Development of an M.I.S. for the prevention of hurricane damage on the Mexican coast

Transcription

1 Development of an M.I.S. for the prevention of hurricane damage on the Mexican coast A. Contreras, R. Silva & G. Diaz Institute* de Ingenieria UNAM, Grupo de Ingenieria de Costas y Puertos. Mexico D.F. Mexico. Abstract This article describes the first stages in the development of an early warning system to alert Mexican authorities of the possible track, intensity and consequences of incoming hurricanes. The MIS was developed to manage the impact of a hurricane, assisting decision-making before and after the event through the forecasting of possible scenarios. Every year over twenty tropical cyclones are generated over the waters surrounding Mexico: 61% in the Pacific Ocean, 39% in the Atlantic. Of these, around four will hit land or pass close by causing tremendous damage to the region economically, ecologically and of course, socially. Along the coast storm surge, surf beat, storm winds and heavy rain may effect the closure of ports even the evacuation of inhabitants. With 11,000 km of vulnerable coastline and a population of over 12 million living along them, Mexico is in dire need of some form of MIS for effective logistical planning to limit the damage caused by this devastating natural phenomenon. The MIS described is based on MAPINFO 5.5 which analyses political boundaries, topographical and bathymetrical features, location of centres of population as well as the infrastructure including that of ports, oil platforms and pipelines. In addition a database from ORACLE 8i gives further analysis such as the type of constructions found in the settlements, detail of telecommunication networks and health services. With this information the probable track of the hurricane can be forecasted as well as possible damage to affected zones. As a result early warning announcements can be given and plans for the emergency made in advance.

2 472 Management Information Systems I. Introduction 1.1 Background Mexico needs an MIS capable of predicting various scenarios caused by hurricanes simply because of the very large number of hurricanes which pass along or close by the Mexican coast generating damage to infrastructure, population or the environment as a whole. Disruptions in port services, reduction in fishing activities or the evacuation of towns are some examples of the effects which may occur due to a storm, cyclonic wind, storm surge or hurricane rain, events which affect the coastal zones of Mexico. 1.2 Goal The Engineering Institute of the National University of Mexico has developed a Hurricane Risk Management System focused on the coastal zones of Mexico in order to predict the path and the effects of hurricanes so that criteria exist allowing authorities to alert, restrict, evacuate or suspend activities in areas at risk. 2. Methodology The system is composed of three modules: a geographic information system, covering all the coastal states of Mexico, a meteorological module based on the effects of hurricanes and tropical storms, and a module for the evaluation of risk to infrastructure and population. The first module details 260 counties within 6 Atlantic coastal states, including 47 ports and the Pacific coasts counties as well. This is coincidentally the most important area of petroleum extraction in the country as well as having a population of around 6 million people. The meteorological phenomenon is based on exhaustive analysis and evaluation of the effects of hurricanes and related events. The risk evaluation is described along the coast, divided into: Cyclonic wind Ocean swell Storm surge At this stage, the system is able to evaluate the risk at petroleum platforms, ports and costal towns and cities. The GIS described above is based on a MAP INFO v. 5.5 environment including the following layers: State division, county division, altitude curves each 50 meters, cities, ports, highways and railroad lines and bathymetry of the zone

3 2.1. Risk criteria Management Information Systems 473 The risk analysis methodology is listed below: Cyclonic intensity. The Saffir-Simpson scale is used to manage the cyclone intensity and the phenomena evolution from a tropical depression, a tropical storm to a hurricane class 5. Table 1. Saffir-Simpson tropical cyclone classification Using the previous scale and extrapolating the possible damages which depend on the distance to the point of interest from the centre intensity is obtained the following chart is derived. Table 2. Effect - intensity - distance effect chart. Intensity w'rr:i»wl -A Influence Diameter [km] Oa60 TD TD TH4 H4 H5 H5 TD The main risk factors considered in this study are the wind, the ocean wave and the storm surge, which affects the shore, causes inundations and infrastructure failure depending on the intensity and on the exposure time. The Table 3 shows the different effects generated by the three factors mentioned above depending on the intensity of the tropical cyclones.

