Deadly Flash Floods On The Rise…Arizona to Cornwall
“Flash flooding from rainfall and runoff are actually two very different flood factors from the “five forms of flooding”
Deadly flash floods in Arizona and Cornwall (U.K.) last week illustrated the increasing problem of flooding from rainfall and runoff, two somewhat different problems. Though we tend to pool all “flooding” as if it were one problem, recognizing the four traditional forms, with the relatively new problem of rising sea level can help us understand, plan and adapt to this rapidly changing era. (BTW coastal erosion is often confused with flooding, but is in fact quite different.) The Five Forms of Flooding as I characterize them are:
- Coastal storms (including hurricanes) with waves and storm surge in confined areas
- Rainfall
- Runoff
- Extreme high tides, a/k/a king tides
- Rising Sea Level
The flash floods this week remind us of the disastrous damage from record rainfall, and the related but quite different issue of runoff. Both will get worse with climate change––or global warming––and can occur far inland, as the flash flood conditions this week in Arizona, Las Vegas, and Minnesota should remind us.
We are seeing RECORD RAINFALL events all over the world, every year. The reason is surprisingly simply but unnoticed by most. The oceans have been warmed about one and a half degrees F (roughly one degree C) this century. The rate of warming is increasing. Warmer oceans will cause more evaporation. Moisture in the atmosphere must come down as rain or snow, depending on the temperature. Hence the record levels of rain and snow. You may also recall that even the Paris Climate Agreement anticipates that even with its goals, we will have double the amount of warming that has already occurred. This will lead to the almost certain effect of even more evaporation and rainfall, an important fact of future life.
RUNOFF is commonly thought of as similar to RAINFALL but is a very different issue in terms of risk and location. While it usually is the result of rainfall, the amount and location can vary dramatically. Rainfall, particularly rapid rainfall, will move to lower areas with deadly impact. It could be down a coastal slope as in Cornwall England this week, as a result of nearly 4 inches (100 mm) of rain in just three hours, or down into a valley in Arizona or Minnesota, or even just to a lower street in relatively flat Miami. Fortunately in England, prompt response by authorities and townspeople prevented any fatalities, though quite a few homes were destroyed. The point is that RUNOFF can amplify the rainfall by ten or even a hundred times. The location of impact could be far from the site of the rainfall and delayed by days as the ground is saturated and ceases to absorb the rainfall, turning into runoff.
As SEA LEVEL continues to rise––which it surely will––it will make the flooding even worse in coastal areas as the regular pattern of high tides is pushed to new record heights, and as the groundwater levels rise ever-higher. Such flooding following the pull of the moon is becoming almost monthly “nuisance flooding.” Because sea level can not rise suddenly however, it is almost invisible, compared to the other four forms of flooding. Sea level rise is stealth flooding, like the drip, drip, drip that fills a bucket.
You may have noticed that our short term weather forecasts are now pretty accurate, even a week in advance. Recall the jokes just a few decades ago about daily weather forecasts often being wrong. Improvements in meteorology and technology have changed that. Yet paradoxically, rainfall is now very hard to predict over the long term, due to warming oceans and changing atmospheric currents, which are literally changing our weather system at a level that is unprecedented in all of civilization. The thawing polar ocean at the far north is one of the biggest change agents. Open ocean where there has been a thick frozen sheet for thousands, even millions of years, presents real challenges to modeling and long term climate / weather forecasting. We simply have never seen such conditions. Fortunately, the models are getting better. Yet the changes are happening so abruptly, that even the models do not fully anticipate the longer-term climate patterns, as contrasted with shorter-term weather forecasts.
Note that we still think of rainfall and flooding in terms of history and probability, often with concepts such as “100 year flood events.” These are occurring far more frequently, even in less than a decade. Such antiquated terminology is part of our culture and needs to be re-defined in this new era.
Flash floods are particularly dangerous as headlines from around the world continue to illustrate. Flash floods should encourage individuals, engineers, and government officials to think again about how water will move from high to low areas. Even the information that experts use to plot and prepare for flash floods may need to be reevaluated, both in coastal areas and far inland. Some of the engineering handbooks and tables need to be revised in light of the new conditions. The same with insurance (actuarial) and financial risk models.
In an era of changing climate we all need to re-examine the baselines that have been fundamental to our understanding. Otherwise our world can be washed away in a flash as occurred this week in Arizona and Cornwall. Tragedy should be a force to help us learn and prepare for the future.
As the oceans continue to warm, and the climate continues to destabilize we will have to contend with the often severe consequences of record heat and rainfall, flash floods, frequent ‘100 year storms’ and accelerating sea level rise. It’s never been more important to prepare for the effects of climate change, including the 5 Forms of Flooding.
In addition to those efforts to adapt to the new reality, I encourage everyone to educate––even to push––our elected officials and corporate leaders to strengthen efforts to slow the warming, by reducing the production of carbon dioxide.