Is Your Stormwater BMP Obeying the Speed Limit?

I think anyone who’s driven a vehicle knows you aren’t supposed to exceed the posted speed limit. If you’ve ever gotten pulled over by a police officer because you exceeded the speed limit, you probably weren’t ecstatic about it, but you probably knew deep down that “you did the crime, so you got the fine” and life goes on. So what does all this have to do with stormwater? I’m glad you asked. Turns out we can apply life lessons learned on the road to properly siting and designing stormwater infiltration best management practices (BMPs). First, a few basics on infiltration BMPs.

Infiltration BMPs work (or at least they’re supposed to work) when they’re designed to take in a certain amount of stormwater that can infiltrate, or move into the soil, at a rate sufficient to dewater the BMP within whatever timeframe has been established (usually 24 to 72 hours after the precipitation event). Seems simple enough, right? To design a BMP to the correct size, you need to know how fast the water can infiltrate into the soil. To figure this out, you could make a wild guess, throw a dart at a board, ask your cousin Eddie, or perhaps measure the infiltration rate of the soil. To put some fancy science jargon to it, we say we’re going to measure the soil’s saturated hydraulic conductivity. Ok, let’s get back to talking about cars.

Although there are certainly plenty of high-end race cars that can go mind-blowing speeds, even the most simple sedan has a speedometer that tops out at 140 to 160 mph, which is likely way faster than most of us have ever driven. Nonetheless, if we really wanted to, we could drive our daily commuter at a pretty high rate of speed… but we don’t. I think most of us realize that even though our cars can go that fast, we drive them on roads that weren’t designed for them to go 140 mph. However, I can tell you that I could probably drive at least 50 mph on the roads that go through the neighborhood I live in, and most of us consider 50 mph still a pretty safe speed to drive. Yet, I have never gone 50 mph on these roads. In fact, I haven’t ever gone substantially over 25 mph on these roads. The reason I’ve capped my speed of travel at a max 25 mph on these roads has nothing to do with my car and what it can or can’t do. I limit my speed on these roads because there’s a sign posted on the road that says, “Speed Limit 25”. The sign itself is really of no consequence, but as a (somewhat) functioning adult, I’m aware that as part of the license issued to me to drive, I’ve agreed to follow things like speed limit signs that tell me what the max rate of speed is for a particular road to keep me, other drivers, my neighbors, etc. safe. If I don’t follow these signs then I know I might cause an unsafe situation. To keep this from happening, sometimes we get friendly reminders from police officers when we exceed the posted speed limits. Sometimes those friendly reminders come with not-so-friendly monetary fines.

Now let’s substitute infiltrating stormwater for your car, and the soil pore network for roads, and restrictive soil horizons for speed limits. The stormwater, just like your car, can move pretty fast in theory, except that it has to travel through the soil pore network that provides some limitations that cause the water to move at a more modest rate. Then sometimes, there are things in the soil profile that cause the water to move at an even slower rate, such as restrictive horizons. The one main difference is that, although I could choose to go 50 mph through that posted 25 mph residential road, water cannot go any faster than the most rate-limiting soil horizon it encounters. The fine for trying to put too much water into a slow soil profile is a malfunctioning BMP.

You may be saying to yourself, “Ok, I sorta get your weird car-stormwater metaphor, but what’s the point?”. My point is that even though we all know we aren’t supposed to speed, police issue tickets everyday, so either we’re choosing to ignore the sign or perhaps we didn’t see the sign. The same thing happens when we design infiltration BMPs. We all know that the design is based on the infiltration rate, but what we miss sometimes is that the sign says 25 mph and we’re still designing for 50 mph. So how are we missing the change?

Most guidance included with regulatory stormwater manuals on measuring stormwater infiltration tells the practitioner to measure the saturated hydraulic conductivity at the proposed infiltration surface of the BMP. That makes sense, you would want to know how quickly a soil can move water at the elevation where the water will ultimately be placed. Let’s stick with our metaphor and say that our measured rate tells us we’re in the 50 mph zone. At this point, we’ve likely satisfied any and all regulatory requirements around infiltration testing. However, what happens if six inches below this elevation there is another soil horizon that is in the 25 mph zone? We’ve met the regulatory obligations and checked all the boxes, but at the end of the day, if we try to drive our stormwater through this 25 mph zone going 50 mph, we’re going to get pulled over and fined with a malfunctioning BMP. When you’re conducting infiltration testing in a 50 mph zone sitting over a 25 mph, the scale of the test apparatus may not equate into a measured rate in line with the actual rate you will eventually see when a full scale BMP is on the landscape filled with water.

All is not lost friends. There is a simple way to avoid these stormwater calamities. When doing soil suitability and infiltration testing, you need to look at the entire soil profile and gain an understanding of how water moves through the entire profile, not just through a small portion of the soil profile at a specific depth. Sometimes you’re going to need to test additional horizons below the proposed infiltration surface, and if warranted, design the BMP based on those more restrictive rates. The point is, don’t put on the blinders and only do what is technically required to get a “number”, because you might find yourself convinced that you should drive 50 mph through a 25 mph speed trap. Unfortunately, it’s a lot harder (and costly) to fix a failed BMP then to pay a speeding ticket.