How to obtain inverts elevation for storm sewer profile? 5

[3Hugger]

I have found my sizes for the storm pipe and would now like to design the profile ie. obtain inverts of pipe at each manhole along the pipe lenght. I am using 0.26% slope and have the upstream starting ground surface elevation. For instance I don't know how to obtain inverts when two different size pipes connect into a manhole. I have searched for infomation and got only " approximate method is to place 0.8 depth of flow point of both pipes at the same elevation," and "invert of larger pipe should be lowered significantly so as to maintain the same energy gradient." Sorry I tried to be brief.

 

[civilperson]

If you match the 0.8 height of the diameter of two different size pipes you have accomplished the intent of setting the larger pipe invert at a lower elevation. This arises from the maximum flow in a pipe, (occurs when 0.8 height is fluid level), thus the hydralic grade line is close to level at the connection through the manhole.

 

[bpattengale]

First off you have sized your pipe and you say you are using a 0.26% slope, have you checked to see if you are getting at least 3 ft/sec velocities for self cleansing in your pipes? Anything smaller than a 18-inch diameter pipe is not going have that velcity with that slope.

As far inverts at manholes with changes of pipe size it is accepted practice to either match the crown(top of pipe)or 0.8 depth of each pipe.

If you don't change pipe size, but have a direction change a 0.1 ft drop in elevation across the manhole structure is acceptable.

 

[cvg]

note that maximum flow at 0.8 is only true when the pipe is not flowing under pressure. If it is surcharged, than you could match either the inverts or the soffits. Matching soffits will minimize depth of trench. Matching inverts provides a smoother channel for lower flows. This decision may be mandated by the agency approving your design (if this is in public right of way.

 

[LCruiser]

Matching inverts also maximizes slope.

 

[nutbutter]

It's common practice in my office to match the crowns of the pipes when going to a larger pipe at a manhole. Additionally, some municipalities like to see a certain amount of "drop" across a manhole. This can vary depending on the manhole configuration....whether there's a change in direction or not. With a 90-degree turn at the manhole, I think we used 0.2 ft. With a straight shot, you don't really need anything, but some municipalities require something.
Peace,
Stoddardvilla

 

[gbam]

I'll chime in on this topic. Matching soffits/crowns is the typical method used due to the hydraulic computations of the past. As CVG stated matching inverts is perfect for open flow systems. In addition, your agency may require additional drops through structures like manholes or catch basins.

With the introduction of HEC22 (FHWA) the hydraulic procedures no longer depend on matching soffits. Now you can lay out the system as necessary. Definately check with your agency for their standards.

 

[cr1973]

It's only storm sewer, don't over analyze it. The water will flow regardless.

Odds are that the contractor in the field won't be dead on if you've got your storm lead inverts say .1' above the outpipe invert in a manhole.

All you need to worry about is trying to keep your inverts within a foot or so of the out pipe, otherwise you'd need an inside or outside drop. This distance is set in stone, check your local standards. They sometime measure from the invert of the incoming pipe to the springline (1/2 height) of the outgoing pipe.

 

[saruman26]

There are a few simple rules for storm sewer design.

1. All inlets/manholes should have a vertical drop of around 1-2 inches (minimum) across the structure between the invert in and invert out.

2. When pipes of different sizes connect to a structure, always try to match crown elevations. (i.e. an inlet with an 18" pipe in and a 24" pipe out should have 0.5' of drop across the structure)

3. If there simply is not enough depth within a structure to match crowns, at least try to match pipe centerlines.

4. In most jurisdictions, the minimum pipe slope is 0.5%. This standard may be relaxed in difficult circumstances, or if the pipe diameter is relatively large (36" and up).

 

[pmorrow92]

The bottom line is to satisfy the requirements of the agencies you will have to obtain permits from and to make it as cost efficient for your client as possible. I find it easier for me to work from both ends at once. If you have your starting ground elevation and the normal water level of your outfall (starting invert) you should be able to deduce how deep your starting invert should be based on 3 main criteria; will I maintain sufficient cover over the pipe for the entire run upon obtaining final grade, do I have enough slope to control pipe velocity per municipality code, and will this design interfere with other utilities (mainly your sanitary sewer).