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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 abo
ve valley flo
or (m
)
Distance along valley floor (m)
Height abo
ve valley flo
or (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 [email protected].
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.
