Catchbasin Invert 3

[chicopee]

I have a catchbasin with an inlet and outlet storm pipe. I need to know where the elevation in measured for the inverts of these two pipes and for the bottom of the catchbasin. Please be specific where these elevations are taken at since these drain pipes have wall thicknesses and the catch basin has thickness at its bottom.
Also is the inlet pipe at a higher or lower elevation than the outlet pipe? would any the these pipes be resting on the bottom of the catch basin or would there be a difference in elevation?.

 

[BobPE]

you want to connect the hydraulic gradient of the water in the pipes. This will mean that the influent pipe is going to be at a higher invert than the effluent pipe. you can put the invert of the effluent pipe at the same elevation of the invert of the basin.

BobPE

 

[dik]

You can have the inverts of the inlet and outlet at the same elevation within the CB with the pipe slope accommodated between adjacent CB's. As an absolute minimum, you may want to have the invert of the outlet equal to the invert of the CB base plus the wall thickness of the pipe. I usually have the outlet at an invert greater than 12" (300mm) above the CB base to allow the CB to 'catch' something and reduce silting of the pipes.

 

[PECPESC]

Invert and flowline are synonyms.
They indicate the lowest point of the inner diameter of the pipe.

The highest point of the inner diameter is known as the overt or crown.

Invert for the catch basin is usually given as the elevation of the lowest pipe (in or out) of the catch basin.

 

[BobPE]

dik:

Connection of pipes at their invert is a common mistake. You really dont have many problems in doing that in most areas of the country thats why it appears to be common pratice. Here in the NE part of the US we have to worry about freezing. When you connect invert to invert, you create turbulance in the catchbasin, this can intern create other hydraulic problems with the result being water splashed out of the basin or surchaging which will tend to freeze on road surfaces in the winter time. Seting the invert of all pipes into or out of a catch basin or manhole or any other type of gravity chamber must be done by the hydraulic grade line of the flow in the pipes if you want to do it correctly.

BobPE

 

[dik]

Thanks for the caution... I usually leave a space of a foot or so between the pipe invert and the CB base invert and with the 4' of cover to the pipes for frost (Lindsay, Ontario, Canada), I've never encountered water splashing out... but, I've used less cover with insulation and I can see there's a potential for problems. Thanks.

Usually use an HGL for setting grades or a minimum slope (as fits). We usually scramble to minimize the depth of trenching and eliminate any difference in inlet and outlet. What do you usually consider as a suitable drop across a catchbasin? and do you generally leave a space below the pipe invert for trapping sediment?

 

[chicopee]

Manhole Invert Related topics:
Must the hydraulic gradient for a storm drain system serving let's a street with so many catch basins remain at a constant gradient throughout the entire street length? or will the HGL become steeper as more and more water from each catch basin is drained throughout the length of the piping system as the piping size increase in diameter to accommodate flow? At what level of pipe fill will there be a change either in pipe size or in installing the pipe in a steeper slope?
Any guideline with this question is greatly appreciated

 

[cvg]

Bobpe - explain the problem with connecting inverts a little further. I am having difficulty understanding why this causes so much turbulence that water is spashing out of the basin. I am visualizing a curb inlet with a catch basin structure maybe 4 or 5 feet deep. Water drops in from the gutter and flows in the lateral from the catch basin to the mainline. Rarely are catch basins flow through. However, if it was flow through and if you maintain the hgl within the basin to a foot below the lip or grate (which is a common requirement in the SW part of the country) then you must be getting a standing wave in the basin over a foot high! Sounds like your velocity is quite high.

 

[KRSServices]

For new installations I prefer precast Cb barrels...that wsy the sump will not leak and cause subsidence problems later. I also prefer 18" to 24" of sump in the CB's for debris catchment. Also, I try to set the outlet invert elevation about 1" lower than the inlet to allow the system to free drain in winter and reduce the possibility of freezing in the pipe.To be honest, with propoerly sized CB's, I have yet to see the "splashing" effect BobPE was referring to. During the Tornado in Edmonton a few years back, I did however witness manhole covers being blown off storm sewer manholes due to surcharging...but by that time, most affected CB's were underwater.

