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.
Try to convince them 🙁
Found them from a google ad.
@Gilbert K. Arnold says: January 3, 2011 at 4:15 am
Yeah, did all that and confirm that it is both interesting and enjoyable.
But I can also confirm, from my observations, that shroudwaving pays a lot better. If you do it without blushing, you can even win a Nobel prize (Gore, Pachauri, Obama…)
steveta_uk
January 3, 2011 at 1:57 am
I guess the situation in New Orleans is similar, in that fixed concrete levees have prevented flooding for many years and hence the city has “sunk” relative to the local environment of the delta.
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For at least forty years, hydrologists, including those with the oft blamed Army Core of Engineers, have been warning of the danger that New Orleans faced, but the blasted liberals were too busy, giving away tax dollars in order to get votes, to hear the message or do anything to mitigate damage. When the obvious disaster strikes, in exactly the same manor as predicted by the most likely scenario ( cat 4 or 5 hurricane), the morons run around screaming their brain-dead heads off about it being Bushes fault!
Triton says, “A growing delta is not in balance, it is growing.”
Most people have trouble with a concept called ‘dynamic equilibrium’. Think of a balloon with a small hole in it, leaking air out. You can keep it the same size by blowing in just enough air to balance the air leaking out. Similarly, ‘balance’ in a delta might mean steady growth (not accelerating or decelerating). Yes, it’s a little loose technically, but it is a useful way to describe steady growth – in balance. What words would you rather use?
Brian,
Jan3, 2011 1am
Mountain ranges like the Hymalayas are formed due to the collision of two tectonic plates. They are in effect wedged into each other. The resulting rather complex make-up can best be described as a series of stacked rock layers, thickening the crust with some slices being pushed up and over , other slices being pushed under. Due to the fact that the Upper Crust rocks are lighter than the Lower Crust and Mantle, the isostatic equilibrium is disturbed and the massive pile of rocks will try to regain equilibrium and thus rise. This is how the mountain ranges and especially the high topographyies are formed; without the rise, the deformation would remain invisible in the sub-surface. Once the range start to rise, the erosion picks-up and create the steep valleys, leaving the most resistant parts as mountain peaks. Only when isostatic equilibrium has been reached will erosion slowly lower the mountains and over many millions, probably billions of years make them disappear. Mountain ranges formed more than 300 million years ago are still recognizable as topographic highs.
Triton
January 3, 2011 at 7:34 am
You must be a lefty as your criticism is idiotic. All you guys do, when confronted with facts, is to distract and confuse, by screaming for reference for the obvious, and twisting about the meaning of what is written so that you can denigrate it. But what you really are doing is denigrating your own misinterpretation. Not being able to understand what the author wrote, makes you look mighty foolish.
BTW, the behavior of deltas as explained here is an accurate assessment of what has been understood by REAL scientist for decades. And just so you know, estuary ecology (including the physical character of deltas) was one of my two main areas of study, before I got disgusted by all of the Marxist nonsense that was being crammed down my throat.
Pal Reviewed papers does not make science. Duplicatable experiments makes science.
I notice that somewhat to the south west of Bangladesh the Assam tea crop is being decimated by global warming.
http://www.independent.co.uk/news/world/asia/how-assams-tea-is-beginning-to-feel-the-strain-of-global-warming-2174678.html
Triton,
Jan3 2011, 7:34
I wrote: in balance with sea level. This means that deltas don’t sink , unless they are starved of sediment, in which case the delta will not be there for very long or would never have been there in the first place. They also don’t get much higer than sea level. It is a vertical or topographic balance. They do built out, again “up to sea level” , not much higher.
Some parts of deltas are periodically starved of sediment and thus sink, or are flooded for prolonged periods. Those areas become prefered deposition centres once rivers break out of there natural levees during spring floods, then prefer to flow to the lowest points, eventually dumping their sediment in those lows. Over time this amounts to some form of balance. But yes, deltas build out and are not in balance geographically. The avulsion process, when rivers change course, is also highly erosional and modifying topography. If you prefer to approach nature that way indeed very few things are in balance.
If you want to read more about Deltas I am sure you can find your way to a fine collection of decent text books.
