Stilling Basin in a Circular Pipe

Hi d_m,

I can't say I've directly any experience with this design but I do alot of water treatment work. The velocity you mention is quite high, especially for such a large pipeline. The water certainly contains a significant amount of momentum energy, you'll have to deal with.

I found this, maybe it'll help (Hydraulic Design of Energy Dissipators) ...

http://www.fhwa.dot.gov/engineering/hydraulics/pubs/06086/hec14.pdf

Regarding the differences between a trapezoidal channel and a circular pipe, for what you are doing (dissipating the energy of the flow) I wouldn't think there would be too much difference (however I'm not familiar with the design you're proposing).

With confined spaces, I can only suggest pushing the flow vertically into a chamber to let gravity and turbulance do the work of dissipating the energy of the flow. This may or may not be a good idea.

Hope this helps.

Kind Regards,

Anthony

I once had to design a "Vortex Drop" on a storm drain which did more or less what you describe. It took the flow from a large pipe, dissipated its energy in a vortex, rather like a whirlpool and then sqirted it down a smaller pipe to reduce the flow into a natural watercourse. The excess water spilled over the upper rim and into a balancing lagoon via a concrete staircase.

For reasons of economy and speed it was designed around standard RC components such as pipes, channels & chambers and it has been working well for the past 35 years.

I think the priciple is fairly well established and you should be able to look it up on the net.

Thanks for the suggestions, the only problem with putting the flow through a drop is that it is already a fairly deep structure (around 40-50 feet below ground surface). This structure controls flow to a deep storage and conveyance tunnel for combined sewage. The orifice plate at the base of the structure helps to control to a specific design flow rate and forces the flow to back up behind it. When the flow backs up high enough it overtops a weir that overflows to the receiving water.

I have never designed a similar facility. So my thoughts do not have a practical basis.

That said, I know Bridgestone makes an inflatable dam that fits inside large diameter pipes. Something like this could be used for force a jump during high flows. Then it could be deflated during low flows. This could prevent the build up of debris in the stilling basin. Water pressure is used to modulate the inflation / deflation of the dam.

Also, Vortex manufactures stilling chambers that essentially spin the water to force it to lose energy. The issue here is access to remove build up of material that is dropped from the current when energy is lost.

Cast-in-place structures that force rapid expansion / contraction of the flow could be utilized to create energy loss. That might required some specialized modeling software or techniques. Again, maintenance may be an issue

Based on the depth you indicated a plunge pool is probably not a good option. But a riser section with a weir spillway might work. However, except for the inflatable dam option, every other option requires routine maintenance.

Anyways, not any good concrete answers here. But maybe some possibilities to investigate.

Good Luck!

Hello D_M:

I think I may help, but I need a clearer picture of what you're trying to accomplish. Is it possible you can post a sketch of the system or even email a AUTOCAD sketch/detail of you are trying to design? I use IntelliCAD and can supply you with my personal email addie if you want to go that route. That would help immensely!

Are you designing this system as a means to control stormwater runoff? Also, regarding the stilling basin inside of the 6 foot RCP pipe: is this pipe to be installed vertically? Not quite sure what you mean as far as a stilling basin is concerned? To control silt and sediment discharges to a downstream water course?

Not sure where or how the orifice fits into the scheme of things. Meant as a flow control/discharge device? I would be extremely concerned with the exit velocity of the water from such a device, especially at 30 fps. You can expect high erosion of any surface it impinges upon. Generally in Civil Works we try to limit the maximum water velocity to 10 fps so as to avoid severe erosion of concrete and soils, and yes, even steels!

Any additional info you could provide would be most useful!

Have a great sunny day!

Sorry it took awhile to get back. Other work was piling up. I have included some quick schematics of the structure. I am not really a drafting/design person at heart so forgive the crude sketches.

I realize there will be issues with debris. There will be a bar rack in the upstream structure to remove large debris, but cleaning will be necessary on a regular basis. The pipe will be a 6-foot diameter RCP pipe.

Hello D_M,

Thanks for pasting the CAD sketches. It's much easier to see how your system is configured.