KRS Services

http://www.krs-services.com

 

[BobPE]

Unfortunately KRSS, catch basin size has nothing to do whith what I was talking about. You should have learned what I was talking about in college Fluid Mechanics I.

A catch basin transfers energy from one pipe to the other. Calculating the HGL in elevation and placing the influent and effluent HGL at the same elevations is common practice amongst all engineers involved with this work. If you do not do this, a jump or turbulence form the discontinuity in HGL will cause what I tallk about. Granted, on small mains, the design errors caused by the lay design will not be noticed however, on larger mains you should know how to do it the way I am talking about or people will get hurt or killed. I know of several cases where lay design caused inlet covers to come off under conditions like you speak of, one person dies on a landing in the catch basin, the other case, the storm main led to a pump station and the man was found on the pump bar screens. In both cases, improper invert design which ignored HGL caused covers to dislodge and the affected people fell into the catchbasins. Now granted, these were larger basins, but the proper principals of design should have been followed. In these events, the designs were performed by people who designed smaller storm systems and never knew how to do it the correct way leading to an unfortunate event.

BobPE

BobPE

 

[KRSServices]

Ok BobPE, I understand what you are speaking about, but no need to get a little personal. My reference thread referred to a simple catchbasin MH...900mm or 1200mm barrel, 200 to 300mm leads, and designed storm pipe forming the system. The pipe is sized with thorough understanding of the HGL, intensities, gradients and catchment areas. I did not need the mild chastisement regarding fluid mechanics 101.

My comments were premised on the fact that once the system is designed, smaller items such as offset invert elevations inlet pipe diameters, outlet pipe diameters, etc., have already been accounted for. Offsetting inverts of the inlet and outlet pipe and sumps are not only good design practices but necessary to maintaining the integrity of the system. I've designed several sytems and have yet to see any splashing effects due to the flow characteristics within a CB manhole or CB barrel. Most are very straightforward, small and simple....as I believe the thread was meant. Not some complex monstrosity wherein a team of specialized design engineers are required to satisfy the design protocol. Nuff said about that. Truce...ok?

With reference to the tornado, the system was surchaging due to the fact that the tornado passed by the storm discharge outlets and the force of the storm (vacuum effect) caused the csp pipes to collapse, which in turn led to the surcharging of stormwater due to the HGL of the catchment areas of higher elevations than the MH covers I observed. KRS Services

http://www.krs-services.com

 

[CarlB]

Thanks for the catch basin information, KRS Services. Our local practice is to use 18" catchment also.

 

[jrfosterjr]

Chicopee,

To give an answer to your original inquiry, here are some thoughts to ponder:

1. Does the local municipality dictate where that invert is measured? For example, we have a one here that specifies that you give one invert, and that is at the center of the box. That, of course, would require that you determine a box invert based on pipe slope, wall thickness, etc, which I'll get into more in a moment. Others require an invert in and an invert out, some saying utilize no drop across the box, some saying utilize a minimum of 0.2' across the box, and others leaving it to the engineer's discretion.

2. In our area (Piedmont NC), it is quite typical that the inverts are required at the invert (of the pipe) in and (invert of the pipe) out. We can set the actual invert of the box at whatever we like to accomodate the wall thickness and slope of pipe, but we grout an invert from one pipe to the other to facilitate smooth flow. If there is more than one incoming pipe, each invert in is grouted to the invert out in a manner allowing for the best possible smooth transfer of flow.
A. To give a specific example, assume you have a 24" RCP (reinforced concrete pipe) coming in at 2% and another 24" RCP leaving at 5%. The box is precast with 4" sidewalls. Assume a 0.2' fall across the box with inverts grouted from one to the other. The pipe coming in has an invert of 100.00' (To keep calcs simple, we'll assume there is no pipe bell) The 24" RCP wall thickness is 2.6" or 0.22'. Minimum invert of the box would be 99.80'-0.22'(wall thickness) - [(5%x0.33') (pipe slope in box wall)]= 99.56'. To not take account of pipe slope in the box wall and pipe wall thickness would cause problems in casting of the box, though it could be done. By not doing so, you are 'pushing' the pipe down into the bottom of the box and casting that is difficult. Typically, these boxes arrive on site with knockouts larger than the pipe diameters and casting the box invert to the invert of the outlet pipe invert would cause problems in the field.

