This document summarizes various coastal hazards. It categorizes hazards as either rapid-onset (e.g. coastal flooding from storm surge or tsunamis) or slow-onset (e.g. coastal erosion, land subsidence, or saltwater intrusion). Rapid-onset hazards are caused by sudden events like earthquakes generating tsunamis or storms producing storm surge, while slow-onset hazards occur gradually over time from erosion, sinking land, or encroaching seawater. The document also discusses how human activities like dams, seawalls, and jetties can disrupt sediment flows and influence coastal changes. Mitigation measures proposed include protective infrastructure, emergency response improvements, and climate information services.

1. Coastal Hazards
Md Asif Hasan

2. 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.
The most serious coastal hazards are the following for
coastal areas-
Introduction
Rapid-Onset Hazards
Coastal Flooding
Storm Surge
Tsunamis
Slow-Onset Hazards
Coastal Erosion
Land Subsidence
Saltwater Intrusion

3. ◦ Sudden disruption of the ocean floor
◦ Entire water column is disrupted by uplift of ocean floor
◦ Nearing the shore line wave length is decreased and the
wave height is increased
◦ Rapid advancing surge of water up to 40 meters high
Tsunamis

4. Storms are a year-round concern for coastal residents. Storms cause
coastal erosion and other shoreline changes due to high winds,
increased ocean water height, and wave action along the coast. During a
storm event like a hurricane, a storm surge forms on top of normal tide
levels. The resulting storm tide can cause extensive coastal inundation if
winds push the ocean water toward the shoreline
Storms

5. Coastal flooding occurs when normally dry, low-lying land
is flooded by seawater. The extent of coastal flooding is a function of
the elevation inland flood waters penetrate which is controlled by
the topography of the coastal land exposed to flooding.
Coastal Flooding

6. 1890
1920
1970
1990
Coastal Erosion
Coastal erosion is the loss or displacement of land, or the long-term
removal of sediment and rocks along the coastline due to the
of waves, currents, tides, wind-driven water, waterborne ice, or
impacts of storms.

7. Saltwater intrusion is the movement of saline water into
freshwater aquifers, which can lead to contamination of
drinking water sources and other consequences. It occurs
naturally to some degree in most coastal aquifers, owing to
the hydraulic connection between groundwater and
Salt Water Intrusion

8. Coastal subsidence is the motion of a coastal surface as it shifts
downward relative to a datum such as sea-level. It is a major concern in
cities near the Coastal region. Subsidence can cause decreases in
property values, changes in surface-water drainage patterns that lead
to flooding, and large areas of inundation caused by tropical storm
surges.
Processes Responsible
 Regional faulting associated with gravity
spreading and/or salt evacuation;
 Sediment load-induced down-warping;
 Groundwater extraction-compaction of
shallow aquifers (sands)
 Oil/gas extraction related-compaction of
sediment layers (sands)
Land Subsidence

9. Dams - built so less sediment reaches the mouth of rivers
◦ Changes beaches
Artificial structures - built to stabilize beaches
◦ Reduces sediment transport down shore
◦ Dumping tons of sand up-current from beach, doesn’t work
Groin – short wall or dam built from a beach to impede longshore
currents and force sand deposition
◦ Problem erosion breaks down the groin
◦ A field of groins are built
Jetties – Built in pairs on either side of a river or harbor entrance
◦ Confines the flow of water into a narrow zone
◦ Keeping the sand in motion and inhibiting its deposition in the
navigation channel
Human Alterations on Coastal Areas

10.  Mitigating some of the large impacts from cyclones and flooding
 Improving emergency response in the coastal region
 Rehabilitation and upgrading of protection polders to protect the
areas from tidal flooding and frequent storm surges
 Well improvements of agricultural production by reducing saline
water intrusion in selected polders
 Building mitigation infrastructure to strengthen emergency
Preparedness and reducing the vulnerability of the coastal
population
 Responding to emergency events and to strengthen systems to
reduce the vulnerability
 Delivering reliable weather, water, and climate information services
and improve access to such services
Mitigation Measures