Detention Storage Table 1

[amb2002]

I have a site with a dry detention basin with an infiltration trench in the bottom. I have modeled this with two storage elements: one for the infiltration trench portion (elevation 916 to 922) and the second above the infiltration (elevation 922 to 925). The surface area of the trench is constant at 3,152 sf. The basin above varies from 3,152 to 15,043 sf.

When I double click on the detention node, switch over to the "Storage" tab, and select the "Table" button, it brings up the elevation and storage area in tabular view as expected. If I then right-click to add report columns and select "Surface", it adds that information to the table. However, the storage reported shows 3,152 sf at all elevations. It is not showing the increasing area from 3,152 to 15,043 sf from elevations 922 to 925.

Is this a reporting error or am I misunderstanding what "Surface" is reporting?

Attached are images showing the detention summary and the tabular storage view.

Thanks for any input that you can provide,

Aaron

 

[psmart]

Per your pond summary, you have the "Basin" storage set to impervious, so it's surface area will be excluded from surface area being used by your exfiltration calculations. This also excludes the area from the storage report.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[amb2002]

Peter,

Thank you for taking the time to respond and provide the clarification that I needed.

When you go to make another program update, I would like to request you consider providing a means to report the calculated water surface elevation, regardless of if any portion of the basin is being utilized for exfiltration or not.

For a workaround to get the reporting that I expected, I looked at setting elevation limits on the exfiltration outlet configuration to cover only the infiltration trench area (elevations 916 to 922), then set the upper portion of the basin (elevations 922 to 925) allow exfiltration:

Then the reported surface area does appears to match my expectations (constant through the trench section; increasing through the upper basin due to sloped sides):

However, when I switch to the discharge tab, it doesn't look like I am getting the exfiltration at the lower depths, only the upper depths, which is the exact opposite of what I intended:

I must be doing something wrong in my outlet configuration. I would appreciate any further insight that you could provide.

Thanks again,

Aaron

 

[psmart]

Changes to "Allow exfiltration" will not effect the reported water surface area. The only way to include the entire surface area (the sum of the two ponds) is to uncheck the "Impervious" box on both storage definitions.

The calculated storage will be the same in both cases. The HydroCAD results are correct, you just have to understand that the surface area reporting is adjusted to allow for the exfiltration calculations.

Since you're using two "stacked" storage definitions, it's important that you not double-count the exfiltration area. That's the purpose of unchecking the impervious box.

You may prefer to define all your storage with a single storage entry, using stage voids to allow for the reduced storage in the stone. This will avoid all these issues.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[amb2002]

Peter,

I will look into the stage voids, that would simplify my model!

I'm not seeing where the "Impervious" checkbox is that you mention though, see below:

Can you point me to the correct location? This will be my last question!

Thanks,

Aaron

 

[psmart]

Sorry, it's labeled "Allow Exfiltration"

Please click the help button for full details.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[ralaimo]

I have a different question related to modeling a dry detention basin with an infiltration area at the bottom. The Pond FAQ suggest that the pond area be set up as a separate subcatchment with a Tc of zero and CN of 98-100. Is this recommendation for a dry basin or just for a wetpond?

 

[psmart]

If the pond is predominantly dry you could use the actual Tc and CN values for the underlying ground.

ralaimo - Please start a new thread for a different topic. And please don't post questions that you have also sent by email. Double-posting takes twice the time since we have to respond to each inquiry. Thanks.

Peter Smart
HydroCAD Software

http://www.hydrocad.net