forcemain discharge to pond

[swoosh172]

I have a pumped stormwater forcemain that discharges into a pond via a 4 inch pipe above the pond water level. The discharge velocity is so powerful that it shoots the water about 10 feet at a fairly high pressure and has started to erode the pond bottom. I have been tasked with fixing this issue. I don't know the flow and the pressure in the forcemain and the pump is constant speed. I am thinking just to increase the last 20 feet of pipe to 18 inch diameter or so to slow the velocity down to basically a standstill and then the water will just fall immediately into a rip rap basin. Do you think this will work or do I need something like an orifice plate in the line as well to reduce the pressure? Any help is appreciated. Thanks.

 

[cvg]

first question should be why is your forcemain pumping at such a high velocity?

an orifice plate will reduce the flow and increase the pump head, probably not useful. 18 inch is probably too large, you need to maintain a minimum velocity of about 2 or 3 fps to move solids. the most common solution is to discharge into an energy dissipater instead of just shooting it into the pond.

 

[swoosh172]

I guess the previous engineer miscalculated the head loss of the forcemain so the pump is operating further down on the curve at a higher flow rate. What kind of energy dissipator are you thinking of? Would a rip rap path prove useful? I'd like to not have to use a concrete structure at the end. Something like 6 inch diameter stones.

 

[Ron]

You probably only need to increase the pipe size to 8 or 10 inches....then use rip-rap to protect.


Another option is to turn discharge up and create aerated discharge...helps for a variety of reasons.

 

[bimr]

cvg is on the right track.

What is usually used in this application is a simple multi-port diffuser. You can fabricate a 4-Inch submerged diffuser 5-6 feet long that should be comprised of approximately 10 ports with each port being a 2-Inch diameter pipe. Attach the submerged diffuser onto the end of the existing pipe.

If you have about 250 gpm in the 4-Inch pipe, that will give you a velocity of approximately 2.5 ft/sec out of each of the diffusers.

Here is a link to examples.

http://www.cormix.info/CORMIX2.php

 

[swoosh172]

bimr...the pond in question is a stormwater retention pond that at times can be dry so I think the submerged diffuser would not be a good option since if there was no water cover it would shoot at a high velocity.

 

[bimr]

For a stormwater retention pond, you should install a small manhole. Transition from force main to 8-Inch to 10-Inch gravity sewer in the force main.

 

[cvg]

I have used a a concrete box catch basin with grate on the top as a bubble up energy dissipater. just make the grate large enough to keep the unit discharge low

 

[beej67]

I would go with Ron's idea of a 90 degree elbow up. Might armor underneath it.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

turning the elbow up may not help if there is no tailwater and that sounds like a problem since the pond is dry at times. You need to do 1 or more of the following: a) reduce the discharge velocity from the end of the pipe or construct a structure on the end that will discharge at lower velocity b) reduce the heigt of the discharge above the bottom of the pond. Acceleration due to gravity can greatly increase the energy before the water impacts the bottom of the pond c) provide erosion protection on the bottom of the pond d) increase the tailwater depth at the discharge point