Raj Bhagat P #Mapper4Life Profile picture
Earth Observer - Tweets on GIS, Remote Sensing, Water Resources, Urban Development, Environment; Working: WRI India; Tweets-Personal; Bengalur/Nagercoil n Tamil
7 Jul 20

Since many had asked this, Writing a short thread on the formation between India and Sri Lanka

Usually many get deceived by "static" satellite images and believe that the Tombolo section as permanent relics of a man made bridge.
Deeper ocean currents don't enter the section between Sri Lanka and India because of the Cont Shelf. The sea surface in this section is dominated by longshore currents in two directions - one from Gulf of Mannar and other from Palk strait and they are in opposite directions
Longshore currents bring a lot of sediments. And at the meeting point of the currents they settle and form these islands. Notice in the satellite images below how these islands change a lot owing to current direction, tides, etc. This is just a short term timelapse (<4 years)
Read 9 tweets
28 Jun 20
#Thread on Koshi and Bihar Floods:

Though there are articles on Koshi River n flooding in Bihar, I prepared this thread to visually explain it!

Image: Topography of #Bihar
Despite being far from sea most of Bihar people live at an elevation lesser than 100m similar to Kerala
R.Kosi is a tributary of Ganga. It's watershed covers Mt.Everest but at it's confluence with Ganges, the elevation is just 25 m above sea level. All this within an aerial distance of 300kms! Along with seasonal rainfall, this has led to massive erosion as well as deposition
Cross-sectional profile of the watershed shows this variation

After flowing in higher gradients cutting through the himalayas, Kosi river exits n meets flatter plains section with very less gradient. This leads to millions of tonnes of silt deposition in this section every year
Read 11 tweets
2 Jan 20

Here's a compilation of some timelapse animations that show different stories from across India over the 3 decades (1989 - 2018) . Created with @googleearth @EarthOutreach #newyear2020

1: Restless #Brahmaputra n tributaries meandering,changing course in #Assam
2. Growth of Brick Kiln industry in the banks of #Hooghly, #Rupnarayan rivers near #Kolkata (brown polygons close to the rivers' banks) #WestBengal
3. Embankment failure in #Kosi River (2008) caused flooding. It also left a sediment footprint that was visible from satellites. #Bihar
Read 21 tweets
31 Oct 19

@nytimes has carried an article how cities would be under water which shows #Mumbai being wiped out

We need to exercise caution before using this data n inferences in Indian context!


@PrinSciAdvGoI @dishashetty20 @rapiduplift @RyanMaue
Currently elevation data like SRTM have a vertical error i.e height data created by satellites don't represent ground height very accurately due to buildings, trees etc. So while doing analysis there's a possibility of understimating areas affected by floods, storm surge etc

Inorder to overcome this issue, the authors of the paper had tried to create an AI model that can correct the elevation bias in the data. The AI model is built with ~23 parameters like population density, vegetation etc to explain the variation in the satellite data.

Read 15 tweets
30 Oct 19
@rapiduplift @JamwalNidhi SRTM does have vertical bias however the correction methodology is not sound enough to overcome that in a more accurate manner.

The problem I see is with ML algorithm which uses LIDAR data and ~23 parameters from developed countries as training and testing datasets
@rapiduplift @JamwalNidhi These parameters come in coarse resolution datasets (some of them as coarse as a km). Also, The relationship between those parameters and Elevation bias would vary between developed and developing countries. However the training data doesn't account this.
@rapiduplift @JamwalNidhi For eg, population density is related to Building heights to some extent. But it cannot be assumed as the same for US and India. So it is likely to exaggerate bias in India

Params like Vegetation r also problematic as they report difficulty in estimating for places like Florida
Read 5 tweets
7 Sep 19
Thread - India's flood plain issue:

There r so many flood incidents this year. One big problem s that we haven't mapped flood plains yet. Many crucial infra (road, railway) are built in flood plains with less care

Kabini Flood simulation shown here

FYI @PrinSciAdvGoI
In this case study, We would look at Nanjangud, a small town in the banks of #Kabini river in #Karnataka

Like #MahalaxmiExpress case, Kabini takes a sharp bend n a highway is present in flood plains

Map shows Kabini river's path near Nanjangud (flood sim. was done for this)
In August this year, Kabini flooded. This satellite image shows the area that was flooded. Black indicates water

Most of the flooded areas were farms

But two sections of the flooded areas have highways n builtup area in them (marked in red)

Data @CopernicusEU @sentinel_hub
Read 8 tweets
1 Jul 19
Thread - Way forward for #Chennai

Some of you might have come across my article on #ChennaiWaterCrisis n solutions that I had proposed. In this thread, I will roughly explain why I chose 5 solutions n discarded 2

Read: ow.ly/tbB450uMvxI

Pl read n RT
Sol 1: #RainWaterHarvesting - Chennai has urbanized pretty fast n percolation into ground is getting reduced. Building wise RWH is one,but we need RWH in public spaces like bus stands etc; Every year Corporates spend a lot of CSR which could be used

Sol 2: #WasteWaterReuse - Currently most of the sewage is going to the sea via Cooum,Adyar rivers, Buckingham canal or ending in the lakes. This water could be reused through decentralized sewage treatment and reuse system

Read 11 tweets