The Andaman & Nicobar Islands and the 1% Flood
The one place in the atlas where the districts that rank lowest are the ones that were destroyed
This atlas ranks the Nicobars as the least flood-exposed district in the territory — 5.6%.
On the morning of 26 December 2004, the Nicobars were among the worst-hit places on Earth. Entire islands went under. Trinket Island was cut in two. Some of the Nancowry group were submerged and communication was lost. On Car Nicobar, 111 Indian Air Force personnel and their families were washed away when the tsunami destroyed the air base. Indira Point — the southernmost tip of India — subsided by about 4.25 metres, and its lighthouse now stands in the sea. Across the Nicobars, roughly 97% of mangrove cover was lost.
Both statements are accurate. Understanding why is the only responsible way to use this map here, so we are putting it first rather than in a footnote.
What this atlas models — and what it does not
This is a rainfall and river flood model. It takes a 1% annual-chance storm, runs it over the terrain, and reports how deep the water gets. On the mainland that captures most of what floods people.
On these islands it captures the smaller half of the problem. Tsunami and cyclone storm surge are not modeled here at all, and those are what have actually killed people in the Andamans and Nicobars. A tsunami is not weather; it is the sea arriving. No rainfall model anticipates it.
So the Nicobars score low because little rain-driven flooding is modeled on steep, small islands with short streams — not because they are safe. They are the clearest demonstration in this entire atlas of the difference between what a map measures and what a place should fear.
The scale
- Of the territory’s 137,217 building footprints, 26,758 — 19.5% — sit in the modeled 1% annual-chance rainfall floodplain. Roughly one building in five.
- The mean depth over exposed buildings is 2.42 m, though this carries the usual steep-terrain caveat: about 10% of exposed buildings model at over 6 m, which reflects a ~30 m elevation model in hilly island country rather than real water. Saddle Peak rises to 732 m.
- 385 km of road — 16.7% of the network — lies in the modeled floodplain. There is no railway.
Where it concentrates
| District | % of buildings exposed | Exposed buildings | Modeled depth |
|---|---|---|---|
| North and Middle Andaman | 34.5% | 17,168 | 2.44 m |
| South Andaman | 11.8% | 8,898 | 2.35 m |
| Nicobars | 5.6% | 692 | 2.82 m |
North and Middle Andaman carries the real rainfall exposure at 34.5% — a third of its buildings. It has the larger valleys, the longer streams and the flatter coastal strips where rain-driven water collects, and it holds the majority of the territory’s exposed buildings.
South Andaman, which contains Port Blair and most of the population, sits at 11.8% but holds the largest total building stock. Port Blair floods from intense rain with some regularity; this is the district where the atlas is most directly useful.
The Nicobars, at 5.6%, are the number to read most carefully — for every reason set out above.
Why the islands flood
- Extremely high rainfall. The archipelago receives roughly 3,000 mm a year across two monsoons — the southwest from May, the northeast from November. Almost no dry season.
- Short, steep catchments. Rain reaches the sea in hours. Flooding is flashy rather than prolonged, which makes it dangerous but brief.
- Settlement on the flat coastal strip. The only level ground is the coastal fringe — which is also where rainfall drains to, where storm surge arrives, and where a tsunami lands. Every hazard converges on the same narrow band of inhabited land.
- Cyclones in the Bay of Bengal, bringing both extreme rain and surge.
- Seismic exposure. These islands sit on the Sunda subduction zone. The 2004 event was a magnitude 9.1 rupture; the geological record in the Andamans shows large tsunamis recurring over millennia. Some islands were uplifted in 2004 and others subsided, permanently changing where the water now reaches.
Reading the numbers honestly
- The 19.5% extent is a genuine rainfall-flood figure, and useful for drainage, siting and road design.
- Depths above about 5 m should be discarded. The 10% of exposed buildings modelling over 6 m is a DEM artifact in hill terrain, the same effect flagged for Arunachal Pradesh and Sikkim.
- The district ranking must not be read as a safety ranking. The Nicobars rank lowest for rainfall flooding and are the most exposed place in India to tsunami. If you take one thing from this page, take that.
- Post-2004 subsidence is not in the terrain model. Parts of the southern Nicobars and Great Nicobar sit 2.5–4 m lower than they did before 2004. Where the elevation data predates that, the modeled water is optimistic.
- District boundaries follow the 2011-census-derived set of three districts, which matches the territory’s current official count.
A strategic approach
Pillar 1 — Use this map for what it is. Rainfall flooding, drainage design, road and culvert sizing, and siting decisions on the coastal strip. It is genuinely useful for those.
Pillar 2 — Use a different map for surge and tsunami. Inundation modelling for seismic and cyclone events is a separate discipline with separate inputs. The territory’s disaster management authority works to tsunami zonation that this atlas does not replace and does not attempt to.
Pillar 3 — Protect the warning chain. After 2004 India built the Indian Tsunami Early Warning Centre, and for a near-field rupture on the Sunda trench the warning time here is minutes. Sirens, drills and rehearsed routes to high ground matter more than any depth map.
Pillar 4 — Re-survey elevation where the land moved. Any hazard mapping in the southern Nicobars that uses pre-2004 terrain is mapping an island that no longer exists at that height.
Pillar 5 — Keep the mangroves. The Nicobars lost about 97% of their mangrove cover in 2004, and mangroves are among the few natural defences that reduce both surge energy and coastal erosion. Their recovery is coastal protection, not only ecology.
The honest bottom line: 19.5% of buildings in the Andaman and Nicobar Islands sit in the modeled 1% rainfall floodplain, concentrated in North and Middle Andaman at 34.5%. Those numbers are real and useful for drainage and siting. But this atlas ranks the Nicobars lowest, at 5.6% — the same islands where whole settlements vanished in 2004, where Trinket Island was split in two, and where Indira Point dropped over four metres into the sea. That is not a contradiction in the data; it is the boundary of what the data covers. On these islands the deadliest water has never come from the sky.
Sources
- 2004 tsunami impact — Wikipedia: Effect of the 2004 Indian Ocean earthquake on India · Directorate of Disaster Management, A&N Administration: Earthquake & Tsunami · IIT Kanpur / Earthquake Spectra: Landscape changes in the Andaman and Nicobar Islands after the 2004 earthquake
- Subsidence and land movement — Nature India: How much did the tsunami really move Andaman? · Scientific Reports: Tsunami records of the last 8000 years in the Andaman Islands
- Ecological loss — Mongabay India: Nicobar Islands lost 97 percent of mangrove cover
- Exposure statistics — India Flood Atlas (modeled 100-year / 1% annual-chance rainfall event), NEER.
Exposure figures are modeled estimates (~30 m) for a 1% annual-chance rainfall and river flood, intended for awareness and prioritisation — not parcel-level certainty, and least reliable in steep terrain. This atlas does not model tsunami, cyclone storm surge or sea-level rise. Building footprints are structures, not people.