4 Dhurandar

Inspiration

Bengaluru was built on a cascade of man-made tanks. Rain fell on high ground, filled the first tank, overflowed down a channel into the next, and so on down the valley. The tanks were the storage, the channels were the plumbing, and together they were a drainage system that needed no pumps.

Over four decades the tanks were drained and built over, and the channels were culverted, narrowed or filled. The water still arrives. The storage no longer does.

What pushed us to build something was noticing where the decision lives. When a neighbourhood floods, the cause is usually a sanction granted a decade earlier, several kilometres uphill, by someone who never saw the consequence. By the time the water is in somebody's ground floor, the evidence that a lake was ever there has been paved over, and every map in the room shows the encroached boundary as if it were the real one.

The record is not gone, though. It is in orbit. Landsat has been photographing this ground since 1984. The question was never whether the evidence exists — it was whether anyone puts it in front of the person signing the approval.

What it does

BlueTrace reconstructs a city's lost water bodies from the satellite record, then works through four questions that form a loop rather than a sequence:

Evidence — what was here, and what happened to it? Two eras of water, one method, with the 1984–99 shoreline drawn over today's ground and the construction standing inside it.
Prevention — may this plot be built on? Click any point for a screening outcome against a selectable buffer regime.
Consequence — where does the water go? It walks the flow-direction field downhill from the plot and names the ground that receives the displaced runoff.
Remedy — what else could go here? The displacement becomes a retention requirement in cubic metres, with the land uses that can discharge it.

It is deliberately not a flood predictor, and it refuses that question — plus damage cost, legality, and owner identity — explicitly, in the answer itself. None of its four outcomes is approve: it screens, and the sanctioning authority decides.

For the catchment we mapped: 71.31 ha of water lost, 35.9% of the blue grid, 3.05 ha already built on former water, 13,818 buildings inside 2,212 ha of flood-prone ground.

How we built it

Extraction happens once, offline. A single Earth Engine script exports 14 layers for the catchment rectangle. Nothing else in the system ever touches Earth Engine, which is why the app has no cloud dependency at runtime.