Surely Looks Like Rain

 

With El Nino ragging thru the southern states and Climate Changing performing as advertised, with more frequent extreme events, it is no wonder that the weather is the main event on the evening news, even with all the Tomfoolery in Washington DC.  The entire ocean is record warm now, not only the Southern Oscillation, as this massive heat sink lag catches up with the systemic heating of our atmosphere.  Huge floods and runoff events have inundated rivers, have filled some local lakes and even added 4-6% capacity to Lake Powell.  (This is nice but Lake Powell needs 10 feet of snow over 100,000 square miles to fill again.  Snowpack fills our big dams.) Some people seem surprised at these events, but they have been prophesied by engineers, meteorologists and hydrologist prognosticators for the last 75 years as maximum possible storms, 40 years as Climate Change and 2 years as El Nino.  

Back in the day we used to design dams for the Probable Maximum Flood (PMF) from the Probable Maximum Precipitation (PMP) because Probable Maximum Storm (PMS) was already taken. There was no rational reason for a high hazard dam, one capable of killing someone, to breach because you couldn’t forecast max floods or you saved money on a cheap emergency overflow spillway.  This PMF was the Deterministic, proverbial line in the sand, million-year flood.  These floods were used to design dangerous large dams, volatile nuclear power plants, prized golf courses and grandma’s rose garden, depending on your priorities.  It took a lot of money and disproportionate resources to provide this level of protection at all dams.  A big dam would not fail in a max storm but all the schools in the floodplain would wash away in the successfully routed flood.  There was lop-sided spending and no one to insure risk consistency between all the hazards we face. 

To calculate the PMP for a dam, we would look at all the biggest historical storms and largest ones the atmosphere of the last 10,000 years could possibly physically produce in a drainage basin.  Then we would maximize them, to account for things like climate change, politics, economics, error bias, and the things that we didn’t know that we didn’t know.  We weren’t even allowed to say the words climate change back then, so we didn’t feel too bad about rounding up.

The PMP for Utah came from a storm near Mogan in 1958 that dropped 6 inches of rain in 45 minutes.  There were full buckets, barrels and hubcaps that proved that it rained hard and anecdotal tales, but there was no paleo evidence of a PMF that would result from rain like that (like a stick, rock or cat stuck high up in a tree).  Sometimes the PMP would not give you the PMF depending on temporal or spatial distribution of the storm, the antecedent saturation or post-apocalyptic burn scars in the watershed.  But the storm, that formed from two colliding thunder cells, was extrapolated by NWS-NOAA on paper to 72 hours over 200 square miles and became the design anchor storm for the intermountain west.  That’s when dam spillways got real expensive

We spent a ton of money on spillways to the point where the new Trial Lake Dam spillway was bigger than the Jordanelle Dam spillway and sometimes the design spillway was bigger than the dam.  Something had to give.  Dam engineers started to look at a more consistent, Probabilistic approach that considered dam beach flood risks with all the other non-voluntary risks in society and tried to spend the money where we could get the most bang for the buck.  We also looked a Significant Incremental Damage Thresholds for inflow floods that were bigger than potential dam breach floods, when no one would care. Terms like ‘acceptable risk’ and the ‘cost-to-save-a-life’ got around the fact that we were putting a value on human life.  This didn’t seem too bad since the probabilities were very small. 

But these rare beach events would happen, and we didn’t want to be the cold, callus, calculating engineer siting on the witness stand telling the widow why her family was killed so we could save money.  So, the burden of determining acceptable risk was put on our elected officials who, of course, chose to punt.  We depend on vague ideas of ‘Industry Standards’, ‘Act of God’ and the But-For defense where a failure would have happened no matter what we had done.  This was a small comfort for engineers and hydrologists who would be hung out to dry anyhow.  Doctors put their mistakes in graveyards, lawyers put theirs in jail, while engineers and hydrologists hang their dirty laundry out on the line for all the world to see.

With the increased frequency of severe storms, as we predicted 40 years ago, we must come to terms with seeing a 100-year flood every 5 years.  We have come through the Stewardship periods of Environmentalism - which then got a bad radical name, Sustainability – which got overused at colleges, Adaptability – implying surrender, and Resilience – implying toughness.  Now we have Discontinuity, Disassociation or Sui Generis – where what is happening now has nothing to do with what happened before.  That is what is happening with our atmosphere, climate and storms.

Our Probabilistic models have no relevant historical data to decipher anymore, and we must go back to our Deterministic models that physically simulate the atmosphere, the drainage basins and the river channels with pixelated, finite elements that make up our hydrologic systems.  Slap some NextRad radar over some GIS terrain models and out pops the flood.  Luckily these models are good enough and computers are fast enough, with Ai assistance, to help model these things as they happen.  We now have 2D and 3D models that help with Computational Fluid Dynamics (CFD).  With quantum computers there will be no coincidences and there should be no surprises with real-time forecasting.  Necessity is the mother….  The proverbial PMP-PMF line in the sand may be moving, figuratively and virtually, but at least we should be able to keep up with it.  Even a blind man knows when it is raining.