Grate Inlet modeling w/ HydroCAD?

[SCEngr1]

Has anyone used HydroCAD to model grate inlets or Catch Basins in order to verify that their capacities are adequate for a given storm event?

HydroCAD tech support keeps sending me back to the general support on their website.

I want to keep the model simple. I'm not sure how to deal with the tailwater issue nor any overflow. Does overflow mean that the grate inlet is undersized and will back up (pond) with storm water?

 

[francesca]

I used manufacturer's guidelines to check grate capacity. As long as the catch basin isn't backing up into the street, you shouldn't need a tailwater condition at your grate. The biggest issue we had with grates was spread on small slopes at interceptor catch basins as the water hadn't concentrated sufficiently in the gutter and was bypassing the catch basins.

HydroCAD should be able to check inlet conditions to make sure that the catch basin headwater depth doesn't get too high, although we used a simple nomograph for headwater depth, which is fairly easy if you don't have too many inlets.

If you have overflow I would suspect that your pipe is too small. Bypass flow is usually a spread problem due to a gentle road slope maintaining sheet flow.

 

[SCEngr1]

Does your model start with inflow from a subcatchment into a "zero-storage" pond node (representing the grate inlet) routed directly into another "zero-storage" pond node and then to a storage pond or reach?

Also, do you have a website link that I can go to for nomographs for headwater and info. on tailwater/headwater?

 

[francesca]

Do you have your city/county/state drainage manual? That's where we got our nomographs from. The Nashville one is fairly comprehensive, though in the process of being updated.

http://www.nashville.gov/stormwater/SwMgt_ManualVol01_1999.htm

I haven't modeled storm drains in HydroCAD except for one or two exercises in a college class. I did find this, though:

How do I model a catch basin?

A catch-basin is best modeled as a pond with a culvert outlet. If the catch-basin provides negligible storage, it can be modeled as a "zero-storage" pond. Or you can evaluate the detention effects by defining the pond storage, including any above-ground storage that is used when the basin overflows. (Always enter enough stage-storage data to prevent a "data exceeded" warning.) This will allow a more accurate culvert analysis to be performed on the pipe, including the effects of headwater and inlet losses. In some cases the Dynamic Storage-Indication routing procedure may be used to handle varying tailwater conditions. More info.

That "More info" link seems pretty useful.

 

[psmart]

SCEngr1, I think you're trying to put too much detail into the model. I agree with francesca: Use the manufacturer's data to check the grate capacity. This will generally be more accurate than any orifice/weir simulation.

Although you CAN do grate/orifice modeling in HydroCAD, this is generally as an outlet control device on a detention pond or storage area. In most cases, the HydroCAD model will include the catch basin with a culvert outlet, but the inlet grate is omitted from the model.

Remember that HydroCAD deals only with "outlet" controls. If you do want to model the "inlet" grate this must be done as an outlet control on an above-grade "pond". Although this is possible, is can get unnecessarily complex, and is rarely done. For complete details please see "What about the grate?" at

http://www.hydrocad.net/sewers.htm

This is the exact question you've asked...

 

[TStringer]

If you want to model an inlet the correct way, that is computing inlet capacity, Q capture, bypass and carryover as well as gutter spreads, you need to use the right tool for the job. Most reviewers are going to expect you to produce those results. Hydro-cad wasn't designed for that. You'll save a lot of time and frustration using dedicated storm sewer modeling software. Try Hydraflow Storm Sewers. It costs a whopping $495.00. For complete details check out

http://www.intelisolve.com/stormsewers_overview.htm.

 

[francesca]

Well I used to use a spreadsheet for my spread calculations. I'm sure that even with Hydraflow you have to input the manufacturer's equations for the grates, so for free you may as well just set up a spreadsheet. Heck, you can even cut Microsoft out of the deal and use Open Office.

 

[geosavvy]

Open Office is excellent, and its free. What's this world coming to...

 

[Callin]

I find that the spreadsheets developed by Urban Drainage and Flood Control District are very useful for calculations at inlets.

You can go to their webpage and download UD-Inlet.

http://www.udfcd.org/downloads/down_technical.htm

Maybe this link will work also.

http://www.udfcd.org/downloads/software/UD-Inlet_v2.14b.zip

Colin McKernan

 

[SCEngr1]

Thanks for the help! Very valuable!