What has happened?
- Residents of two buildings on Marine Drive, the iconic 3-km promenade in south Mumbai, complained of “unusual vibrations” during high tide over the past weekend.
- The residents wrote to Brihanmumbai Municipal Corporation (BMC) Commissioner Iqbal Singh Chahal and Additional Municipal Commissioner Ashwini Bhide to say that,
- The unusual vibrations, coinciding with high-tide times, were the result of the relocation of tetrapods as part of the ongoing Coastal Road Project (MCRP).
What are tetrapods?
- Tetra pod in Greek means four-legged. These are four-legged concrete structures that are placed along coastlines to prevent erosion and water damage.
- Tetrapods were first used in France in the late 1940s to protect the shore from the sea.
- They are typically placed together to form an interlocking but porous barrier that dissipates the power of waves and currents.
- These are large structures, sometimes weighing up to 10 tonnes, and interlocked tetra pods act as a barrier that remains stable against the rocks when buffeted by waves.
- Tetrapods, each weighing about 2 tonnes, were placed along Marine Drive in the late 1990s to break and dissipate waves and maintain the reclaimed shoreline in South Mumbai.
Why were tetrapods removed?
- They were temporarily removed to help carry out reclamation for the ongoing Coastal Road Project — 10.58 km of coastal road from Princess Street in Marine Drive to the Worli end of the Bandra-Worli Sea Link.
- There are over 6,000 tetrapods along Marine drive alone.
How bad were the vibrations?
- Residents said there was “no doubt” that the vibrations were felt by “almost all” who live in the two buildings.
- “I would like to bring to your urgent notice that for the last few days, many residents of Shreeniketan and Gobind Mahal buildings (sea-facing buildings between G&F Road on Marine Drive) have been feeling vibrations,” the residents said in their letter to the BMC.
- “The vibrations”, they said, “come for 1 second and in intervals for a period of ~30-60 minutes in the afternoons.
- These vibrations feel like tremors akin to an earthquake.
- Many of us searched if there was an earthquake in Mumbai.
- Now imagine earthquake-like tremors 20-30 times in an hour.”
But how do we know tetrapods are the reason?
- The BMC has provided vibration monitoring instruments at the site to study the impact of the phenomenon.
- While the corporation has not officially stated that the removal of the tetrapods caused the vibrations, it has agreed to re-install the structures.
- They would be put back over the next two-three days during low tide, the BMC has said.
- “To allay the fears of citizens of the said societies, BMC has asked the contractor to put the tetrapods back in place.
- The contractor has started the work of placing tetra pods at the said location,” the civic body has said.
- On Monday, a team of representatives of the Coastal Road contractor and management consultant, along with civic staff, visited the site.
- “It was discussed that the reason for the vibrations last Friday was high tide, furious high waves, and the speed of the gusty winds,” an official said.
Q) Which of the following earthquake waves is first recorded on the Seismograph?
- Rayleigh waves
- Love waves