Catch Basin to Catch Basin Flow

[lenny189]

Can anyone give me a reason to not allow catch basin to catch basin flow in a storm drain system? Our county code and surrounding areas do not allow it, however, no one can give me a reason why.

I want to pipe from catch basin to catch basin to a retention pond,from a street along a property line into a golf course. The catch basins are behind the curb, curb inlet only, with a circular lid. The county engineer is asking for hydraulic calcs showing it works, but I'm not sure how to model this. Any suggestions?

 

[CarlB]

There shouldn't be a problem hydraulically. Catch basins may be viewed as more likely to clog with sediment (if not cleaned regularly) or more prone to freezing (since shallower). Perhaps you could make the second CB deeper, call it a "catch basin manhole" and then get by the regulation.

To model it I would determine the peak design flow, work from downstream, determine water elevation in CB's to provide necessary hydraulic head, include pipe losses and manhole/sstructure junction losses, and if water levels don't rise in structures to some unacceptable level then it shows that it "works".

 

[cvg]

it can work, but you need to maintain sufficient depth in your basins to account for the additional headloss that you will introduce into your storm drain. The key is to properly estimate the headloss that occurs at each basin as flow enters from the pipe, expands and then flow from the inlet above flows in and combines.

 

[Hoagie]

Good responses..Also, there may be a minimum HGL freeboard requirement at structures (commonly one-foot in my locus)..check with your regulator/s.

 

[gbam]

lenny189
Look for FHWA's Hydraulic Engineering Circular No. 22, HEC22. The approach outlined in chapter 7 defines methodology to determine headloss and water profiles using "Access" Holes of any shape.

http://www.fhwa.dot.gov/engineering/hydraulics/library_arc.cfm?pub_number=22

BTW, the only reason I can think of why not to connect CB to CB would be access for maintenance. Manholes are more accessible.

 

[francesca]

It was standard practise to connect catchbasins where I worked in Middle TN. As long as the downstream pipe could convey the cumulative flow from the upstream pipe and the inflow from the catchbasin, there shouldn't be a problem. (Perhaps a headwater depth calculation would work here.)

As a rule, we didn't have a large, flat pipe flow into a small, steep pipe, but maintained pipe diameter or increased it. The former city engineer and I debated this principle with sewer pipes and he believed that it was acceptable to have a downstream pipe with a smaller diameter if its gradient was sufficient to achieve scour velocity and to convey the flow. It's hard to argue with a city engineer with 30 years' experience, but I never did call for a smaller downstream pipe!

 

[JasonG]

Where I am, in New England, every storm drain system is CB to CB. I'm interested to know how it's done otherwise?

To answer the modeling question, you would use the rational method to calculate the peak flow to each catch basin. This flow would be used to size the drain pipe that is fed by that basin using the manning equation (open channel flow).

Since each pipe is sized to handle the incoming flow then no hydraulic calculations are needed. Each pipe operates independently.

As a general practice, each down stream pipe should not be smaller then the upstream pipes. Though, as francesca points out it could be done.

 

[cvg]

no hydraulic calculations?
always open channel flow?

this sounds too good to be true!

 

[JasonG]

From my perspective, the need for hydraulic calculations sound too complex to be true!

 

[lenny189]

Thanks for all the input. I'm located in Nevada, with a very arid climate and see little rainfall. We are typically required to design our conveyance systems for a 5-year event and onsite retention for a 25-year event. I typically design my pipe systems to flow as near to half full as possible for a 5-year event, with a minimum cleaning velocity of 3 ft/s. Generally even with a 25-year event my storm drains remain in open-flow conditions. We typically run our storm drain systems down the center of our streets allowing for a manhole between catch basins, however, in this case we are not.

So, as long as the pipes are sized to remain in open flow conditions any headloss due to flow thru a catch basin are not applicable, and showing inflow calcs for each catch basin, and flow calcs for each pipe should be adequate. Agreed?

 

[JasonG]

I see what you mean. We usually put the drain along the side of the road CB to CB.

I agree with your summary.

 

[cvg]

most of my clients require us to design the pipe flowing nearly full. Designing for half full conditions would require significant extra money to oversize the drains. in addition, many areas (around here) just don't have sufficient slope to maintain open channel flow conditions in the pipe. Outfalls are also commonly designed with a tailwater which can cause water to back up in the storm drain system. Therefor, many storm drains are designed under pressure flow conditions.

In addition, you will have an inlet loss from the basin into the pipe. We are required to have 12 inches of freeboard between the lip of the inlet and the water surface in the catch basin. With larger flow through basins, you need to maintain a deep enough profile to provide this freeboard.

 

[JasonG]

My favorite thing about this forum is learning how different "standard" engineering practices are in different parts of the country, and the globe!

I also size the pipes to flow "full". A pipe's most efficient cross section is something like 95% full. Which is still considered "open channel flow".

We are generally required to use a minimum slope of 1%. I can see how hydraulic calcs could be warrented with a flat slope that would result in a full pipe under pressure.

We are also required to have 3 feet of cover over the pipes, so freeboard isn't usually an issue.

My suggestion to Lenny189 then is to decide if your scenerio is more like that which CVG described or like what I described. That will determine how to approach the problem.

You could always use HydroCAD, or similar, to model the system and see how it works out. Good luck.

 

[TerraSouth]

A minimum slope of 1% !!

Wow, I am constantly designing pipes with 0.3% slopes,
and as long as I can get a velocity of 2 fps, I design them as flat as possible. I design for near full flow for the 25 year storm event (11 inches in 24 hours)

I work with minimums every day. Minimum slope and minimum cover. It is get it done as cheap as possible, while meeting the regulator requirements. It's darn flat around here, and the cost of materials is going throught the roof.

I design systems catch basin to catch basin also, just make sure I put a juction box between runs longer than 300ft for 24" or less pipes trying to keep maintanence in mind. Get yourself a copy of Hydro-Cad stormwater modeling software or something simular and you can model the system to your hearts content. You can make it work.

 

[saruman26]

You should be using a software program to model and analyze the storm sewer network. After having worked with multiple software packages, I think the best two are StormCAD by Haestad Methods, and Hydraflow StormSewers 2005 by Intelisolve.

Of course, each as pros and cons.

StormCAD for 25 inlet structures is rather expensive, but has the most intuitive interface. It allows to you look at your storm sewer network from an inlet-to-inlet viewpoint, and allows you to specifiy your own structure numbers for all objects. Also, it interfaces very well with the Pipes database tools in the Civil Design modules of AutoCAD Land Desktop. You can import/export the network both directions (ACAD to StormCAD, StormCAD to ACAD).

The Hydraflow StormSewers is a less expensive and somewhat less intuitive option that works well, too. It cannot interface with the Land Desktop software, and does not allow you to assign your own structure numbers or line details to the analysis runs. Instead, it examines each pipe and the accompanying upstream inlet as a "line", and sequentially numbers each line. The data can be difficult to track sometimes because of this.

Both packages allow you to define inlet characteristics (size, configuration, drainage areas, c-factors, etc.) and pipe parameters, and also has design features that can size pipes and inlets through an iterative process, based upon user-entered and pre-programmed constraints.

Regardless of what you choose, eachn will work for perfectly well for hydraulic modeling of a conveyance network.

 

[JasonG]

HydroCAD is also a good software. One advantage of HydroCAD is that it uses the SCS methdologies (TR-55, TR-20) which is required by our local agencies.