Estimate of uncertain cohesive suspended sediment deposition rate from uncertain floc size in Meghna estuary, Bangladesh

Highlights

• Uncertainty in sedimentation rate in the Meghna estuary is substantial.

• Numerical derived distribution approach gives rapid estimate of uncertainty.

• Suspended sediment erosion/deposition is strongly linked to local flow speed.

• Flocs deposit in areas where the current speed approaches zero.

Abstract

Suspended sediment in the Meghna estuary, Bangladesh, typically consists of fine to medium silt near the water surface, silty sand at increasing depth, and sandy silt close to the bed. The behavior of fine, cohesive sediment in a complex environment with multiple drivers, such as river and tidal flows, is comparatively little understood because the deposition and erosion processes depend on many chemical, biological, and physical factors. This article examines the propagation of uncertainty from input floc size to output sedimentation rate in the Meghna estuary, Bangladesh, using a fine-sediment hydro-morphodynamic model that utilizes the cohesive sediment transport module in Delft3D. We assume that sediment particles and flocs are both single-sized throughout the solution domain. The effect of uncertainty in floc size on output sediment transport statistics is examined at three sites of interest located in the Meghna estuary using a novel numerical derived distribution approach. After deriving the probability distribution of suspended cohesive sediment, we find the coefficient of variation to range from 20% to 38% across the three locations. Planners therefore need to consider substantial uncertainty in cohesive sediment transport estimates for the coastal zone of Bangladesh, especially given the increased risk of flooding in deposition-prone areas as they become shallower. The methodology may be readily extended to the estimation of uncertainty in land reclamation and erosion control planning studies.

Introduction

The Bengal Basin, also known as the Ganges-Brahmaputra-Meghna Basin, consists of Bangladesh and parts of three eastern states of India (West Bengal, Assam and Tripura). According to Siddique-E-Akbar et al. (2011), more than 90% of the river flow passing through Bangladesh originates in upstream countries such as India and Nepal, whereas Bangladesh occupies only 7% of the total Ganges-Brahmaputra-Meghna basin area. The riverine discharge through the Meghna estuary is the fourth largest in the world (Milliman and Meade, 1983; Mukherjee et al., 2009), and the sediment discharge rate of ∼1 × 10⁹ t/yr is the highest (Goodbred and Kuehl, 2000a; Mukherjee et al., 2009). 30% of the sediment flux is deposited on floodplains, 40% in the marine area, and 10% in the Sundarbans and active Bengal delta, with the remaining 20% washed into the Bay of Bengal (Goodbred and Kuehl, 1999; Rogers et al., 2013; Seijger et al., 2019).

The Meghna estuary covers the zone of transformation of the Meghna River as it flows to the Bay of Bengal over a shallow shelf to a deeper basin. Several channels have formed in the estuary and they carry the river discharge to the bay. Large islands such as Bhola, Hatia, and Sandwip are located at the mouth of the estuary. According to Jakobsen et al. (2002), the Meghna Estuary is a ‘Coastal Plain’ estuary, implying that the estuary is very sensitive to its drivers, including tidal conditions, river discharge, and wind speed (Fischer et al., 1979). Local hydrodynamic conditions dominate sediment transport in the Meghna estuary, affecting grain size distribution, suspended sediment concentration, bed composition, and morphology. Sediment moves back and forth in the estuary, because monsoon river discharges bring sediment downstream to the estuary whereas the daily tidal prism pushes sediment back upstream (i.e. inland) through tidal channels (Barua et al., 1994; Goodbred and Kuehl, 2000b; Rogers et al., 2013; Seijger et al., 2019). Fig. 1 shows the elevation of the Bengal basin, which mainly comprises flat land of altitude about 5–10 m above mean sea level.

Bangladesh's distinct monsoon and dry seasons produce correspondingly different magnitudes of rainfall, river flow, and sediment yield throughout the year. During the monsoon season, rainfall is very intense and consequently the river flow greatly increases. Heavy rainfall in the Himalaya region raises the sediment yield of its constituent rivers. The combined effect of flow and sediment dominates the characteristics of the rivers, and hence the character of the Meghna estuary, where complex interactions also take place between river and tide (Akter et al., 2016). Erosion and deposition of sediment depend on local currents in the river and estuary; in fast currents there is sufficient shear to erode the bed with sediment particles transported predominantly in suspension (Brammer, 2014). The banks and bed level of the Meghna estuary have changed almost continuously over the centuries, with extensive erosion and deposition occurring near the islands. River erosion is a perennial problem in Bangladesh, which contains a network of about 230 rivers. The scarcity of land exposes residents living in the high-risk zone to natural disasters.

In the 1960s, extensive embankment construction commenced in the coastal area of Bangladesh. The works provided flood protection by decreasing land submergence and prohibited sediment from reaching the delta front by reducing sediment input to the delta. Dikes substantially altered the hydro-morphodynamic characteristics of the delta by raising water levels in the diked channels and reducing sediment input, which in turn enhanced changes to the local erosion-accretion pattern in the channels (Mikhailov and Dotsenko, 2007). By considering a map overlay of shoreline position from the Lloyd's survey in 1840 and the LANDSAT image in 1984, Allison (1998) (cited by Mikhailov and Dotsenko, 2007), observed that significant land erosion and accretion occurred near the mouth of the Meghna Estuary during the intervening time. Allison (1998) calculated the volume of eroded and accreted land at various locations from different maps over several time periods and found a trend of net sediment accumulation at the delta front. With the help of satellite images, Mahmood et al. (2020) produced a shoreline movement map for 1980–2016 that demonstrates the entire mouth of the Meghna estuary is geographically very dynamic.

