atlas 14 rainfall probabilities

How to Find and Verify Flood-Related Information

This will be the first of a two-part series on how to find and verify flood-related information. Today’s post will focus on flooding itself. Tomorrow’s will focus on how some developers can affect flooding. Together, they should help you protect yourself whether you are a homebuyer, homeowner, or community activist.

Triangulating on Truth

The world is awash in misinformation. Most of it results from people simply repeating things they’ve heard but haven’t verified. And some of it is intentional, i.e., for political or financial gain.

Getting an accurate picture of the world around you often involves investigation and “triangulating on truth” by looking at multiple perspectives. 

Below are sources of information you may find useful. They represent my go-to sources. I often supplement them with interviews, but I usually start with these.

Local Sources 

The Harris County Flood Control District’s website,, is a wealth of information about what’s happening where. It is organized by watershed. Click one to see an overview of issues there, flood mitigation projects that address them, maps, risks, costs, pending grants, and more. Note: Flood Control manages hundreds of projects. Sometimes the projects move faster than website updates. So it’s always good to verify the status of projects by laying eyes on them. 

The Harris County Flood Warning System,, gives you real-time information during floods. It also gives you historical information about rainfall, gage heights, and flooding at locations throughout the region. You can use this site to explore when, if, or how often a channel came out of its banks and by how much.

Note: Before 2010 you may find suspicious data because of the type of gages in use during that period. Pressure transducers frequently clogged with floating debris and reported false information. So, if you see a hundred-foot flood that lasted 15 minutes, you’re likely looking at error. Cross check the reading against rainfall at the same gage. Also check the readings immediately up and downstream.

Using this information, you can help narrow down the source of flooding. If a neighborhood flooded, but the channel didn’t come out of its banks, chances are that 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.

Drive Around and Talk to Residents

To confirm whether street flooding is your issue, drive around and look at the ditches and storm drains. Even if you clean out your ditch but a neighbor doesn’t, water could be trapped in your neighborhood. The photo below shows ditches in three areas that report frequent flooding: Kashmere Gardens, Trinity Gardens, and East Aldine. They are symbolic of a problem that exists in many other areas.. 

Ditches blocked by silt in Kashmere Gardens, Trinity Gardens, and East Aldine

Report blocked drains and ditches to authorities. Who you report them to will depend on where you live. Inside the City of Houston or another municipality, report them to the city. If you live outside a city but inside Harris County, report them to your precinct commissioner. The Harris County Flood Control District is not involved in roadside ditches. Flood Control only works on channels, bayous and rivers.

Ditch blocked by garbage in Kashmere Gardens, a neighborhood that experiences frequent flooding.

It’s often hard to see Flood Control projects from the ground. Construction happens behind tall fences and trees in remote areas. However, you can spot projects easily from the air with drones or helicopters.

Lauder Detention Basin on Greens Bayou as of 10/12/2021
Phase 1 of the new Lauder Detention Basin on Greens Bayou is virtually invisible from streets. Yet many Greens Bayou residents are convinced nothing is being done to protect them. 

The Harris County Flood Education Mapping Tool at was developed after Tropical Storm Allison in 2001. It shows channels and watershed boundaries. Zoom in to find your neighborhood. That activates the ponding button. You can then see areas likely to flood from ponding or, alternatively, floodplains. Note: Most of the current flood maps for Harris County are based on Allison but are now being updated. 

Coastal Flood Exposure Mapper 

This particular tool is geared toward coastal areas but also covers all of Harris County. It includes FEMA’s flood hazard layers (see below), plus dozens of other visualization tools, all in one website. It shows development density, wetlands, emergency infrastructure, and much, much more. The site also lets you vary opacity of different layers and save maps.

City of Houston Water Flood Hazards 

City of Houston maintains a GIS site that shows the extent of flood hazards for many smaller streams and ditches in neighborhoods that are not covered in county or national maps.

Texas Sources of Flood-Related Information ­

A one-stop shop for flood preparedness anywhere in Texas. brings together local information from all over the state. Check everything from stream gages to the status of evacuation routes. It even lets you see the spread of floodwaters and the structures that will be inundated when a gage reaches a certain height. Hosted by the Texas Water Development Board.

Texas Flood Viewer 

Texas Flood Viewer shows gages throughout Texas. Click on a dot and you can see current water level relative to various flood stages.

Texas Watershed Viewer 

Not exactly a flood map, Texas Watershed Viewer is useful in figuring out where water comes from and how it converges. This also lets you see how streams may have been altered. For instance, a part of North Kingwood Forest that used to drain into Mills Branch now drains into Taylor Gully where hundreds of homes flooded in 2019.