Joshua says:
January 3, 2011 at 7:57 am
I gotta say “People living in deltas should learn to swim” is a flippant comment that seems pretty insensitive to the thousands of people who have died in Bangladesh flooding. —-the Bangladeshis should be educated and allowed to make their own choices, not mocked. In a way.
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Joshua. Gee – aren’t you the politically correct babe! What’s wrong with making observations like this? – and what’s with this crap about educating the Bangladeshis? My guess is that they are well aware of THEIR climatic conditions and are in no need of being educated – and I bet most of them know about swimming too. Sheesh!
Douglas
“Jimbo says:
January 3, 2011 at 4:34 am ”
“Cassandra King says:
January 3, 2011 at 7:02 am”
Thanks for the links and comentary however, I knew what “position” the MSM, both in Aus and the UK, would take. Snow, well almost everywhere, and 100 year floods in Aus. Its proof of AGW, although 100 – 150 years ago, similar floods in the region. Live in a flood plain, expect a flood! Must be due to SUVs in the US and patio heaters in the UK right? Strange though we are not having a usual “summer” here in Aus, well not on the east coast, Sydney anyway, just like the last 3 summers, COLD and wet. Sounds familiar, although inverted.
“Triton says:
January 3, 2011 at 7:34 am”
Check out what Cairo was 2000 years ago. Hint: it was a sea port. Where is it now?
Triton & Hoser
re: balance
Triton is correct that Quaternero’s original sentence is awkward. ,“[Deltas] will always be in balance with sea level but almost by definition increase in size, if rivers are allowed to follow their course.”
To my mind, a balanced delta would be one that oscillated around an average land area. i.e. sometimes the sea would eat away some of the delta, sometimes the rivers would deposit more new land. However, the whole point of a delta is that it’s growing, creating horizontal land out of vertical land (mountains). I still support Quaternero’s point that as sea levels rise, deltas will switch from generation of horizontal land to generation of vertical land. When an equilibrium is reached however, it’s back to the generation of square footage.
“Joshua says:
January 3, 2011 at 7:57 am”
They need to understand what “living on a delta” means. Its like living at the foot of Vesuvious, or Mt St Helens. Those who understand know one day, maybe not in your/our lifetime, but one day sure enough nature will act, and we can’t stop it.
Most, who live in detal regions, don’t understand and that is why so many die when “water” takes its course. This is a “water world”, and nothing to do with the BS film “Waterworld”.
What does this portend for New Orleans? From what I understand, the land under the city is sinking, and since the city, being below sea level, is protected with levies the river is not allowed to deposit silt. Consequently, the situation in New Orleans will get worse and worse.
People living in deltas do understand the potential dangers, just as do those living in Los Angeles, for example. It is simply a matter of what the individual sees as practicalities.
“bob says:
January 3, 2011 at 10:34 am”
Of course. Or Naples, 16m people at the foot of Vesuvious. Istanbul, another 16m people living on a ver active fault.
Sure “it” will be “worse” as more and more prople live in these zones.
Stated more simply, Bangaladesh IS a delta. Its geography is thus nothing much like a bedrock-based locale, and the lifestyles of the residents is entirely centered around exploiting the many features of deltas. They are very productive, but rather dicey places to build permanent structures!
Ken Harvey says:
January 3, 2011 at 9:38 am
“I notice that somewhat to the south west of Bangladesh the Assam tea crop is being decimated by global warming.”
Assam is north east of Bangladesh. And in the linked article we learn that rainfall has decreased by 20%, and that last years crop was damaged by to much rain, both presumably due to CAGW.
Peter Plail says:
January 3, 2011 at 1:31 am
Does anyone know how much deposition of sediments from rivers across the world, and dust and sand blown in by storms, contributes to sea level rise?
—-
River sedimentyation contributes between .02-.03mm/yr of sea level rise.
area of oceans:
3.61 X 1014 m2
annual sediment tranport:
15-20Gtons
iex x 10^12 kg
density:
2700kg/m3
->volume of sediment:
7.4 x 10^9 m3
->sealevelrise:
2.05 x 10^-5 m
.0000205 m
.0205 mm
I dont even need to read this post to know what the answer is, its high school level geology. As an environmental engineer who specialises in hydraulics, including sediment transport it obvious that the delta would be a constantly evolving, expanding and rising area of land and will not be affected by small rates of sea level rise. Why anyone would say otherwise is beyond me, they probably just got a digital elevation model for the earth and raising the sea by 0.5m to see where would flood, ignorant of the constant changing surface of the earth, or the large errors such a DEM would contain. But thats Al Gore IPCC Greenpeace science for you…
The only risk to the Delta is engineered flood schemes like large canalised channels which would instead move the sediment out to sea, disturbing the natural processes of sediment accumulation.
