Introduction
The entire design of a storm water system is summed up in a riddle: How does rain pass through without damaging anything?
I’m sure you’ve interacted with storm water systems before. If not, you are missing out on a great opportunity. They are quite a fun mental test. Unlike other elements who drive most of the initial design (see horizontal and vertical alignments), the storm water system interacts with many neighboring elements and is mostly a consequence of all previous design decisions. This, of course, creates quite the migraine if the road lacks any forward thought of handling storm water. In those scenarios, the designer will find themselves kicking their own seat or, more likely, their coworker’s.
While there are many steps in the design of storm water systems, the one step I’d like to share here is placing storm water inlets within the roadway. It’s the introduction most engineers receive when starting out, though, it is not the first step of storm water design. Below are my best practices for this step.
Best Practices
- Place all necessary inlets first
- Inlets in low spots within the road are non-negotiable. Place them and their flanking inlets first. Always. Use your state’s drainage manuals to determine the spacing of your flanking inlets.
- Any superelevation will have a point with 0% cross slope. Those need inlets right at or before here.
- Inlets directly upstream of pedestrian crosswalks are always needed. Remember to not place them within the curb ramp – you can’t have a curb inlet where the curb is transitioning down to a lowered position.
- Avoid overdesign (too many inlets!)
- Too many times have I seen systems with unnecessary additional inlets. Most of these unnecessary inlets are tossed in because the designer is nervous about the downstream inlet’s spread becoming too close to the maximum allowable spread. Here’s my take: if the calculated spread is lower than the maximum allowable spread, it’s fine. Stop worrying about it. There may be legitimate reasons, but this is the most common. There are no bonus points for your spread being 1 foot less than the maximum versus 0.3 feet less.
- Be aware of your geometry
- After setting your inlets so far, look and see how they may connect to each other at intersections. I’ve seen many designs whose inlet connections are too skewed (I won’t go too into this here but you can get into Right-of-Way trouble). If you can, consider adding a manhole that helps tee-up the skew to be more like a 90 degree angle. Of course, place the manhole outside the wheel path of the vehicles.
- Read the manuals!
- Storm water design, hydraulics, and hydrology have the best published manuals of any design element I’ve ever read. They are full of incredible theories, best practices, and strategies. I’ve received the best instruction when I had a flesh and blood mentor next to me. These are the next best alternative.
Great Reading for Storm Water Design
FHWA’s HEC 22: Urban Drainage Design
Anyways
Storm water design is a rewarding exercise. I hope some of these best practices I follow help you too.
