Exfiltration - start above an elevation?

[Horaz]

Can HydroCAD delay exfiltration from a basin until the water within it gets to a certain depth? The "Only above invert" is not the answer as that then excludes the bottom area as impervious. I only want to delay the start as required by MA DEP "static method" regs.

Therefore, a basin with invert 100, is required to fill to 102 THEN exfiltration can begin over the entire surface area. Can this be modeled?

Thanks

 

[psmart]

HydroCAD doesn't have an obvious way to handle this because it's not a physically realistic situation. Unless you have an active gate or other flow control setup, no system will behave this way.

However, you should be able to model this situation using a "pump" outlet, which allows you to set a "turn on" elevation. You'll also need to use the DSI routing procedure to handle the change in the stage-discharge relationship when the exfiltration starts. But it can be done.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

Other options:

1) If you can model your exfiltration with a constant flow (in CFS), then you would be able to set the exfiltration invert elevation anywhere you wish with effecting the flow rate.

2) If your primary objective is to model the pond draw-down, you could eliminate the inflow hydrograph entirely and set the starting elevation instead.




Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[psmart]

Finally, why not use one of the "dynamic" methods described by MA DEP? These alternatives are intended for situations where you have a numerical routing procedure like HydroCAD. They are based on the Saturated Hydraulic Conductivity, which can be entered directly with HydroCAD 9.0 and later. (Note the HydroCAD output which is used to illustrate some of the regs.)

http://www.mass.gov/dep/water/laws/v3c1f.doc

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[Horaz]

Thanks for the input Peter. I may switch to simple dynamic or may take a manual approach. If I model a basin with exfiltration at all elevations. I could then manually print the discharge table and take those CFS volumes at say every 0.50ft and plug them into a special user defined outlet. Utilizing zero for the elevations I need to satisfy the static method, then just entering the head and CFS at the rest of the heights for the basin. The results and hydrograph seem to appear correct and may err further on the conservative side if anything.

Do you feel that would work? I only have two small basins so this technique is fine for this particular project.

 

[psmart]

If I'm understanding correctly, I don't think that reentering the exfiltration stage-discharge values as a special outlet will give you any additional capabilities. A special outlet will not let you delay the "turn-on" until a higher elevation. Only a pump outlet has the ability to provide that (hysteresis) behavior, and it's a bit of a stretch to use it for this application.

As happens with the "simplified" procedures in many regulations, they're really intended for manual evaluation, and often do not lend themselves to a simulation program like HydroCAD. Fortunately, the Mass DEP regs do provide "dynamic" alternatives that are better suited for numerical simulation.

Peter Smart
HydroCAD Software

http://www.hydrocad.net