Water Main Pressure Relief - Drop Structure to Storm Water Channel

[ivanlocke]

I am designing pressure relief off a 60" water main as it crosses a storm water channel. The 60" runs under one sidewalk and I am looking at the discharge pipe running parallel under the opposite sidewalk with 2 or 3 sets of pressure relief and isolation valves running between the two. We are looking at each valve releasing 10,000 gpm, so max would be 30,000gpm.

My question is about the outlet. The bottom 6 feet of the channel are lined and my outlet would need to come out just above this. The roadway and other pipes are about 20' above this. I am looking at using a drop structure. This would serve as an air gap and if the outlet is large enough I won't have any head from the 20' drop.

If I use a 20" into the drop structure the water would be coming out at 30ft/s. How can I get this in without erroding everything? Could I put a 90 bend on it aimed down and have energy dissapators down the length of the manhole? What are energy dissapators used in drop manholes - grating, rebar, plates with holes? As far as the outlet I am at the bottom I am thinking of a 30" pipe with a flapper on the end to a flared concrete end with some energy dissapators (just to spread the flow out across the flare). Any better ideas?

Thanks in advance.

 

[CivAlan]

I would think that any kind of energy disipator made of metal would deteriate very quickly. Any finish on it would be washed off by water moving 30ft per second.

I know there used to be a engergy disipator shown in the Green book of standard plans for Public works. It was intended for use with storm drains. However, I dont have the current edition, so I couldnt tell you where to find it. If I remember correctly, it was made up of alternating concrete post.

You may want to consider droping it into the channel at a 45 degree angle against the flow of the channel. Just make sure it will drop water into the lined section of the channel. If it an overflow, and doesn't get used a whole lot, it may not be cost effective to do an elaberate structure that might get used once a year.

 

[cvg]

try to dissipate the energy in the drop structure and don't worry about additional dissipation and flare in the channel. This will only mess up the hydraulics in the channel.

As far as the outlet, normally this is not done in a manhole, but an open channel energy dissipater. See Bureau of Reclamation Engineering Monograph No. 25 - Hydraulic Design of Stilling Basins and Energy Dissipators. Drops, baffles and roughness elements force the flow into a hydraulic jump.

In a manhole, you may run the risk of pressurizing the manhole so I would advise a grated air vent on the top to reduce that risk. You could use a 20" x 30" increaser to reduce the velocity somewhat before it discharges into the drop structure. Turn it downward and provide suitable thrust restraint. If the manhole and outlet drain to the channel are properly sized, the manhole will fill up partially with water and the jet discharging into the tailwater will dissipate without impacting the walls. It will impact the floor every time the pressure relief valve opens, but only for a short time until tailwater builds up. I'm not sure why a flapgate would be necessary.

 

[KRSServices]

Just an idea here. Has consideration been given to a larger diameter discharge pipe, flared at the discharge end. The hyraulic analysis would have to be calculated of course, to determine the energy of the water at discharge and feasibility of the idea. At the discharge point, large baffles, or better yet, hefty rip rap could line a channel to dissipate the energy in the water further until it reaches a stage where you are comfortable with it's flow characteristics prior to entering into the storm channel. This outlet will have to be protected with fencing to prevent children or animals from the dangerous discharge.

I have heard of aging drop structures with vertical drops in excess of 15 feet completely destroying the bottom of manholes over time. I also have not seen any liturature on constructed baffles within the manhole for energy dissipation, but I would think they could easily be desinged, but would be a nightmare if ever the manhole required servicing from the inside. There are likely many variations and different themes along these lines. Just my thoughts as an initial option.

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