The Impacts of Natural Hazards » Tropical Cyclone Physical Impacts

What you'll learn this session

Study time: 30 minutes

The formation and structure of tropical cyclones
Primary physical impacts of tropical cyclones
How wind, storm surge and rainfall cause damage
Regional variations in tropical cyclone impacts
Case studies of significant tropical cyclones
How physical impacts lead to secondary effects

Introduction to Tropical Cyclone Physical Impacts

Tropical cyclones (also known as hurricanes or typhoons) are among the most destructive natural hazards on Earth. These powerful storm systems form over warm ocean waters and can cause devastating physical impacts when they make landfall. Understanding these impacts is crucial for disaster preparedness and mitigation.

Key Definitions:

Tropical Cyclone: A rotating, organised system of clouds and thunderstorms that originates over tropical or subtropical waters and has a closed low-level circulation.
Storm Surge: An abnormal rise in seawater level during a storm, measured as the height of the water above the normal predicted astronomical tide.
Eye: The calm, clear centre of a tropical cyclone.
Eye Wall: The ring of thunderstorms that surrounds the eye, where the most severe weather occurs.

Formation and Structure of Tropical Cyclones

Before we explore their impacts, it's important to understand how tropical cyclones form and their structure.

🌊 Formation Requirements

Tropical cyclones need specific conditions to form:

Warm ocean water (at least 26°C to a depth of 50m)
Atmospheric instability
High humidity in the lower to middle levels of the troposphere
Sufficient Coriolis force to develop rotation
A pre-existing near-surface disturbance
Low vertical wind shear

🌪 Cyclone Structure

A tropical cyclone has several key parts:

Eye: Calm, clear centre (typically 30-65km wide)
Eye Wall: Ring of intense thunderstorms with strongest winds
Rain Bands: Spiral bands of clouds, rain and thunderstorms extending from the eyewall
Outflow: Upper-level clouds moving away from the storm centre

Primary Physical Impacts of Tropical Cyclones

Tropical cyclones cause damage through three main mechanisms: extreme winds, storm surge and heavy rainfall. These primary physical impacts can devastate landscapes and infrastructure.

💨 Extreme Winds

Wind speeds in tropical cyclones can exceed 250 km/h, causing:

Structural damage to buildings
Uprooted trees and vegetation
Flying debris becoming dangerous projectiles
Power line damage leading to widespread outages
Complete destruction of poorly constructed buildings

🌊 Storm Surge

Often the deadliest aspect of tropical cyclones:

Wall of water pushing inland (can reach 6+ metres high)
Coastal flooding and erosion
Destruction of coastal structures
Saltwater contamination of freshwater sources
Reshaping of coastlines and barrier islands

🌧 Heavy Rainfall

Cyclones can produce extreme precipitation:

Widespread flooding, even far inland
Landslides and mudslides in hilly areas
River flooding and dam failures
Soil saturation and agricultural damage
Some cyclones can drop 500+ mm of rain in 24 hours

How Physical Impacts Vary by Location

The physical impacts of tropical cyclones aren't uniform across affected areas. Several factors influence the severity and type of impacts experienced:

Geographical Factors Affecting Impact Severity

Coastal Topography: Shallow, gently sloping continental shelves can produce higher storm surges than steep coastlines.
Land Elevation: Low-lying areas are more vulnerable to flooding from both storm surge and rainfall.
Distance from Landfall: The most intense winds are typically experienced near the eyewall, with decreasing intensity further away.
Landscape Features: Mountains can enhance rainfall on windward slopes but may protect areas on leeward sides.
Built Environment: Urban areas with extensive concrete and poor drainage often experience worse flooding.

Case Study: Typhoon Haiyan (2013)

One of the strongest tropical cyclones ever recorded, Typhoon Haiyan devastated the Philippines in November 2013:

Wind Impacts: Sustained winds of 315 km/h destroyed over 1 million homes
Storm Surge: A 5-7 metre storm surge inundated coastal communities in Tacloban City
Physical Destruction: Completely altered coastlines, destroyed forests and created new channels through barrier islands
Scale of Impact: Affected over 16 million people and caused approximately 6,300 deaths
Why So Severe? The funnel-shaped bay near Tacloban amplified the storm surge, while the city's low elevation and dense population increased vulnerability

Detailed Physical Impact Analysis

Wind Damage Mechanisms

Wind damage occurs through several processes:

