This volume calculation also excluded material displaced by mass flows or failures not captured in the sonar survey, and therefore represents a minimum.
The estimated volume of material displaced by Cyclone Sidr is 385 x 106 m3, which is equivalent to a mass failure of 578 x 106 tons of sediment (Table 5). Compared to previous
total volume displaced (m3) 3.85E+8
total mass displaced (t) 5.78E+8
years of total annual SoNG input removed by Sidr-
related failures >1.93
fraction of total annual GB sediment load displaced by
Sidr 0.58
Table 5. Results of failure volume calcuations based on a sediment bulk density of 1.5 g cm-3 budget estimates of annual sedimentation to the entire canyon (300 x 106 tons yr-1) these failures compare to about two years worth of average canyon sedimentation, or 58% of the entire annual discharge for the G-B system. The previous estimate of 300 x 106 tons yr-1 of sediment discharged to the SoNG was quantified using sedimentation rates of up to 50 cm yr-1 determined from 137Cs and 228Ra-dating of cores taken on the upper canyon floor in water depths of 228-564 m (Michels et al., 1998). However, the failure volume calculated in the present study is restricted to a 400 km2 area around the canyon head at water depths of <150 m. Compared to the size of the G-B drainage basin (841.6 x 106 km2), these failures are an example of intensely focused transport through the system and provide strong evidence for direct connection between the Bengal Basin and Bengal Fan.
To further illustrate the magnitude of these failures, an earlier sediment budget based on an average sedimentation rate of 20 cm yr-1 determined that the upper canyon stores 90 x 106 t yr-1 of sediment, or 9% of the 109 tons of annual river discharge (Michels et al., 2003). Compared to this budget estimate from the upper canyon floor, the volume of material removed by Cyclone Sidr in 2007 was equivalent to 6.4 years of sediment input. However, using a lower sedimentation rate of 5-10 cm yr-1 obtained from 137Cs-dated core material from the canyon rim near to the present study would indicate that the storm removed about 10-20 years’ worth of sediment accumulation (Kuehl et al., 1997).
If average accretion rates in the canyon head account for 5-10 cm yr-1 of new
sedimentation on the canyon head clinothem, then the 20-30 m deep failure valleys around the canyon head are no more than several hundreds of years old. Evidence for up to three failure sequences can be documented in the canyon head in the chirp sonar data used in this study (e.g.
Fig. 6). Based on the sedimentation rates and maximum depth of observable failure scars, it is estimated that storm events of similar magnitude to that of Cyclone Sidr have a maximum recurrence interval of <100 years, assuming the buried failure scars were caused by storms. The budget estimates above indicate that the equivalent of ~2-20 years of sedimentation at the canyon head was removed by the mass failures associated with the passage of Cyclone Sidr. The results of this research suggest that the balance of sediment flux into and out of the canyon throughout the Holocene has been maintained by recurrent storm-related mass failures on the time scale of 10-100 years. Storms of Cyclone Sidr’s magnitude are known to have occurred historically in the Bay of Bengal every 3-30 years (e.g. Alam et al., 2003). Assuming that large storms track near the SoNG canyon head once a decade, it is suggested that mass failure due to storms is the principal mechanism maintaining the upper canyon head despite extremely high sedimentation rates.
Conclusions
The enormous load of G-B sediment discharged to the shelf (700 x 106 tons) is partitioned between the rapidly prograding subaqueous clinothem and the canyon. The results of this study illustrate that sediment transport linking the widely-separated river mouth and canyon head occurs via three principal mechanisms: 1) rapid progradation of the clinothem foresets into the eastern and northern canyon head; 2) erosion from mass wasting of canyon banks, and 3) funneling of fluid muds formed on the inner shelf into the canyon by two bypass gully systems.
The morphologic features representing each of these processes overlap in the SoNG canyon head.
This research also demonstrates a new model of canyon head stability in a high-discharge marine
environment whereby high rates of sedimentation are balanced by mass failures that transport material down-canyon. The rapid infilling and flushing out of material in the SoNG canyon head has allowed the canyon to maintain its position on the shelf since the mid-Holocene.
The results of this study show that individual storm events in the Bay of Bengal can generate widespread mass failure that displaces a large percentage of the total river discharge sequestered to the canyon head. Finally, the identification of an active gully system terminating at the mouth of a major tidal channel suggests a possible connection between the canyon and the lower subaerial delta plain, which was shown in Chapter 2 to be another major depocenter for G-B sediment discharge.
