The Reason We are Seeing 100-year Rain, Wind and Seismic Events
What is a 50-year Rain Storm? What is a 100-year Storm?
California Civil Engineer here. When a 50-year storm hits, it means there is a 2% chance of that event in any given year. There is a 1% chance for a 100-year event to occur this year.
These events are predicted based upon records of past events. All large cities and counties have their own records of rain fall. Some cities have 50 years worth of data. Some of the longest record keeping goes back more than 100 years. These rainfall levels are usually recorded at airports and public engineering departments. It is possible for two 50-year rain events to dump water on the same city in the same year. Statistically, it is a reasonable series of events; it is not statistically improbable.
Each city keeps its own data and produces its own tables to predict rainfall. Most cities’ new underground storm water systems are designed to move a 50-year rain event. Other municipalities have some common problems: older cities have very old pipes underground. Many of these are too small. Another common problem is lack of elevation available to create slope to move water quickly. Increased velocity moves more water.
If two large events arrive together, more flooding results. The first downpour soaks the soil. The second event, with reduced ground absorption, produces more runoff.
For example, the annual rainfall, and the maximum event that has a 2% chance of occurring in Riyadh, Saudi Arabia, is much smaller than for Seattle, Washington. A 100-year event in Riyadh is likely a 10-year event in Seattle. Is this beginning to make sense?
When someone says there is a "record" rain storm, or "the biggest in 20 years", it doesn't really mean much without knowing other factors.
Rains in Excess of Natural and Man-made Systems Create Floods
Wind is Measured and Predicted Same as Rain
Wind predictions, similarly, are predicted based upon historical events. Airports famously have excellent wind data. Additionally, a wind design may be based upon locally collected wind data, and not on national tables provided by professional organizations such as the American Society of Civil Engineers (ASCE) and the International Building Code (IBC).
The phrase, “a fifty year event” is professional engineering jargon. To have major events occur suddenly together is nothing to alarm the people.
Why are Recent Rain and Wind Events Different than in Past Decades?
This is what should receive attention: These events are only infrequent in the places where they are happening. Rain events in the proportions being reported in news channels as 100-year events in New York are much more common in other regions and other latitudes. Events change according to geography. For example, mountains cause clouds to rise as they pass over. This condenses the moisture and creates rain and snow. This is why California is green and Nevada is desert chaparral. Clouds moving in from the ocean dump on the western side, and carry very little to the east of the Sierra Nevadas. A common rain storm in a California foothill town might be very rare in a Nevada city east of the mountain range.
Rainfall levels also change according to the latitude & longitude of a location such as a city. Today, the earth is tilting off the axis it has been on for thousands of years. The latitude and longitude of locations is changing. Because of this relocation of cities within the lat-long matrix, weather events are going to change.
What is the Engineering Community Doing?
Within the engineering community, this natural phenomenon is largely going unnoticed. However, there are some engineers who recognize that seismic, wind, and rain/flood events are becoming slightly less predictable. I think the major organizations will begin to discuss this in the next decade. However, it is unlikely that the ASCE or the IBC will address this before then. For these leading design procedure codifiers to institute new processes of design, some substantial data, theory, and remedies will need to be proposed in the engineering community.
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