Just In Time for Tropical Wave: Harris County Flood Control Announces Near-Real-Time Inundation Mapping

Approaching Storm

Harris County Flood Control District (HCFCD) has introduced a new, near-real-time, inundation mapping tool – just in time for a tropical disturbance predicted to affect the Houston area from this Saturday through Monday. The NOAA map below (created 6/13/18) shows expected rainfall totals associated with the disturbance which is currently centered just south of the Yucatan. Depending on how far north you live in the Houston area, the storm could dump from 3 to 7 inches of rain on you.

Jeff Lindner, Director of Hydrologic Operations for HCFCD cautions that rainfall totals will be adjusted in coming days and that locally heavy rainfall within the storm could create flash flooding with rains of 4 inches in one hour.  All of which leads me to the second part of this post…

New Inundation Mapping Tool

The Harris County Flood Control District has added inundation mapping to the Harris County Flood Warning System, providing a new tool to help you make safety decisions during flooding events.  Information is posted on the Harris County Flood Warning System website (www.harriscountyfws.org).

The inundation mapping system even contains a historical feature. If you like, you can go back to floods in 2015, 2016, and 2017. The image below is from Harvey and gives you an idea of what the mapping feature would potentially show during an actual event.

15-Minute Updates

Lindner says, “Moving forward, this will be available every 15 minutes based off of the current gage data along our creeks and bayous.”

The new inundation mapping tool generates a map of flooded areas adjacent to Harris County bayous and creeks where the Flood Warning System has water level gages. The tool runs continuously to provide live inundation mapping during a flood, though the results may be delayed as much as 15 minutes. That’s due to the time required to generate maps from gage data.

Please share this with your neighbors and friends so they can use this tool during heavy rains and flooding.

How Inundation Mapping Works

Inundation mapping provides information about areas of current bayou and channel flooding, based on information collected by Flood Warning System gage stations strategically placed along Harris County bayous and their tributaries.

During heavy rains, to monitor inundation levels near your home, school, work place, relatives, or commuting routes, just type in an address at or near your area of interest.

Inundation mapping can zoom from county-wide down to 500 feet. The map depicts the current extent of flooding, but does not include water depth. To experiment with it:

• Go to harriscountyfws.org on your desktop or mobile device.
• The “MAP VIEW OPTIONS” section at the top left allows you to add inundation mapping and other features to the county map. (When there has been no rainfall, the inundation mapping feature will not be available.)
• Go to the “ADDRESS SEARCH” box in the lower left column and type your address.
• The map will zoom to the gage station nearest to your location of interest. The icon shows the amount of rainfall (in inches) received at that location in the past 24 hours.
• Click on the gage station icon for more detailed information.
• Other “MAP VIEW OPTIONS” allow you to add watershed boundaries, drainage channels and channel status icons to the county map. You can also change the setting in the “RAINFALL DATA” section to see rainfall during various time periods.

An instructional video that shows how to use the entire Flood Warning System, including the new inundation mapping tool, is available on our YouTube page at:https://youtu.be/eb4AFIspQSQ

Experiment

A personal review.

I typed in my address as instructed above and zoomed into my neighborhood as closely as I could. Then I used the historical feature to type in different dates and times during “Harvey Week” starting with 8/25/17 and going to 9/1/17.

The results were not perfectly accurate down to the household level, but they certainly were to the block level.  With this tool, a weather forecast and upstream gage data, people could have seen the flood coming at them and made better decisions that would have protected their property and safety.

Limitations of Inundation Mapping System

This is NOT a notification system for flood watches, warnings or evacuations. It’s a tool to help make critical decisions that may ultimately reduce risk.

The mapping tool will only generate a new map if the gage data for water levels changes by one-half foot. Also, damage to gages during a flood may affect the accuracy of the flooding estimate and inundation mapping in the vicinity of the damaged gages.

The new mapping tool does not show all flooding that may occur.  This map will not depict flooding from:

• Channels and tributaries without gages
• Ponding, which may occur during or after intense rainfall when water gathers in low-lying areas, such as in streets when the capacity of the storm sewer is exceeded.
• Sheet flow, which occurs when intense local rainfall flows overland to reach a channel. Frequently, this exists when water “ponds” in streets deep enough to flood residences that are not even near a creek of bayou. The water will seek a path to the channel by flowing overland, flooding residences and other structures that are in that path.

Posted 6/13/18 by Bob Rehak

288 Days since Hurricane Harvey