Direct Pressure: Wind exerts force on structures, potentially causing collapse when it exceeds design tolerances
Uplift: Strong winds flowing over roofs create low pressure zones that can lift roofing materials
Oscillation: Fluctuating winds can cause structures to sway and eventually fail from material fatigue
Projectiles: Debris carried by wind becomes dangerous missiles that can penetrate buildings

Storm Surge Mechanics

Storm surge is influenced by several factors:

Wind Stress: Sustained winds push water toward the shore
Low Pressure: The low atmospheric pressure in the cyclone centre causes water to bulge upward (about 1cm rise per 1mb pressure drop)
Coastline Shape: Bays and inlets can funnel water, amplifying surge height
Timing: Surge impacts are worst when coinciding with high tide (creating a "storm tide")
Continental Shelf: Shallow, gently sloping sea floors allow for higher surge than deep, steep approaches

🌎 Regional Variations

Tropical cyclones have different names and characteristics in different regions:

North Atlantic & Northeast Pacific: Called hurricanes, season June-November
Northwest Pacific: Called typhoons, can occur year-round but peak July-October
North Indian Ocean: Called cyclonic storms, peaks April-June and October-November
South Indian & South Pacific: Called tropical cyclones, season November-April

📊 Measuring Intensity

Different scales are used to classify tropical cyclones:

Saffir-Simpson Scale: Used for Atlantic hurricanes (Categories 1-5)
Australian Tropical Cyclone Scale: Categories 1-5
Indian Meteorological Department Scale: Categories from Depression to Super Cyclonic Storm
Higher category storms generally cause more severe physical impacts

From Physical Impacts to Secondary Effects

The immediate physical impacts of tropical cyclones trigger a cascade of secondary effects that can persist long after the storm has passed:

Environmental Changes: Coastal erosion, wetland loss, saltwater intrusion into freshwater systems
Infrastructure Damage: Disruption to transportation networks, power systems and water supply
Agricultural Impacts: Crop destruction, soil erosion, livestock losses
Public Health Concerns: Contaminated water supplies, disease outbreaks, limited access to medical care
Economic Consequences: Business disruption, property damage, insurance losses

Case Study: Hurricane Katrina (2005)

Hurricane Katrina demonstrated the catastrophic physical impacts possible when multiple factors align:

Storm Surge: A surge of 8+ metres overwhelmed the levee system protecting New Orleans
Flooding: 80% of New Orleans was underwater, with some areas under 6+ metres of water
Wind Damage: Widespread destruction across 230,000 square kilometres
Physical Infrastructure: Destroyed 30 oil platforms, damaged 50 others
Environmental Impact: Damaged 320 million trees, created 118 million cubic metres of debris
Why So Severe? New Orleans' below-sea-level location, failure of engineered defences and the storm's path created a "perfect storm" of physical impacts

Comparing Tropical Cyclone Impacts Globally

The physical impacts of tropical cyclones vary significantly between regions due to differences in vulnerability, preparedness and cyclone characteristics:

🌏 Developed Regions

In places like the United States, Japan and Australia:

Better building codes reduce structural wind damage
Advanced warning systems allow for evacuation
Higher economic losses but typically lower death tolls
More resources for recovery and rebuilding
Examples: Hurricane Harvey (2017), Typhoon Jebi (2018)

🌍 Developing Regions

In places like Bangladesh, Philippines and Caribbean islands:

Higher population densities in vulnerable coastal areas
Less robust infrastructure more susceptible to damage
Limited evacuation options and emergency resources
Higher death tolls but often lower economic costs
Examples: Cyclone Nargis (2008), Typhoon Haiyan (2013)

Summary: The Physical Impact Chain

Tropical cyclones create a chain of physical impacts that can be understood as a sequence:

Initial Contact: Wind and water interact with the landscape
Primary Physical Impacts: Wind damage, storm surge flooding, rainfall flooding
Landscape Modification: Erosion, deposition, vegetation damage
Infrastructure Disruption: Damage to buildings, roads, utilities
Environmental Changes: Water contamination, habitat destruction
Long-term Physical Alterations: Changed coastlines, river channels and landforms

Understanding these physical impacts is essential for developing effective mitigation strategies and building more resilient communities in cyclone-prone regions.

🧠 Test Your Knowledge!