Belated First Flush Warning   Leave a comment

This past weekend I was driving at the very beginning of a rain event, and it inspired me to write this post…only I didn’t get a chance to write it for a couple of days. We are all told that we need to be a little bit more careful when driving in the rain, but sometimes I think that the message is too general and doesn’t highlight the dangers of a first flush rain event. While I was driving this weekend during the first rain in a few weeks, I felt my tires skid twice, and I was very glad that I was paying extra attention to my driving.

A first flush rain event is the first rain event after an extended dry period. In stormwater sampling, 3 days is often considered the minimum period of time between rain events to qualify a rain event as a first flush event. In dry conditions stuff builds up on the road surface. Stuff like oils. Here in Sacramento where we have a very dry season and a rather wet season, we’ll have months with no rain and during that time a significant amount of oil can build up on the road surfaces. You don’t have to be able to see the buildup for it to be significant. At the very beginning of the first rain event, those oils slick on top of the fallen water. Ever noticed a slight sheen on the water running off of a street or parking lot? That’s the oil buildup. The layers of oil and water don’t have to be very thick to reduce the friction between your car tires and the road, and without enough friction the tire spins freely and you skid.

The most dangerous time to be on the road during a rainstorm is the very beginning. Starting when the street barely looks wet is when roadways are the most slippery. The danger is compounded by the fact that most of us don’t drive more carefully at that time. I think we don’t get the “drive careful” signal in our brains until the windshield wipers are on, or at least I find that to be the case often for myself. Not that it’s really safe to drive in pouring rain, either.

So be careful out there, especially if you’re on the road during the very beginning of a rain event.

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Posted October 19, 2010 by mayakey in environment, musings, travel

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