Erosion and accretion processes have been transferring sediment around Bhola, Sandwip, and Hatia islands. Barua (1997) reported that the erosion rate at the northeast bank boundary of Bhola had an average value of about 150 m per year between 1940 and 1963. Using LANDSAT satellite images from 1989 to 2018, Anwar and Rahman (2021) found that the northeast face of Bhola had a shore erosion rate of 139 m/year, and the southeast part of the island experienced a shore erosion rate of 40 m/year. Brammer (2014) compared land boundaries between 1984 and 2007, finding that about 40% of the eastern side of Sandwip island had been eroded, though there was a net accretion of 451 km² in the Meghna estuary. Considerable erosion and accretion occurred simultaneously at Hatiya island, another dynamic island in the Meghna estuary, with maximum shoreline shift rates of 138.5 m/year seaward and 285.4 m/year landward (Kabir et al., 2020). Kabir et al. also observed that the tidal flats around this island are very active, and predominantly accreting sediment. To the west of the estuary, the Sundarbans area is accretion prone, with the bed level increasing with rising sea level because of sediment deposition (Rogers et al., 2013). In recent years, Bomer et al. (2020) investigated bed surface elevation change and sediment accretion near the Sundarbans for the years 2014–2019, and confirmed that bed elevation rise is occurring, with the rate of elevation gain exceeding the rate of relative sea level rise. Active land formation is also evident at newly formed land (locally named “Char”) in the riverine and coastal areas (Sarker et al., 2003).

The bed of the Meghna Estuary consists mostly of very fine sand and silt (Anwar and Rahman, 2021), whereas the suspended sediment typically consists of fine to medium silt near the water surface, silty sand at increasing depth, and sandy silt close to the bed (Borromeo et al., 2019). As would be expected, coarser particles were moved by near-bed transport processes, whereas fine particles were predominantly transported as suspended material by the current. More than 70% of the suspended sediment in the upstream rivers had particle diameter smaller than 63 μm. Kuehl et al. (1989) observed that sediment in the estuary mainly comprised fine material. Borromeo et al. (2019) collected silt from the Bengal Shelf and found that particles of size <5 μm occupied from 65 to 80% of the total sediment by weight at the locations considered. Barua (1990) reported that the turbidity maximum was generally located at or near the head of the salt intrusion where salinity is 1–5 ppt, and that its location fluctuated seasonally throughout the estuary.

One of the biggest challenges in studying the hydro-morphodynamic behaviour of the riverine and coastal system of Bangladesh is the unavailability of field data. Bricheno et al. (2016) mentioned Bangladesh as “notoriously data-poor” and reported that validation of a hydrodynamic model for this region is very difficult. Moreover, the quality of collected data is not good. Sediment data are scarce. During the field measurement campaign, data on suspended sediment data from Bangladesh Inland Water Transport Authority (BIWTA) were only obtained for 5 random days in 2007. By its very nature, sediment transport is a chaotic phenomenon depending on very complicated processes that are imprecisely understood. Even the best methods can calculate sediment transport rates to an accuracy of a factor of 2 in only 70% of cases in river engineering, and may not achieve accuracy of a factor of 5 in 70% of cases in coastal engineering (Soulsby, 1997).

Physical parameters and field data relevant to the floc size of suspended cohesive sediment in Meghna estuary are subject to considerable uncertainty. This in turn has a significant impact on estimates of sediment transport and morphological change. In practice, it is important to quantify such uncertainty and to understand how it affects the output parameters of hydro-morphological models, including sediment deposition and erosion. This paper describes use of the cohesive sediment transport version of Delft3D, an established hydro-morphological model, along with the numerical derived distribution approach to determine the propagation of uncertainty in floc size to uncertainty in sediment transport rate and bed morphological change at selected locations in the Meghna estuary, Bangladesh.

The paper aims to estimate uncertainty in sediment deposition rate at selected locations in the Meghna estuary, due to the underlying uncertainty in floc size (composed of 5 μm particles) using a numerical version of the derived distribution method. To the best of the authors’ knowledge, this is the first time the numerical derived distribution method has been applied to uncertainty in coastal sediment transport processes. Here, we use a state-of-the-art, hydro-morphodynamic, regional-scale model to generate sediment transport scenarios in the Meghna Estuary and the Bay of Bengal. The paper is structured as follows. Section 2 compares different methods for assessing uncertainty propagation, provides a rationale as to why the numerical derived distribution approach is used herein, and describes the numerical derived distribution method used for uncertainty propagation. Section 3 outlines the Delft 3D hydro-morphodynamic model. Sections 4 Results, 5 Discussion respectively present and discuss the uncertainty propagation results. The main findings are listed in Section 6.