Transcript Coastal Hazards - Indian Institute of Technology Bombay

Civil Engineering along the coast
-environmental problems and solutions
Dr. M. C. Deo
IIT Bombay
Contents
Introduction
- types of problems, damage
Structural measures
- dykes, sea walls, bulkheads, revetments, groins,
storm surge barriers
Non-structural measures- sand dunes, beach nourishment,
bio-shields (mangroves, coral reefs)
Closure
Introduction
• Indian coastline (~7500 km)
• 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,..)
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
Tsunami
Primary
:
Earthquake
 Cause
Secondary :
Landslides, volcanic activities
Wave ht. @ 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
Warning system:
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.
• 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
Structural measures for hazards reduction
Sea Dykes
Protect low-lying areas against flooding
Mound of sand and clay – sloping towards sea to reduce wave run up
Slope surface armored with grass, asphalt, stones, or concrete slabs
Shore protection manual, 1984, Coastal Engineering Manual, 2003
Sea walls
Parallel to shoreline
Sloping faced (armored with concrete slabs
or armor units)
Vertical faced (concrete or stone filled)
Problem -toe erosion
(use with
groins)
Revetments -- protection from erosion
Bulkheads -- basically for slope stability
Groins
Walls Perpendicular to shoreline; rubble mound or sheet piles
- Fixed or adjustable
- High or low height
- Permeable or impermeable
Updrift  accretion
Downdrift  erosion
 saw-tooth shoreline
Storm surge barriers
 A series of movable gates (sliding or rotating)
that prevent water intrusion in low lying areas
- open – normally;
but closed – in storm surges
Non-structural measures
--Building sand dunes and growing vegetation around them
--Artificial beach nourishment – removal of sand from one place and
and transportation to some other place
upstream where erosion feared
--Providing bio-shields
Bio-shields
•
•
•
•
•
•
•
Mangroves
Coral reefs
Sea grass
Sea weeds
Animal habitats
Marine parks
Marine sanctuaries
Mangroves
In recent Gujarat and Orissa cyclones,
presence of mangrove buffers  less destruction
 Located in inter-tidal regions, can grow in saline water
and in sheltered places like creeks and estuaries
 Act as buffer against storm surges arrest erosion, trap
sediments, harbor fish
 But certain species are sensitive to excessive
sedimentation, stagnation, oil spills
(Source: www.deh.gov.au/.../ protect/images/mangroves.jpg)
Coral Reefs
-natural barriers against erosion and storm surge
(Source:mbgnet.m
obot.org/salt/
coral/faq.htm)
 corals  slow growing colonies of animals
- growth rate = 1, 10 cm / yr.
- occur in shallow tropical areas
- sea water should be clean, clear and warm
- provide habitat for a large variety of
animals and plants
destruction - outbreak of reef-destroying animals,
storms,
depletion of essential symbiotants
chemical pollution,
mechanical damage,
nutrient or sediment loading
 necessary to quantify the
protection the bio-shields provide
and determine limits beyond which
they are not so effective
Oil spill control
 Mechanical Containment
– Booms and barriers >
– Skimmers
– Sorbents (oil collecting
sponges)
 Chemical and Biological
dispersants
-- disintegrate oil
-- Used in conjunction with
mechanical means