Pipe versus Open Channel

mfalati · Mar 1, 2004

I need help trying to explain to lay people that a much larger open channel ditch that may hold water, mosiquito breeding ground, have weeds, bank erosion (i.e. maintenance headache) that outfalls into a canal, that has a water surface elevation sometimes 3 feet higher than the invert of the ditch, is compared to a smaller smooth closed pipe that has a consistent slope.

The idea of the greater difference in surface area between the two confuses people to think that the ditch can handle more flow.

Bare in mind, I am working for a developer of a property adjacent to an existing subdivision that drains into the ditches that we will be inclosing. (Don't develop in my backyard syndrome)

Another thing to bare in mind is dentention is not an option. I live in New Orleans. We are below Sea Level. We pump water out not hold it in.

Any ideas on visual presentations that can convince the existing residents that a closed system will be better for them.

Thanks...in advance.

Matt

LHA · Mar 1, 2004

I think the layperson can understand the concept of resistance to flow behind Manning's "n" values. No need to numb them with Hydraulic Radius and so forth. I think I would just photocopy a one page table with many Manning's "n" values from an authoritative source that people recognize (the Army Corp would be good, your own state's DEP or DOT would probably be better). Then explain why smooth -lined pipe is around 0.012 and grass ranges from 0.3 to 0.2 (again leave out the d/D theory and such).

It sounds like you are more worried that the adjoiners are actually opposed to the development more than enclosing the ditch (why should they oppose closing a ditch, it will look better?). If this is the case, don't waste too much time on it...just dutifully put together a presentation for your meeting, and present it as politely and professionally as you can. They are likely to use any stall tactic they can find (IF they are opposed to development).

TerryScan · Mar 2, 2004

If all things were equal, I'd rather enclose a ditch that adjoined my property. But sometimes there are other things to consider:

On one hand, it IS possible for a pipe with a smaller cross sectional area may be able to convey as much or more than a larger open ditch.

On the other hand, the surface area is not necessarily a point of confusion. In the scenario described, the additional "storage" in the ditch may attenuate flows in a beneficial way. Piping could cause downstream flooding.

Also, the open ditch could provide a "water quality" component that the piped system would not.

gbam · Mar 3, 2004

Lets all remember that now-a-days everything leans towards aesthetics. So under-grounding a conveyance system may have benefits other than cost and such. So one approach would be adressing the "Looks" of the system and the ability to use the area above the Pipe.

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Pipe versus Open Channel

mfalati

Civil/Environmental

LHA

Civil/Environmental

TerryScan

Civil/Environmental

gbam

Civil/Environmental

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