I believe I know what your trying to achieve here. I am assuming that you are controlling the Post-Development Peak Stormwater Discharge Rates for multiple storm events equal to, or less than, the Pre-Development Peak Stormwater Discharge Rates respectively. Am I correct in that assumption?

Now, in regard to the very high velocity resulting from outflow discharge of the orifice plate: The primary reason you have a high velocity is that you have a high hydraulic Head acting on the orifice. If you want to reduce that velocity to below 10 fps, then I suggest you do the following corrections. Decrease the operating upstream head and provide more storage volume in the structure, or other words stretch the basin structure width and length to provide more area while at the same time reducing the vertical Head height. You are going to have to run multiple trial and error scenarios to get things to work out. Make sure that the orifice planar area (in the vertical) is large enough to decrease the exiting velocity significantly...that is your primary goal! My suggestion is to limit the exiting velocity to 4 or 5 fps maximum, may be more, so as to reduce the erosion effects of the water jet on the downstream discharge pipe, and therefore you can eliminate the energy dissipators such as the chute blocks, etc. If this is for non-municipal client, then you can count on no regular maintenance being performed on the system to remove accumulated debris...that's the way it is. Private companies for the most part don't want to spend the time and money for maintenance work on civil works......once it's in the ground they want to forget all about it...out of sight and out of mind. It really is a sad commentary how our society and economic thought process works.

BTW, why such a huge discharge pipe????? As long as you maintain the design flow (the lesser storm event) and at least provide a minimum of 3 fps to keep fines and other solids suspendedin the outflow, then you can reduce the size of that pipe considerably. The goal is to prevent siltation of your pipes conveying the stormwater.

Also, at the pipe (entering on the left) discharging the storm flows into the large basin (Stilling Basin?), why not attach a Standard Tee fitting here with a solid rubber clean-out cap (threaded with operator's handle) and a vertical drop pipe....an arrangement just like that used in a Sanitary Sewer Drop Manhole. First, you don't have to worry about the water jet exiting the pipe and impinging on a wall or floor below, and secondly, you can forget the concrete flow trough inside the structure between pipe in and pipe out. Provide instead a 2 or 3 foot deep sump so that sand and silts can settle out and not be carried through the discharge pipelines to the water body you will be ultimately discharging into. You'll find that your water quality in the discharge will improve considerably if you do this.

Hope this dialog was of some use to you. Any further questions, please feel free to ask away, okay? Have a great sunny day!

CaptMoosie,

It would not be possible to make the structure large enough to achieve an outflow rate of 4-5 ft/sec. We are talking about volumes in the millions of gallons and these structures are to be built in extremely urbanized areas where there is only a small amount of room to work. This structure is to control Combined Sewer Overflow such that no overflows occur during a typical year and that the peak flow rate to the tunnel (downstream of the orifice plate) is only 10% greater than the typical year peak design rate. That is why the orifice is creating such high velocities. There is really no way around the hydraulic issues, I was just hoping that maybe someone had experience with designing a similar structure (to create a hydraulic jump in a pipe). I have come to realize over the last couple of days that not many people have. I work in a company of 46,000 people and so far no one has designed anything quite like this.

Thanks for your help.

Hello D_M,

Now I see where your coming from. Many thanks for the additional information.

I'd still be concerned about that 30 fps orifice jet flow if I were you. I've designed hydraulic jumps several times before for Combined Sewer overflow structures in an urban setting, but inside large underground vaults that where fairly long where we had room to do so. I can appreciate where your coming from because you have to deal with all the underground utilities, right-of-way issues and property issues, let alone buildings and other structures in a dense urban setting. Sounds like you're in downtown Chicago or somewhere similar!

I've never designed a hydraulic jump for placement inside a large diameter pipe, just in vaults. Looks to me like you've painted yourself into a corner, so to speak.

Tell me, how do you envision the "Stilling Basin" (I think that's what you've named it....the structure where the orifice is installed) to operate? Is it's function only to split the flows, or to store the wet weather volume, both? I guess I cannot see how you envision how this entire scheme needs to work and needs much more explanation to be understood. I guess I really don't understand the need for such a deep structure in the first place if you're primary aim is to split the flows, where that can be accomplished in a shallow structure too.

