Sub-Driveway Infiltration Bed
Sub-Driveway Infiltration Bed
(OP)
I'm designing an infiltration area below a large, looped driveway area so that the post-developed 10-year flow will equal the pre-developed 2-year flow.
I am placing two inlets in the driveway area so stormwater can flow into the infiltration bed more quickly, rather than just seeping through the ground surface. I tried to model the system as follows: subcatchment to catch basin (with 2 grates as my primary outlets and a broad-crested weir to represent overland flow as my secondary outlet) to an infiltration bed, where infiltration is the only outlet.
I keep getting an error message because infiltration is my final outlet and no calculations are completed. Even when I add a "dummy" outlet (1" diameter orifice) from my infiltration bed, all the flow goes through the dummy outlet.
Another issue with this system is using the broad-crested weir to model the runoff that does not flow into the infiltration system. Is there a better way to quantify this flow? This is the flow rate I need to show that the SWM system meets the 10-year to 2-year requirement.
Can anyone out there help me figure out how to model this so I won't get any error or warning messages?
I am placing two inlets in the driveway area so stormwater can flow into the infiltration bed more quickly, rather than just seeping through the ground surface. I tried to model the system as follows: subcatchment to catch basin (with 2 grates as my primary outlets and a broad-crested weir to represent overland flow as my secondary outlet) to an infiltration bed, where infiltration is the only outlet.
I keep getting an error message because infiltration is my final outlet and no calculations are completed. Even when I add a "dummy" outlet (1" diameter orifice) from my infiltration bed, all the flow goes through the dummy outlet.
Another issue with this system is using the broad-crested weir to model the runoff that does not flow into the infiltration system. Is there a better way to quantify this flow? This is the flow rate I need to show that the SWM system meets the 10-year to 2-year requirement.
Can anyone out there help me figure out how to model this so I won't get any error or warning messages?
RE: Sub-Driveway Infiltration Bed
Is this underground infiltration or an infiltration basin?
Have you defined an Exfiltration device on Outlets tab of your storage?
RE: Sub-Driveway Infiltration Bed
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.
RE: Sub-Driveway Infiltration Bed
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.
RE: Sub-Driveway Infiltration Bed
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.
RE: Sub-Driveway Infiltration Bed
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 www.hydrocad.net/exfilt.htm
RE: Sub-Driveway Infiltration Bed