Pipe drop at Catch Basin
Pipe drop at Catch Basin
(OP)
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.
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.





RE: Pipe drop at Catch Basin
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?
RE: Pipe drop at Catch Basin
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.
RE: Pipe drop at Catch Basin
Mike McCann
McCann Engineering
RE: Pipe drop at Catch Basin
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
RE: Pipe drop at Catch Basin
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
RE: Pipe drop at Catch Basin
RE: Pipe drop at Catch Basin
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
RE: Pipe drop at Catch Basin
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.
RE: Pipe drop at Catch Basin
RE: Pipe drop at Catch Basin
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