Coastal Hazards Guide

Coastal Hazards
Md. Yousuf Gazi, Lecturer, Department of Geology, University of Dhaka (yousuf.geo@du.ac.bd)

Sea Level RiseSaltwater IntrusionCoastal Erosion
Floods Storm SurgeStorms Coastal Flooding
Land Subsidence
▪ Coastal hazards are natural and human-made hazards that occur at the interface between the
ocean and the shoreline. Coastal hazards are categorized as rapid-onset (fast moving) or slow-
onset hazards
Rapid-Onset Hazards
Storms
Flooding
Storm Surge
Tsunamis
Slow-Onset Hazards
Shoreline Erosion
Sea Level Rise
Saltwater Intrusion
Land Subsidence

vulnerable to hazards:
Normal ➔ wind, waves, tides, currents, sediment movement
Extreme ➔ cyclones, storm surges, (~6 / yr) tsunami, (1, 2/century), oil spills.
▪ Impact depends on:
-Density of population, (25 % ➔ within 50 km)
-Design of structures and quality of construction,
-Environment (presence of mangrove forests,..)

Hazard
Vulnerability
Coping
Capacity
Risk Loss
In lives, health
status,
livelihoods, assets
and services
Potential loss
which may
occur due to
disaster
Phenomenon
resulting in loss
Conditions
increasing
susceptibility to
disaster
Ability of people
and organizations
to manage
impacts of
disasters
+
-
Understanding the occurrence of a Disaster:

Cyclones & Storm surges:
▪ Tropical cyclone➔ a rotating wind (low pressure disturbance generally with heavy rainfall)
▪ As cyclone approaches coastal area, strong on-shore winds create
➔ water current circulation, together with lowering of pressures
➔ rise of several meters in sea level (Storm surge)
➔ flooding large areas of the coast

▪ A low-pressure system develops as air rises over warm ocean water (at least 800 F to depths of 150 ft
with no vertical wind shear).
▪ Because of the Coriolis Effect, air begins to spiral toward the center of the low-pressure cell, increasing
in velocity as it approaches the eye-wall.
▪ The incoming air rises and condenses as it loses heat, creating an area of high pressure at the center,
which becomes the “eye” of the storm.
▪ The storm continues to grow as it moves across warm ocean surface water.
▪ Upon landfall or passing over cold water, the cycle breaks, and the storm begins to wane.
Tropical Cyclone Formation:

❑ Hurricane
Impact:
• High winds
• Heavy rain
• Storm surge
▪ Cyclones are centers of low atmospheric pressure
characterized by inward rotating winds. The eye of the
cyclone is an area of high pressure at the center of the
storm. It is characterized by clear, calm skies.

▪ The only difference between a hurricane, a cyclone, and a typhoon is the location where the storm
occurs. In the Atlantic and Northeast Pacific, the term “hurricane” is used. The same type of
disturbance in the Northwest Pacific is called a “typhoon” and “cyclones” occur in the South
Pacific and Indian Ocean.
Difference between a hurricane, a cyclone, and a typhoon:

▪ As a cyclone approaches land, sea-level rises (storm tide) due to low pressure conditions. This
combines with wind-driven surface waves to produce high, surging waves along the coastline.
The surge is greatest along the “front right” margin.
Storm Surges

Google Earth- JTWC Historical Track & Forecast Track of
Cyclonic Storm Mahasen

Primary Cause: Earthquake
Secondary Cause: Landslides, volcanic activities
Wave height at the shore : 20m or more
Time of oscillation
Tsunami: 30 to 60 min
Wind generated gravity waves: 3 to 20 sec
Tides: 12 or 24 hr
Tsunami:

▪ Occurrence of the tsunami-generating earthquake can be recorded on seismographs and
communicated via satellites to the warning center, where computer-based models calculate tsunami
heights and travel times.
▪ This could be verified by a series of wave rider buoys and accordingly tsunami warning can be
issued. Such warning systems exist in Pacific countries including the U.S. and Canada since 1964.
Tsunami Warning system:

Tsunami:

▪ Near the eye-wall, precipitation rates can exceed 10 in/hr, leading to severe flooding,
including far inland from coastal regions.
Inland Flooding

Damage due to water coming from the sea:
- Associated hydrostatic, hydrodynamic forces,
- Impact of objects being carried by the attacking water mass,
- High speed currents, overtopping,
- Resulting flooding and current induced erosion.

Coastal Erosion:
- by breaking waves and associated currents

Oil Spills:
Caused by accidents involving tankers, barges, pipelines, storage facilities can harm marine life, which
in turn can harm human beings through food

▪ Saltwater intrusion is the movement of saline water into freshwater aquifers, which can lead to
contamination of drinking water sources and other consequences. Certain human activities, especially
groundwater pumping from coastal freshwater wells, have increased saltwater intrusion in many coastal
areas.
Saltwater intrusion:

Coastal Hazards Guide

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