Tough engineering type question

[Snake Pliskin]

I dont know if this can be answered here but we have some very educated members so here goes. This pertains to water drainage. This question specifically pertains to a pipe that drains a ditch under a highway to an open area on the opposite side of the highway. Here is a description of what I have.

An 18" diameter concrete drainage pipe 20 feet long at 5 degree slope. I will assume this pipe has a rated capacity and the length and slope cannot be changed. Now the question.

Can the capacity of this pipe be increased without the use of a pump of some type? Can inlet size or design increase this pipes capacity? Any input would be appreciated or links to this specific type of application.
Thanks in advance.

 

[thebestofindica]

Fluid mechanics. Q=A x V, where Q is the flow rate, A is the cross sectional area and V is the velocity of the fluid. To increase Q you must increase the area of the pipe or the velocity. I'm a licensed professional engineer. A little more info on the problem would help me.

 

[Snake Pliskin]

Let me know what info would be helpful and I will try to get it for you. Also if the "rated capacity" of this pipe can be increased is it possible to quantify the increase in some way. Appreciate the response.

 

[hockeylover86]

You must be thinking of adding some kind of bell-mouth on the inlet end, that might help a little, but I'm not a fluids expert.

Unless you can increase the pressure on the inlet or decrease the pressure on the outlet side, I can't think of any other way.

If the inside is rough, maybe a smooth lining or sleeve to help reduce turbulence.

 

[thebestofindica]

Let me know what info would be helpful and I will try to get it for you. Also if the "rated capacity" of this pipe can be increased is it possible to quantify the increase in some way. Appreciate the response.

I'm not sure what is going on. What is causing the need for the increased flow? Is the inflow and outflow on private or public property?

 

[oldmodman]

Pressure will increase flow rate. Build a dam on the supply side. Deeper the water the faster the flow. Up to the point where the water pressure will blow the pipe out.

But this would most likely defeat the purpose of trying to drain the up stream side of the ditch.

 

[Blk04L]

Hmm. Been trying to think of some questions to help understand the scenario but I'm mentally beat. If the area can't be increased, and the upstream pipe is fully submerged...

 

[svtaddict]

Rifle it.

 

[MovingZen]

What type of pipe? Reduce the roughness coefficient to increase flow rates. Also, what's happening at the inlet and outfall? Make sure there isn't any water backup at the outfall. Gotta know what type of pipe you have.. concrete? corrugated metal?

 

[Blk04L]

What type of pipe? Reduce the roughness coefficient to increase flow rates. Also, what's happening at the inlet and outfall? Make sure there isn't any water backup at the outfall. Gotta know what type of pipe you have.. concrete? corrugated metal?

It's concrete. I overlooked that part at first too. I haven't looked to much into this, but there's no real feasible option in reducing the friction for a concrete pipe, especially a 20 ft 18" pipe?

 

[thebestofindica]

Changing manning's n value from 0.015 for concrete to roughly 0.012 for PVC would increase the flow rate from 20-25 cfs. I know this isn't feasible, but more for instance. A circular conduit will have peak flow when flowing at 93% of the height.

 

[Snake Pliskin]

I'm not sure what is going on. What is causing the need for the increased flow? Is the inflow and outflow on private or public property?

The original design was for conditions around the year 1950. Due to development in the area the increase in debris and water is more than the design can manage 1-3 times per year which is causing some flooding of private property. Its on state property and its doubtful that it would be sufficient even if functioning at 100%. It appears that a 20" inlet has been added to this particular pipe but it does not appear to be oriented in any way that could increase incoming flow or pressure.

 

[MovingZen]

It's concrete. I overlooked that part at first too. I haven't looked to much into this, but there's no real feasible option in reducing the friction for a concrete pipe, especially a 20 ft 18" pipe?

If it were corrugated metal would there be a feasible answer? I am not an engineer but I have dealt with shitty drainage. The things we did were replace with a larger pipe if the fall allowed, add another pipe if the fall didn't allow, and make sure the outfall was clear so the water could go away and not back up. If I couldn't change the pipe I looked for ways to divert some water to another drain.. last but not least I did this :shrug: or pumped when I had to get rid of the water fast. Since it was a 27 hole golf course I pumped a lot of water :nonono:

This nine is closed for a while. :shrug: lol Sometimes it just rains way too much, and no easy answer is going to fix that.

 

[GT Premi]

Like somebody said, you can try to reduce the pressure on the down side of the pipe. Only way I know to do that is make the sump deeper and/or wider assuming the outlet is partially or completely submerged. An ideal would be a delta shape that gets deeper on the back end. This would create low pressure (suction) at the mouth of the pipe to increase flow. But you haven't said how much space you're working with on the other side of the road.

If the outlet is above the water level, you'll need to create some higher pressure on the inlet side of the pipe. Like somebody said, your only option there, based on what info you've given, is to build a small dam to channel the water toward the inlet since you can't/don't want to use a pump. Or as was also mentioned, install a bell/horn mouth on the inlet which would increase velocity.

Then again, I suppose you could install a reducer on the inlet side and as the flow area expanded inside the pipe, it would create suction at the inlet. That would be a much simpler and cheaper option that accomplishes the same as digging a bigger sump.

I don't know. :shrug: I'm not an engineer. I just have basic/rudimentary knowledge of fluid dynamics. Start quoting math formulas and I'm out.

 

[thomas91169]

Add a second pipe.

 

[JeronimoJC]

Can the capacity of this pipe be increased without the use of a pump of some type? Can inlet size or design increase this pipes capacity?

The short, practical answer is no to both questions.

To increase flow you would have to bore the pipe to a larger diameter and/or install a smooth liner to reduce friction. None of these seem practical.

Pumping?

 

[ElscottHavoc]

I may not be understanding the situation correctly, but is it possible that the areas upstream of the pipe is in fact the cause of drainage problems regardless of increasing the pipes flow capacity.

For instance, is the design of the ditch not provoking the water to move downstream with any greater momentum. Assuming you were able to increase the capacity of the pipe, if the ditch itself isn't sloped enough toward the pipe, you may still end up with problems. Not sure on how much flooding you have or exactly what the situation is, but bringing some dirt in that provides a greater taper toward the pipe could encourage quicker drainage earlier on before water levels get to a point that exceeds what the pipe can handle.

On the other hand, if I'm not understanding the scenario that'd be a terrible idea as you'd just be making things worse and lessening the volume of the ditch in its ability to hold more water!!! Perhaps maybe just increasing the volume of the ditches in the area would help. You wouldnt necessarily be moving the water out of the area any faster, but at least then if the ditches were slightly deeper and wider more water would be contained and water unable to be drained via the pipe in a timely fashion would soak into the ground slowly naturally move across the highway through the soil underneath or evaporate.

I do more with structural design than civil engineering so take that with a grain of salt.

 

[Snake Pliskin]

The short, practical answer is no to both questions.

I believe this is the answer. Thanks for the information guys.

 

[mc01svt]

What type of pipe? Reduce the roughness coefficient to increase flow rates. Also, what's happening at the inlet and outfall? Make sure there isn't any water backup at the outfall. Gotta know what type of pipe you have.. concrete? corrugated metal?

^this

Without increasing the flow area the only other logical solution is to decrease the frictional losses at the wall of the pipe. Although im guessing these losses will be minimal for this particular application.

Paint the inside of the pipe and then coat with teflon and liberal amounts of KY. oke:

 

[HYBRED]

Can the capacity of this pipe be increased without the use of a pump of some type?

Unless you're using a compressible fluid, the CAPACITY of the pipe cannot be changed. Sounds like a trick question to me.