Flood Decision Support Toolbox

A Texas Water Development Board/USGS site. Click on a river gage, select a flood depth, and see how far the waters would spread. Clicking on a location within the flooded area will also show you the estimated depth at that point. You can also turn on a layer that shows flooded buildings. Unfortunately, however, the number of gages is limited, and most are in northern Harris and southern Montgomery Counties.

National Sources of Flood-Related Information

FEMA Estimated Base Flood Elevation Viewer 

This site shows not only the extent but also the estimated depth of floods. It maps many areas not included in the National Flood Hazard Layer Viewer. See below.

FEMA National Flood Hazard Layer Viewer 

FEMA’s National Flood Hazard Layer Viewer lets you zoom into any part of the country. Wait a few seconds. And outlines for the floodway, 100-year flood plain and 500-year floodplains will appear. This website has amazing investigative potential. With it, you can tell how far your home or business is from flood threats.

FEMA Flood Map Service Center 

FEMA’s Flood Map Service Center lets you plug in any address and instantly see where your property stands in relation to floodplains that may exist around it.

USGS National Map Viewer 

The USGS National Map Viewer lets you find elevations and slopes everywhere in the US and works down to the individual property level.  Find the elevation of your slab, the slope of your street, your elevation above street level, and more. Best of its kind. Here’s a post that explains more about how to use it. Realtors and people who want to buy homes with minimal flood risk will find this useful.

US Fish and Wildlife Service Wetlands Mapper 

Want to know if a house was built on wetlands? Check the US Fish and Wildlife Service Wetlands Mapper. Some developers fill in wetlands to build more homes. But those homes are subject to foundation shifting and driveway cracking. Also, water will often collect in former wetlands after a storm. See below. Also note how the homes are built within feet of a major ditch. These factors should send signals to homebuyers, who may want to request discounts to compensate for increased risk.

High-density neighborhood built over wetlands near East Little York
NOAA’s Precipitation Data Frequency Server ­

Enter your address or zip code in NOAA’s Precipitation Data Frequency Server and see the Atlas-14 precipitation frequency estimates for your neighborhood, in graphical or tabular formats. This will tell you what constitutes a 1-, 2-, 5-, 10-, 25-, 50-, 100-, 200-, 500-, or 1000-year rain in your area. The rates vary with distance from the coast.

Precipitation Frequency Estimates for 77339. 17.3 inches in 24 hours constitutes a 100-year rain. You can see how storm sewers designed to hold a 1-year rain could easily become overwhelmed in a climate like Houston’s.

If your neighborhood floods on less than the 1- to 2-year amounts shown above, you may need to investigate the cause.

USGS High-Water Marks

High water marks validate the extent of flooding. After Harvey, the US Geological Survey measured them at 74 selected points throughout SE Texas and published them in this study. Check out this event viewer to learn about other floods and how they affected various neighborhoods or this post to learn how high-water marks fill gaps in flood-related information.

Imelda left a 13-foot high bathtub ring around around East End Park in Kingwood. With this knowledge and the elevation of a home’s slab from a survey or the USGS National Map, you could determine whether a particular home flooded during Imelda.

Other Valuable Sources for Flood-Related Information

Google Earth Pro

Using the History Function in Google Earth Pro (a Free App Download) lets you scroll back through aerial and satellite images of an area at different points in time. With it, you can see how neighborhoods filled in floodplains, rivers migrated, deltas formed, and more. The app also contains powerful measurement tools to calculate area and distance. Using this gives you a greater appreciation for the difficulty of building flood mitigation projects where people build too close to bayous, ditches, and rivers. Whole neighborhoods must be bought out before construction can begin. Check out, for instance, the detention ponds on both sides of US59 at Halls Bayou.  

FOIA and TPIA Requests 

All people are entitled to request government information under the Freedom of Information Act or Texas Public Information Act. Both are valuable tools for getting at information that may not be published. Make sure you put FOIA or TPIA in the subject line of your email and state your request as succinctly as possible. Certain records are exempt (such as personnel files and correspondence with lawyers). But bids, plans, reports, and spending information are all fair requests. Generally, a government agency has 10 business days to respond. Different agencies have different procedures and starting points. So you may want to call or google before submitting your request.

Caveat Emptor

Buyer Beware. The ancient slogan applies to flood-related information as well as homes. The more you know, the safer you are.

To see the second part of this series which focuses on developers’ plans, click here.

Posted by Bob Rehak on 11/28/21

1552 Days since Hurricane Harvey