Saturday, December 17, 2016

What is Lagtime, and Why Should You Care? Checking out Dry Creek

Dry Creek at 9AM this morning, just a few cubic feet per second
I wonder how many Dry Creeks there are in the country? I know of two of them within a thirty mile radius of my home, including this one just two miles to the north. The name itself imposes a sort of insignificance to the waterway. The Dry Creek of my town could not be mistaken for an important river. Instead of having headwaters in the high country of glaciers and granite like the nearby Tuolumne and Merced Rivers, it begins in the Sierra Nevada foothills in dry gullies west of Highway 49. It has no natural source of flow other than rainfall. It does run for much of the year due to of irrigation overflow from agricultural fields upstream. But it could never be mistaken for a major waterway.

Except during intense storms...

Dry Creek is one of the few waterways in the Sierra Nevada with no reservoirs or flood control measures. When big storms roll through, the water can rise quickly and overflow its banks. We had such a storm last night. Rain throughout California from Eureka to San Diego. I saw that some coastal areas received 7" or more. Less rain fell in the arid Central Valley, with about 0.70" in Modesto and 1.21" in the rain gauge in my backyard. But several inches fell upstream in the Dry Creek drainage, and I expected to see some high water today. I headed out this morning for a look and...nothing. I was briefly surprised, but then I remembered that I teach about this stuff. It's a perfect example of flood lagtime. Should you care? Yes, if you live anywhere near a river.
Dry Creek at 2,000 cubic feet per second this afternoon.
Rain doesn't fall into rivers, at least not much of it. If falls all over the landscape. In gently sloping landscapes like the Sierra Nevada foothills, it takes time for water to gather into rills and small channels so there is a delay in the rise of the river downstream. That's what lagtime is, the difference between the height of the storm and the height of the runoff measured at some point downstream. I checked on the NOAA website for the latest predictions and could see that the river was slated to begin rapidly rising in the late afternoon, peaking about 8PM at 4,000 cubic feet per second. Sure enough, when I left Modesto about 2:00PM, the river hadn't changed, but by the time I crossed it again 10 miles to the east, the channel was full and beginning to spill over into some of the surrounding fields. It was running at about 2,000 cubic feet per second.
Dry Creek at 4,000 cubic feet per second, as expected after nightfall tonight (March, 2011)
So, how can hydrographers predict floods with such accuracy? They have been monitoring all the major streams in the country for upwards of a century, so there is a huge database to draw from. They watch the pattern of the storm as it progresses, and compare it to those of the past. They can then forecast the onset of flooding, the height of the water, and the cessation of the flood for areas downstream. The greater the lagtime, the more time people downstream have to prepare for the deluge.

Accurate scientific data is fundamental to the health of our society. There is not a single place in our country (and indeed the world) that is free of geological hazards. Hydrologists analyze the storm data and report to government officials who then take action to protect the populace from flooding. Seismologists analyze fault activity in hopes of minimizing the damage from major earthquakes. Volcanologists monitor the dangers of active volcanoes. Climate scientists track the effects of global warming.

In a healthy society, the politicians and government officials accept the findings of those who know the dangers best, and act accordingly. I hope that the new administration will come to understand this. If they were to deny the existence of earthquakes or volcanoes (however strange that might seem; Bobby Jindal once famously complained about "volcano monitoring"), disaster would happen. And likewise, if those in government deny global warming, then it would be like a politician predicting that the rivers won't rise, despite the massive storm upstream.

No comments: