Ever wonder how to find the depth of flooding on a bayou, river or stream near you? Here’s a simple way. But this only works for those in and around Harris County, Texas, and those who live on streams with gages.

Step-By-Step Guide

Go to HarrisCountyFWS.org.

FWS stands for Flood Warning System. The main function of this website is to alert you when streams are in danger of coming out of their banks. But the site also shows historical information for dozens of gages that blanket Harris and surrounding counties. That information includes flood peaks and bank elevations. By subtracting the bank elevation from the peak, you can easily determine the height of a flood and compare the height in your area to other areas.

For this exercise, start by selecting ALL gages in the left column. Gages across the region will pop up. See below.

By default, the map shows how much rainfall all those gages received in the last 24 hours. But there’s much more information behind them. Next…

Click on Any Gage

Another box pops up that is the gateway to historical information about that location. In this example, I clicked on the gage at the San Jacinto West Fork and US59.

Click on the hyperlink that says, “More information and alert signup.” Then, in the next screen…

Click on the Stream Elevation Tab

Scroll Down

You should see a box that looks like the one below.

The red “Flooding Likely” line represents the top of bank in that area. They say “Likely” because bank heights may vary slightly around a gage. But for most locations in a flat area like ours, that’s the point at which water starts to come out of the banks. Near this gage, the river starts coming out at 49.33 feet above sea level.

Below that box, three more boxes show:

• Gage readings during the current time period (or any historical time period if you specify one).
• Flood frequency for that gage location. For instance, the height of a 10-, 50-, 100- or 500-year flood.
• Historical records for major storms.

This last box contains the information you want (if you’re looking for the Harvey peak). For instance, you can see that at this location, the West Fork reached 69.6 feet.

Subtract Flooding-Likely Elevation from the Flood Height

Subtracting the flooding-likely elevation from the high-water mark tells you the depth of flooding, i.e., how high the water got above the banks. At this location, that was more than 20 feet! (69.6 minus 49.33)

Step and Repeat

To compare the depth of flooding at other locations, repeat the same process. To visualize the differences, it helps to develop a spreadsheet with four columns: Location, Flooding Likely, Peak, Difference. Then you can then easily create a graph that looks like the one below.

In this case, you can see that the San Jacinto, Spring and Cypress Creeks had the deepest floodwaters in the northern part of the county during Harvey. Some gages at other locations show that water didn’t even come out of banks.

What About Minor Floods?

The Harris County Flood Warning System contains information about stream levels going back 20+ years. If you’re looking for information about a flood not shown in the Historical Record box, you can search for it by specifying a time period and date range above the stream elevation and rainfall tabs.

Caution

Usually, Harris County Flood Control District personnel manually verify the historical records. So, you can trust the information. But if you’re researching smaller floods by inputting your own dates and time periods in the search fields, you may run into a problem.

Before 2010, sometimes gages recorded faulty readings. Gages during that period used pressure transducers, which could clog with floating debris and report false information.

So, if you see a hundred-foot flood that lasted 15 minutes, you’re looking at error. Cross check such readings against rainfall at the same gage. Also, check the readings immediately up- and downstream to rule out spurious readings.

The graph above shows wide ranges in the depth of flooding on the same bayous. Don’t assume that because a flood was 20 feet deep at one location that it will be the same along the entire stream. The topography could narrow, widen, deepen or flatten. All could affect the depth of flooding. So could other factors, such as the amount of surrounding development or previous flood mitigation efforts in an area.

Identifying Causes of Flooding

Using information from the Flood Warning System, you can help narrow down the source of flooding. If a neighborhood flooded, but the channel didn’t come out of its banks, you’re looking at a street-flooding issue. Most storm sewers and roadside ditches in Harris County and Houston are sized to handle a two-year rain. But older ones may have only a one-year level of service. And many become clogged over time. See below.

For More Information

To learn how to find and verify other flood-related information, make sure you check out this post.

Posted by Bob Rehak on 10/29/22 with thanks to the Harris County Flood Control District

1887 Days since Hurricane Harvey