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# Infiltration Trench Modeling On a Slope

## Infiltration Trench Modeling On a Slope

(OP)
What is the best way to model the storage component of an infiltration trench that is on a slope, i.e. running inside a swale? At a particular elevation there would start to be two different void percentages, 40% for the stone and 100% for the open swale. And the higher the elevation, the 100% void space surface area becomes larger, why the 40% void space surface area becomes smaller.

### RE: Infiltration Trench Modeling On a Slope

If the trench has a rectangular cross-section, you could use a box pipe to model the (sloped) trench storage. Otherwise you would need to use the custom storage option to provide externally calculated stage-storage data.

Peter Smart

### RE: Infiltration Trench Modeling On a Slope

(OP)
Great suggestion for the box, which would be the easiest solution I believe. And HydroCAD would not "look" at the top of the box as being a solid slab per say or maybe that doesn't matter since the water level would surround that top layer anyway? Or am I just way over-thinking it?

BUT, if I went the custom storage option, would there be two entries for the same elevation with one being a surface area of 40% voids and the other being a surface area of 100% voids?

THANKS!!!!!

### RE: Infiltration Trench Modeling On a Slope

You could extend the height of the "box pipe" above the top of the trench, so that the water would never reach the top of the box. If you use the custom storage option, I would calculate the combined storage for the stone and open space, and enter that total into HydroCAD.

Remember that the whole system is going to overflow as soon as the WSE exceeds the lower end of the trench. Even though your trench may be sloped, pond modeling is based on a level pool, and not a sloped water water surface profile.

Peter Smart

### RE: Infiltration Trench Modeling On a Slope

(OP)
If I extended the box higher, how would I separate out the 2 different void %'s?

If I did the custom storage option with a combined storage number, you would definitely not use a surface area component with void %, correct? Basically, "hand" calculate a volumetric number to put in the custom storage table?

And I get ya' about the level pool aspect.

### RE: Infiltration Trench Modeling On a Slope

I really need a sketch to understand what you are trying to model.

Peter Smart

### RE: Infiltration Trench Modeling On a Slope

(OP)

Hopefully this works....

### RE: Infiltration Trench Modeling On a Slope

Assuming a rectangular cross-section, you could stack two box pipes, one with 40% voids and the other with 100% voids. On the upper storage definition, de-select the option to "Allow exfiltration". Remember to define a weir for any possible overflow at 26 feet.

Peter Smart

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