Why Harvey’s flood is worse than previous storms

Harvey, Houston, flooding
A man floats past a truck submerged on a freeway flooded by Tropical Storm Harvey on Sunday, Aug. 27, 2017, near downtown Houston. The remnants of Hurricane Harvey sent devastating floods pouring into Houston Sunday as rising water chased thousands of people to rooftops or higher ground.

GRAND RAPIDS, Mich. (WOOD) — Superlatives abound with Harvey: The storm has crushed rainfall records across southern Texas and much more rain is on the way. On the high end, 50 inches is possible.

The storm will be once in a lifetime for many in the South, but it probably won’t go down as the heaviest rain storm or hurricane for Texas. In 1979, Tropical Storm Claudette dropped 42 inches of rain in Alvin, Texas in one day, which remains the biggest single-day rainfall total in U.S. history.

Tropical Storm Claudette
A 1979 file photo of Tropical Storm Claudette.

In the last century and a half, five other Category 4 hurricanes have made landfall in Texas.

But Harvey will be different and may rival or exceed 2005’s Katrina in damage because modern day floods drain more slowly due to impervious surfaces like roads, parking lots and buildings. Deep floodwaters inundate storm drains and can’t soak into the ground.

Houston impervious surfaces
This map shows the difference between impervious surfaces in the metro Houston area in 1940, when the population was around 500,000, and today, when it’s nearly 7 million. Yellow, orange and red are areas where water simply cannot enter the ground.

Enough rain has fallen on southern Texas to fill Reeds Lake in East Grand Rapids 3,000 times — and the total in Texas may double before Harvey is done.