Guest post by: B.Quartero
Bangladesh, the largest Delta in the world, has been the poster child of a scary sea level rise story ever since “An Inconvenient Truth”. There is much to be concerned about in Bangladesh, and flooding is most certainly one of the seasonal hardships Bangladesh has to put up with. Sea level rise just happens to be not one of them.
The Ganges and Brahmaputra rivers flow through the Himalaya Foredeep and end-up dumping their sediment load in the Gulf of Bengal, forming a huge delta at the ocean edge. Deltas have one common characteristic in that they are actually formed by the rivers bringing the sediment to the ocean, in this case a huge load coming off the largest and highest mountain range in the world, the Himalayas. The proto Ganges and Brahmaputra have been active for millions of years, and have been filling the fore deep, a sinking part of the crust caused by tectonic loading of the Himalaya thrust belt. The collision of the Indian Plate with the Asian Plate has resulted in a structural complex deformation of the rock layers, which in itself is a most fascinating and only partially understood process. The net result however is a pile of rocks (The Himalayas) on the north side of the Indian plate, bending this plate down under its weight. The resulting trough is almost simultaneously filled with sediment eroded from this same pile of rocks. The mechanism of deposition is mainly by fluvial processes (river sediments) and alluvial fans directly shedding off the incipient mountain range into the fore deep. The rivers have been finding their way, following the natural law of water flowing to the lowest point. In this case the bay of Bengal, where the subsidence of the earth crust is also influenced by the Arakan-Yoma foldbelt of Myanmar. The resulting depression is filled with sediments transported for more than 20 million years by the proto Ganges and Brahmaputra rivers, and has possibly been an active centre of sediment deposition for more than 60 million years.
All this sounds very impressive, but what deltas in essence are is the place where a large amount of fine grained sediments are being deposited due to a significant reduction in flow rate when the river flows into a wider oceanic basin. This deposition is cyclical yet continuous in such sense that it has been a continuous process for millions of years but also very much seasonal and thus cyclical. The Himalayas are subject to monsoon rains as well as seasonal snow melt, resulting in variable yet yearly, predictable jumps in run-off, generally resulting in flooding of the“flood plains” that in the dry season are well above water level. Those flood plains are protected by modest natural levees. Natural levees are formed when a river overflows and loses its coarsest sediment first, in proximity to the main channel, thus building up natural high ridges along the main river body. These natural levees have been recognized as effective dikes by some and have occasionally been enhanced and built up by human inhabitants of flood plains (e.g. the “summer dikes” in The Netherlands). Levees are, however, seldom high and strong enough to withstand large floods, in which case they break through with resulting widespread seasonal flooding.
During flooding the fine muds in the now rapidly decelerating river (the same volume of water now flows over a much wider area and even appears to stand still for some time) are deposited and when the flood waters recede, there is a fine layer of mud left behind. This annual or rather frequent flooding allows a delta to build “up” during floods. 1 mm/year still adds up to 1 meter every 1000 years, which is approximately equivalent to the annual subsidence in the Gulf of Bengal, also known as the Patuakhali Depression.
One of the interesting things about deltas is that they are very dynamic and by their very nature are building up and out rather than drown and disappear.
When sea level drops, the rivers tend to by-pass their most recently built sediment wedge and incise deeper valleys in their old river beds, then dump their sediment further out into the ocean and build-up a new addition to the Delta complex. When sea level rises, the delta builds-up rather than out into the ocean and thus stays more or less balanced with sea level. When sea level rises very rapidly, and the sediment load can not keep up, the Delta will find a new equilibrium further back, where the available accommodation space balances the sediment load. The Ganges-Brahmaputra delta happens to have not only survived one of the most rapid sea level rises in geological time, post Pleistocene, but has built and built for millions of years thanks to being endowed with one of the largest sediment loads on earth. More than 16 km (vertically) of sediments derived from the Himalayas has been deposited and consistently built and maintained a delta environment. The sea encroaches where the rivers are not, due to sediment compaction; the fine muds deposited away from the main channels during seasonal floods initially hold a lot of water and over time this water is expelled. Rivers change their course when low areas become the preferred place to flow to. This constant shifting of rivers and river mouths to the lowest areas, forms the distributing process by which a delta spreads and builds. Unless sea level rise and the resulting increase in water depth and accommodation space exceeds the sediment supply, deltas will never drown. Bangladesh and the Ganges-Brahmaputra delta will be there for as long as the Himalayas deliver the gravels, sands and mud.
So what about the flooding? The seasonal flooding is of course a direct result of the dramatic increase of run-off during monsoon and annual snow melt. The rivers are literally constricting the excessive water flow. It can not unload its water fast enough and as a result the water level rises. The levees overflow and/or break through and the floodplains in between the rivers are flooded. Water level sometimes rises by two or more meters. Sea level has nothing to do with it. One mm more or less has no influence on the massive seasonal run-off. The flooding and resulting deposition of a film of mud actually completely compensates for the estimated annual global sea level rise. The lower the flood plain, the higher the flood water column, the more silt and mud is deposited. It all evens out, hence the very uniform flat nature of delta plains.
Of course deltas can only build up so much, and are therefore always more or less in equilibrium with flood and sea level. Flooding will be a regular occurrence, for as long as there are monsoon seasons. Unless people build dikes and dams to regulate the flow, floods will happen. While dikes seem to be a good idea, it does modify the dynamics of a system that is in balance with sea level and sediment supply. The Netherlands (effectively the Rhine delta) have been building dikes for more than 1000 years and as a result many old settlements are now well below high river level. The absence of regular sediments added to the floodplain requires ever stronger and higher dikes. Sooner or later the imbalance will no longer be sustainable. Wisely, new settlements are now mostly built with adding thick layers of sand, not only to strengthen the foundation of the new housing, but with an added benefit of artificially elevating the country. Al Gore had it partially right to flag the Netherlands as being threatened to drown, not so much by rising sea level as well by having engineered a safer environment from river flooding, thus starving the flood plains of balancing sedimentation.
Deltas are formed at the boundary of rivers and oceans. The rivers that build deltas flow to low and slowly sinking parts of the crust, where large volumes of sediment are being deposited. They will always be in balance with sea level but almost by definition increase in size, if rivers are allowed to follow their course. Deltas, by their very nature are building out and up. They also tend to flood frequently and seasonally, often with disastrous effects on the inhabitants. People living in deltas should learn to swim, have a boat and generally be aware of what can happen. Sea level rise is not an issue in large deltas; they have been proven to be able to keep up with any sea level rise. Flooding disasters are seasonally the result of excessive run-off, and occasionally due to unfortunate storm surges that result in breaks through natural barriers, but this has nothing to do with sea level rise. Bangladesh will be there, even if all the ice in the world has melted, with its people still fighting floods while farming the fertile floodplains.