3. We typically do not put sumps in the boxes due primarily to possible problems with mosquitoes inbetween storm events. In concept, catchments are a smart idea, but if they are not maintained, then all a sump does is give you a little bit longer a time before a problem from debris occurs.

4. In 14 years, I don't think I've ever worked on a storm system that matched pipe crowns, though I would see it as a more optimum method, though possibly a bit more difficult to construct. We are required to maintain HGL within the box and below grade at all points between boxes, but not within the pipes. This puts pipe in pressure flow conditions at times, which can cause scour in misfitted pipe joints, but there has not been enough of this to cause a change in the rules. We provide a minimum of 0.5' of 'buffer' in our computations as a protection measure. PECPESC's comments on HGL and engineering practice are certainly worthy of note.

In summary, local conditions, regulations, pipe material, box construction, soil, etc. affect the overall design and the points made by all are worth consideration, but do not supercede. My comments reflect my own experience and are not meant to impune other's methods, experience or comments.

Hope this helps. Jeff Foster, PE
CE Group, Inc.
Apex, NC

 

[PECPESC]

From Pg 145 of the 1970 (note date) "WPCF Manual of Practice No. 9 Design and Construction of Sanitary and Storm Sewers"

Quote
It was FORMERLY (Emphasis mine) ordinary practice to allow an arbitrary drop of 0.1 ft in the invert across the manhole, or a slope of 0.025 (2.5 cm/m), regardless of the slope of the adjacent pipeline. .... The drop is, in fact, objectionable for it causes excessive turbulence just where it is least desirable.... etc
End Quote

This practice was found invalid prior to 1967 but a lot of communities still adhere to this because that's they way it has always been done. Lay the pipe straight through if the in/out are the same size.

 

[gibfrog]

A most excellent discussion. Is is my standard practice to vertically offset the pipes to account for the minor losses due the inlet/out losses due to the end finish/geometry (I always use a rounded bell type), horozontal alignment angular losses (ie bend losses) and sudden enlargement/contraction in the MH chamber. These losses are very small due to the low velocities achieved on the Poorly Drained Coastal Lowlands of Florida (aka swamp land). Usually a drop of 0.02'-0.04' is sufficient at velocities of 2-4 fps.

I have nothing really to add to the HGL discussion. Good advice. Kudos to all. Clifford H Laubstein
FL Certified PE #58662

 

[sgwpe1]

Hi. I thought I posted this before, but perhaps did not do it right, so I'll try again. Quite some time ago I was sitting in my car in a VERY heavy rain. While waiting for the storm to slacken, I observed a series of inlets on a steep street. Water was erupting from each inlet and actually rising, what I recall as several feet into the air, I think with each geyser a little higher going down hill, if i recall correctly. My friend asked what that was all about, and I indicated that I believed connecting the crests of each little geyser would describe the HGL of the surcharged system. Quite the eye openner, as opposed to seeing it on paper.

 

[cvg]

you are correct - the HGL (and the EGL) of the surcharged system is above the gutter elevation. However, it is not likely that this was caused by some small offset in the catch basin as discussed above. It's likely that the pipe capacity was too small therefore the HGL was above the gutter elevation - this caused the water to "erupt".

Suggested better design would be to increase the size of the pipe and possibly flatten the slope. The HGL should be maintained a foot or more below the grate or lip of the inlet. Plot the EGL and compare it to the street elevation. If it is much higher than the HGL, than consider methods to reduce the velocity. Consider limiting your velocity to no more than 10 or 15 feet per second. This could be done by a series of drop manholes.