Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Using Hydrocad for L.I.D. items?

Using Hydrocad for L.I.D. items?

Using Hydrocad for L.I.D. items?

OK, I've entered into the green world of "L.I.D." Or "Low Impact Development" techniques.

A common "LID" BMP is a "rain garden" or sometimes refered to as a "bio-retention" cell.

My question regarding Hydrocad...
In respect to the bio-retention cell, this facility is designed based on the water quality volume, which is close to the "first flush" theory of stormwater.  

The designer of a bio-retention cell must specify special media (hardwood mulch, for instance) that is supposed to retain and treat that 'water quality volume.'

A specification I have seen is the transmisivity of that media,  I have yet seen any designs that specify the percent voids of the media - see where I'm going with this?  

One method to incorporate this bio-retention cell into a hydrocad model would be to model it as a pond, with only a small void space, and the outlet being exfiltration.

Has anyone done this, submitted it to a regulator, and have that regulator approve your numerical model?

How did you approach the LID concept in Hydrocad?

Craig T. Bailey, PE

RE: Using Hydrocad for L.I.D. items?

Craig - I agree, it's basicaly a small detention pond with the appropriate exfiltration rate, and often an overflow device.  A lot of these have been modeled with HydroCAD.  The exact details depend on the local regulations.  I'd certainly be interested to hear how others have approached this.

Peter Smart
HydroCAD Software

RE: Using Hydrocad for L.I.D. items?

Hi Peter: Yes, I totally agree that the exact details will depend on local regulations.

Here's the trouble:  NHDES has no regulations regarding these types of storm water treatment/control facilities. (Yet.)

I have proposed one in a central New Hampshire city, and the local regulators were over-joyed to hear of a designer who was willing to incorporate LID.  The ironic thing is... they had never actually seen LID be incorporated into a site plan.  So I was on my own.

I fear that we here in the Northeast are behind the 8-ball when it comes to LID techniques.

I have used the Model Regulations that the Maryland Department of Environmental Services has proposed, and I like the outcome, but I did not attempt to model the true performance of the bio-retention cell.  I just ignored any retetion due to the filter media.

HydroCAD has become the De facto modeling system in New Hampshire for storm water analysis and runoff, and the writing is on the wall that LID, and the "Water Quality Volume" will be implemented in New Hampshire. I believe Vermont has implemented the "WQV" method, and possibly Maine.

LID is in it's infancy, and I plan to continue using HydroCAD.  I'm very interested in hearing from other designers who implement LID and model them with HydroCAD.

Thanks very much,


Craig T. Bailey, PE

RE: Using Hydrocad for L.I.D. items?

Here is the Hydrocad print from a bioretention facility I had approved.  It was an existing dry pond that we expanded and coverted.  It has underdrains.  The 35% voids is the stone in the underdain.  The 40% voids was acquired from a study of bioretention soil.  The infiltration rate through the biosoil was from our state manual.

198C002 POST BR                  Type II 24-hr QUALITY  Rainfall=2.00"
                          Summary for Pond BIOR:
       Inflow Area =      1.350 ac, 55.56% Impervious,  Inflow Depth =  0.65"    for
       Inflow    =      0.74 cfs @  12.09 hrs,  Volume=        0.073 af
       Outflow   =      0.10 cfs @  13.35 hrs,  Volume=        0.073 af,  Atten= 86%,
       Primary   =      0.10 cfs @  13.35 hrs,  Volume=        0.073 af
       Secondary =      0.00 cfs @   0.00 hrs,  Volume=        0.000 af
       Routing by Stor-Ind method, Time Span= 0.00-36.00 hrs, dt= 0.01 hrs
       Peak Elev= 87.64' @ 13.35 hrs   Surf.Area= 1,562 sf   Storage= 1,274 cf
       Plug-Flow detention time= 130.5 min calculated for 0.073 af (100% of inflow)
       Center-of-Mass det. time= 130.5 min ( 1,005.4 - 875.0 )
       Volume     Invert   Avail.Storage  Storage Description
          #1      85.00'        4,473 cf  Custom Stage Data (Pyramidal) Listed below
       Elevation       Surf.Area  Voids       Inc.Store       Cum.Store        Wet.Ar
          (feet)         (sq-ft)    (%)    (cubic-feet)    (cubic-feet)         (sq-f
           85.00             854   35.0               0               0             8
           86.00           1,100   40.0             390             390           1,1
           87.00           1,371   40.0             493             883           1,4
           88.00           1,668   40.0             607           1,490           1,7
           89.00           1,991   40.0             731           2,221           2,1
           90.00           2,524  100.0           2,252           4,473           2,7
       Device  Routing          Invert  Outlet Devices
          #1   Primary          84.92'  15.0"  x 46.0' long Culvert   
                                        CMP, projecting, no headwall,  Ke=
                                        Outlet Invert= 84.26'   S= 0.0143
                                        '/'   Cc= 0.900   n= 0.024   
          #2   Device 1         85.00'  2.830 in/hr Exfiltration over Surface area   
          #3   Device 1         89.50'  24.0" Horiz. Orifice/Grate   
                                          Limited to weir flow   C= 0.600   
          #4   Secondary        89.72'  176.0 deg x 21.0' long Sharp-Crested Vee/Trap
                                        C= 2.46   
       Primary OutFlow  Max=0.10 cfs @ 13.35 hrs  HW=87.64'   (Free Discharge)
         1=Culvert  (Passes 0.10 cfs of 6.10 cfs potential flow)
           2=Exfiltration  (Exfiltration Controls 0.10 cfs)
           3=Orifice/Grate  ( Controls 0.00 cfs)
       Secondary OutFlow  Max=0.00 cfs @ 0.00 hrs  HW=85.00'   (Free Discharge)
         4=Sharp-Crested Vee/Trap Weir  ( Controls 0.00 cfs)

