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Modeling a reach through a flood plain

Modeling a reach through a flood plain

Modeling a reach through a flood plain

(OP)
I have a situation in which each of 3 subcatchments drains through a respective culvert and into a 4th subcatchment, and then that 4th subcatchment drains through a 4th culvert.  This 4th basin is a large flood plain and would fill in during larger events.  When this happens, the "reach" (trapezoidal section) fills in and I get an error message saying that the (4th) culvert exceeds the reach inflow depth by 1.96 feet (warning message #63).  Intuitively, it makes sense that the reach would fill in, and then the entire subcatchment would continue fill in.  Do I seem to have modeled this situation correctly?  It's an existing-conditions model.

RE: Modeling a reach through a flood plain

A reach always operates under normal Manning's flow as discussed at www.hydrocad.net/reach.htm  The warning message indicates a tailwater effect that is not being handled by the model.  You might be able to model the flood plain as a pond with a culvert outlet, but this assumes that the flood plain is effectively a level pool.  If there are more complex tailwater issues, you may need to do a full surface water profile with a model like HEC-RAS.  It really depends: What is the ultimate objective of your analysis?

You also didn't indicate if your pipes are being modeled as a circular reach, or as a culvert outlet.  For details please see www.hydrocad.net/pipes.htm

RE: Modeling a reach through a flood plain

(OP)
Thank you for your quick reply!  The ultimate goal of this model is to determine (a) whether the existing culvert is appropriately sized or needs to be replaced with a larger one, and (b) to determine whether runoff will increase from pre-development to post-development conditions at the analysis point (which is at a property line).  I've analyzed the culvert as a pond with culvert outlet.  It's an 18" cmp (INV 99.8)at s=0.0040 and the model is showing that the peak elevation behind the culvert is up to 102.10.  The roadway is at 102.70, so the roadway is not being overtopped.  Qin is at about 125 cfs and Qout is at about 7 cfs.  Even for such a slight slope, the Qout does not make sense, especially with the head built up behind the culvert.  The total inflow area is about 45 acres and the culvert has about 17 acre-feet of storage behind it.  I'm not sure why I'm getting such a low Qout...

RE: Modeling a reach through a flood plain

I'm not sure what you mean by "doesn't make sense".  Are there any warning messages?  Also check the pond summary carefully to verify all your input.  Otherwise, it sounds like you've got a very effective detention pond.  If you have specific questions you can send your HydroCAD .HCP file to support@hydrocad.net

RE: Modeling a reach through a flood plain

(OP)
Thank you again for your reply -

The warning message is that the "culvert exceeds the reach inflow depth by 1.96 feet (warning message #63)."  Should I model the culvert differently, or does my approach seem ok? Ultimately the problem is that a subdivision is planned for within this flood plain!  It is a requirement for this particular town to model the 100-year (6.7") storm event.

RE: Modeling a reach through a flood plain

You may be able to use a pond with a culvert outlet, but a reach is never taiwlater-sensitive.  Please see my first reply...

RE: Modeling a reach through a flood plain

gwool,

What are the tailwater conditions downstream of the 4th culvert?

What is the length, depth and slope of the channel section through the 4th subcatchment?

It certainly sounds like it should be modeled as a pond with culvert.  I just wonder if you are taking all necessary factors into account.

RE: Modeling a reach through a flood plain

(OP)
Hi TerryScan -

The 4th culvert empties into another smaller floodplain, which I have modeled as a Reach: L=335', s=.0031, trapezoidal channel 75' wide, 1' deep, side slopes 20/1. Again, these are existing conditions and everything seems to be working properly as-is, so I'm not sure what the error is indicating.  The data is limited, which could account for some problems - I'm working with a 2' contour aerial survey.  Also, I have not modeled the culvert to which this final smaller drainage basin discharges.  I had added it briefly to see if that might clear the error message, but it just added more.  

RE: Modeling a reach through a flood plain

Message #63 indicates that the reach is subject to a tailwater condition, and that the reach routing is not able to respond to tailwater.  (For full details, click the message or see page 115 of the Owner's Manual.)

A possible solution is to model the reach as a pond with a culvert outlet.  This assumes that the culvert restriction is sufficient to create a level-pool scenario.  In this case, you would use a single node (a pond) to model the reach and culvert.  In your current setup you are expecting the reach to "back up" because of the culvert, but the assumption of normal Manning's flow makes this impossible.  To model the tailwater effect you would need to use a pond, or do a complete water surface profile with a tool like HEC-RAS.

RE: Modeling a reach through a flood plain

(OP)
Hi psmart -

I think I understand what you're saying and it makes sense.  The bottom line is that the reach upstream of the culvert is, by definition, under Manning's flow, whereas by adding a pond with culvert outlet I'm telling the model I don't WANT Manning's flow.  To correct this I should model the entire reach as a pond with a culvert outlet (as one node) rather than having a reach followed by a pond with culvert outlet (two separate nodes).  It this correct?  Thanks!

RE: Modeling a reach through a flood plain

Hi gwool,

Yes, that's what I was trying to get at.  The stage-discharge relationship for a reach is calculated strictly by Manning's equation, so it can't respond to the culvert restriction.  But if the water impounded by the culvert creates a level pool, you can model this as a pond with a culvert outlet.

RE: Modeling a reach through a flood plain

(OP)
Great!  Thank you!

RE: Modeling a reach through a flood plain

(OP)
psmart -

I just had one more question:  I replaced the reach + pond/culvert with just the pond/culvert.  It worked!  I started thinking...isn't the travel time affected now?  The reach I replaced was quite long - about 1000 ft.  The travel time for the reach I removed was about 12 minutes.  Is there a way to account for some travel time now that I've removed the reach?  Perhaps somehow put in a direct entry?

Thanks again!

RE: Modeling a reach through a flood plain

Yes, you may want to include a separate reach to allow for the travel time and possible attenuation. The length should reflect the part of the channel that is not inundated by the culvert.

RE: Modeling a reach through a flood plain

(OP)
psmart -

Thank you so much for all your help!

I looked at adding back a shorter reach and it seemed almost trivial because the length would be so short ~ the inundation level behind the culvert extends for a considerable distance.  I think I'll leave the model as is...

I'll be sure to post my questions again in the future - this has really been a valuable learning tool for me!  Many thanks again!

RE: Modeling a reach through a flood plain

gwool,

Also be sure to review the articles at www.hydrocad.net/support.htm  You'll find lots of useful information.

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