HydroCAD Infiltration Cell Modeling

[craleigh]

I have a storm water scenario I'm having trouble with. I have a wet pond, nwl with an 8" outlet at elevation 100. At 102, the pond overtops (weir flow) into an infiltration cell whose bottom is 101. So at elevation 102.1, my wet pond is 2.1' above the outlet and there is 1.1' of water in the infiltration basin. I have 27" horizontal orifice overflow at elevation 103. Let's just say my wet pond surface areas are as follows: 100=10000 sf, 101=11000 sf, 102=12000 sf, 103=20000 sf (wet+ area above infiltration), 104=22000 sf (wet+area above infiltration), 105=24000 sf (wet+area above infiltration). The infiltration cell areas are 101=5000 sf, 102=6000 sf.

How do I model this in HydroCAD?

Here's what I've tried: I used 2 ponds (1 wet, 1 infiltration). My wet pond had a primary orifice (8") at 100, a secondary outfall (weir) at 102, and another primary outfall (27" horiz. orifice) at 103. My infiltration pond only had exfiltration as an outlet because what doesn't exfiltrate will keep on filling up with the wet pond. However, when my infiltration pond filled up, I couldn't model it to go "back" into the wet pond as it was a circular reference.

So then I tried modeling it as one big pond. 100=10000 sf, 101=11000sf+5000sf, 102=18000sf, 103=20000sf, 104=22000sf, 105=24000sf. I had my 8" outlet at 100, my 27" outlet at 103, then exfiltration between elevations 100 and 102 and I used a discharge multiplier of 0.3333 (since the infilt pond was about 1/3 of the size of the whole pond system.) This at least 'worked' in the model but it didn't give me very high infiltration volumes so I question if that was the correct way to do it.

Does anyone know the 'right' way for a scenario like this? I know HydroCAD has provisions for embedded storage but I'm not sure if that is what this would be.

 

[psmart]

When you tried the 2-pond solution, were there any warning messages? I'm guessing you got a tailwater warning. If you click the message for details it recommends that you need a trailwater-sensitive routing procedure, such as DSI. This will allow the overflow from the wet pond to decrease as the WSE rises in the infiltration area. The water doesn't actually "go back", so you don't need a reverse outlet. It just flows more slowly due to tailwater.

Modeling as 1 vs 2 ponds depends on the relative volume of the areas. If the wet pond provides significant detention below the weir, you will need 2 ponds to properly simulate that effect. But if most of the volume is in the infiltration area, the storage of the wet pond may not have a significant effect on the hydrograph routing, and you may be able to use a simpler 1-pond solution.

As far as the exfiltration areas, rather than taking one-third of the exfiltration, just un-check the "allow exfiltration" box for the wet pond storage definition.

It's also important that you address any other warnings. Most problems (and their solution) are identified by warning messages. It's always the first thing I look at...


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

Thanks for your response but I'm not completely following you. When I ran the infiltration pond, I got the message that the pond wasn't calculated because the storage volume had exceeded the 102. In one of my trials I got a message to try sim-route and I gave that a whirl too but it didn't model it correctly. (The infilt basin elev and the wet pond elev should be the same above 102 and they weren't.)

There is significant detention in the wet pond below 102 so in your second paragraph it sounds like I would want to model it as 2 ponds. And I realize it doesn't "go back" into the wet after filling the infilt but I wasn't sure how to describe that. Past 102 they need to fill up together and I just can't get it to do that. Thanks for the response!

I'm not sure about your 3rd paragraph. I want to have exfiltration in the infilt basin part of the pond so I think I need to allow that?

Thanks!

 

[psmart]

You must define the storage volume ABOVE the weir in order to get an accurate routing. That's the reason for the error. For details click the Help button on the error box and see

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

Scroll down to the section about weirs.

Your initial approach is reasonable, but you need to resolve each specific error or warning according to the details. (just click the message or Help button) Jumping to another approach is not helping you solve the problem.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

I do understand all of that but there is no storage above the weir...above the weir, it's all part of the same big wet pond. I attempted having 2 ponds and splitting the storage areas...imaginary wall between infiltration and wet but those two really act together above the weir so I don't think that is the right way to do it. I'm not positive we're on the same page...wish I could attach a sketch or something...thanks for the help anyway!

 

[psmart]

Since you're modeling as two separate ponds, you need to include the storage in each pond above the weir. As you suggested, use an imaginary wall above the weir crest to delineate the extent of each pond. Make sure you extend the data ABOVE the maximum WSE.

