Trench Drain Modeling

[arbitrader]

I am new to Hydrocad and am looking for recommendation as to best way to model a rectangular trench drain.

Total drainage basin is 36' wide by 600' long with the trench drain running N/S down the center for the full 600' length.

Paving is sloped from the west, and the east at 1% to the trench drain.

Any suggestions are greatly appreciated.

 

[psmart]

How you setup the model depends on your objectives. What aspects of the trench drain are you trying to model? Storage and detention? Infiltration? Underdrains? Overflow pipe?


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

I forgot to include another option: The trench drain could also be modeled as a flow segment in the subcatchment Tc path.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[arbitrader]

I want to ensure trench dimensions are sufficient to prevent overflow, and also size discharge pipe. There is no infiltration/exfiltration.

Inflow to the trench is entirely from surface runoff, discharge pipe will be located at one end.

 

[psmart]

In that case, you can model the trench as a pond with a culvert outlet.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

Peter - I'm not sure that's the best approach. There's conveyance in the trench drain. If he makes his outlet culvert big, but his trench drain small, the trench drain itself could control and HydroCAD wouldn't pick up on it if he models it as a pond. Do you agree, or am I missing something? There's an HGL in the trench drain, and there isn't one in a pond, and that HGL can be quite steep in narrow prefab trench drains. Nobody wants to install a big one because they cost so much more.

The easy way to make sure you're good is to calculate your peak flow with the rational method, consider that to be design flow for the trench, and size the trench width and depth to carry that flow with the manning's equation for trapezoidal channels, assuming vertical walls. Then size the outlet culvert with manning's as well. It also couldn't hurt to take a look at the grate.

I'm sure there's a way to do a more detailed analysis in HydroCAD as well, and Peter would probably have some better ideas about that.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[psmart]

beej67 - I agree, it all depends on the flow regime on the trench. My pond scenario assumes the outlet is sufficiently small in relation to the flow rate and trench volume so as to create a level water surface. Otherwise the trench conveyance could be the dominant effect.

In order to answer the original question we really need more info about the trench and pipe. But I suspect the pipe capacity is probably less than the capacity of the trench itself, due to the smaller cross-section and pipe losses. (Assuming a typical arrangement where the pipe connects to a fitting on the side of the trench) If so, this brings us back to a pond/culvert analysis in order to assess the capacity of the pipe and the headwater that will exist for a given design flow.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[beej67]

I hear you Peter. He really probably needs to check both.

Most of the time when owners hear "trench drain" they're thinking 4 inch. If he uses a 4 inch wide trench drain at 1% slope and 1 foot depth, he can only convey around 1.2 cfs. (mannings) That's about a 1 year storm down here.

My experience with trench drains is that your client always wants to put in one smaller than you need, and when you're dealing with drainage areas of half an acre or up, you usually end up with the biggest one in the catalog. My experience is also that whenever you get the things sized, the client doesn't want to pay for what an adequate one will cost, and you end up changing your grading to push surface flow to an inlet instead.

From the top post, it sounds like 300 feet of pad on either side of a trench drain, at 1%. That's 3 feet of fall across the pad. If he can manage another 4 inches of fall, and if grade changes are an option, the thing to do is probably pitch the center back to one curb and use a drop inlet instead. If he's trying to stick this in as a retrofit to an existing drainage problem, then the trench drain is likely the best way to go.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com