Rethinking sediments, tidal rivers and delta livelihoods: tidal river management as a strategic innovation in Bangladesh

Scientists and practitioners around the world are searching for more sustainable development trajectories for delta regions (the relative flat, fertile plains located between rivers and the coast) as many urbanised deltas face development challenges that are driven by rapidly growing economies and populations, and by the prospects of a changing climate and rising sea levels (Nicholls et al., 1999; Syvitski et al., 2009; Renaud et al., 2013; Giosan et al., 2014; van Staveren et al., 2017a). Strategic delta planning has been introduced as a relatively novel approach to more sustainable development (Norgaard et al., 2009; Seijger et al., 2017; Zegwaard et al., under review). It represents a particular form of strategic planning (Albrechts, 2004) and is defined as a public-sector led process through which a long-term vision (the strategic delta plan) and actions and means for implementation are produced that shape and frame what a sustainable delta is and may become (Seijger et al., 2017). A strategic delta plan sets strategic priorities for development and sketches alternative development options on how a delta could cope with growing economies, populations and the impacts of climate change. Innovations play an important role in delta planning across sectors (e.g. transport, water, food, agriculture) as they give shape to adaptations for a changing environment (Nicholls et al., 2016; Tran et al., 2018a) as opposed to retaining business as usual in a changing environment. Innovations in a delta planning perspective can cover technical and institutional changes, be diffused bottom-up or top-down and transferred from one region to another (Norgaard et al., 2009; Vinke-De Kruijf et al., 2012; Seijger et al., 2017).

This paper explores how innovations in land and water management could enable a shift to more sustainable strategies and livelihoods, in line with directions outlined in a strategic delta plan. The paper focuses on tidal river management (TRM) as an innovation to cope with waterlogging, salinity and congested rivers in the Bangladesh delta (Tutu, 2005; Shampa, 2012). The objective of the paper is to explore the strategic value of TRM and trace how it has been appreciated by different groups of people and in various strategic plans. We study TRM as a strategic innovation (Schlegelmilch et al., 2003) in the context of strategic spatial planning and delta planning. We thus offer a novel perspective on TRM by studying its strategic aspects of reconceptualising business models, values and markets. We therefore supplement a growing body of TRM literature that has covered hydrodynamic modelling (Shampa, 2012; Amir et al., 2013), initial experiences with TRM (Tutu, 2005), historical perspectives (Nowreen et al., 2014; van Staveren et al., 2017b), social learning and conflicts (Mutuhara, 2018), local participation and transdisciplinary implementation (Haque et al., 2015; Gain et al., 2017), and sustainability indicators (Masud et al., 2018).

In Section 2 the importance of silt in the southwest of Bangladesh is summarised and the principal technical features of TRM are introduced. Section 3 reviews theories on innovation and explains the research design and methods. The case analysis of TRM as a strategic innovation is presented in Section 4, and implications of our findings are discussed in Section 5. Section 6 presents conclusions on the role of a strategic innovation such as TRM in re-thinking Bangladesh’s delta management.

Deltas are formed and elevated over time as sediments accumulate in the delta plain, instead of being dispersed by waves, tides and ocean currents. The formation of the Ganges Brahmaputra Delta, or Bengal Delta, is characterised by a huge sediment load, tectonic subsidence and a near shore canyon system that resulted in widespread sediment distribution across the delta (Goodbred Jr & Kuehl, 2000; Datta et al., 2008). Two major carriers of sediment are the monsoon river discharges and the daily tidal prisms carrying sediment 120 km inland through tidal channels (Barua et al., 1994; Goodbred Jr & Kuehl, 2000; Rogers et al., 2013). The Ganges and Brahmaputra rivers jointly produce a total sediment flux of 1,060 MT per year, making it the third-largest sediment flux in river systems worldwide, after the Amazon and Huanghe rivers (Milliman et al., 1995). Of the 1,060 MT per year, c.30% is deposited in the floodplains, c.40% is deposited in the marine part, c.10% is deposited in the Sundarbans and immature Bengal Delta, and the remaining c.20% is washed away into the Bay of Bengal (Goodbred Jr & Kuehl, 1999; Rogers et al., 2013). Active land formation in the delta, driven by sediment influx and river bank erosion, is not delimited to the Sundarbans mangroves that elevate with sea level rise (Rogers et al., 2013), but also evident in the thousands of new (is)lands, chars, formed in the rivers and coastal edges (Sarker et al., 2003).

We recognise the diverse biophysical classifications of the Bengal Delta. Rashid (1991) distinguished between the Maribund Delta, Immature Delta, Mature Delta and Active Delta. Brammer (1996) distinguished the Ganges River Floodplain and the Ganges Tidal Floodplain. Our research area covers a number of these classes and we therefore refer to the more generic term ‘Lower Bengal Delta’ in southwestern Bangladesh (following Rogers & Overeem, 2017).

TRM in the Lower Bengal Delta can be considered as a partial return to the age-old practice of ‘temporal overflow irrigation’ (Willcocks, 1930; Tutu, 2005; van Staveren et al., 2017b). Yet, instead of inundating the fields with fresh water and sediments, TRM foresees the use of brackish and saline water and sediments to inundate the fields. The shift to tidal waters was needed in the Lower Bengal Delta as upstream interventions like the Farakka Barrage and irrigation projects significantly decreased fresh water flow into the area (Gain et al., 2017). TRM (Figure 1) enables the natural movement of sediment-loaded tidal river water into an embanked low-lying area (a ‘beel’) during high tide. This leads to sediment deposition in the beel as flow energy is significantly reduced. During low tide, the outgoing sediment-free water picks up river sediment, erodes the riverbed and increases the drainage capacity of tidal rivers (Mutahara et al., 2017). TRM requires a cut in the river embankment, or a link-canal between the river and the beel, to daily transport water and sediments into the low-lying area for a period of 3–5 years (Shampa, 2012). During these years, the beel is daily flooded with brackish water and sediments. Formal and informal TRM projects in the Lower Bengal Delta in the southwest of Bangladesh are and have been conducted; see Figure 2 and Table 1. As Table 1 shows, TRM is capable of elevating land within the beel between 0.2 m (at the far end) and 2 m (near the cutting point), and increasing river flow as the tidal river deepens (by 9–12 meters) and widens (by 2–8 times the pre-TRM width).