Please review my previous link regarding weirs and the storage definition. Although this example deals with storage on only one side of the weir, describing storage on both sides is exactly the same.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

I reviewed all of the info in that link. I also defined storage on both sides of my imaginary wall up to 105. The problem is when I analyze the same event in each pond, the water surface is reaching 102.94 in my infiltration pond and 102.29 in my wet pond. Anything over 102 should be at the same level in both ponds.

 

[psmart]

Are there any warning messages?


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

There are no warnings when I run the wet pond. The warning on the infiltration pond is that I'm exceeding the wet pond by 2.07'.

 

[psmart]

What is the EXACT message? Did you click the message for details and recommendations?


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

(81) Warning pond 3P (infilt) exceeded pond 2P (wet) by 2.07' at 19.95 hours and I clicked the message for recommendations. Trying sim-route now...

 

[psmart]

Why are you using SimRoute? You should use DSI, which is more stable. Please read the help topic that is attached to the message carefully...

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

The help message said to try a different routing method. I was using DSI, that was when I was having trouble. So I tried sim-route. I feel like I'm getting closer with that...the pond elevations in the 2 year event I've been modeling were very close to the same. Also the help menus with I'm getting on the sim-route warnings seem to fit my scenario much better. Too much headache for a Friday afternoon...I'll revisit on Monday.

 

[psmart]

Sorry, but message 81 only occurs when you're using the default Storage-Indication routing procedure. You're not giving me accurate information...


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

I think you are misunderstanding me. I was using DSI when I started this and when I got warning 81. I double clicked on the warning to get into the Help menu. Once I was in the Help menu for that warning, I read the following "At some point during the routing, the node's elevation has exceeded the elevation of an inflowing pond, indicating a possible tailwater dependency. The message shows the maximum amount of reverse head and the time at which it occurred. Important: The pond routing is not altered by this situation, even though the higher tailwater may in reality cause a reduced discharge. To remedy the situation, select a pond routing method that is able to handle tailwater effects" So then I decided to try Sim-Route. I've done a 'save-as' so I can compare the results of the different routing methods.

 

[psmart]

I understood. But your information is wrong. Message 81 only occurs when using the Storage-Indication method. If/when you get message 81 the recommended solution is to switch to DSI. After changing to DSI you may still have warnings (perhaps 80?) but it won't be 81.

In any case, DSI is the recommended routing procedure for your situation. For details please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

.... I attempted having 2 ponds and splitting the storage areas...imaginary wall between infiltration and wet but those two really act together above the weir so I don't think that is the right way to do it. I'm not positive we're on the same page...wish I could attach a sketch or something...thanks for the help anyway!

I believe this IS how you want to model this in HydroCAD. Two ponds with the "imaginary" wall between for storage calculations.

However, I think there IS the possibility you need to use Sim-Routing. During a larger storm event, if the storage rises above the connecting weir, and the "wet" side is discharging significantly quicker than the infiltration discharge, there will be a REVERSE FLOW from the infiltration node back to the "wet" node. This can only be accomplished with a sim-route. (see "Reverse Flows" in help and

http://hydrocad.net/simroute.htm)

 

[psmart]

TerryScan: Good point. From the information given it wasn't clear if the "wet" side could discharge faster. But if this should occur, HydroCAD will alert you to the reverse-head condition, and you can consider other modeling options, as you suggest. I just don't want to encourage folks to jump to SimRoute until/unless it's really necessary. Most tailwater situations do not involve a reversing flow and are better handled with the Dynamic Storage-indication method.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[craleigh]

I haven't had much of an opportunity to work on this model today but I just took another look at it. psmart when you were talking about the routing methods and I was reading help files and kept seeing Default Stor Ind, I thought that that is what DSI must stand for. I am now realizing you meant Dyn Stor Ind when talking DSI. TerryScan, thanks for taking a look at this as well and you are hitting the nail on the head. The wet side is discharging way faster than the infiltration discharge and that's what I meant when saying that in the large events, the infiltration flow will "go back" into the wet pond.

Moving forward - Dyn Stor Ind or Sim-Route...what's the best method?

 

[psmart]

Start with DSI and review the messages. Be sure to fix any "storage exceeded" conditions. Use SimRoute only if a significant reverse-head is reported.



Peter Smart
HydroCAD Software

http://www.hydrocad.net