Pipe drop at Catch Basin

[itk1]

I'm performing a val/eng review of a road/drain design and have come accross numerous catch basins in a row that the invert out is substantially lower than the invert in (8'-10' drop). The road profiles are steep 7-12% and the design calls for the pipe(s) to step down at successive CB's as it moves down each hill. I don't have the hydraulic calcs yet so I don't know the flows, but the pipe size range from 18" to 36" rcp.

My initial reaction is that the CB's are going to be subjected to quite a bit of scour and therefore will create some longterm maint. problems. Are there any other problems that this type of design might cause? Are there any special hydraulic considerations that should be in the hydraulic analysis?

Thanks for your help.

 

[PELS]

If the pipes were parallel with the ground surface, the velocities would be too high. The stair-stepped pipe design apparently is to address the need to minimize the velocity of the flows in the pipe. This causes a need for more frequent catch basins that are also unusally deep.

Depending on the slope of the pipes, there could be some drastic hydraulic jumps in the system. This needs to be carefully analyzed.

Can any portion of the system be an open channel or pools with a series of waterfalls?

 

[itk1]

There are a number of open roadside swales throughout the site. I suspect that the municipal codes/erosion control requirements call for closed drainage though.

I know for sanitary sewer systems I was always required to add a riser pipe when the invert elevation difference was greater than 4'. I'm not sure if the same would apply to storm drains. Prelim design/cost estimate has standard CB's and MH's without any additional cost. Costs also do not appear to account for the deep excavation required for initial construction. The developer will be responsible for the long term maint, therefore I am trying to consider all factors so that the client has a true cost.

 

[msquared48]

If the idea here is to dissipate energy, then the invert should be lower than the outlet so that the water spills into a basin of water from underneath the water surface rather than spilling onto the surface itself. Essentially a drop catch basin.

Mike McCann
McCann Engineering

 

[tsgrue]

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I don't see the problem, but I may not be understanding it either. I've seen many 40+ year old concrete pipes, catch basins, and stilling basins that have experienced relatively frequent high velocities, heavy flows, and large drops (respectively) with a fair amount of sediment (generally up to coarse sand sized) without any significant scour degradation of the concrete material.

Can you post a diagram?

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com

 

[msquared48]

In addition to the diagram, is this an area that will see a lot of storms and high flows? Is the likelihood more like once a year or once a month?

Will there be a lot of sediment in the runoff to clog the catch basins?

Not to seem ridiculous here, but if there is a lot of flow, and often, with little sediment, and since this is a Value Engineering project, why not propose installing small turbines in the system to generate energy and reduce the energy gradient? Could be feasible. It may seem ridiculous, but has been proposed in the past.



Mike McCann
McCann Engineering

 

[TerryScan]

FYI, the "pipe riser" you refer to in sanitary applications is commonly termed a "drop manhole". It's purpose is not to eliminate scour. It is to allow a person to enter the manhole for maintenance without being showered with effluent, since sanitary flows can be continuous. One can usually choose to access storm structures at times when there is no flow.

 

[Erich870]

One distinction that should be made about sanitary sewer drop manholes is that the drop is also designed to prevent the solids from separation from the liquids. I.E. if the effluent were allowed to freefall the solids would impact the bottom and splatter on the sides, thus building up into a maintenance nightmare.

Because storm sewers do not typically see the same sort of "solids" the energy from impact in a structure with a sump full of water is greatly reduced. As long as the inlet stream is not at such a velocity that it clears the structure and hits the outlet pipe (which I've seen before) the drop of 8'-10' shouldn’t pose any excessive maintenance issues. Storm catch basins will always be wet inside and as long as they are sealed properly at the joints it shouldn't be a problem.

Erich

 

[PELS]

If the road grades range from 7%-12%, I suspect the pipe slopes are somewhere below 7% down to maybe 3%-4% or thereabouts. Pipe slopes in that range would yield self-cleasing velocities so I don't really think sediment removal will be an issue. The velocities will move the sediment that gets into the catch basins along with the flows.

You need to review the hydraulic calcs to see if there is transition from supercritical to subcritical flow, which results in hydraulic jumps, which are characterized by violent turbulence, eddying, air entrainment, and surface undulations. You want a steady flow in your conduit system to pass the maximum volume.

This type of design does have higher construction costs due to a greater number and depth of the structures, but maintenance should be not be a problem. In fact, flat land drainage systems require more sediment maintenance.

 

[itk1]

Thankyou for the info. There are several larger brooks and wetlands throughout the property, but I don't believe that flood flows will be that significant even though the commercial portion of the site will add a significant amount of pavement. A 25y frequency was used in the design (noted on plans) and I will check the hydraulics once I receive the report. I see that maint/cleaning may not be a concern. Given the initial estimates of structure construction though, I'm certain that no provissions have been made for the deep construction cost (shoring, dewatering, ledge, etc).

 

[tsgrue]

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Have you looked at dispersing smaller infiltration areas across the site to handle the water instead of piping it all? At a minimum, the infiltration areas could reduce the pipe sizes by reducing the runoff.

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tsgrue: site engineering, stormwater
management, landscape design, ecosystem
rehabilitation, mathematical simulation

http://hhwq.blogspot.com