Myanmar Engineers are experiencing a transition between traditional way of development and sustainable development approaches. For that reason, a 12 week course on Dredging Engineering has been created and delivered free of charge. This is the third lecture.

1. DREDGING ENGINEERING (6 Jun – 23 Aug 2020)
Lecture (3), Saturday 13th Jun 2020
8:00 – 10:00 am, Yangon, Myanmar
Prof. Dr. Khin Ni Ni Thein, Secretary of the Advisory Group, Member of the
National Water Resources Committee (NWRC), Advisory Group Member of the
National Disaster Management Committee, and Hydro-Informatics Centre
(HIC) Lead
The National Water Resources Committee
The Hydro-Informatics Centre (HIC)
Myanmar Water Academy (MyanWA)
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2. Appropriate Extraction Methods
(Slide 1)
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3. Appropriate Extraction Methods
(Slide 2)
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4. Appropriate Extraction Methods
(Slide 3)
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5. Appropriate Extraction Methods
(Slide 4)
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6. Appropriate Extraction Methods
(Slide 5)
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7. Appropriate Extraction Methods
(Slide 6)
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8. Appropriate Extraction Methods
(Slide 7)
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9. Appropriate Extraction Methods
(Slide 8)
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10. Appropriate Extraction Methods
(Slide 9)
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11. Appropriate Extraction Methods
(Slide 10)
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12. Appropriate Extraction Methods
(Slide 11)
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13. Appropriate Extraction Methods
(Slide 12)
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14. Appropriate Extraction Methods
(Slide 13)
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15. Appropriate Extraction Methods
(Slide 14)
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16. Appropriate Extraction Methods
(Slide 15)
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17. Appropriate Extraction Methods
(Slide 16)
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18. Appropriate Extraction Methods
(Slide 17)
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19. Appropriate Extraction Methods
(Slide 18)
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20. Varieties of Dredging Equipment
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21. Impacts of Gravel Mining
(In channel gravel mining and also on a sand bar)
Video Clip
• https://www.youtube.com/watch?v=Se5HzG8MPKk
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22. Mode of Sediment Transport in Rivers
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Source: (Nalluri & Featherstone, 2001)
Bed Load: Some sediment particles
roll or slide along the bed intermittently
and some others saltate (hopping or
bouncing along the bed). The material
transported in one or both of these
modes is called ‘bed load’.
Suspended load: Finer particles (with
low fall velocities) are entrained in
suspension by the fluid turbulence
and transported along the channel
in suspension. This mode of transport
is called ‘suspended load’.
Wash load: Sometimes finer particles
from upland catchment (sizes which are
not present in the bed material), called
‘wash load’, are also transported in
suspension.
Total load: The combined bed material
and wash load is called ‘total load’.

23. Notes for Meander Cutoff Ratio
Page (1)
• A meander cutoff, the natural form of a cutting or cut in a river occurs when a pronounced meander
(hook) in a river is breached by a flow that connects the two closest parts of the hook to form a new
channel, a full loop.
• The steeper drop in gradient (slope) causes the river flow gradually to abandon the meander which will
silt up with sediment from deposition.
• Cutoffs are a natural part of the evolution of a meandering river. Rivers form meanders as they flow
laterally downstream, see sinuosity.
• Meandering rivers flow higher and hence with more total flow, pressure and erosion on the outside of
their bends due to forming a vortex as in a stirred coffee cup and consequently the river erodes more the
outer bank.
• On the inside bend of a river, the level is lower, secondary flow moves sand and gravel across the river
bed creating shallows and point bars, and friction of air and perturbances of the bed act against a higher
proportion of the column of water, being shorter, slowing the water to varying degrees.
• Rivers are commonly described and interpreted by their sinuosity. The term is equally used to describe
the actual incidence of and potential tendency of a river to curve or meander over its length.
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24. • Rivers are commonly described and interpreted by their sinuosity. The term is equally
used to describe the actual incidence of and potential tendency of a river to curve or
meander over its length.
• The sinuosity is expressed as the ratio of the distance between two distant points in a
river following the middle-of-the-river course of the river as compared with the straight
distance between those points.
• Three conventional categorizations of rivers or their reaches exist. Straight, Meandering
and Braided Rivers.
• A sinuosity value of less than 1.1 is a “straight” river.
• Meandering rivers have a sinuosity value/ratio of greater than 1.5. Mostly between 1.5
and 4.0
• Between these values, a river is described as sinuous which describes those in a
transitory state between the two states.
• Braided rivers do not follow this same convention. Meandering rivers trend in the
direction of increasing sinuosity.
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Notes for Meander Cutoff
Page (1)

25. Notes for Meander Cutoff
Page (2)
Meander Cutoff
• A meander cutoff, the natural form of a cutting or cut in a river occurs when a
pronounced meander (hook) in a river is breached by a flow that connects the two
closest parts of the hook to form a new channel, a full loop.
• The steeper drop in gradient (slope) causes the river flow gradually to abandon the
meander which will silt up with sediment from deposition.
• Cutoffs are a natural part of the evolution of a meandering river.
• Meandering rivers flow higher and hence with more total flow, pressure and
erosion on the outside of their bends due to forming a vortex as in a stirred coffee
cup and consequently the river erodes more the outer bank.
• On the inside bend of a river, the level is lower, secondary flow moves sand and
gravel across the river bed creating shallows and point bars, and friction of air and
perturbances of the bed act against a higher proportion of the column of water,
being shorter, slowing the water to varying degrees.
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26. • https://en.wikipedia.org/wiki/Meander_cutoff#/media/File:OxbowAnimation.gif
(Attribution: Theo886 at en.wikipedia)
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• Cutoffs => Neck Cutoff and Chute Cutoff
• Long-term dynamics are characterized by the irregular occurrence of cutoffs.
Neck cutoff occurs when a meander becomes very tortuous and the water
crosses the thin neck of the loop, giving rise to the formation of oxbow lakes.
Chute cutoff, instead, occurs during heavy floods, when the overflow scours a
new reach in the floodplain that by passes a large loop, sometimes occupying
the swales of newly deposited and unvegetated scroll bars.
• Cutoff ratio is sometimes used as the ratio of the distance between two distant
points in a river following the middle-of-the-river course of the river as
compared with the straight distance between those points.
• In sedimentary geology and fluvial geomorphology, avulsion (river) is the rapid
abandonment of a river channel and the formation of a new river channel.
Avulsions occur as a result of channel slopes that are much less steep than the
slope that the river could travel if it took a new course.
• An example of a minor avulsion is known as a meander cutoff, where the high-
sinuosity meander bend is abandoned in favour of the high-slope (i.e. Large
bending meander has river cut through a straighter course, and the meander has
water drain away) This occurs when the ratio between the channel slope and the
potential slope after an avulsion is less than about 1/5. 26
Notes for Meander Cutoff
Page (3)

27. Lecture (3) closing remark
• Engineers and sand-mining operators must acquire a
good understanding of the theory of sediment
transport process that determines the sand
replenishment rate and hence the volume of sand that
can be extracted from the reach of the river channel.
• The application of annual replenishment concept is key
to ensuring long-term river channel stability as well the
health of the aquatic and riparian habitats by allowing
only a sustainable volume of sand based on the natural
sediment transport process to be extracted.
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