What causes street flooding? At the risk of clarifying the obvious, rain accumulates faster than storm sewers and drainage ditches can carry it away.

A Lack-Of-Capacity Issue

Most streets are actually designed to be part of the flood retention system in any community. That’s because most storm sewers can only handle a two-year rain (about 2 inches per hour). When we get more than that – say a 10-, 25-, 50- or 100-year rain – water is stored in the street until capacity opens up in the storm sewers, ditches and creeks.

As you can see from the new Atlas-14 rainfall chart below, a 2-year rain in this area is 2.23 inches/hour; a 25-year rain 3.88 inches/hour; and a 100-year rain 4.88.

When evaluating rainfalls, look at the storm totals AND shorter intervals, such as 15, 30 and 60 minutes.

Street flooding usually results from short, high-intensity downpours caused by slow-moving or training thunderstorms.

From a street-flooding perspective, getting 4 inches of rain in one day is not the same as getting 4 inches in one hour.

If you get 2 inches of rain in 30 minutes, you’re already at a 5-year rain. That’s well beyond the design capacity of storm sewers. You can expect water to back up into the street at that point, even if there are no blockages in the storm sewers.

That’s why builders elevate most homes several feet above street level and above the 100-year flood plain. It gives you an additional margin of safety.

How To Determine Intensity of Rainfalls

If you flooded from your street, first determine whether the cause was simply overwhelming rainfall or whether complicating factors existed.

How can you determine how much rain you got in any given time interval during a storm? Follow these simple steps:

• Go to HarrisCountyFWS.org. (Harris County’s Flood Warning System)
• Click on the gage nearest you. (For me, that’s Gage #755 at the San Jacinto West Fork and West Lake Houston Parkway. I will use that in the example below.)
• In the pop-up window, click on the “For More Information” button.
• At the top of the next window, select date and time intervals. The Time Interval varies from One Hour to One Year. I selected September 19, 2019 (the day of Imelda) and 24 hours. That shows me 24 1-hour intervals. From this and the table above, you can see that we had three very intense hours in a row during Imelda.

From the two charts above, correlate the actual precipitation with the recurrence intervals. You can see that…

We had a 10-year rain, followed by a 5-year rain, followed by a 2-year rain – all in three hours!

Every single one of those hours met or exceeded the maximum capacity of the storm sewers. So it’s easy to see WHY we had street flooding.

When Street Flooding Turns into Home Flooding

In a small percentage of cases, street flooding turns into HOME flooding – when there simply isn’t enough backup capacity in the streets. (In the following discussion, I’m EXCLUDING homes that flooded from rivers, streams, or overland sheet flow during Imelda, i.e., Ben’s Branch, Elm Grove, etc.).

Extreme events reveal the weaknesses in any system. If your home was:

• At a low point on the street…
• Near a clogged storm drain…
• A foot or two lower than surrounding homes…
• At the bottom of a hill…
• In an area where water collected or converged…
• Near an outfall pipe that collapsed or was blocked…
• Upstream from a ditch that was blocked…

…you may have flooded.

And then there are the bizarre cases.

I visited one man in Trailwood at the bottom of a hill that had NO storm drains. Inexplicably, someone placed the nearest drain in the middle of the hill – about half a block ABOVE his home.

Another man called me who lived near Village Park Drive next to a tributary of Ben’s Branch. The Community Association had erected a fence between the end of the street and the tributary. They built the fence so low to the ground that it became clogged with weeds and grass clippings during Imelda and formed a dam. In the heavy rain, water could not get under it and backed up into his home.

What Can You Do?

Short of praying or digging up every street in Houston to enlarge the storm sewers, homeowners DO have some remedies.

• Keep storm drains clear. Keep yard waste out of them.
• Participate in the City’s Adopt-A-Drain program.
• Call 311 for a storm-drain inspection if you suspect yours have become clogged. The City is currently inspecting ALL drains in Kingwood subdivisions that had street flooding last year.
• Inspect outfall pipes where your storm drains enter the nearest ditch to ensure they have not collapsed or become blocked.
• Look out for new construction, such as the fence above, that may back water up. Remove or elevate the horizontal rot board if it blocks the overflow of water from your street.
• If the problem recurs in less extreme events, consider flood proofing or elevating your home.
• Make sure you have flood insurance; that it’s up to date; and that it reflects the true replacement value of your home.

Great Options Where Possible

If your area floods repeatedly, you may also be interested in lobbying the City or County to build an overflow spillway or detention pond between your street and the nearest drainage channel. Obviously, geographic circumstances may rule this possibility out for many. But if you have a vacant lot in your neighborhood and a nearby ditch…

Posted by Bob Rehak on 2/22/2020

907 Days after Hurricane Harvey and 156 after Imelda