Assessing Erosion and Accretion in Meghna estuary
Deciding climate change adaptation and mitigation measures
A very severe earthquake of the magnitude of 8.6 on the Richter scale occurred in August 1950 with its epicenter close to the border of Assam in India and Tibet in China. This earthquake caused huge landslides in the Himalayas, estimated at about 45 billion cubic meter. Within a few years, most of the debris generated from the landslides entered into the Brahmaputra river through numerous tributaries in Assam. The fine fraction of sediment mainly consisting of silt and clay very quickly transported through the Brahmaputra/Jamuna river and deposited in the Meghna estuary area. It moved through the river slowly like a sand wave and caused significant changes in river morphology. Some recent research (by this writer) describes the responses of the rivers through widening and increasing of erosive capacity during the propagation of the sand wave. That was the main reason why the erosion along the Jamuna and Padma rivers was higher in the 1980s and 1990s than in the previous and later periods. As the silt and clay fraction of sediment entered within a few years after the 1950 earthquake, it caused very rapid accretion of land in the Meghna estuary area. On the other hand, the leading edge of sand wave entered in the mid 1970s and contributed incremental accretion in the following decades. It took more than 50 years for all the extra sand fraction of sediment generated by the earthquake to enter into the Bay of Bengal from Assam. Erosion and accretion in the Meghna estuary area
A topographic map was created from a survey conducted 70 to 90 years ago, but the landform was revised in the aerial photographs of 1943 and 1944. This map provides the shorelines, locations and sizes of off-shore islands. Time-series satellite images covering the Meghna estuary area are available in CEGIS (Center for Environmental and Geographic Information Services) archives from which images of 1973 and 2008 were used to estimate the erosion and accretion. Maps and satellite images were georeferenced with a common projection system. To avoid errors due to tidal variations during the imaging period, a set of criteria was developed to define the shoreline of the estuary. The long-term net accretion in the estuary has been estimated by different scientists by comparing different historical maps like James Rennel's map (1776) or Commander Lloyd's chart (1840) with the modern surveyed maps. The results of the estimates for the long-term net accretion in the Meghna estuary area are found to vary from 4 to 10 km2 per year. Comparing the map of 1943 with satellite images shows that from 1943 to 1973 net accretion was more than 1200 km2. If it is assumed that most of the accretion occurred between 1950 and 1973, the net annual rate of accretion becomes more than 50 km2/year, which is 5 times higher than the long-term average rate. Later, the rate of net accretion slowed down and from 1973 to 2008 it became about 17 km2 per year. The overall land gain after the earthquake of 1943 is about 1800 km2. In 1957 and 1963 two cross-dams were built in Noakhali, which contributed to the rapid land formation and joining of the islands with the mainland. Nevertheless, the source of the huge sediment that increased net accretion more than 5 times than in other periods the massive landslides caused by the Assam earthquake in 1950. The rapid accretion in the following two decades was mainly due to the rapid transportation of silt and clay to the Meghna estuary. Noakhali district gains the most
As the bay penetrates several kilometers into the mainland of Noakhali in the northeast corner of the estuary, it is likely that the delta building process would be concentrated in Noakhali district. During the 1930s, most of Noakhali district town had been washed away by the Meghna river. After the earthquake the main branch of the Meghna river flowing though Noakhali rapidly silted up -- Char Alexander and Char Jabbar became part of the mainland causing it to advance about 40 km towards the south. Construction of cross-dams expedited this process. Accretion in Noakhali district was about 1000 km2 which was nearly 5 times of erosion. Thus, over the last 60 years net gain of land has been about 800 km2. Next to Noakhali district, accretion has been more than 4 times of erosion in Patuakhali district -- and thus net gain of land has been about 325 km2 over the 60 years. Sandwip, Hatiya and Bhola islands lose the most
Although net gain of land after the earthquake was very high, southward movement of the mainland caused huge erosion of the three large islands Sandwip, Hatiya and Bhola. For Sandwip net loss of land has been 160 km2, for Hatiya it has been 35 km2 and for Bhola island it has been 230 km2. However, Bhola district as a whole has gained about 180 km2land. Adverse effects
Rapid advance of the mainland of Noakhali caused drainage congestion in the south of Comilla and north of Noakhali. Huge erosion of old and fertile land in Sandwip, Hatiya and Bhola can also be partly attributed to the rapid advancement of the mainland. What next
The long-term sediment yield in the three great rivers -- the Jamuna, the Ganges and the Meghna -- is about one billion tons per year. All extra sediment generated by the Assam earthquake has already reached the Bay. It is likely that the rapid accretion rate will slow down and come closer to the long-term net accretion rate i.e. less than 10 km2 per year. While construction of many flood embankments and polders during the last 40 years has restricted floodplain sedimentation, sea level rise would certainly reduce the rate of net accretion. Effects of climate change and sea level rise
Global climate is changing over time, but due to human activities the expeditious rate of climate change and subsequent rise of sea level have become evident now. There are, however, uncertainties about the rapidity of climate change and magnitude of sea level rise in the next 100 years. Whatever might be the rate of sea level rise, there is no doubt that Bangladesh would be the worst victim of climate change. Due to low land elevation, one meter rise in sea level within the next 100 years may cause inundation or enhance drainage congestion in significant parts of Bangladesh. While assessing the extent of inundation or flooding due to different probable magnitudes of sea level rise and rainfall intensity, it is common practice in Bangladesh to consider that the bed level of the estuary, riverbed and river bank levels, floodplain and tidal plain levels would be constant in the future. Recent research of CEGIS indicates that as the rivers of Bangladesh carry huge amounts of sediment, the levels of riverbeds and banks as well as the bed and unpoldered tidal plains of the Meghna estuary will be raised as well considerably. Proper sediment management based on better understanding on the sediment distribution processes in the floodplain, tidal plain and Meghna estuary may partly compensate for the inundation of the coastal area, but the difference between these two processes, however, depends on the rate of sea level rise in the coming decades. On the other hand, any adaptation measures without considering appropriate sediment management may further aggravate the situation by causing more drainage congestion and erosion in the coastal areas. Priorities for adaptation and mitigation
The real situation in terms of extent of inundation and flooding due to climate change would be far more complex and different than shown by different studies. Research should be taken up immediately to assess the morphological responses to climate change in the Meghna estuary as well as other estuaries, rivers, floodplains and tidal plains. Such research would help to generate more reliable extent of inundation and flooding pattern based on which adaptation and mitigation measures could be planned. More research should be directed to (i) finding out the most efficient way of injecting sediment into the existing polders to cope partly with sea level rise where sediment is abundant in the surrounding rivers and (ii) diverting sediment laden flow to the most vulnerable and deeply flooded back swamp areas in southwest and south-central Bangladesh. Acknowledgement: Mr. Giauddin Ahmed Choudhury, Executive Director, CEGIS, Mr. Sonkor Chandra Sinh, CEGIS and Ms. Fahmida Nur, CEGIS.
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