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Why does a growing city flood more? Two short pieces on the same question. Read both before you answer anything.
Text A
Rain that lands on undeveloped ground mostly goes into it and reaches the streams over the following days. Asphalt, roof tile and the fill compacted under both are close to impervious, so most of what falls on them stays on top and moves in minutes.
Every drain is built to a design intensity, a rainfall rate it is meant to carry. Chennai's older storm water drains were sized for 31.39 mm per hour, the figure in the 1999 CPHEEO manual for a two-year return interval. Newer drains in some zones take roughly 68 to 72. Which figure a street got depends on when its drain was last rebuilt, and most of the network has not been.
Capacity is one half of it. The catchment is the other. Chennai has grown onto floodplain and wetland, flat ground that is cheap to build on and that used to be part of the drainage. The same storm now delivers a larger volume to a network sized for a smaller one.
Neither of those changes is a change in rainfall. Whether a given street floods depends on that street's drain and its catchment, which is why a road that stayed dry in the 2015 floods is not evidence that its drain is adequate.
Text B
A city does not flood only because it rains.
When rain lands on soil, most of it soaks in. Some evaporates. Whatever is left runs across the surface. Pave that soil and the balance breaks. Asphalt absorbs almost nothing, so water that used to go down goes sideways instead, fast, toward whatever is lowest.
Cities tend to build on what is lowest. Flat ground beside a marsh is cheap and easy to develop, which is why it gets developed. Pallikaranai marsh covered about 5,500 hectares in the 1960s. It is under 700 now. A marsh that size held an enormous volume of water through a monsoon and let it go slowly. That capacity did not move somewhere else when the marsh was built on. It stopped existing.
Storm drains are meant to make up the difference. They are sized against the rainfall records available when they were designed, which means they are sized against a past that is no longer a good guide.
Rain is the trigger. The surface, and what used to be under it, decide what the rain does.
Four minutes, on your own. For each text, decide: was this written by a person, or with AI? What made you decide: point at a specific sentence in each one, not a general feeling. How sure are you about each: certain, fairly sure, or guessing? Write it down before you hear anyone else's answer, because you will not be able to unhear it.
What the authors said about AI use. Open this only after you have written your answer.
Text A: nothing. Text A came with no statement about AI use.
Text B: "AI use: this text was drafted with an AI language model, then checked and edited by the author."
The written reflection for this one is in the Reflection Journal, opened in class.