Keeping drain pipes.... draining. 3

[TomDOT]

I see a fair number of bridges, and often see badly plugged drains. I've seen a 2" DBH tree growing out of what is evidently a long-clogged drain. I'm aware of one bridge with eleven (yes, 11) 90-degree elbows in a single run of drain pipe. Admittedly it was PVC - but when the whole drain system got a full cleanout, it was clogged again in a matter of weeks.

So, what design limits do you think are reasonable to put on drain pipes? Limit turns to no more than 45 degrees? Limit total bends to no more than 180 degrees? I'm partial to deck scuppers with a single 45 degree bend to direct water away from the bridge, and freefall a few feet after the bend (just enough to prevent splashback on the structure) - these rarely clog, absent a very large chunk of debris stuck in the mouth. Unfortunately the environmental folks tell me that's no longer allowed for new build.

 

[cvg]

the drainage design rules should be similar to any other drain design such as the sewer piping in your house. unfortunately bridge designers generally do not follow those rules. so minimum slope (2% or better), minimize bends, horizontal 90 degree bends should be avoided, 45 or 22 1/2 degree are much better, keep the debris out of the inlets and provide cleanouts at every angle point. perhaps a good solution would be to install a debris / sediment / oil water separator / trap at the inlet so that only clear water is discharged.

 

[bridgebuster]

About 20 years ago NYSDOT switched from 8" diameter pipe to 12" diameter. I'm not saying it's the silver bullet, but it has helped. I'm not a fan of 90 degree bends either but so rimes they can't be avoided.

 

[3DDave]

Use non-galvanized thin wall steel pipes and let the salt crews do the rest.

Here's the sort of problem you face:

http://www.engineeringtoolbox.com/slurry-transport-velocity-d_236.html

The problem for a bridge is that there is no guarantee that water is included with the sediment. Typical sanitary drains expect a certain amount of bath, toilet, and clothes washing machine water to help move things that are generally neutrally buoyant along. Much of the material on road surfaces is not neutrally buoyant and there isn't always much water. Some sand, dust and a little bit of moisture can form a pretty good plug before there is enough rain to get the speed up.

If it was dependable, float operated valves would only open the drain when enough water was present to flush the system, but I can imagine that not being reliable in freezing weather. Similarly an over-balance bucket that can fill slowly, and tips and dumps with plenty of velocity. Plus the bucket is a mosquito breeding ground.

How about perforated pipe? Don't mention the holes or use unprotected pipe and just wait for the salt trucks.

 

[pikku14]

Coming at it from a different angle: the other option if feasible/acceptable, is to restrict the flow width within the shoulder and collect using off-structure drainage