticlock - case B norot - case B Port number along section - 1 Leeward Bottom slab clock -case A half - case A clock -case B half - case B Fig. 1. Distribution of C p along Section-1 for cases A and B for θ = 0 (Model ).

11 Design of Cyclone Shelters Based on Wind Tunnel Studies K-09 Spectral Ordinate-Pressure no rot clock anti half Spectral Ordinate-Pressure anti clock no rot half Frequency,Hz (a) Frequency,Hz (b) Fig. 13. Spectral characteristics of the oncoming flow for (a) case A and (b) case B. coefficients along Section-1 for cases A and B in cyclone shelter Models 1 and, respectively. Increase in rms of pressure coefficients has been observed under simulated cyclonic condition, compared to normal wind condition. Fig. 13 shows the spectral characteristics of the oncoming flow for case A and case B. It can be observed from the spectrum that there is increase in low frequency content for rotation of the auxiliary device in clockwise (clock), anti-clockwise (anticlock) directions, in both directions simultaneously for half circular rotation (half) with respect to without any rotation (norot), as reported in literature. Conclusions More numbers of cyclone form in the Bay of Bengal and in Arabian Sea. It has been observed that even though damage to property is continuously increasing, loss of life shows a declining trend due to improved early warning system, mitigation measures and improved response mechanisms. An important aspect of cyclone disaster mitigation measures is to ensure availability of adequate numbers of shelters, which are primarily utilised for moving people from vulnerable areas to safety. Storm surge is considered to be more responsible for the loss of lives since tropical cyclones in the Bay of Bengal usually produce a higher storm surge as compared to elsewhere in the world because of the nature of the coastline, shallow coastal bathymetry and characteristics of tides as reported in the National Disaster Management Guidelines, issued by NDMA. Hence, the cyclone shelters are to be protected from storm surge. To allow the storm water to pass through during the event, the cyclone shelters are preferred to be of stilted type. This however leads to additional wind loading on the bottom floor slab for which, currently there are no Codal provisions for computing wind loads using pressure coefficients on such kind of special structures. Hence, CSIR-SERC has carried out pressure measurement studies on 1:50 scale models of two existing configurations of cyclone shelters. Experiments are conducted under simulated cyclonic conditions using an auxiliary device developed in-house. The device successfully demonstrated the feasibility of reproduction of increased turbulence intensities with higher energy at low-frequencies as reported in literature. The increase in intensity of turbulence that can be achieved is primarily dependant on the spacing between the centre to centre distance between the device and the model. With increasing distance the increase achieved in spectral ordinates tends to dissipate. An increase in turbulence intensity of the order of about 50% could be achieved when the performance of the system was

12 K-10 S. Selvi Rajan fine tuned. Based on the wind tunnel pressure measurement studies on the two models (Models 1 and ) of cyclone shelters, it is observed that a maximum value of mean positive pressure has been observed in the windward. And maximum values of mean suction pressure coefficients have been observed for the roof slab and bottom slab. Increased mean pressure coefficients have been observed for both the cyclone shelter models in cyclonic condition, in comparison with normal condition (no rotation of the device). This increased value of pressure coefficients and the maximum value of suction pressure coefficients on the bottom slabs, showing tendency to develop large suction pressures for wind direction perpendicular to the longer side of the building have to be considered while designing the cyclone shelters. The rms pressure coefficients are found to be more under simulated cyclonic wind characteristics compared to normal wind conditions, as expected. Acknowledgment Sincere and grateful acknowledgements are due to Dr. N. Lakshmanan (Former Director and Project Advisor) and Dr. S. Arunachalam (Former Chief Scientist). Acknowledgements are due to Ms. Keerthana, Scientist and Ms.Manimekala, Project Assistant, who helped in the analysis of data. This paper has been published with the approval of Director, CSIR-SERC. References Abraham, A., Selvi Rajan, S., Ramesh Babu, G., Harikrishna, P and Chitra Ganapathi, S., Auxiliary device to create cyclonic wind spectra in wind tunnel studies, Proc. of the International Conference on Disaster Management, Chennai, January 014 (Accepted for publication). Harikrishna, P., Selvi Rajan, S., Ramesh Babu, G. Arunachalam, S. Lakshmanan. N., Abraham, A., Chitra Ganapathi, S., and Sankar, S., Simulation of Cyclonic Turbulence Intensities in Boundary Layer Wind tunnel, SERC Research Report No.6, MLP 14041, April 010. Guidelines for Design and Construction of Cyclone/Tsunami Shelters (006), GoI-UNDP Disaster Risk Management Programme, Ministry of Home Affairs, Government of India. Wilson, K.J. Characteristics of the sub cloud layer wind structure in tropical cyclones, Proceedings of the International Conference On Tropical Cyclones, Perth Australia, Murthy T.S., Flather R.A., and Henry R.F. (1986), The storm surge problem in Bay of Bengal, Prog. Oceanog., Vol.16, Lakshmanan, N., S. Selvi Rajan, S. Arunachalam, A. Abraham, G. Ramesh Babu, and P. Harikrishna, "Auxiliary device to create cyclonic wind spectra in wind tunnel studies", Report No.11, MLP , CSIR-SERC, February 011. Black, P.G. Evolution of maximum wind estimates in typhoons, Proceedings of the ICSU/WMO International Conference on Tropical Cyclone Disasters, Beijing, China, October 1, 18, 199, Beijing University Press, 1993, pp Ramesh Babu, G., Harikrishna, P., Abraham, A., Selvi Rajan, S., Keerthana, M., Nagesh R. Iyer and Arunachalam, S., Field measurement of wind characteristics during cyclone "Nilam", Proc. of VI National Conference on Wind Engineering, New Delhi, December 01, pp Schroeder, J.L., and Smith, D.A., (003), Hurricane wind flow characteristics as determined from WEMITE, Jl. of Wind Engg and Industrial Aerodynamics, 91, Selvi Rajan S., M. Keerthana, P. Harikrishna, A. Abraham, G. Ramesh Babu, M. Kameshwaran, Wind Tunnel Pressure Measurement Studies on Models of Cyclone Shelters under Simulated Cyclonic Wind Characteristic, Proc. of the International Conference on Disaster Management, Chennai, January 014 (Accepted for publication). Simiu, E and Scanlan, R. H. (1996), Wind Effects on Structures, 3rd Ed., John Wiley & Sons, New York, USA. Yu, B.O., Choudhary, A.G., and Masters, F.J., (008), Hurricane wind power spectra, Co spectra and integral length scales, Boundary Layer Meteorology, 9: