Flooding is local

Rain does not flood places. Catchments do.

Lesson 2 gave you the rainfall. This lesson is about what the ground does with it, and the honest headline is that three inches of rain in an hour means something completely different in five different parts of the country. In one it is a wall of water down a canyon inside twenty minutes. In another it is a street that stands under a foot of water for six hours and then drains. In a third it is nothing at all.

Which means the most common failure in flood spotting is applying a rule of thumb learned somewhere else. Most national training material is written around terrain, because terrain floods dramatically and photographs well, and a spotter on a coastal plain or in a flat agricultural county reads it and concludes their area does not really flood. It does. It just does it differently.

Five regimes

Three things that are true everywhere

• Antecedent soil moistureHow wet the ground was before the rain started. It decides how much of the rain becomes runoff, which is why the same storm floods a catchment one week and not the next. decides the share that runs off. The same storm floods a catchment one week and not the next, and nothing about the storm changed.
• Flat drains slowly. Water that arrives fast and leaves slowly stands deeper and longer than water in a valley that empties itself. Slow flooding is not mild flooding.
• Soil type is overwhelmed, not decisive. Sand drains until the rainfall rate exceeds its infiltration rate, which a heavy convective rate does in minutes. After that what the soil is made of stops mattering.

Compound flooding, the coastal special case

One regime has a failure mode the others do not, and it is worth a section because it breaks the arithmetic people use. Compound flooding is heavy rainfall arriving at the same time as elevated coastal water. The rain has to go somewhere, and the somewhere is already above its normal level, so it does not go.

The NHCNational Hurricane CenterThe national office for tropical cyclone forecasts and warnings. report on Hurricane Sally puts it in one sentence: "Due to the incoming storm surgeAn abnormal rise of water generated by a storm, over and above the predicted astronomical tide. It is the storm-driven part on its own, which is why it can be quoted as a height without saying anything about how deep the water actually got., extreme rainfall produced by Sally had nowhere to drain, which resulted in widespread flash and river flooding as well."

The same mechanism, without a tide, appears inland wherever a tributary meets a main river that is already high. The tributary cannot discharge and backs up, flooding ground the main river never reaches. If you live at a confluence, this lesson is about you too.

What floods first, wherever you are

• Low-water crossings. Built to be overtopped, unmarked as often as not, and the single most common place people drive into moving water.
• Creek, bayou and arroyo banks, where the rise is fast and the banks are low.
• Underpasses and low points in the street grid, which collect urban runoff and give no indication of depth from either approach.
• Confluences and outlets, where the water has to get past something that may not be letting it.
• The place that flooded last time. Local flood history is the best predictor there is, and it is free.