Flash flooding in Arizona

Great before and after photos of flash flooding in Arizona. What's up? I thought Arizona was in a dry climate because of its interior continental location and rainshadow effects. What's with all the water? Why is flash flooding a relatively common hazard?

Read the full article below, from http://epod.usra.edu/blog/2014/11/flash-flood-at-galloway-wash.html

Credits for photography and summary: Thomas McGuire

Seven of the ten U.S. states with the most flood fatalities per capita are in the dry areas of the West; especially the desert southwest. How can these relatively dry regions have so many flood deaths? They're subject to summer monsoon storms that can dump several inches of rain in just a couple of hours. Soils are thin and rocky, so there's relatively little vegetative ground cover and rainwater runs offquickly. With on-going recent plate tectonic activity, streams have steep gradients and flow fast, especially when at flood stage. Fortunately, local zoning has been relatively successful in preventing damage to homes and properties. The major cause of these fatalities has been drivers foolishly (and often illegally) attempting to cross flooded stream-level crossings.

The above photos were taken from the same position at Cave Creek, Arizona, just one week apart. The stream volume in this flood is estimated to be roughly equal to half the flow of the Colorado River through the Grand Canyon. Yet, Galloway Wash is bone dry about 95 percent of the time. This is a privately owned bridge to just one property. At the public road, just 300 ft (97 m) downstream from here, stream level traffic was halted a few hours. There are about a thousand citizens who are completely blocked from access to their homes by flooding at this crossing. But it seems most of them don’t want a bridge as they consider this a normal part of life in Cave Creek. Photos taken on August 19, 2014 (top) and August 26, 2014 (bottom)

Great before and after photos of flash flooding in Arizona. What's up? I thought Arizona was in a dry climate because of its interior continental location and rainshadow effects. What's with all the water? Why is flash flooding a relatively common hazard? Read the full article below, from http://epod.usra.edu/blog/2014/11/flash-flood-at-galloway-wash.html

Photographer: Thomas McGuire Summary Author: Thomas McGuire

Seven of the ten U.S. states with the most flood fatalities per capita are in the dry areas of the West; especially the desert southwest. How can these relatively dry regions have so many flood deaths? They're subject to summer monsoon storms that can dump several inches of rain in just a couple of hours. Soils are thin and rocky, so there's relatively little vegetative ground cover and rainwater runs offquickly. With on-going recent plate tectonic activity, streams have steep gradients and flow fast, especially when at flood stage. Fortunately, local zoning has been relatively successful in preventing damage to homes and properties. The major cause of these fatalities has been drivers foolishly (and often illegally) attempting to cross flooded stream-level crossings.

The above photos were taken from the same position at Cave Creek, Arizona, just one week apart. The stream volume in this flood is estimated to be roughly equal to half the flow of the Colorado River through the Grand Canyon. Yet, Galloway Wash is bone dry about 95 percent of the time. This is a privately owned bridge to just one property. At the public road, just 300 ft (97 m) downstream from here, stream level traffic was halted a few hours. There are about a thousand citizens who are completely blocked from access to their homes by flooding at this crossing. But it seems most of them don’t want a bridge as they consider this a normal part of life in Cave Creek. Photos taken on August 19, 2014 (top) and August 26, 2014 (bottom)