Then there is the issue of planning, if an area is going undersea or is prone to flooding – dont let people build there, or if they do, make sure the building floor levels are above the flooding and they have a stash of backup food etc… or tell them “live there at your own risk, we wont help you”. I am often shocked at the poor planning of even councils in the western world that let people build in ephemeral lakes, functional flood plain, below perched canals / levies / leats or even over a river valley blocking the rivers flow path and reducing its capacity!
Man is normally to blame when there is tidal or fluvial inundation, but its not due to co2 emissions, its normally as simple as a poor decision to build there, or the 10,000 ha of forest upstream has been stripped leaving bare-fields instead (trees are very good at reducing run-off from land as they massively increase evapotranspiration and store a lot of rainfall on their leaves and in the leaf litter and buffer out peaks in rainfall).
Joshua says:
January 3, 2011 at 7:57 am
I gotta say “People living in deltas should learn to swim” is a flippant comment that seems pretty insensitive to the thousands of people who have died in Bangladesh flooding. That’s like telling the okies in the 30s they need to learn how to farm with less water.
Are you by nature overly sensitive or just about people living in deltas? Good advice is good advice period.
I guess I’m pretty sensitive in general, though I’ve rarely been called politically correct in RL. I just think that if Anthony jumps on the alarmists for their exploding people ad, that that kind of sets the tone.
I think that telling people who are suffering from a natural disaster to learn how to swim is inappropriate, yea though it be a repetitive disaster in nature.
To me, it’s the equivalent of telling Katrina victims – “Learn how to swim!”
Mexicans dying in the drug war “Learn how to defend yourself.”
Californians laboring under the tax yoke “Figure out how to live with less money!”
etc.
stumpy says:
January 3, 2011 at 1:06 pm
I mostly agree with those words, though of course some inundations are entirely ‘natural’ – 100 year precipitation events and all that!
The biggest issue with flooding is the constant man made alterations of rivers along their length, mostly undertaken to gain or protect building land of course. But all any river defenses/alterations do is upset the natural balance and push the problem (i.e. the occasional excess water) somewhere else along the river!
Not withstanding this natural phenomenon, Bangladesh has for more than 80 years been messing up the natural watercourses of the country, now nature is catching up. As the Poster Child Bangladesh has a dubious history…
Bangladesh (was East Pakistan) had for years manipulated their geography (with soviet assistance) using widespread flood control, re-routing natural drainage, building large scale irrigation schemes, land recovery projects, and damming rivers for hydroelectric schemes, etc, etc.
() changed to
http://www.mowr.gov.bd/about_bwdb.htm and re-edited in the last year to remove most references to the soviet assistance.
Bangladesh also has an unsustainable population growth when the high population density is considered.
https://www.cia.gov/library/publications/the-world-factbook/geos/bg.html
Cassandra King says:
January 3, 2011 at 7:02 am
Patrick Davis says:
January 3, 2011 at 12:29 am
Interesting. However, I wonder how long it’ll be before alarmists and politicians attribute the flooding in Queensland, Australia to rising sea levels/rainfall as a result of man-made Co2 induced global warming?
__________________________________
The BBC are already trying just that, any weather event is simply a propaganda tool for the BBC to use. Fortunately the Australian commentator actually told the truth to the BBC anchor, it is a natural cyclic post drought event and nothing more. You can imagine that this particular guest will not be receiving another BBC invite any time before the next millennium. The BBC researchers will be looking for anyone regardless of qualifications who will be more ‘on message’ next time.
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Just to add to this: A letter appeared in the “Australian” a few days ago that climatologists having been saying for decades that AGW would cause “extreme events” There you go it didn’t take long did it.!
Yes folks cold snaps, massive flooding, bushfires, low clientel numbers in Bolivian brothels – all due to climate disruption.