subsurface detention

[pjhealy]

I would like to model subsurface detention basin (crushed stone layer) under a parking lot that is sloped. There are a number of inlets through pervious pavement into the layer, so I thinnk they should be multiple Ponds. I added low (6" high) dams below each inlet, so that the infiltration will contact the maximum subsurface area. How can I model the flow over those subsurface dams in the crushed stone layer? It seems that sharp crested weir is not approporiate due to the stone/void ratio. Sharp crested weirs require open air, right? How would you recommend modeling the flow over the buried weirs?

 

[gbam]

Darcy's equation.

 

[pjhealy]

Thanks, gbam, I see how I could model a custom outlet using Darcy's to model flow laterally under the weirs, but I am looking to model it over the weirs. PJ

 

[psmart]

You could use a weir with a reduced capacity to account for the stone voids. If the stone voids are 40%, you could set a discharge multiplier of 0.4 for the weir. It's not exact, but pond routing is often quite insensitive to the exact weir coefficients. You can do a quick sensitivity analysis to verify this.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[gbam]

If you know the permeability of the stone you should be able to use Darcy's Equation to model flow over the weir. Assume that there is no lateral flow through the weir. That was my intent.

 

[psmart]

gbam, can you clarify, how would you employ Darcy's equation in conjunction with weir flow?

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[gbam]

My thought is that the flow over ther weir would be modelled using Darcy's and the area below the top of the weir would be ineffective and infiltrate into the subgrade. I have attached a sketch.

 

[pjhealy]

gbam, you understand exactly what I am trying to model: subsurface berms to encourage infiltration of most storms, but modelling the lateral flow through the stone above the berms for larger runoff. At this point, I have a series of three subsurface berms, each modelled as a pond, with exfiltration through the subgrade. Larger flows go from one pond to the next, using 33% flow over a weir, as suggested by Peter. Peter, how would I model the Darcy's flow in Hydrocad? a custom outlet perhaps? and what value would you use for 1-1/2" stone with 40% voids? Thank you both! PJ

 

[psmart]

You could use a custom outlet to enter an externally-derived stage-discharge curve. But I would have a close look at your actual flows to see how much depth over the weir is really going to occur, and how sensitive the routing results are to this calculation.

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[pjhealy]

Peter, the flows would be very shallow (<0.1 ft) in most storms, so it would seem more trouble than it's worth to create custom outflow curves. With the 140'+ widths of the weirs that are shown in the model I posted, some very spiky flows occur. I just cut and pasted these particular ponds out of my overall flow diagram to show you what I was doing, but in the full model some extreme oscillations occur. Not sure why exactly - I checked the storms, rainfall, times of concentration, calculation settings, dt, hours, and "finer routing" and they all seem to match. Thank you, PJ

 

[psmart]

If you've only got 1/10 foot of depth over the weir, I believe the flow is best approximated as weir flow, with a suitable reduction for the voids.

The oscillations definitely need to be resolved. There are a number of possible causes. Check ALL your warning messages. If you still have questions you can post your file or sent it to HydroCAD support.

Peter Smart
HydroCAD Software

http://www.hydrocad.net