Sedimentation Patterns in the Ganges- Brahmaputra Delta System

Transcription

1 Sedimentation Patterns in the Ganges- Brahmaputra Delta System SRTM-topography >10 m Irina Overeem, CSDMS, University of Colorado at Boulder K. Rogers, S. Goodbred, S. Higgins, K. Matin, Z. Kahn, S. Cohen, M. Steckler, M.Hossein Himalayan hinterland and dynamic coastal drivers cause a high sediment flux Depositional setting 1. Bengal Basin: tectonically deformed from continental collision of India into Eurasia 2. Asian monsoon: 80% of Q w and 95% of Q s from May- Sept. Peaks in August. 3. Large sediment discharge: Q s ~ 992 x 10 6 tons/y 4. Tidal range: 2-6 m Tidal velocities: 1-4 m/s 5. Recurring cyclones every ~2 y 1

2 144 million people live on the GBD Rice farming in polders Clothing manufacturing in urban areas Shrimp farming in polders Photos: Irina Overeem Flooding Flood Maps based on MODIS satellite imagery (1999-now) From: Flood Observatory, Brakenridge and Kettner,

3 Embankments in Coastal Zone 139 polders constructed over last 50 years, ~ 6,000 km of embankment or dike, 2900 regulator and flushing inlets were constructed (From Worldbank, 2013). Embankment Maintenance Photos: Irina Overeem 3

4 elevation in mm Relative Sea level Rise in Deltas Global SLR =1.8 mm/yr Ganges Mississippi Mekong Chao Phraya Global Average Relative sea level rose 2-3 times faster within deltas than globally. Controls on Delta Elevation Δ RSL = Vertical change in delta surface elevation (m/yr) A = Aggradation Rate (m/yr) ΔE = Eustatic Sea Level Rise (m/yr) Cn = Natural Compaction (m/yr) Ca = Accelerated Compaction (m/yr) M = Crustal Vertical Movement (m/yr) Can the Ganges-Brahmaputra delta system keep up with sea level rise and subsidence? Equation from Syvitski et al., Nature Geoscience,

5 Map Subsidence with InSAR From Higgins, Overeem et al., JGR 2014 Focus Area with GPS control Subsidence Results - Compaction varies between 0-18 mm/yr - Shallow stratigraphy controls compaction - Abandoned, former Ganges and Meghna channel fills compact rapidly (upto 18 mm/yr) - Pleistocene Madhupur terrace clay is stable (0 mm/yr) 5

6 Elevation (m) Comparison to GPS results Retrieved elevations from SBAS-InSAR Higgins et al., In-SAR trend over is slightly less steep but similar to GPS trend In-SAR can not pick up the seasonal cyle of land movement in the GB delta the wet monsoonal season does yield no useable imagery Modeling Results from Steckler & Nooner, in prep. 6

7 Sedimentation Budget Terms? 1000 Floodplain storage of 300 million ton/year Existing estimates based on stratigraphic analyses, geochronologic dating of core material (Kottke et al. 2003; Goodbred and Kuehl 1999; Allison 1998; Michels et al. 1998; Rogers et al., 2013) Model the fluvial-dominated delta Sea level (t) Flowpath (t) Variable Discharge and Sediment input (t) WBM model dictates magnitude and variability of annual water and sediment mass Simple approach: models flood events only, no tides, no ocean storms. 7

8 AquaTellUs Model H x t F in (0) = F x Topography (H) depends on sediment flux (F) F x er k m c( x, t ) S( x, t ) Q( x) ( t ) Erosion depends on slope (S) and discharge (Q) in fluvial domain, grainsize-independent F in (x) F depo (x) F out (x) F ero (x) x - 1 x x + 1 downstream stroomafwaarts F sed ( x, t ) x k u sed ( x, t ) F Sedimentation depends on sediment flux (F) en de streampower (u), k sed is grainsize dependent. ( x, t ) Lateral Sedimentation Basic principles of sedimentation across channel belt and floodplain: exponential with distance from channel (Pizutto, 1987; Goodbred & Kuehl, 2000). Variability in floods creates Gaussian distribution; and error function solution (Paola, 2000; Overeem, 2005). F(y) = -(y-m) 1 2 2ps e 2s 2 erf (y) = 2 p ò e -t 2 dt y = horizontal distance normal to channelbelt σ = standard deviation across sedimentation zone μ = position of flowpath axis 8

9 90 km Flood deposition maps- 50 yrs 60 km X = 10 X = 40 X = 90 2 nd order channels (1-2 km) 1) Lateral sedimentation >2500 m, h ~ 1.5m 2) Rapid sedimentation causes natural avulsions, 50 yr deposition >1.5m Overbank sands amalgamate X = yr deposition ~ 0.2-1m 2 nd order channels disconnected X = yr deposition ~ m 2 nd order channels disconnected X = 90 9

10 Observations from Sediment Traps 2012 mixed tidal-fluvial delta plain sediment trap sites Sediment Trap Set-Up Before monsoon, May 2008 After monsoon, October

11 Mass accumulation (g cm -2 ) 1.1.A 1.1.B 1.1.C 1.1.D 1.2.A 1.2.B 1.2.C 1.2.D 1.3.A 1.3.B 1.3.C 1.3.D 2.1.A 2.1.B 2.1.C 2.1.D 2.2.A 2.2.B 2.2.C 2.2.D 2.3.A 2.3.B 2.3.C 2.3.D 3.1.A 3.1.B 3.1.C 3.1.D 3.2.B 3.2.C 3.2.D 3.3.A 3.3.B 3.3.C 3.3.D 4.1.A 4.1.B 4.1.C 4.2.A 4.2.B 4.2.C 4.2.D 4.3.A 4.3.B 4.3.C 4.3.D Natural Sedimentation Rates 6,00 5,00 4,00 3,00 2,00 1,00 mass flux Be7 inventory 2,0 1,8 1,6 1,4 1,2 1,0 0,8 0,6 0,4 0,2 7 Be Inventory (dpm g -1 cm -2 ) 0,00 0,0 Overall mean mass accumulation (dashed line): 1.3 ±1.1 g cm -2 Regional vertical accretion: 1.1±0.9 cm y -1 11

12 Mass accumulation (g cm -2 ) Human-Impacted Sedimentation Rates mass flux 7 Be inventory 1,4 1,2 1 0,8 0,6 0,4 0,2 7 Be inventory (dpm g -1 cm -2 ) D 5.2.D 5.3.B 7.1.A 7.2.D 7.3.A 8.1.A 8.1.C 0 Regional vertical accretion: 2.3±0.9 cm y -1 A: sediment aggradation ΔE: eustatic sea level rise C: compaction (n = natural, a = anthropogenic) M: tectonic movement/lithospheric flexure Pristine Human-Modified A (cm/yr) 1.0 a (prelim) ΔE (cm/yr) -0.3 b -0.3 b C n (cm/yr) -0.4* -0.4* C a (cm/yr) * M (cm/yr) -0.3 c -0.3 c ΔRSL (cm/yr) to +0.1 ΔRSL since 1960s (cm) 0 cm -100cm to +5cm a Allison and Kepple, 2001; Rogers, 2012; b Church and White, 2006; c Sarker et al. c Steckler et al., in prep; *Higgins et al.,

13 Conclusions and Future Steps Subsidence rates strongly variable: 0-18 mm/yr. Maps of subsidence are essential but D- InSAR will still be really unreliable without in-situ GPS control. Sedimentation rates in tidal delta are ~1.1 cm/yr. Preliminary results for fluvial-dominated part of delta plain are as much as 2.3 cm/yr. More extensive observational network essential. Modeled sedimentation rates over 50 yrs are consistent with small field dataset decline with distance from fluvial channelbelt. In future: design experiments with physic-based models (Delft3D), ANUGA to better constrain patterns in floodplain sedimentation, experiments on embankment stability. A large research agenda for Bangladesh and international community! 13