Big floods more often

Some German researchers conclude about climate change and floods:

New research by Brunner and her colleagues shows the occurrence and intensity of extreme flood events will increase, but smaller and more moderate floods will probably decline.

“There is extensive evidence that climate change is increasing the frequency and intensity of extreme precipitation events around the world, but much less evidence that flood events have increased over the same period,” wrote Daniel Swain, a climate scientist at University of California at Los Angeles and a co-author of the paper.

“This new research demonstrates how climate change could actually have divergent effects for very large but rare floods versus smaller but more common floods,” he said.

Swain said additional work would be needed to confirm that this flooding behavior applies more broadly around the world but suspects it probably does....

In line with previous research, the team found that precipitation will increase and intensify in a warming climate. The largest increases in precipitation occurred in the most extreme and rare events.

By the end of this century, the authors found, intense precipitation events that would typically occur two times per century would occur twice as often. Events that would occur once every 200 years would become four times as frequent.

Flooding trends were more nuanced.

In general, they found that moderate flooding events depended on land-surface processes, such as soil moisture or the amount of snow in the watershed.

For instance, if soil moisture was high, perhaps from previous recent rains, then even a small amount of additional precipitation could cause the area to flood. Brunner also said that water falling over impermeable surfaces, such as concrete sidewalks in cities, could cause flooding even from relatively small amounts of rain. Both of these were contributing factors to the disastrous flooding in New York in early September.

All sounds pretty plausible.  And as I keep saying - actually a very expensive and difficult thing to deal with in infrastructure terms - re-engineering storm water drainage for an entire city to expand its capacity by a substantial amount must be very pricey.