4 474 Management Information Systems Table 3. Effect - cause relation depending on hurricane intensity. Effect Difficulties in fishing and port operation Suspension offishingand unloading activities Damage to the wrench and small structures, glass breaking Severe damage to wrenches, towers and electric posts Damage to warehouse ceilings Severe damage to homes and buildings Some suspension in fishing and unloading activities Mayor suspensions infishingand unloading activities Damage to ports and small boats. Damage to medium sized vessels and total destruction of smaller ones. Damage to breakwaters, large boats, shore and harbours. Collapse of structures, severe damage to docks and harbours Flooding and shore erosion. Flooding of highways Flooding of low lying zones and rivers Flooding of natural faults, docks and marinas Severe flooding and damage to structures and ships Very severe flooding and destruction of large ships 2.2. Wind-pressure model description-hurac The first stage in evaluating the sea wave is to obtain the pressure field of the hurricane which yields the value and distribution of the cyclonic winds, which commonly generate the sea waves around and inside the hurricane. This model presents series of concentric circles representing lines of equal pressure. The slope of the pressure configuration yields the pressure gradient that is used on the hurricane wind equation. The mathematic form of the pressure field is the following: - = Ae -B(Rlr) (1) Where PO is the pressure at the hurricane centre, P^ is the pressure within a distance r from the centre point, P* is the pressure at an infinite distance and R is the maximum cyclonic wind ratio. Four pressure models are presented, all with the same base. Table 4 shows the evaluation equations for each model to give the pressure at any distance r, the pressure gradient and the maximum cyclonic wind Re. (2)

5 Table 4. Four different pressure models Management Information Systems 475 Hydrometeorology Model HI NOAA Model-I [21,*. 2ln Fuiita Model-I Bretschneider Bret Model-X [ Sea wave prediction model over deep water-hidromet The simplified model known as HIDROMET [4] was originally developed by Bretschneider and it has the capability to obtain the significant sea wave height and its associate period based on the cyclonic wind. The required data for this model is listed below: Know the best-fit pressure model. Calculate the pressure at the hurricane centre and the pressure outside the influence zone of the phenomena. Know the radius of the maximum pressure gradient. Know the exact position of the hurricane centre. Have an estimation of the movement and velocity of the hurricane. The significant sea wave height depends on a K' value which in turn depends on a Nc value and is obtained though the next equation: #*,_* =*'V*cA/>o (3) At this point, any sea wave height value can be obtained as well as any wind velocity within a radius r. The sea wave field for the hurricane is obtained with the following equation: (4) J Where Vp is the hurricane advance speed and p is the angle between the hurricane and the windu,.. The velocityfieldfor winds 10 meters over the mean sea water level is obtained as follows: V^ = 0.865(17, + 0.5V, cos(/j)) (5)

