Thread by @rajbhagatt

#Thread
#Marina beach is one of the most iconic destinations in Chennai and one of the most visited beaches in the world. However the beach as we see today didn't exist a century ago.

This thread explains the formation of #Chennai's Marina beach
1/n

This map below shows the approximate original coastline of Chennai during British era. It was much closer to the Kamarajar road. A promenade was developed during the british era and that is when it got the name, "Marina". However the big sandy expanse itself didnt exist
2/n

Seas are very dynamic in nature. Whenever the waves hit the beach which is called as "swash", they bring in sand and when the water goes back ("backwash"), it removes the sand. This balance is critical for beach existence.

The sand which the beaches need are brought by the sea from other places and one of the biggest suppliers of sand are our rivers. The rivers bring a lot of sand particularly during rainy seaons and these help in providing sand for the beaches

But these waves when they hit the land, they dont actually move straight. Becaue of the winds prevailing in that season, they have a slight angle to them. This means that it takes sand from one place and moves to another place.

For Chennai, during the winter months of November, December, January, the drift is from north to south. Because of this and moves from north to south in that time period

This can be clearly observed in this satellite image where the plume of Cooum river can be seen going towards the south. The plume (black in colour) is clearly identifiable because of the difference in the nature of water brought by Cooum river

But in the rest of the months of the year, the drift moves from south to north

This can be observed in this satellite image from April where the plume from Cooum river is moving to the north direction

Though both the direction exist in different months, the south to north one is dominant in Chennai. It is more like "2 steps forward and 1 step back".

So in this context, when the breakwaters for the Port of Chennai were constructed, it began obstructing the drift. The sudden disruption meant that sediment movement was not even anymore

Acting as a wall the port began stopping the sediment flow. This resulted in the settling of sand to the south of the port. The land expanded into the sea slowly and thus forming the Marina beach

So the sediment was settled in south side, rest taken to deeper sea. This means that the swash hitting on the north side near Kasimedu didnt have enough sand. The incoming wave didnt bring sand but the backwash began taking sand. This led to erosion and loss of land

In this image we can see how the coastline had existed in British era, the Chenai port, the marina beach and the areas eroded close to the north of Chennai

The modern Marina beach hence is much more wider closer to the port and is narrower near lighthouse and much smaller near the mouths of Adyar river. It is in these wide areas were the memorials for leaders have been built.

It doesnt stop here. Satellite image below shows the mouth of Cooum river. There is a good amount of sediment settlement in the mouth which typically reduces the capacity of the river (and was cited as a problem during flooding episodes)

So the government decided to build new groynes in the mouth to prevent the siltation last year. This means that another hard wall is built further into the sea

Satellite image animation shows the changes to the beach near the cooum river mouth since the construction

These Groynes have resulted in further growth of the beach. The Marina would grow further which would be a few hundred metres near the groyne.

In short, the Marina beach in Chennai was created by the obstruction of the longshore drift by the breakwaters of Chennai port. As a consequence, it also caused the erosion of land in the northern side. Based on current trends it is expected to grow further

End of Thread :)

Photo taken from Light House. The Kamarajar road is to the left of the image and the green arrow is where the port is (far end). Area to the right of the red line was all created by this process. This includes all the memorials for the leaders, the shops, etc.

Some similar threads for further reading in related subjects

https://x.com/rajbhagatt/status/1493225683786502148

https://x.com/rajbhagatt/status/1535624093218635776

Enmore - Kaatupalli

https://x.com/rajbhagatt/status/1067061136254631938