Part 3 of a series on tracking landscape changes in Bangladesh. Read part 1 and part 2 here.
We are finishing the last two sites for this part of my trip. We have GNSS (the general term for GPS) instruments on buildings to measure the subsidence below them, and RSET-MH (Rod Surface Elevation Table-Marker Horizon) to measure compaction/subsidence above a rod driven deep into the ground. On this trip we are also sampling the upper meters of sediments using an auger. In this part of the trip, we are traveling by boat through the interconnected waterways of the Sundarban Mangrove Forest.
On our way north from Hiron Point, our last stop, we overnighted at a place called Kaga Dubaki still in the Sundarbans Mangrove Forest. We spent a lot of the next day continuing to sail north. As we left the Sundarbans, cellular service returned and we rushed to download emails and make phone calls. We arrived at our next site, Jorshing in the afternoon.
While the RSET and auger teams went to their stations, Zohur and I went to the school/cyclone shelter where the GNSS receiver is. Since it is Friday, the headmaster and caretaker met us to let us in. The equipment box is in an unused former toilet off the headmaster’s office. This site has not sent data for a year. When we checked the batteries, they were fine, the problem was with the SIM card and modem. The SIM card was locked and the modem still had the older VPN set up. However, since it was the middle of the night, I was unable to reach my contacts in the U.S. for help.
Luckily, the RSET team could only do one RSET, so they have to do the other in the morning. They are in pairs, one inside and one outside the embankments that protect the area, but also put it at risk. Inside the polder (embanked island) flooding and sedimentation is restricted, so these areas have been subsiding and losing elevation without sufficient sediments. That created a disaster for our last sites on Polder 32. There, Cyclone Aila breached the polder (embankment) in 2009 and flooded the island for almost two years until the embankment was successfully rebuilt. We measured the land there as 1-1.5 m below the natural level outside the polder. Around 10,000 families were displaced by the disaster. This is why understanding the subsidence, sedimentation and elevation change is so important here.
Zohur and I joined that RESET and auger teams as they were finishing up. Then I joined a volleyball game in the school courtyard while we waited for the country boat to take us back to the Bawali. The players were very good and a fraction of my age; I did my best to keep up.
In the morning, Zohur and I returned to the GNSS and called Ken Austin, who will join me in Bangladesh in less than a week. He led me through upgrading the VPN, but the SIM was still a problem. Zohur went out and got a new one, while I waited. When he arrived with a new SIM card, it was able to connect to the cellular network. Ken still wasn’t able to connect to it, there is a little more work to do that can be done remotely from the U.S. Just in case, Zohur practiced connecting to the GNSS and downloading data. This way the RSET team can download data as needed from recalcitrant GNSS during their twice yearly RSET service runs. The speedboat came to take us to the RSET site, but by the time we got there, the others had finished.
After we finished, we headed north to Polder 32. Since this was a focus of a previous project, there are 8 RSETs here, 4 in the Sundarbans at Suterkhali and 4 inside the Polder at Sreenager, both on the south side of the island. Plus, I learned there is also an SSET (Shallow Surface Elevation Table). There were two, but one was vandalized. The RSETs are in pairs, close to the embankment or shore, and in the interior of the polder or forest. My GNSS is distant at Gunari on the NW corner of the island. It will take 2 days to do all these RSETs. We have been studying the subsidence at Polder 32 since 2012. It became a focus because of the disaster there, but many of the embanked islands are facing the same issues of subsidence and water logging, because the interior is now lower than the river level.
We started with the Sundarbans sites. We all got up early, but fog prevented us from going into the forest until 10 am. A forest with Royal Bengal Tigers is not safe with no visibility. The first site was near a forest ranger station and we did a deep auger hole while the RSET team did an RSET and the SSET. The second site a longer walk behind it and we all pitched in for a quick RSET measurement. To get to the next RSET site we took the country boat up a tidal creek. When we finished that, we went back to the Bawali for lunch.
Now the tide was high enough for us to go several kilometers up a tidal channel to the last RSET site. We saw a number birds on the way, egrets, kingfishers, and a stork. We also did an auger here. The soft mud in the Sundarbans is much easier to go through and we exceeded 4 meters in depth. We played loud music to keep the tigers away. It made our last Sundarbans site more fun. We saw signs of deer and wild boar, but no pugmarks (tiger footprints). After returning to the Bawali, we sailed the short distance to Sreenagar.
The next morning, the RSET and auger teams took the country boat to the shore, while Zohur and I, along with Rifat and Iyasin from the Bawali took the speedboat to Gunari, where my GNSS is, circumnavigating half the island. When we arrived, the ghat (dock) was gone. I wasn’t very disappointed as the walkway from the ghat to shore was always very scary. Instead, we just scrambled up the sandbags protecting the shore. Rifat came along, thankfully carrying the heavy 80 Ah battery. The caretaker let us in and battery switch did not take long. A quick call back to the U.S. confirmed that the GNSS was still online after our work.
Returning to Sreenagar, we rejoined the other teams for their “interior” sites. With the tide low, we had to walk through thick mud to get to the island. Joining them, we set a new record for ourselves, reaching 5.85 m (19.2 ft). At one of the RSET stations, the very nice family that owns the land added mud to improve the walkway to the site, thereby altering the elevation of one side. As Sazzad put it “Too much hospitality ruined the data collection”. When we finished, the family treated us to fresh tamarind and boroi (small fruit similar to a cross between an apple and a pear. These boroi were sour rather than sweet; apparently this is the original fruit, before human modification made them sweet.
We have now finished all the data collection and equipment servicing in coastal Bangladesh. The Bawali is heading back to Khulna. Tomorrow morning, we will leave the boat and Rafiq will meet us with his van and drive us back to Dhaka, together with all the data we collected and 100-150 sediment samples from the augering. The next day Austin will return to New York. The following day, my other postdoc, Lin Shen, and a geodetic engineer, Ken Austin, will arrive. In between, I have a few more meetings scheduled in Dhaka. The three of us, along with Sanju Singha will head east and north with Babu, our long term driver, to the part of Bangladesh affected by earthquakes. Here we will repair, service and install GNSS for another week. Meanwhile, I can bask in the successful completion of the first part of the fieldwork during the 4 hour trip back to Khulna.
I had worked for 3 years in Jorshing (Polder 13-14/2) after cyclone Aila of 2009 and experienced that the breach in embankment caused heavy loss and damage in one hand and on the other hand it contributed a lot to offset the land subsidence by continuous flow of tidal waters and silt deposits for 3 years. It is estimated that the land surface level has been elevated by at least 1m on an average. But unfortunately, we do not know how much was the subsidence since the polder was built stopping tidal silt deposits. Association for Climate Refugees in Khulna city is going to establish the Sundarban School of Climate Science to deal with impacts of climate change with particular emphasis on land subsidence. I had also worked in Polder 32 and have similar experience. Would love to know the amount of subsidence of these 2 Polders during the pre-Aila period.
Thank you Mike for sharing your field journal in a beautifully written blog post illustrated with lots of photos. Measuring land subsidence in the Bengal Delta like other Asian Mega-deltas is crucial to understand the impact of slowly rising sea levels in the region. Many people simply think that the rising water levels in coastal rivers are due to rising sea levels. However, a significant proportion of observed rising trend in river water levels can be attributed to land subsidence as measurement sites (i.e., river gauging stations) are often located within the tidal river banks that are subjected to slow but steady land subsidence.