A major river in the mid-Atlantic is expected to crest on Saturday at 65 ft. But, what does that mean? Is it 65 ft above its banks? If so, how could you measure this when the water would be overrunning its banks…not piling up like snow!
It means 65 ft above the local datum of the gauging station. The exact height of this is arbitrary, but typically about the bottom of the stream/river at the gaging station (which means that reported levels will essentially always be positive).
You use a gaging station. This is typically located clear of the highest possible water, so it continues to report during flooding (when data is most needed).
I’ve typically heard something like “feet above flood stage”, not just feet. Still somewhat arbitrary.
USGS at Science in Your Watershed - General Introduction and Hydrologic Definitions has a definition of flood stage. I did not see crest or cresting but I didn’t spend too much time at the site.
The 0 level is sometimes sea level, but as Xema pointed out, it is usally some arbitrary height.
I wonder if defining flood stage as zero was ever considered. Then you wouldn’t hear “the river is at 14 feet, which is two feet above flood stage”
But then people would have to deal with negative number
Brian
Good article on flood forecasting from American Scientist http://www.americanscientist.org/issues/feature/real-time-flood-forecasting/1
The river in my neck in the woods, the Susquehanna just hit a new record high of 26.71 feet beating the previous high set in 2006 of 25 feet. 24 hours before it was at 7 feet. The record low was back in 1908 at 1.8 feet.
Nowadays you can look up what the ‘0’ reading corresponds to according to some datum, such as the National Geodetic Vertical Datum, Mean Sea Level, or what have you. Back in the day it wasn’t as easy to establish what the relationship was.
The local USGS gaging stations on the White River are mounted on the side of bridges. What shows is a solar panel and an antenna. By watching the collection of readings along the river, you can see where the biggest glut of water is. Upstream of it, the level falls, and downstream, it’s still rising.
In Anderson, at flood stage, Edgewater Park and Grand Avenue start to be underwater. That’s merely inconvenient. Higher than that, it gets more troublesome.