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CHAPTER V
IMPLICATIONS FOR SEDIMENT TRANSFER FROM LAND TO SEA:
SUMMARY AND LOOKING FORWARD
The results of this study demonstrate that tides and storms distribute sediment discharged from the Ganges-Brahmaputra river mouth to the coastal delta plain and Swatch of No Ground canyon head. The study also reveals that the extremely high sedimentation rates on the prograding subaqueous delta foresets are enabling net sediment aggradation at the canyon head despite recurring mass failures. Several questions emerged from this project regarding the exchange of sediment across the land-sea boundary and the continental margin, and about modern submarine canyon stability.
This chapter is a summary of key questions arising from this research.
In chapter 2, seasonal sedimentation patterns on the moribund subaerial delta were
discussed. The additional questions arising from the mass flux and general sedimentological results of this chapter pertain to:
• the role of tidal island morphology and stream order in controlling the spatial distribution of sediment
• influence of microtopography and subsidence on sedimentation patterns
The results of this study indicate that accretion in the Bangladeshi Sundarbans (~1.0 cm yr-1) is presently keeping pace with regional rates of sea level rise (~0.5-1.0 cm yr-1). While regional sedimentation results were consistent across the lower delta plain, heterogeneities in mass flux patterns exist at the local scale. The high rate of sedimentation is maintained in part by sediment transport and distribution via smaller order streams, though sediment trap placement did not specifically target these smaller streams. In order to better constrain the contribution of these small
creeks to the overall sediment flux, a focused sampling strategy should be aimed at choosing sites according to their proximity to creeks of pre-specified stream orders. Remotely sensed data or local maps would be ideal for mapping a targeted sampling approach. However, at the time of the writing of this project, no remote sensing technology is capable of imaging through the thick vegetation canopy such as the Sundarbans, nor are satellite sensors able to resolve the land surface at a scale feasible to detect the first and second order streams that are transporting sediment to the interior of tidal islands.
Likewise, microtopography partially controls where sediment is routed on the mangrove platform after it is brought in on the flood tide (Furukawa et al., 1997; Wolanski, 1995). In the Sundarbans, elevation is lower in the west than in the east, based on measurements by Ellison et al.
(2000) and the sedimentation results of the current research. A detailed topographic elevation survey would be an inexpensive way of quantifying overall elevation differences between the eastern and western portions of the Sundarbans, as well as local variations that may be influencing sedimentation patterns. Survey results could be matched with sedimentation data in a GIS to confirm the
relationship between microtopography and sedimentation patterns. One caveat to this approach is that in many areas of the Sundarbans, vegetation is thick and may hinder survey attempts. A possible solution is to couple ground surveys with an airborne LiDAR (Light Detection and Ranging) survey of the coastal zone. While LiDAR does not see through vegetation, the frequent canopy openings in the Sundarbans forest would allow the beam of the system to strike the mangrove platform and provide for a more accurate elevation contour map of forest floor microtopography (Night et al., 2009).
Chapters 3 focused on mass failures around the canyon head associated with the passage of a large storm, and discussed characteristics of the underlying strata that might
predispose some areas to failure and other areas to resist mass wasting. The main question arising from this chapter regards the first-order controls on canyon and shelf morphology in the canyon head:
• How does sea level lowstand morphology define highstand margin processes?
This question also arises from the research in Chapter 4. During the Last Glacial
Maximum, sea level was ~ 120 m lower than its present position, and many rivers formed incisional valleys on the surface of the continental margin as their base levels changed (Clark and Mix, 2002;
Talling, 1998; Blum and Törnqvist, 2002). On the modern Bengal shelf, the position of the Late Pleistocene shoreline is marked by a series of shallow, laterally extensive oolitic beach ridges at about
~120-130 meters below present sea level (Wiedicke et al., 1999). The absence of fine-grained
sediment in the composition of these ridges indicates that the muddy suspended load of the G-B rivers bypassed the Bengal shelf when the ridges were forming. Low stand braided channels, similar to those of the modern Jamuna/Brahmaputra, extended across the shelf during the Late Pleistocene, feeding directly into the canyon (Goodbred, 2003). These channel complexes were draped with wet, unconsolidated muds during the Holocene sea level transgression. Storm wave pumping during Cyclone Sidr initiated failure that led to the evacuation of Holocene sediments overlying the low stand surface, and in some places the dimensions of Sidr-related failures mimic those of the modern Brahmaputra braidbelt. While the lowstand surface is presently buried under 60-100 m of sediment, repeated failure at several locations throughout the canyon head is observed to the lowest depth resolution of the chirp sonar data. The repeated cycles of failure-aggradation discussed in Chapter 4 may continue deeper in the subsurface, and could be related to the location of buried sand-filled low stand channels.
Finally, chapter 4 discusses the interplay of depositional, erosional and bypass features in maintaining the position of the canyon head throughout the Holocene. The key questions that emerged from this chapter are: