1. Figure
1:
Annotated
physiographic
map
of
northern
Bangladesh
from
Pickering
et
al.,
2013,
with
loca>ons
of
sites
A
&
B
(see
Figure
2)
marked.
Figure
2:
Results
from
numerical
model
for
the
early
Holocene
floods.
A)
At
this
loca>on,
overtopping
and
spillover
is
plausible,
as
the
floodwaters
would
exceed
130%
of
bankfull
flow,
possibly
leading
to
par>al
avulsions
into
Sylhet
Basin.
B)
In
contrast,
the
Jamuna
valley
downstream
could
easily
accommodate
the
floods,
which
would
only
reach
40%
of
bankfull
flow
(45
m)
without
spillover
and
only
30%
when
accoun>ng
for
spillover
(shown).
I)
Numerical
Modeling
Using
Manning’s
Equa=on:
Manning’s
equa>on
relates
discharge
to
channel
dimensions
and
parameters
under
the
assump>ons
of
uniform
flow.
Manning’s
equa>on
is
given
by:
Q
=
n-­‐1·∙A5/3·∙P-­‐2/3·∙S1/2
where:
• Q
=
discharge
[m3/s]
• n
=
Manning’s
number
[dimensionless]
• A
=
channel
area
[m2]
• P
=
we[ed
perimeter
[m]
• S
=
slope
[rad]
Calcula>ons
were
run
assuming
a
discharge
of
5
x
106
m3/s.1
Valley
floor
topography
was
es>mated
based
on
drill
core
evidence
from
Transect
A
(see
map).2
Manning’s
n
of
.06
and
.05
for
the
valley
walls
and
floor,
respec>vely,
were
chosen
to
reflect
roughness
due
to
abundant
vegeta>on
and
gravel.
Height
above
valley
floor
(m)
Distance
along
valley
floor
(m)
Height
above
valley
floor
(m)
68
m
Distance
along
valley
floor
(m)
40
m
Poten=al
Impacts
of
Tsangpo
Lake-­‐burst
Megafloods
and
their
Preserva=on
in
the
Bengal
Basin
and
Delta
System
Michael
Diamond1,
Steven
Goodbred1,
Luisa
Palamenghi2,
Saddam
Hossain3,
Jennifer
Pickering1,
Ryan
Sincavage1,
Volkhard
Spiess2,
Lauren
Williams4
(1)
Earth
and
Environmental
Sciences,
Vanderbilt
University,
Nashville
TN,
USA
(2)
Department
of
Geosciences,
University
of
Bremen,
Bremen,
Germany,
(3)
Department
of
Geology,
Dhaka
University,
Dhaka,
Bangladesh,
(4)
Departhment
of
Earth
and
Environmental
Sciences,
University
of
Rochester,
Rochester
NY,
USA
EP13B-­‐3511
Abstract:
Large,
glacially-­‐dammed
lakes
formed
via
the
impoundment
of
the
Tsangpo
River
in
Tibet
led
to
lake-­‐burst
floods
during
the
late
Pleistocene
and
at
least
two
intervals
in
the
early
and
late
Holocene.
We
present
the
first
cri>cal
examina>on
of
the
poten>al
effects
that
the
Holocene
lake
drainages
had
on
the
downstream
Bengal
delta
and
their
preserva>on
in
the
geologic
record.
Based
on
stra>graphic
evidence
from
cores
drilled
across
the
delta,
digital
eleva>on
models,
seismic
data,
and
hydraulic
flow
calcula>ons,
we
propose
that
lake-­‐burst
floods
could
be
responsible
for
I)
triggering
short-­‐lived
avulsion
events
of
the
Brahmaputra
River
into
the
Sylhet
basin,
II)
genera>on
of
a
10
m
thick
gravel
layer
flooring
the
Jamuna
valley,
III)
the
forma>on
of
two
apparent
overflow
channels
on
the
Madhupur
Terrace,
and
IV)
the
deposi>on
of
a
large,
mass
transport
deposit
in
the
submarine
Swatch
of
No
Ground
canyon
system.
Comparing
the
early
and
late
Holocene
events,
we
expect
the
distribu>on
of
the
floodwaters
and
their
deposits
in
the
two
intervals
to
differ
sharply
owing
to
major
differences
in
flood
volume
and
the
paleotopography
of
the
delta.
