Modeling a culvert with groundwater always present in channel

[Akel]

I'm trying to design a culvert for a road that will cross an existing concrete storm drainage channel. The ground water table is above the channel bottom so groundwater is always present in the channel and occupies 20%~50% of the channel depth.

How do I approach this problem? How do I account for the water present in the channel?. How will it affect my design flow, manning's n value, channel dimensions etc... and what software could be used for design ?

 

[TerryScan]

Yes, one should account for water in the channel. My initial inclination would be to model it as base flow. I would also be inclineded to contact the reviewing agency to see what software they want used for the analysis.

 

[Ekenny_G]

If groundwater is always present, it should be modeled as base flow. What modeling software are you using?

 

[Akel]

The reviewing agency does not put restrictions on what software to use as long as I can provide readable results.
In my office, we use Bentley CivilStorm/SewerGems as well as HY-8.

 

[ACtrafficengr]

Could the channel owner provide existing flow rates?

My glass has a v/c ratio of 0.5

Maybe the tyranny of Murphy is the penalty for hubris. -

http://xkcd.com/319/

 

[Ekenny_G]

If the base flow is known (cfs), you can add it to your design flow to size your culvert. However, if only the groundwater depth is known, you can adjust your required Freeboard or Headwater elevation (HW) to account for the groundwater flow depth.

 

[Akel]

No flow rates are not known, only water depths. So if I understand correctly, I will design the culvert using standard HDS-5 procedure and assume there is no groundwater in the channel. Then I'll add additional freeboard equal to the groundwater depth in the channel?

 

[cvg]

you cant assume there is no groundwater in the channel, at the very least you would assume 20% as you stated originally

 

[gbam]

If you know the configuration of the existing channel you can approximate the base flow using flow depths. Typically, when base flow is present in a channel, I design the culvert for the appropriate return period flood event plus the base flow. This way the culvert can handle the combined flow rate.

 

[Akel]

Thank you for your help everyone.