A Quick Way to Assess Flood Risk In Your Neighborhood During Storms
For decades, weather services have forecasted flood warnings, watches and alerts for general areas, such as the Houston region. But what is the risk to your particular neighborhood? Web-based, interactive tools now make it possible to forecast flood risk near you. However, they require some “do-it-yourself” interpretation. Hence, this post.
YESTERDAY morning (TUESDAY 6/19), I woke up and saw standing water in my backyard. The sky was black. I heard thunder. I remembered the forecast from Monday night about storms training across the area. My heart started racing as I fired up my laptop.
Here is what I did. (NOTE: TODAY’S RAINFALL IS DIFFERENT; THE INFORMATION BELOW IS AN EXAMPLE ONLY TO ILLUSTRATE A PROCEDURE.)
- I first went to Space City Weather to get a good feeling for the big picture and learn of any National Weather Service warnings or Harris County Flood Control District insights; it’s always a good idea to consult the professionals first. The threat appeared both east and west of Lake Houston; it seemed as though we might have threaded the needle with this storm. However, forecasters felt the storm over Beaumont at the time might move west during the day.
- So next, I wanted to see how much capacity the San Jacinto river and Lake Houston had. To figure that out, I went to USGS to find the level of Lake Houston. The spillway is at 42.5 feet shown at the top of this graph. The blue line represents the actual water level.
You can see from the widening gap between those two lines how the city lowered the lake to create extra capacity before the storm. However, you can also see how the blue line started to turn up at the far right.
- Next, I wanted to see if a huge upstream rainfall was rushing toward Lake Houston. So I went to the SJRA site to check the level of Lake Conroe. I determined that the threat from the west was minimal. Lake Conroe was also below its normal level.
The lake level had only risen a few hundredths of an inch since the day before.
- Next, I followed another link on the SJRA website to the Lake Operations and Rainfall Dashboard. It is located right below the information in #3 above. Montgomery County gages showed that not much rain had fallen to our north and west. Only one of 14 gages showed more than an inch of rain. Most showed less than a half inch. At this point, I felt that the threat was more in the future than the present.
- To see what was happening with that two inches that fell on Lake Creek, I went to the Harris County Flood Warning System. I could see from the home page that the gage at US59 had received 1.36 inches of rain and the one at West Lake Houston Parkway 1.44 inches. Not a huge threat! But rainfall doesn’t correlate perfectly with flood levels.
- I still needed to see how much the San Jacinto was below its banks. So I clicked on the gage at West Lake Houston Parkway for more information. That’s the gage nearest me. The link took me to a page that showed a breakdown of rainfall at that location. Right next to it was a tab called Stream Elevation. In the graph, I could see that the river was near 41 one feet. The banks were six feet higher! Better, there was no sharp rise in the river level. I let out a big “Whew!”
All of this took about 5 minutes. I could have waited for a weather report on TV or checked the weather app on my iPhone. However, they would have only told me what was happening in the region, but not at my exact location. Try it for yourself the next time you have a pitter patter panic.
Had the river been coming out of its banks, I could have accessed the new, near-real-time, inundation mapping system on the Harris County Flood Warning System home page. It is updated every 15 minutes. The map allows you to zoom into your neighborhood and see where flood waters are predicted to go based on the Flood Control District’s models and the river’s height.
Diane Cooper, a Kingwood resident who has 20+ years of forecasting experience with the National Weather Service, also suggested this shortcut. It lets you look at upstream and downstream rainfall over the entire region all at once. My thanks to Diane.
Posted 6/20/2018 by Bob Rehak
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