Despite
much
higher
discharge,
the
early
Holocene
floods
were
largely
accommodated
within
the
vast
lowstand
valley
of
the
Brahmaputra,
with
some
spillover
into
the
Sylhet
basin.
In
contrast,
the
late
Holocene
floods
likely
spread
over
a
larger
area
due
to
the
rela>vely
even,
low-­‐gradient
topography.
Offshore,
a
40
m
thick,
chao>c,
semi-­‐transparent
seismic
facies
observed
in
the
canyon
corresponds
temporally
with
the
early
Holocene
floods
and
is
interpreted
as
a
subaqueous
mass
debris
flow
generated
by
the
flood
pulse
directed
to
the
canyon
via
the
lowstand
river
valley.
Methods:
Sediment
cores
were
drilled
in
16
transects
across
the
delta
using
a
local
drill
method
and
shipped
to
Vanderbilt
University
for
the
following
analyses:
• Grain
size
was
measured
on
a
Malvern
Mastersizer
2000E
• Magne>c
suscep>bility
was
measured
on
a
Bar>ngton
MS2E
High
Resolu>on
Surface
Scanning
Sensor
• Stron>um
(Sr),
silica
(SiO2),
and
calcium
(CaO)
concentra>ons
were
measured
via
X-­‐ray
fluorescence
(XRF)
on
a
benchtop
Oxford
Instruments
MDX
1080
+
XRF
Spectrometer
Digital
eleva>on
models
(DEMs)
were
used
for
visual
inspec>on
of
delta
morphology.
Enthought
Canopy,
a
Python
analysis
environment,
was
used
for
numerical
modeling
of
the
floods.
Seismic
data
from
a
marine
mul>channel
seismic
survey
was
analyzed
using
the
HIS
Kingdom
suite
of
soqware
and
GEDCO
Vista.
References:
1. Montgomery,
David
R.,
et
al.
(2004),
Evidence
for
Holocene
megafloods
down
the
Tsangpo
River
gorge,
southeastern
Tibet,
Quaternary
Research
(vol.
62),
pp.
201–207.
2. Pickering,
J.L.,
et
al.
(2013),
Late
Quaternary
sediment
record
and
Holocene
channel
avulsions
of
the
Jamuna
and
Old
Brahmaputra
River
valleys
in
the
upper
Bengal
delta
plain,
Geomorphology,
DOI:
10.1016/j.geomorph.2013.09.021.
Acknowledgements
and
Correspondence:
We
would
like
to
thank
the
en>re
BanglaPIRE
team,
past
and
present,
for
their
support
and
assistance.
In
par>cular,
this
project
has
benefi[ed
immeasurably
from
conversa>ons
and
correspondence
with
Carol
Wilson,
Jonathan
Gilligan,
Chris
Paola,
and
Jean-­‐Louis
Grimaud.
Financial
support
for
undergraduate
student
travel
was
generously
given
by
the
Vanderbilt
University
College
of
Arts
and
Science.
BanglaPIRE
funded
by
NSF
Grant
#
0968354.
Correspondence
can
be
sent
to
Michael
Diamond
at
michael.s.diamond@vanderbilt.edu.
10
m
Bangladesh
200
km
Sylhet
Basin
Madhupur
Terrace
Namche
Barwa
India
Swatch
of
No
Ground
canyon
Tibet
Burma
Shillong
Massif
30
18
6
Loca=on
A
B
Slope
.0002
.00025
Valley
width
25
km
58.8
km
Valley
depth
(max)
59
m
67
m
Frac>on
of
5
Sv
flood
discharge
accommodated
77%
246%
SONG
deposit
Figure
5:
Stra>graphic
columns
of
boreholes
shown
in
Figure
4E.
III)
Madhupur
Terrace:
Two
prominent,
symmetric
channels
(“scars”)
cut
through
the
Madhupur
Terrace.
Three
plausible
hypotheses
can
explain
their
forma>on:
1. They
were
carved
by
the
Brahmaputra-­‐Jamuna
River
as
it
avulsed
across
the
delta;
2. Megafloods
excavated
the
scars
in
discrete,
violent
events;
and
3. Local
drainage
carved
the
channels
over
millennia.
We
reject
the
first
hypothesis
because
there
are
not
meters
of
Holocene
sand
underlying
the
modern
floodplain,
as
would
be
expected
with
a
Brahmaputra-­‐origin,
and
the
only
Holocene
sand
underlying
the
modern
channel
has
a
Sr
concentra>on
of
~80
ppm,
well
outside
the
typical
Brahmaputra
range
of
140-­‐180
ppm.
The
sharpness
of
the
boundaries
between
terrace
and
scar
and
the
size
of
the
incisions
are
difficult
to
explain
with
local
drainage
alone,
sugges>ng
a
poten>al
role
for
floods
as
the
primary
morphological
agent.
Key:
Holocene-­‐Pleistocene
boundary
10
m
Figure
4:
A)
Bangladesh
in
context
of
south
Asia,
with
Namche
Barwa
indicated
(Google
Earth
image).
B)
DEM
image
of
Bangladesh
(scale
in
meters)
with
loca>ons
of
interest
labeled.
C)
Reconstruc>on
of
Tsangpo
paleolake,
with
ice
dam
at
Namche
Barwa,
from
Montgomery
et
al.,
2004.
D)
Loca>ons
of
boreholes
drilled
for
Transect
A.
E)
Loca>ons
of
Transect
D
&
E
boreholes
drilled
around
Madhupur
Terrace,
which
is
highlighted.
F)
Stra>graphic
cross-­‐sec>on
of
Transect
A
from
Pickering
et
al.,
2013.
Floods
may
have
had
a
role
in
carving
the
Old
Brahmaputra
Valleys’
strikingly
different
dimensions
with
respect
to
the
main
Brahmaputra-­‐Jamuna
course.
Shillong
Madhupur
II)
Gravel
Layer:
A
~10
m
thick
gravel
layer
extends
at
least
200
km
down
the
delta.
Such
a
thick
gravel
surface
is
rare
in
fluvial
systems
––
it
requires
a
significantly
different
hydrologic
regime
than
what
is
present
today.
Since
it
is
well-­‐established
that
monsoon
discharge
was
reduced
during
the
last
glacial,
it
is
implausible
that
such
an
extensive
gravel
layer
would
develop
from
the
river
alone.
Figure
3:
A)
Seismic
data
taken
via
ship
along
the
northern
Jamuna
river
shows
a
~10
m
gravel
layer,
which
has
been
corroborated
by
field
evidence
from
the
local
drill
teams.
B)
Loca>on
of
seismic
cruise
in
rela>on
to
the
Shillong
massif
and
Madhupur
Terrace.
Scale
is
0
m
(purple)
to
40
m
(red)
above
sea
level.
100
m
below
water
level
(125
ms
TWT)
50
m
below
water
level
(76
ms
TWT)
Gravel
layer
(approximate)
17.3
km
15.2
km
Madhupur
Terrace
Jamuna
River
Jamuna
River
Shillong
Massif
Transect
E
Transect
D
Transect
A
Pleistocene
sediments
IV)
Swatch
of
No
Ground
Canyon:
There
is
a
~40
m
thick
deposit
in
the
Swatch
of
No
Ground
canyon
characterized
by
oversized
event
beds
of
coarse
or
mixed
grain
size.
The
age
of
this
surface
could
be
es>mated
between
ca.
14
ka
as
it
is
conformable
with
the
transgressive
surface
of
erosion
associated
with
early
deglacia>on
and
ca.
2
ky
from
the
surface
sedimenta>on
rate
(25
cm/yr),
with
ages
closer
to
the
former
figure
more
likely.
Internal
reflec>ons
within
the
unit
suggest
it
was
deposited
in
a
>mescale
on
the
order
of
days,
which
would
be
expected
if
the
deposit
originated
as
a
subaqueous
mass
debris
flow
from
the
early
Holocene
floods.
Isopach
images
of
the
deposit
reveal
it
to
be
much
thicker
and
more
extensive
than
ordinary
slumping
events
due
to
earthquakes
and
other
factors.
Figure
6:
A)
Mapping
of
units
in
the
Swatch
of
No
Ground
canyon
from
seismic
data.
The
unit
in
red
corresponds
to
the
mass
transport
deposit
that
may
be
linked
to
the
early
Holocene
megafloods.
B)
Isopach
map
of
mass
transport
deposit
that
may
be
linked
to
the
early
Holocene
megafloods.