Flooding is local
- View
The same rainfall floods five places five different ways. Which one you live in decides what to watch, what to expect, and which rules of thumb will mislead you.
In this lesson you will
- Explain why the catchment, not the rainfall, decides the flood response.
- Characterise the main US flood regimes and identify which one describes your own area.
- Define compound flooding and explain why coastal plains are acute for it.
- Identify the local settings that flood first, in any regime.
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
| Regime | How it floods | What misleads people |
|---|---|---|
| Steep terrain: Appalachians, Ozarks, interior West | Very fast. Small catchments concentrate runoff into narrow valleys in tens of minutes, often from rain nobody in the valley saw fall. | That you are safe because it is not raining on you. The rain was upstream. |
| Burn scar, western and increasingly elsewhere | Fastest of all, and it is not only water. Hydrophobic soil sheds almost everything, producing debris flows dense enough to carry boulders. | That a modest rainfall rate is safe. Post-fire thresholds are far lower than pre-fire ones, for years. |
| Coastal plain: Gulf and south Atlantic | Slowly and widely, and then it will not leave. Flat ground drains slowly and a high water table means the soil is already full. Compound flooding is the acute case, below. | That sandy, well-draining soil does not flood, and that flat country floods gently. |
| Arid Southwest | Monsoon rain on baked ground that sheds it like pavement, down arroyos that are dry 360 days a year. | That a dry channel is a road. It is a channel. |
| Cold season and snowmelt: northern tier, upper Midwest | Slowly and predictably in spring melt, and very fast when rain falls onto snow or onto frozen ground that cannot absorb it. Ice jams flood upstream of the jam with no rain at all. | That flooding is a summer convective problem. |
Soft boundaries, and plenty of places are two of these at once. The row that matters is the one your own ground is in, and the point of reading the other four is knowing which borrowed rules to distrust.
Urban. Impervious surface converts rain to runoff immediately and delivers it to a drain system sized for a design storm, and every regime above has cities in it. Urban street flooding is usually not a maintenance failure: it is the system working as specified against rain outside the specification. It is also the most under-reported flooding there is, because it looks ordinary.
Three things that are true everywhere
- Antecedent soil moisture 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 NHC report on Hurricane Sally puts it in one sentence: "Due to the incoming storm surge, extreme rainfall produced by Sally had nowhere to drain, which resulted in widespread flash and river flooding as well."
Rainfall flooding and surge flooding are forecast separately, warned separately and filed separately, and on a low coastline they are frequently the same event. A compound flood can be worse than the sum of the two forecasts, because the surge does not add water inland, it removes the drainage. It is also why rainfall-based radar products will not corroborate the inland part of it, which lesson 6 returns to.
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.
Learn your own list before the season. Which regime you are in, the four or five spots on your patch that go under first, and what they are called. A report that names a known crossing is worth far more than one describing an anonymous flooded road, because the forecaster already knows what that crossing means.
Check yourself
Four inches of rain falls on sandy, well-draining soil over flat ground two miles from a bay, with a two-foot storm surge running on a high tide. Which factor most increases the flood risk?
- The surge, because it removes the drainage the rainfall depends on
- The sandy soil, because sand becomes impermeable once wet
- The flat terrain, because water accelerates over smooth ground
- None of them. Four inches is below any flood criterion
Hint
One of these four changes what happens to the water after it has already reached the ground.
Answer
The surge, because it removes the drainage the rainfall depends on
This is compound flooding. The rain is unremarkable and the surge is unremarkable, and together they flood ground neither would reach alone, because the raised water level blocks the outlet the runoff needs. Sand does not become impermeable, it is simply overwhelmed, and flat ground drains slowly rather than quickly.