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.
Some Geology:
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.
Sea level rise and deltas
Deltas have been extensively studied for many years, partially motivated by pure self-preservation, partially because abundant oil and gas has been found in delta sediments.
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.
Flooding
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.
In summary:
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.
Great to see a well explained article. I have previously lamented the loss of physical geography from the education system. It astounds me that simple principles like “fast rivers erode, slow rivers deposit” have been so easily forgotten. Just a few questions and observations. What role does vegetation (mangroves etc) play in moderating levels of sediments and temporary island construction? Most photos of the Sunderbans show mangroves. Does it enhance deposition and reduce erosion during flood events? Consolidation of sediments is understood in engineering foundation design including the use of preloading to accellerate consolidation. Does the height of deposition similarly have a limit?
The other half of the equation is the reality that people have always settled near to water for a number of reasons including security of water supply, trade and transportation and not coincidently, deep alluvial soils are great for agriculture. So it is no coincidence that flood plains make attractive areas of settlements. It is just unfortunate (for the inhabitants) that they happen to FLOOD ocassionally – or in the case of Bangladesh, seasonally with the monsoons.
The current floods in Australia are not unusual on a historical scale and largely a result of the current La Nina phase. Some of the flooding is actually LESS severe that it would otherwise be due to the existance of man made dams that moderate peak flood levels. River gauges on the Burnett River that flooded Bundaberg actually show flood peaks of 10000 cumecs upstream of the (enviromentally unpopular) Paradise Dam but no less than 4000 cumecs below the dam showing the benefits of temporary flood storage. This significantly reduced the scale of the floods and no doubt the quantum of “climate change ;-)” refugees.
It is also unsurprising that areas of western Queensland currently flooding are actually described as “large areas of fine-textured alluvial plains associated with the main tributaries of the Condamine River system”. In other words, nothing unprecedented here, just nature doing what it does.
Ken Harvey>
“I notice that somewhat to the south west of Bangladesh the Assam tea crop is being decimated by global warming.”
There’s an unusually blatant piece of spin underpinning that claim. I thought I’d check the statistics for historical yields, and in fact, we’re talking about a single year small drop in yield which is well within the bounds of ordinary year-on-year fluctuations. The only way you can get a drop is to measure over a period of a couple of years, since any longer period shows an increase.
Joshua, MKelly,Stumpy, Anna v
I wrote: “people living in deltas (in general) should learn how to swim”. This is ment to be entirely pre-emptive. It also includes people in the Mississippi delta, the Nile Delta, the Rhine delta etc. . It is the simplest way of avoiding completely unnecessary drowning due to entirely predictable and seasonal events and to recognize the dangers of your environment. I would be surprised if more than 25% of the people in Bangladesh can actually swim. I suspect that none of the female half and less than half of the male population can actually swim. It also would also surprise me if swimming lessons are part of the school curriculum in New Orleans, while I know it is part of the school curriculum in the Netherlands, for a good reason. If someone wants to regard this comment as flippant, that is their prerogative. I was actually quite serious about this, including having boats. Inhabitants of the Mekong delta prefer to live on boats. The danger lurks when dealing with such an immense delta as Bangladesh, where flooding occurs hundreds of miles away from the coast line, in a flood plain that can be reasonably well protected by natural levees for dozens of years and then suddenly floods when a levee breaks. The most effective option is to build settlements on man-made elevated “highs” rather than to dam or control the run-off. Fixing the enormous rivers and their sediment load according to human design is a monumental task, likely to fail in the long run. The population growth rate in Bangladesh however guarantees that only the elite will be able to secure the highs, leaving a large percentage to fend for themselves on the least favorable lows. I really don’t know what else to advise. The points about the Vesuvius are indeed quite analogous. Fertile soils will always attract people, never mind the dangers. Naples will eventually become the next Pompeii, but my advice to them would be similar: have a boat to get out of there, highways will be plugged when it matters.
Swimming however detracts from the main argument. Although Stumpy believes thata this is “high school geology”, I believe that insufficient geology is actually taught at high schools, or else Al Gore would have realized his complete ignorance. Anna v said itwell: try to convince them..
Excellent post!
Chris
Norfolk, VA, USA
Atolls flooded by sea level rise, what a joke. Atolls are formed when islands sink into the ocean, either by sea level rise, or more likely because the weight of the island presses down on the ocean floor and causes the crust to deform. As the water rises, the coral grows around the perimeter of the island. Eventually the island sinks from view, leaving a ring of coral.
Global warming is not a threat to coral. Quite the opposite. In general cold water corals are rare. Typically coral needs warm water. Coral bleaching results from temperature changes in the water. As the water heats up, different species of polyps are advantaged, and take over the reef. Bleaching is what we observe during the change in ownership. The bleaching does not persist. Come back in a year and the coral will be healthy once again, as the new owners are hard at work growing the reef.
Cold, fresh water and sediment will kill reefs, as will over fishing. The reef needs coral eating fish to remain healthy, otherwise algae will soon smother the polyps and kill the reef.
Perhaps I’m not allowed a tiny bow here. I have been entreating someone here to debunk the Bangladesh flooding due to CO2 for over a year – making a small contribution to how the system works on the Ganges, Mississippi, etc. but, as a Precambrian geologist (Billion+ yrs old rx) and mining consultant, hoped a good sedimentologist would come forward for this important job. Thank you B. Quartero – an excellent job, and lets hope climatologist practioners who did not know the dynamics of river deltas will benefit and relax the alarm level in this sector. By the way do you notice in the flooded street above – there is no panic, people holding hands, carying their babies and goods calmly down the flooded street – no alarm here.