Rain Garden - Darcy's Law Outlet Control

[oregonpe]

Has anybody set up an outlet control for a rain garden (or flow-through planter)using a stage/discharge outlet control based on Darcy's law? Darcy's Law solves for flow rate given hydraulic conductivity (permeability), hydraulic gradient and cross sectional area or depth of the filter medium. Runoff entering a ponding area will slowly percolate through a soil bed based on its depth, permeability and applied hydraulic head. In my case Ive sized the under-drain to freely pass the flow without restriction. I'm also assuming a permeability of 1.0 feet per day which seems to be a fairly conservative value for a 75% sand, 25% compost filter. Anybody disagree?

The standard method is to assume a total treatment volume and an average depth of water in the ponding area (say half depth) and then size the pond area based on the time allotted to completely discharge the design flow (say 24 hours). But this method doesn't route a variable inflow or tell you much about the depth of ponding area required nor does it allow you to check the design of the overflow device.

 

[psmart]

I'm not clear about your exact question, but perhaps this background info will help...

Common design guidelines for bioretention areas tend to address the high-frequency low-volume rainfall events they are primarily designed to treat. This is often based on retaining and infiltrating 1" of runoff, or something to that effect. If enough storage is provided to hold the entire 1" runoff, the exact exfiltration rate may not be of great concern, as long as the system will drain before the next. Since exfiltration rates are subject to considerable uncertainly, the system must be able to handle a considerable margin of error in the dewatering calculations.

Modeling of larger storm events is a different matter. This analysis tends to be concerned with a much shorter time period (such as 24-hours) and the runoff volume often exceeds the storage or exfiltration capacity of a bioretention system, so provisions must be made to handle the excess with additional storage and/or overflow devices. Infiltration effects are also less significant during this shorter time period, and may have a minimal effect on the hydrograph routing, especially in comparison to other storage and overflow effects.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

For further modeling details please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

oregonpe,

There was a related thread that may provide some tips here:

http://www.eng-tips.com/viewthread.cfm?qid=209756