The words "Act of God" get bandied around in most cases involving floods and landslides. By their nature, floods and landslides often occur, in part, because of a large storm event or a series of storms. When this defense is raised, we remind judges or juries that floods and landslides don't happen in dry conditions in the middle of summer.
Because we live in the wet Northwest, a certain level of storm is foreseeable here. First, we will turn to the science behind this term and then tackle the cases in Washington that address it.
In defining the size of a storm event, hydrologists use regional maps called "isopluvials," which were promulgated by the National Oceanographic and Atmospheric Administration ("NOAA").1 The regional map for Seattle is the map for Western Washington, since Eastern Washington has markedly different weather and rain patterns. These maps provide the statistical reoccurrence of various storms given the inches of rain that fall in a 24-hour period.
A small storm would be a "two-year" event. This designation means that statistically scientists expect to see the amount of rain that fell every two years. The largest event mapped by NOAA is the 24-hour "100-year event." In the Seattle area, about four inches of rain must fall in a 24-hour period to statistically be a 100-year storm.
Because it is a statistic of return, when you have a 100-year storm, it does not mean that you are now safe for 99 years. We seem to have 100-year storms every five years or so in the wet Northwest. The incidence of recurrent large storms will increase over time if the predictions of global warming are correct.
It is important to note that the normal "envelope" used by NOAA for storm intervals is 24 hours. Under more modern surface water regulations, other envelopes are used so that current drainage systems are being designed to withstand larger storm events. For instance, today, retention detention facilities may be sized to hold the runoff from a 100-year "seven-day" envelope.2 Attorneys need to pay attention to the size of the envelope used, especially by trial experts, because varying the size of the envelope can hurt or help your case.
In the Holiday Storm of 1996–1997, several storms occurred in the course of a week. There were two 25-year storms in a 24-hour period. We then had approximately four feet of snow fall on the ground, which equates to four inches of rain.3
...login to read the rest of this article.