RE: Using Hydrocad for L.I.D. items?

TerryScan is using a custom storage definition with the "Stage Voids" option.  This allows the voids to be set separately for each horizontal slice of storage.  (stone, soil, open water, etc.)

Note that each value specifies the voids from the previous elevation up to the current elevation.  Since there is no storage below the first stage, the first void value has no effect on the results.  You can verify this by changing the first value, and noticing that the storage is unchanged.

Peter Smart
HydroCAD Software

RE: Using Hydrocad for L.I.D. items?

Now that looks promising!

Terry reports that that approach was approved by a regulatory body.  I think this might just work for me.

Thanks Terry!

Peter - I've never turned on that "stage voids" setting.  It's like you were quite the prophet with that dialog box.  graduate

Thank you.

I now have ammunition.  thumbsup2


Craig T. Bailey, PE

RE: Using Hydrocad for L.I.D. items?

You can also vary the voids by "layering" any of the standard storage options, one directly on top of another.  This works best if you have a regular shape, such as a section of a cone.  However, you have to use the "Allow exfiltration" settings carefully in order to avoid double-counting the exfiltration area.  This complication is avoided by using custom storage and stage voids.

Peter Smart
HydroCAD Software

RE: Using Hydrocad for L.I.D. items?


Thank you for pointing out my error in entering the voids.  The intention was to have the first 0.5' use 35% depicting the stone underdrain.

Could you expand upon the comment regarding double counting the exfiltration area?  The problem is not immediately apparent to me.

RE: Using Hydrocad for L.I.D. items?

TerryScan, regarding exfiltration calculations:

When you specify an exfiltrtion velocity, you have the option to apply exfiltration to all surface or wetted areas.  When you use two or more storage volumes, HydroCAD uses the total area from all the volumes, assuming that the exfiltration velocity can be independently applied to each volume.

But if the volumes are "stacked" on top of each other, the lower surface area is completely covered by upper area, resulting in a double-counting.  To prevent this, you can remove the option to "allow exfiltration" on one of the volumes.

If in doubt, you can examine the surface-area or wetted-area curve on the stage-storage plot.  This will always reflect the exact area that is being used for exfiltration.

Peter Smart
HydroCAD Software

RE: Using Hydrocad for L.I.D. items?


In your example, you are using the bioretention facility for storm water treatment only. You do not use the infiltration as part of a groundwater recharge calculation because the stone is infiltrating to the outlet pipe?

We are placing our subdrain a foot above the bottom of the stone, routing the subdrain to the main outlet device, this way we can use the recharge volume below the subdrain as part of our overall recharge calculation for the site.

We are just doing our first bioretention area, and I'm just throwing this out here for any thoughts on recharge in this calculation.

RE: Using Hydrocad for L.I.D. items?


If infiltration testing indicates I can recharge my "water quality" volume, I would not likely have an underdrain at all.  One foot does not seem like it would necessarily provide much storage for the recharge process.  I would install a riser slightly above the bioretention surface sized to handle storm events larger than the "quality" event.

As long as the infiltration tests indicate a design rate that is more restrictive than the bioretention media, I would not use the exfiltration outlet to model the bioretention media.  Rather, I would use the exfiltration outlet to model the actual recharge.

RE: Using Hydrocad for L.I.D. items?


I wish to get clarification on what you are saying here.  By your description, I understand that I one has more than one item on the Storage tab of the Edit Pond dialog, exfiltration will be routed through all entries. (and if they were stacked entries exfiltration would be double counted.)

However, in my example, it is one entry on the Storage tab which simply has Stage Voids checked.  Are you saying this creates double counting of exfiltrations? (I would not expect this).

RE: Using Hydrocad for L.I.D. items?


A single storage defintion (like your stage-voids example) will always be OK.  You only have to watch out for double-counting the area when you have multiple definitions that are stacked on top of each other.

Peter Smart
HydroCAD Software

RE: Using Hydrocad for L.I.D. items?


Thanks for the reply, your answer is very helpful to me.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close