This is pretty much my #1 pet peeve in Civil Design. This is a cultural difference between design methodologies from east coast to west coast. It is so ubiquitous in San Diego, you’d think there’s only one designer in the whole county placing these wherever they can squeeze one in. I think they came about where there was not a piped system, these would substitute in streets as the drainage system. They would direct flow, and even store some runoff. Unfortunately, it’s still bad design and not needed any more in my opinion. What am I talking about? The concrete swale, or ribbon gutter. It makes me upset just typing it! Here is the perpitrator:
Hmm. Doesn’t look so bad does it? Here’s what’s wrong with them though:
- They concentrate flow – While this isn’t bad in a piped system, concentrated flows create hazards on the surface for hydroplaning, and ponding.
- They’re annoying – When driving in a parking lot you shouldn’t notice big bumps while driving around. It temporarily distracts drivers, potential for lost traction if you go over it fast enough, splashing pedestrians during a rain event, and a potential for scraping the undercarriage of your car.
- Maintenance – It breaks down faster than just an asphalt surface. Whenever you have a gap between different surface types water will infiltrate in. When water gets in, it starts it’s corrosive process and accelerates the areas breaking down.
- Hydrology – When doing hydrology calculations, we assume an infiltration rate of the pavement to be up to 10% because asphalt is porous. If it is gently sheet flowing along, it will slowly infiltrate into the ground, which has added benefits of cleaning the water, and reducing downstream runoff. When it’s placed in a concrete ribbon gutter it increases flow speed, and does not infiltrate.
We’ve done plenty of designs that drain wonderfully without the use of these. Keep an eye out, see if they bug you as much as they do us. Like them? Have an indispensable use for them? Let us know in the comments.
