water Article A Century of Riverbank Protection and River Training in Bangladesh Knut Oberhagemann 1,*, A. M. Aminul Haque 2 and Angela Thompson 1 1 Northwest Hydraulic Consultants, Edmonton, AB T6X 0E3, Canada; [email protected] 2 Bangladesh Water Development Board, Dhaka 1205, Bangladesh; [email protected] * Correspondence: [email protected] Received: 27 September 2020; Accepted: 25 October 2020; Published: 27 October 2020 Abstract: Protecting against riverbank erosion along the world’s largest rivers is challenging. The Bangladesh Delta, bisected by the Brahmaputra River (also called the Jamuna River), is rife with complexity. Here, an emerging middle-income country with the world’s highest population density coexists with the world’s most unpredictable and largest braided, sand-bed river. Bangladesh has struggled over decades to protect against the onslaught of a continuously widening river corridor. Many of the principles implemented successfully in other parts of the world failed in Bangladesh. To this end, Bangladesh embarked on intensive knowledge-based developments and piloted new technologies. After two decades, successful, sustainable, low-cost riverbank protection technology was developed, suitable for the challenging river conditions. It was necessary to accept that no construction is permanent in this morphologically dynamic environment. What was initially born out of fund shortages became a cost-effective, systematic and adaptive approach to riverbank protection using improved knowledge, new materials, and new techniques, in the form of geobag revetments. This article provides an overview of the challenges faced when attempting to stabilize the riverbanks of the mighty rivers of Bangladesh. An overview of the construction of the major bridge crossings as well as riverbank protection schemes is detailed. Finally, a summary of lessons learned concludes the impressive progress made. Keywords: Brahmaputra; Bangladesh; riverbank protection; river training; geobag revetments; adaptive approach 1. Introduction 1.1. The Disaster Prone Ganges and Brahmaputra Delta “He had heard it said, by one who knew them well, that the longer one studied the Bengal rivers the less one understood them; dread of their vagaries certainly increased with closer acquaintance.” Mr. J.N.D. La Touche in correspondence with Sir Robert Richard Gales on the Hardinge Bridge over the Lower Ganges at Sara, reported by Gales in 1917 [1]. The development of river training works and riverbank protection in Bangladesh is closely linked to how the fast-growing and developing population attempts to deal with one of the most hazard-prone deltaic landscapes on earth. The Ganges and Brahmaputra (named the Jamuna in Bangladesh) are among the ten largest rivers of the world [2]. Both rivers join in Bangladesh to form the Padma River (Figure1). The Padma River has a discharge of approximately 150,000 m 3/s during a 100 year flood event [3] and an estimated annual average sediment load between 0.5 and 1 billion tons [4]. With an annual average discharge of 30,000 m3/s, the Padma is the third largest river in the world (in terms of discharge), only surpassed by the Congo and Amazon. Water 2020, 12, 3018; doi:10.3390/w12113018 www.mdpi.com/journal/water Water 2020, 12, 3018 2 of 30 Water 2020, 12, x FOR PEER REVIEW 2 of 33 These immense rivers flow through intensively used land; Bangladesh leads the world in populationThese density immense of area rivers states flow with through more intensively than 1100 personsused land;/km Bangladesh2. In comparison, leads the the world population in densitypopulation of both density The Netherlands,of area states with the mostmore than populated 1100 persons/km country in2. In Europe, comparison, and Indiathe population is just over 400density persons of/km both2. ThatThe Netherlands, of the US is lessthe thanmost 40populate personsd /countrykm2. Over in Europe, 90% of and the catchmentIndia is just areas over of400 both riverspersons/km lie outside2. That of Bangladesh. of the US is less than 40 persons/km2. Over 90% of the catchment areas of both riversFloods, lie outside cyclones, of Bangladesh. and erosion (river and coastal) contribute to nearly 60% of all disaster damages in Bangladesh,Floods, cyclones, costing USDand erosion 2.5 billion (river betweenand coastal) 2009 contribute and 2014 to nearly [5]. Nearly 60% of threequartersall disaster damages of these water-relatedin Bangladesh, losses costing were USD associated 2.5 billion with between floods, with2009 and the two2014 largest[5]. Nearly components threequarters related of these to crops water-related losses were associated with floods, with the two largest components related to crops (36%) and houses (17%). Another one-quarter of these water-related