Today, readers sent me links to two more websites that help you understand drainage and flood risk. The first by Texas Parks & Wildlife is called the Texas Watershed Viewer. The second is a FEMA site that estimates base flood elevations based on USGS data.

FEMA defines base flood elevation as “The elevation of surface water resulting from a flood that has a 1% chance of equaling or exceeding that level in any given year.” In other words, it’s how deep the water would be in a 100-year flood at any given spot.

Let’s take a look at each.

Texas Watershed Viewer

The Texas Watershed Viewer lets users identify local watersheds, sub-watersheds, river basins, and river sub-basins throughout the State of Texas.

To find your watershed and river basin, simply type your address into the search bar and press enter. The map will zoom into the address. From here, click anywhere on the map and the name of the sub watershed will appear. If you click the next arrow on the feature label, the name of the watershed will appear. If you click the next arrow again, the name of the river sub basin will appear followed by the larger river basin.

This lets you quickly visualize the extent of a watershed so you can see where water is coming from and going to.

After you click on map to see the feature’s name, you can view the geographic extent of the sub watershed, watershed, river sub basin, and river basin, by clicking the minus sign on the top left corner to zoom out from the address level to the boundaries of the other features. The boundaries of these features will be light blue. 

Other Texas Watershed Viewer tools

Zoom: You can zoom in on your neighborhood or zoom out to the entire state of Texas.

Layers: adds the layers window in the top right corner. You can turn the layers on and off by click on the check box.

Basemap gallery: lets you change the basemap of the viewer. The topographic map, for instance, is a useful layer because river, lakes, and streams are labeled.

Measure: lets you measure the distance from your home to a water feature.

Share: lets you show your friends what you see on social media.

Print: lets you print out a copy for your records.

Uses

This site helps viewers understand where water comes from and how it converges. As land is cleared and leveled, it also helps you understand where streams used to flow. (Note: This feature only works until background maps are updated, however.)

One reader used this feature to show how a developer had filled in natural drainage on the developer’s property and blocked off drainage from the reader’s subdivision. With three potential tropical systems moving in our direction at this moment, that information could be very useful if his home floods and he needs to call a lawyer.

Using the topographic base layer, you can also predict where and how runoff will flow during a flood. Many homes near the East Fork flooded during Imelda when Caney Creek captured the Triple PG mine and started flowing south through an area where several other creeks converge. Homeowners report being flooded from overland flow before the creek rose. The topographic feature shows the path that the water likely took.

Those who have a passion for understanding the physical world around them could spend days exploring this website.

FEMA Estimated Base Flood Elevation Viewer

Like most flood maps of this sort, you can turn layers on and off and change base maps.

For instance, by clicking buttons, you can have it show the estimated flood extent and depths for a 1%-chance flood and a .02%-chance flood. You can also view stream center lines, cross sections, and view detailed information on flood insurance rate maps.

You can even activate a split screen mode and compare different features side by side, i.e., ten and hundred year flood extents.

The point of this whole site is to understand not just the extent of floods, but their DEPTH as well.

Uses

FEMA says information from this site helps:

• Inform personal risk decisions related to the purchase of flood insurance and coverage levels.
• Inform local and individual building and construction approaches.
• Prepare local risk assessments, Hazard Mitigation Plans, Land Use Plans, etc.
• Prepare information for Letter of Map Amendment (LOMA) submittals.

Helpful Where Flood Maps Not Yet Available

The BLE (Base Level Engineering) Data in this website provides flood hazard information where flood insurance rate maps may not yet be available. We saw this, for instance, in Woodridge Village (north of Elm Grove) where flood maps stopped at the Harris/Montgomery county line. LJA Engineering claimed there were no floodplain issues on the Montgomery County side of the line. In fact, most of the Woodridge Village was in a flood plain as you can clearly see below; it just had not been mapped yet.

Compare that to FEMA’s Flood Hazard Layer Viewer below and you will immediately see the difference.

FEMA’s estimated base-flood elevation viewer helps reputable land developers identify flood risk, expected flood elevation, and estimated flood depth where Base Level Engineering has been prepared (i.e., as in the Lake Houston Area).

Reportedly, the information in this tool is not yet Atlas-14 compliant. But it’s still better than nothing.

“Buyer Aware”

The more tools you have to evaluate the purchase of insurance and property, the safer you will be.

No one tool can do everything. But together, the can make you “buyer aware.”

Posted by Bob Rehak on 8/20/2020

1087 Days since Hurricane Harvey and 335 since Imelda

The thoughts expressed in this post represent opinions on matters of public concern and safety. They are protected by the First Amendment of the US Constitution and the Anti-SLAPP Statute of the Great State of Texas.