Pipes at 0% 3

[uncivilized]

I am working with a very flat site (.5%) I am proposing a series of ponds to detain the water. Each pond will have the same initial water level. The ponds will be several hundred feet apart (>500'). So if my inlet starts at the initial water elevation then the outlet into the next pond will be a couple of feet below the water level if the pipe has a slope. Is there any reason to have a sloped pipe if it is outfalling in a submerged condition like this? Why not just keep the pipe on a 0% grade. I have seen this done before but wanted to get other's thoughts.

 

[RWF7437]

The first problem may be that at low flows you may have very low velocities (<2 or 2.5 ft/sec ). At these low velocities the pipes will not be self cleaning. Unless this is not a problem for you, why not put the pipes in at minimum slopes resulting in a velocity of say 2.5 ft/sec when flowing full or half full. ( Remember that the velocity in a circular pipe is the same flowing full or half full ?)

good luck

 

[francesca]

Theoretically, if your pipe serves only to link two ponds together for additional storage, it doesn't matter if the slope is positive, zero or adverse, because the hydraulic grade will control the flow direction.

In practise, however, if your pipes do not have some slope to them, water will invariably pond inside your pipe once the need for joint storage has been eliminated. If your design is for your pond to hold water, and the pipes will be submerged at all times, then this shouldn't worry you.

I would suggest that you have a slope, however small, to keep the water moving during times of drought.

 

[uncivilized]

Francesca your response brings forth a question. The ponds are designed to hold water. The hydraulic head will be the difference between the ponds water surface and the slope of the pipe will not affect the capacity. You said my pipes will be submerged at all times but do they have to be? If my pond elevations are at elevation 80 shouldnt all of the inlet inverts be at 80 as well. Certainly you could set the inlet of the final outfall pipe in the last pond at 80 and then submerge all of the connecting pipes of the interior system. Would there be an advantage in doing so?

 

[bltseattle]

An advantage of sloping a submerged pipe is that you can direct settled material to one end or the other, and it may be easier to clean if that is ever necessary.

 

[uncivilized]

I visited a site yesterday where all of the connecting pipes are submerged except the outfall pipe. One result is that the ponds looked better without all the pipes. I also ran some culvertmaster calculations and noticed that the submerged pipes have more capacity for certain conditions.

 

[TerryScan]

I am in a coastal plain where flat slopes and interconnected ponds are quite common.

While the ponds may look better when all of the pipes are hidden, one should also consider maintenance. Here, due to feedback from inspectors, regulators frown upon submerged pipes because they are difficult to inspect and maintain.

 

[uncivilized]

TerryScan

What software program do you use to model your interconnected ponds? I use xpswmm but find it somewhat cumbersome to use.

 

[TerryScan]

I use ICPR from Streamline Technologies:

http://www.streamnologies.com/

Once one gets used to the interface, it is very easy to use.

 

[uncivilized]

I looked at the website - I am interested. It sounds like you are also in the southeast. One more question: how are you estimating Times of Concentration. I use the TR55 method: sheet flow, shallow concentrated flow and channel but I dont know how applicable it is to my flat area. For instance take 300' of pre-development flow. Id break it down into 2 sections: 150' of sheet flow and 150' of shallow concentrated flow.

Sheet flow
n=.4 for woods
slope = .5%
2 year rain = 4.4 inches
Tc=44 minutes

Shallow concentrated flow
slope = .5%
surface description = unpaved
Tc = 2.2 minutes

The first seems excessivly large, the second seems too small. Any insight on time of concentration would be appreciated.

 

[TerryScan]

I also use TR55 methods for Tc. The only flaw I see in your example is the unlikelyhood of having sheet flow for even 150 feet in a flat wooded area. The regulators in my local are very picky about sheet flow. Generally, they do not want it to exceed 100'. The only time 150' would be accepted would be on paving. The flatter and more irregular the surface, the more likely it is for flow to collect and concentrate.

A good way to get a feel for some of this stuff is to observe runoff during significant rain events on different surfaces.

Shortening the sheet flow to 100' in your example would reduce Tc from 46 min to 35 min. Still, I am trying to imagine being in a extremely flat wooded area during a heavy rain and observing flow. Hopefully, you know the woods that you speak of better than I, but I have observed this type of area having likely sheet flows less than 50 feet.

 

[cvg]

I would agree that shallow concentrated flow is more likely. Most agencies allow a minimum value for Tc of 5 or 10 minutes - an estimate of 2.2 minutes would round up to 10.

 

[RWF7437]

How did this question about pipes at 0% slope devolve into a discussion of time of concentration ?

 

[uncivilized]

What you are saying makes sense. I remember the old TR55 manual said that the maximum sheet flow is 300' and then they moved that down to 150'. 50' probably sounds more reasonable. So if I divide the 300' as you recommend:


50' of Sheet flow
n = .4
slope = .5%
2 year rain = 4.4"
Tc=18 min.
velocity = .05 fps

250' of shallow concentrated flow
slope = .5%
surface description = unpaved
Tc=3.7 min
velocity = 1.1 fps

Total Tc = 18 min. + 3.7 min = 21.7 min.

21.7 minutes sounds a lot more reasonable than 46 minutes which I had before. However it seems unreasonable that the velocity jumps from .05 fps to 1.1 fps. Perhaps my "n" value is too large.