When the country was celebrating independence day on August 15, Kerala, the southern state of India was in a panic. The heavy rain caused all of the dams to fill up to the neck. For the first time ever in the history of Kerala shutters of 35 dams where opened. The rivers where overloaded causing the deluge. Losses are severe, Roads and bridges are destroyed and Cochin international airport was also flooded causing huge loss. Death toll nears 400 and cost crossed more than 20,000 crores.
Now post flood analysis is very important, the areas inundated should be identified and mapped for references. No one was aware of the reach of flood water this time. There were reports that people refuse to evacuate because they believed that water won’t enter into their houses. So post flood analysis is very important to calculate the losses and for our future generation, there should be a reference to compare.
As you know remote sensing is a powerful tool for this situation, Satellite imagery can be used to analyze the case. But there are some barriers also.
- Optical sensor satellites are not effective because during rain atmosphere was heavily clouded. Next option is SAR satellites, which is actually better than optical sensors for this application. SAR means “Synthetic Aperture Radar” and its most advantage is that it is an active remote sensing platform, It can capture the image during the night and penetrate through clouds also. Microwave transmitted is the energy source. It transmits the microwave signal towards the area of interest, receives the portion of transmitted energy as backscattered from the AOI, It then observes the strength and time delay of the returned signal. Generally, bright areas in the image are highly reflective surfaces like buildings and black areas with less or no reflection is water bodies. Depending upon Radar signal wavelength it can penetrate the tree canopy also.
|X Band||12.5-8 GHz||2.4-3.75 cm||Widely used for military reconnaissance, mapping and surveillance (TerraSAR-X,|
|C Band||8-4 GHz||3.75-7.5 cm||Penetration capability of vegetation or solids is limited and restricted to the top|
layers. Useful for sea-ice surveillance (RADARSAT, ERS-1)
|4-2 GHz||7.5-15 cm||Used for medium-range meteorological applications—e.g., rainfall measurement,|
|L Band||2-1 GHz||15-30 cm||Penetrates vegetation to support observation applications over vegetated surfaces and for monitoring ice sheet and glacier dynamics (ALOS PALSAR)|
|P Band||1-0.3 GHz||30-100 cm||To date only used for research and experimental applications. Significant penetration capabilities regarding vegetation canopy (|
vegetation biomass), sea ice, soil, glaciers
- Sun-synchronous satellites needed because
Geo-synchronoussatellites won’t give you much spatial resolution. The downside of Sun-synchronous satellites are, it may not be available above the location at required days. There is a temporal resolution for every Sun-synchronous satellites. That is, the satellites will take some days to revisit the same place again. Temporal resolution varies for each satellite, so satellite data of different space agencies have to be used.
To overcome this barrier and for effective disaster management there is a charter with worldwide collaborations named as “The International Charter Space and Major Disasters”. ISRO(Indian Space Research Organisation) is also a member and contributor of this charter. The charter is a worldwide collaboration, through which satellite data is made available for the purpose of disaster management for free of cost. Earth observation assets from different space agencies are made available so that most suitable and recent acquisitions can be used for the purpose. Thus ISRO/NRSC has acquired the data and analyzed it.
The inundation due to the flood was started from August 8 and the highest water level of Periyar river was recorded in August 16 evening at Kalady. Then by August 17 evening, the flood water started receding.
The available SAR satellite data was of
- ALOS 2 PALSAR, Dated August 17, 1530Hrs
- Radarsat-2, Dated August 18, 1900Hrs
- TerraSAR-X, Dated August 18, 0400Hrs
NRSC(National Remote sensing Center) has produced the maps using the satellite images received under the international charter. Also, IWMI(International Water Management Institute) has released some maps using ALOS 2 Data.
ALOS 2 PALSAR
ALOS means “Advanced Land Observation Satellite”. It was launched on May 24, 2014, by JAXA(Japanese Aerospace Exploration Agency). Its primary purpose is to monitor disasters. ALOS 2 is specialized for SAR(Synthetic Aperture Radar). PALSAR frequency is 1.2 GHz(L band) and
Radarsat 2 is a joint mission of CSA(Canadian Space Agency) and MDA(MacDonalds Dettwiler Associates Ltd.). It was launched on de 14,2007. The imaging frequency is C band (5.405 GHz). Spatial resolution is 3-100m and temporal resolution is 24 days.
TerraSAR-X is a German SAR satellite mission supported by BMBF(Germanministry of education and science) and managed by DLR(German Aerospace Center). It was launched on 15 June 2007. It has a revisit time of 11 days and spatial resolution up to 1M.
As I said, Flood inundated area should be mapped accurately and it should be made available to future generation as a reference, otherwise same mistakes will repeat again.
The satellite data available during the peek flood days are all paid ones. Open accessible free Sentinel 1 SAR data is only available at August 9 and August 21st.
The state is trying to recover from losses, a Huge fund is needed for that. Now it is our duty to help the affected rebuild their lives.So please contribute to chief ministers relief fund
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