IF it's sole function is to split the flows, then can you not raise the invert the pipe outlet (w/ orifice) much higher, therefore lessening the design head acting on it?

I'd be very careful trying to construct energy disapators inside a large diameter pipe so as to slow down the velocities. You may be better off doing that in a wide flat bottom vault and then restricting it back down to a pipe again.

Good luck D_M! I'll be back online tomorrow sometime if you need further discussion or help. I have several important meeting to attend today and will be out of town w/ clients.

May be you just need to walk away from that project for a few hours and work on something else, but think through various options inside your head in the meantime. I've found that works for me sometimes when I find myself inside a "Brain Box". Try thinking OUTSIDE the "box" with a series of "what if's scenarios", okay? I always say there's a million ways to skin a cat (apologies to Mr. Del the Cat), and there's never only one way to a solution!!!! Think abstractly, but logically at the same time amd you may find a way to fix your problem at hand.......

Have a great sunny day!

Hello,

The reason it needs to be so deep is that we are connecting to the existing infrastructure and so we are starting about 15-20 feet deep to begin with. The additional drop occurs because of the peak flow rate restriction. In order to minimize the increase in the peak flow rate to the tunnel during the 5-year design storm we need to make the differential head above the orifice plate as small as possible. So, if the control storm has 2 feet of head on the orifice plate and the 5-year storm is 4 feet above the orifice so this would create a X% increase in flow. But if the control storm is under ten feet of head and the 5-year design storm is under 12 feet of head then the difference in head is the same but the percent difference is much smaller.

Ultimately, the majority of the flow is captured in a deep storage and conveyance tunnel.

Good Morning D_M,

Thank you for the additional information. Please bare with me this morning as I haven't had enough coffee IV'ed into my veins yet this early in the AM. Eventually the caffeine will start triggering the gray matter in my noggin'. Thank God my wife is a RN and just rig me up to the coffee IV bag!!! ***LOL***

From your sketches that you have posted, it appears that you're providing a fairly steep concrete flow channel. or trough, shown as the double reversible curve between your Invert In and your Invert (Orifice PL.) Out, correct?

Just a suggestion how you could possibly pickup more storage volume, hence lower the operating Head on the Orifice plate so as to lower the exiting velocity: remove the concrete benches and the flow trough and replace them with a Vertical Drop Pipe w/ a Tee fitting at the top and a 90-degree Long Sweep Elbow at the base that will transition into a much shallower concrete flow trough.

I'd also try to tweak your plannimetric are of the structure any way you could to increase it's size to help increase the volume of the structure. I know you're in a tight area, but investigate the depths of the other utilities that could or are affecting the size of your Stilling Basin.....you may find that they're shallow enough for you to construct a larger structure below them (ie, big box) with access to the surface with large diameter Manhole Structures placed on top of the box....it will take some bit of structural engineering designing a cast-in-place reinforced concrete structure, but may be well worth it. Just build the Stilling Basin box UNDER the other utilities if at all possible especially if the top of it can be lowered and they're shallow enough, OR PARTIALLY REROUTE them around the basin.....that can easily be done with water mains and buried electrical and fiber optic lines. Really, you need not have the top of the entire structure accessible at ground level, only a portion of it.

I hope this is of some help for you. I will be in and out of the office/home most of the day, but will check here occasionally if you have any questions or need further assistance.

Good luck! Please have a sunny day!

===Mark (aka CaptMoosie)

PS: IMHO, I'd try to stay away from the energy dissipator idea all together as it'll cause severe maintenance headaches later on for the sewer district crews....like how are they going to be able remove entrapped debris and sludge deposits from it in a safe manner? See where I am going with this? You also have to worry about excessive wear on the concrete blocks that comprise the energy dissipators....they will wear much much faster that you can imagine with a 30 FPS fluid velocity, especially if the fluid contains even small amounts of sand and other particles and solids.