6 476 Management Information Systems 2.4. Sea wave prediction model over shallow water-refract This is a method for the resolution of the wave refraction phenomena through a computational model, which solves the wave height and propagation angle from deep to shallow water including wave breaking. The program considers the shoaling and refraction effects, wave-currents interaction and the non-linearity assumption. The numerical model is based in two principles: The irrotationality of the wave number, which is reduced to the Snell's law for planar bathymetry and the conservation of wave action, which is reduced to the energy conservation concept when no current interaction occurs. The two-step Lax-Wendroff differential method was used to calculate and solve the entire computational grid. This model is similar to those developed by Dean [5] and Diaz [6] Storm surge model [7] This model was used to take account of the flooded zones as a result of the storm surge phenomena. This model uses the data yielded by the previous hurricane wind models in order to solve the equation scheme shown in table 5. Table 5. Equation scheme for the forecast of storm surge high tide. Equations: = JV ^-mg ar ^r af = -fu -mg *- dt dy dt dx\ m, ^ -ml U + V j-, yj r _ - tx r -ml U + V + - V ^ dx dy ^ d (VH dy\ m \u ly, surface wind stress components '("+<r)" Where: U and V are the depth averaged currents in the x and y directions respectively H is the total water depth, t, is the sea surface elevation, / is the Coriolis parameter, g is the acceleration due to gravity, m is the map factor, P is the atmospheric surface pressure, p^ is the water density, v is the coefficient of viscosity and TSX and TSV represent the surface wind stress and Tbx and Tbv the bottom frictional stress in the x and y directions, respectively. 3. System integration As stated earlier, the system is composed of three modules; the first takes hurricane date taken from internet by the user, the second generates the risk maps created by the hurricane data analysis, these maps are divide into wind, sea wave and storm surge maps for actual data as well as future data. Finally on the last module, the vulnerability grids generated by the previous modules are adapted to the GIS obtaining the risk maps for ports, platforms, and coastal zones in general.

7 Management Information Systems 477 module, the vulnerability grids generated by the previous modules are adapted to the GIS obtaining the risk maps for ports, platforms, and coastal zones in general. 4. Example Hurricane Gilbert, September 1998, was chosen for the application of the model. This hurricane caused damage running to millions of dollars and some 259 fatalities. Gilbert was the most intense hurricane registered last century, reaching the category of hurricane type 5 on Safflr - Simpson scale, registering a wind speed of 290 Km/h and a pressure of 888 mb at the centre. Gilbert brought total closure to all the ports on the Mexican Atlantic. Along the coast of Quintana Roo Gilbert generated a 3.8 m storm surge which caused massive structural damage in the city and port of Cancun. Figure 1. The path and hurricane intensity defined by Gilbert,September 1998

8 478 Management Information Systems Year: 1988 Month: September Day: 14 Hour: 12:00 Longitude: Latitude: 20.4 Wind Speed: km/h Central Pressure: 892 mba Figure 2. Hurricane Gilbert study point showing the damage areas according to the Saffir - Simpson Scale * 87.2" 86.8" 86.4" Longltul? Vl Figure 3. Deep waves forecast due to Hurricane Gilbert off Cancun.

9 Latitud [*] 21.6V Management Information Systems 479 Depth [m] ' -87A' 47.6' -87.4*.87.2" -87.0'.86.8' -86.6' -86.4* 46.2* 46.0* -85.8* 85.6'LongMlMl Figure 4. Map of equal wind speeds (Isotacas), due to the Gilbert Hurricane off Cancun. Wave Height t =i8m Depth Scale Im] Figure 5. Bathymetry and sea wave refracted on the zone of Cancun

10 480 Management Information Systems 5. Conclusions This system presents great advantages, especially in the specific management for different risk situations. The system allows confident analysis in order to alert the authorities to avoid structured damage and casualties in the areas at risk. The system creates an analysis for each population at risk which then provides the criteria for evacuation if necessary. The system may be extended to consider the flooding inland over the mountain zone. 6. References 1. Lorete, J.M Meteorology. Edit. Labor S.A. de C.V., Mexico, pp Massel, S.R., Ocean Surface Waves: Their Physics and Prediction. Advanced Series on Coastal Engineering -Volume 11. World Scientific Publishing Co., NJ USA, Bretschneider, C Tropical cyclones. Gulf Publishing. Handbook of Coastal and Ocean Engineering, Chapter CSIRO, Division of Atmospheric Research, Australia. Journal of Coastal Research, Dean, R. y Dalrymple R "Water wave mechanics for engineers and scientists." Advanced Series on Ocean Engineering, volume 2, World Scientific. 6. Diaz G. "Sea Wave refraction due to bathymetry variations and currents, UN AM, Mexico Graeme D. Hubbert and Kathleen L. Mclnnes. A Storm Surge Model for Coastal Planning and Impact Studies