Sub-Driveway Infiltration Bed

I am a bit confused by your description. You have 2 drainage inlets but you say you are modeling them as outlets? Outlets from what? Same question for the weir...outlet from what?

Is this underground infiltration or an infiltration basin?

Have you defined an Exfiltration device on Outlets tab of your storage?

My apologies for the confusion.

The infiltration bed is below the driveway (about a foot underground). The inlets serve as openings so water that is above the ground can get into the infiltration bed faster than it would if it were to just seep through the driveway.

One of the main problems I am having in modeling this is how to quantify the flow rate of water that does not flow into the infiltration system; the stormwater that just bypasses the inlets and continues to flow down the hill. That's where the broad-crested weir came into play, but using that seems to be a really inaccurate way to find the flow rate of excess runoff.

And yes, I defined exfiltration as a device in my Outlets tab. However, whenever exfiltration is the only outlet in a system, I get an error message and no calculations are made.

It sounds like you may need to determine your inflow using other methods such as HEC-22 type inlet capture computations if you do not believe you will capture 100% of the flow from the drainage area.

The exfiltration is another matter. What is the error message? Be sure to check how your elevations are setup in the Exfiltration device, and the "Allow Exfiltration" option.

As an aside: If you were anticipating water seeping through the driveway, am I to assume the drive is not paved?

Are you providing any pretreatment of the runoff before it enters the underground system? If not, it sounds like you are setting it up to fail when the underground storage clogs from sediment.

The key to modeling these situations is to have a clear definition for your pond. Then you can determine how to define the storage and outlet control(s). Remember that HydroCAD deals only with outlet controls. Any inlet controls would need to be modeled as outlets on another upstream "pond".

ONE APPROACH to your situation would be model a below-ground pond, with exfiltration and overflow outlets. This assumes that all the flow enters the "pond", but allows some of it to overflow if the level is high enough.

To define your storage, select an appropriate shape for the underground volume, setting the correct voids for the stone. To enhance the storage, you can embed storage chambers, pipes, or other open volumes in the stone. You should also define the above-grade storage that would be occupied if the stone overfills. This might be a parking lot that can fill to the curb level. Anything that would be part of the same level-pool can properly be included.

Then create an exfiltration outlet for the below grade outflow. This can be based on your choice of wetted area, surface area, or constant flow. To model any overflow, you could use a weir or other device.

In this scenario, the entrance grates don't appear in the model at all, since we're assuming that they are able to pass all the runoff into the pond without diverting any flow. However, if the inflow exceeds the storage and exfiltration capacity and the level rises above the overflow device, the flow will be correctly split between the exfiltration and overflow outlets.

For further details and illustrations please see

PS: If you're getting any error messages, please click the help button for a detailed explanation. You will find answers to 99% of all problems in the linked help system.