Storms Happen

Extreme Storms happen.  Every day.  Somewhere.  Maybe not on your backyard or dam, but around the country and the world there is always an unbelievable storm raging.  Sometimes we see them in the news or on TV and sometimes these extreme storms get names like Katrina, Irene, Sandy, Boulder or Snow-mageddon.  Sometimes no one hears about them at all.  But they happen, all the time, and they could happen to you.  Live long enough and they will.

Why should I care about extreme weather?

Hydro-meteorological experts put all the historical extreme storms, at all their locations, into a big data base to determine how often they happen, how big they can get and what is the threat at individual locations.  We all know about the weather in our own back yard but these storm experts consider hundreds of years of data at thousands of locations and have a much broader understanding of the size, scope and statistics of extreme events.  They know that extreme storms happen.


We have all heard of the ‘100 year storm’ as the typical flood design standard in the USA.  Things that are replaceable, such as a culvert, minor flood plain encroachment, or a low hazard dam, are designed for the 100 year level of risk.  Things that are not replaceable, such as a high hazard dam, bridge  or a nuclear power plant, where human life is at risk, are designed for a much bigger storm because of the high consequences of a failure.  With that kind of high risk and consequence there is no excuse for an acceptable or foreseeable failure and the design standard is the ‘non exceedance event’, no matter how rare that event is.  Design standards should never compromise human life and safety.

This kind of non exceedance event has been determined by the National Weather Service (NWS) to be the Probable Maximum Precipitation (PMP) in the USA.  These PMP events are defined by the NWS as, “theoretically, the greatest depth of precipitation for a given duration that is physically possible over a given size storm area at a particular geographic location during a certain time of year”.  By combining these storm events with the most severe hydrologic conditions that are reasonably possible in a given drainage basin determine the Probable Maximum Flood (PMF) that is the national industry standard for high hazard dam design.

The fact that these extreme storms happen, are predictable and quantifiable, make them the design standards for critical infrastructure and where human lives are at risk in the USA.  They are not random, unpredictable acts of God that surprise designers and owners with their ferocity.  We know they can happen and do happen, even though we personally may not have experienced them.  They are one of the most critical and elusive design components of a dam because they are the weather.  They must be considered like we consider all the dam loading factors and forces such as earthquakes, wind, and gravity, and material properties such as soil steel and concrete.

Dam designers and owners are therefore responsible and liable for all of these accepted loading conditions and their consequence, both legally and ethically. The standard of legal care, in a court of law for these instances, is ‘the actions of a reasonable man’ and a reasonable man.  A reasonable dam man should know the proper loading conditions, and must design against exceedance and failure under these foreseeable loads. 

In conclusion, the impoundment of water is a hazardous undertaking and those who benefit from its storage must also be responsible for its containment.  We never fail to underestimate the power, persistence and the patience of stored water and we must diligently guard against its catastrophic release.  To do anything less, knowing the potential loading conditions of extreme storms, would be ethically irresponsible at best and gross negligence at worst.