losses were attributed to erosion. (36%) and houses (17%). Another one-quarter of these water-related losses were attributed to erosion. Despite these challenges, Bangladesh is currently moving from the status of among the least developed Despite these challenges, Bangladesh is currently moving from the status of among the least countries to a middle-income country. Economic interests, food security, and the increasing assets of a developed countries to a middle-income country. Economic interests, food security, and the fast-growingincreasing populationassets of a fast-growing are the main population drivers for ar infrastructuree the main drivers developments for infrastructure that address developments the natural hazardsthat address of the low-lying, the natural flat hazards deltaic of landscapethe low-lying, (Figure flat deltaic2). landscape (Figure 2). TheThe Jamuna Jamuna River River is theis the single single most most significant significant cause cause of of natural natural hazards. hazards. This This isis largelylargely becausebecause the Jamunathe Jamuna River hasRiver been has widening.been widening. This This widening widening is a is consequence a consequence of of the the Great Great AssamAssam Earthquake in 1950,in which1950, which dislodged dislodged some 45some billion 45 billion cubic meterscubic meters of sediment of sediment from the from eastern the eastern Himalayan Himalayan mountain slopesmountain [6,7]. Inslopes Bangladesh, [6,7]. In Bangladesh, this sediment this overloadingsediment overloading triggered triggered an increase an increase in the averagein the average width of thewidth Jamuna of fromthe Jamuna 8 to 12 from km, 8 while to 12 thekm, Padmawhile the River Padma widened River widened from 6.5 from to nearly 6.5 to 11nearly km from11 km the from 1970s to thethe 2010s.1970s to During the 2010s. this During period, this the period, Jamuna the turned Jamuna into turned a braided into a river;braided however, river; however, it still had it still some meanderinghad some and meandering anabranched and anabranched reaches as recently reaches as as the recently mid-1980s as the (Figure mid-1980s3). The (Figure braided 3). The characteristic braided is stronglycharacteristic associated is strongly with ever-changingassociated with channel ever-changing patterns, channel which patterns, result in severalwhich result thousand in several hectares thousand hectares of annual erosion. Local erosion eats laterally into the floodplain by as much as 1 of annual erosion. Local erosion eats laterally into the floodplain by as much as 1 km in one season. km in one season. The extent and location of riverbank erosion along the Jamuna can be reasonably The extent and location of riverbank erosion along the Jamuna can be reasonably predicted only one predicted only one year in advance [8]. The Padma River, despite being more powerful, is more year in advance [8]. The Padma River, despite being more powerful, is more predictable. This is predictable. This is because some of its banks consist of more erosion-resistant clay, making its becauseplanform some swing of its between banks consist anabranched of more and erosion-resistant meandering at a clay,slower, making more predictable its planform pace. swing Since between the anabranchedmid-1990s, andthe annual meandering average at erosion a slower, rates more along predictable the Jamuna pace. and Padma Since Rivers the mid-1990s, have halved the from annual averageapproximately erosion rates 6300 along ha (from the 1973 Jamuna to 1997) and Padmato approximately Rivers have 3300 halved ha (thereafter from approximately until 2019) (Figure 6300 ha (from3). 1973The toGanges 1997) River, to approximately being a meandering 3300 ha (thereafterriver, is also until more 2019) predictable. (Figure3). Downstream, The Ganges River,the Lower being a meanderingMeghna debouches river, is also the more combined predictable. flow of Downstream,the Ganges, Jamuna, the Lower and MeghnaUpper Meghna debouches into thethe combinedBay of flowBengal. of the Ganges,The Lower Jamuna, Meghna and (as Upper well as Meghna the Upper into Meghna the Bay ofduring Bengal. the Thedry Lowerseason) Meghna is tidal (asand well its as thelength Upper is Meghna almost entirely during part the dryof the season) estuary. is tidal and its length is almost entirely part of the estuary. FigureFigure 1. 1.Bangladesh Bangladesh with with mainmain riversrivers and locations locations relevant relevant to to this this article. article. Water 2020, 12, 3018 3 of 30 Water 2020, 12, x FOR PEER REVIEW 3 of 33 Water 2020, 12, x FOR PEER REVIEW 3 of 33 FigureFigure 2. 2. The natural environment of Bangladesh expressed in area flooded (in percentage of Figure 2.The The naturalnatural environmentenvironment