Earthquakes, floods, sea-level rise and sudden shifts in river courses threaten many of the 150 million Bangladeshis living in the low-lying Brahmaputra River delta. Scientists from Lamont-Doherty, Dhaka University and other institutions have begun a five-year project to understand the hazards and the possible hidden links among them. Lamont geophysicist Michael Steckler keeps us up to date on the work.
Geohazards in Bangladesh
Hiron Point Once More
After visiting the Sundarban Mangrove Forest, we next went to the Forest Ranger Station at Hiron Point in the Sundarbans near the Bay of Bengal. The Bawali cannot cross the 12 km (8 mi) wide mouth of the Pusur River in the summer. Open to the nearby sea, it is too rough for our river boat. In the morning, Bachchu, our boat owner, arrived with a Mongla Port Authority vessel.
We transferred everything we needed to the new vessel, including overnight bags in case we needed to stay over to finish. We brought everything we might possibly need as there was no going back to the Bawali for anything. The crew brought all their cooking supplies as well as mattresses and pillows, in case we needed to stay.
Indeed, the two-hour crossing was rough and Barkat threw up. When John saw the tide gauge station that measures water level, he suggested we make a GPS monument there to tie the tide gauge to the global network and directly measure its subsidence. Tide gauges measure the water level off the fluctuating river level for navigation. Long-term tide gauge records are used to measure sea level rise. However, they also see a mixture of sea level and land subsidence, but cannot separate the two.
Since a GPS measures absolute elevation, the combination allows them to be separated. This is why we put a GPS at the Forest Ranger station in 2012. Putting a GPS monument on the tide gauge, we would now get data from at the same point rather than 500 meters apart. When we reached the Forest Station, and offloaded the boat, John, Sanju, Nahid and I remained as it sailed to the Mongla Port Authority dock.
We walked over the boardwalk to inspect the tide gauge. It was better than we thought. The steel columns supporting the gauge were filled with concrete. We could drill and epoxy in a short piece of threaded stainless steel rod. We wired up a receiver in a waterproof box with a battery, attached the antenna and we were done by 11:30 am. We left most of our equipment on the boat and carried what we thought we would need to the Forest Ranger Station.
Carol and her team had already taken a country boat across the river to scout and install two SETs to measure shallow subsidence and sedimentation.
At the Forest Station, we removed the obsolete communication equipment that never worked in this remote site. We were too far from any cell tower. I have had to come here every year or two to collect the data manually. Recently, a cell tower for one company was installed nearby, so we would try to use that.
We also decided to rewire the ground wire and lightning protector to a better configuration. The work went slowly and we twice had to go back to the Mongla Port Authority dock to fetch things from the ship that carried us.
Things went very well for the SET team and they finished the two installations across the tidal channel by 3:30 pm.
The SET team had now developed an efficient scheme for preparing the rods. They could pound them in without the problems they had encountered earlier. Critical was cleaning and testing the screwing together of the rods in advance. We were close to done, but could not connect to the internet with our modem. Sanju made several calls to customer service, we tried Nahid’s SIM card, but nothing worked. We needed to be done by 5pm to be able to cross the Pusur before dark. With the deadline approaching, we decide to take the modem with us.
We trained several people on how to reinstall the modem by slipping it into its zip tie space and connecting the antenna, power and internet cables. A SET team must return every 6 months to make measurements. Now, we could finally eat the lunch that the crew had been cooking all day outside the room with the GPS. We took a group photo and John, Sanju and I ran most of the way to the tide gauge to disconnect the GPS.
John wirelessly downloaded the data and Sanju and I walked out to the gauge. We joined the ship and took it to the Forest Station dock to pick up everyone else and all the equipment. We started across at 5:30 and watched the sunset from the ship. We had done it all in one day.
We thought we were catching up to our schedule, but found out that the trip to the next site would be much longer than I anticipated.
The Bawali could not cross the Pusur River further upstream where it meets the Shibsha River. We would have to go much farther upstream and then travel west and south around the edge of the Sundarbans to reach the Koira district. Now completing all our tasks while we had the Bawali was becoming questionable. In case we could not finish our work in time, Bachchu offered use of his small boat, the Mowali, but it only sleeps five. Still, it might work for the trip to our last stop.
We spent most of the day traveling upstream. We then continued west to the crossing of the Shibsa. While it was windy, the waves were small and we crossed and stopped for the night. The next morning we took a short cut through the Sundarbans and arrived at the next site. The school I had picked from the maps looked excellent.
The GPS team got off while the SET team shifted upriver to a place where new sediments were being deposited. Where we were, the coast of the island was eroding. At each site there are two SETs, one inside the embankments protecting the island and one outside near the river.
After a wait for school officials, we got permission to install the GPS equipment with many friendly hellos from the children. The work went smoothly and we finished around 3pm. We went to join the SET team at their second site in a rice field near the school, but they had just finished, too. We met the Hindu family hosting the SET and helped carry equipment back to the boat. We could now start for the next site. Despite our worries, we had finally caught up to our schedule; the power drill had once again shaved a lot of time off the SET installation.