Storm Sewer Inlets: Often Overlooked in Cases of Street Flooding

We step on them. Spit on them. Park on them. Ride on them. But we rarely think about them. If there’s anything we truly take for granted, it’s the lowly storm sewer inlet.

Critical Rate-Limiting Factor in Drainage

I’ve always known they were there. But I never realized how important they were until I posted about street flooding last week, omitted them, and an expert called them to my attention. Unless properly sized or designed, they can limit how quickly streets drain in heavy rains.

Think of storm sewer inlets like fire exits. Better have enough capacity in the right locations when you need it!

How tall is your inlet?
How wide is it?
Does it have a grate or a plate in it?
Or is it just a hole in the concrete between the street and the curb?
What is the slope of the street towards it?
Can it capture all the water that runs toward it?
Or does some get by?
What’s the spacing between inlets?

Have you ever really thought about these things? Who on earth does?!!!

Visual Inventory of Drain Types

Fortunately, engineers DO. And standards constantly evolve as they come up with better designs. Here are a few variations I spotted as I drove around.

*Rectangular grate design in North Woodland Hills.*

*…but 7.5 inches tall. That’s 50% more surge capacity than a 5-inch high inlet.*

Inlet Trivia

In researching this topic, I discovered many fascinating pieces of trivia.

Thirty years ago, Sugar Land decided to enlarge all its storm sewer inlets. Street flooding is very rare there, almost unheard of.

Storm sewer inlet size and design varies by the type of street.

Thoroughfares usually get bigger inlets than feeders.
Feeders usually get bigger inlets than residential streets.

Hills and busy intersections often get special attention.

*Double-wide inlet at the bottom of Kingsway Court by Kingwood High School. Below hill* *with approximate 10% grade at end of cul de sac. Better not let the water get by this one!.*

Many municipalities (including Houston) frown on grates. While they theoretically offer higher capture rates, they also clog easily and require constant maintenance, increasing costs.

The slope of the street (from the highest point of the crown to the lowest point of the gutter) can radically affect the capture rate of water flowing down the gutter. The slope “forces” the water into the inlet.

Inlet spacing is a function of gutter slope! (See page 137 of COH Infrastructure Design Manual).

The geometry of grates can also radically affect capture efficiency (measured as percent of water intercepted). Below is a video by the U.S. Department of Transportation that shows the efficiency of different geometries for grate designs in a table-top experiment. (Attention science teachers: this is a “must see” video that will fascinate students and could even intrigue some enough to pursue careers in engineering.)

Produced by US Department of Transportation

City of Houston and Other Resources

For those who want to learn even more, here is a link to the current City of Houston infrastructure design standards for stormwater runoff. Page 139 shows you how much the capacity of inlet types can vary. Some inlets can handle four times more cubic feet per second than others.

For COH construction standards, click here.

For budding engineers, here’s a primer on the design of inlets and storm drains.

Buyer Awareness

When looking for a new home, storm sewer inlets may be the last thing you think about. But just like plumbing, maybe they should be one of the first.

Posted by Bob Rehak on 3/3/2020

917 Days after Hurricane Harvey and 166 after Imelda