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(OP)
I am finding conflicting results in calculating prismatic storage volumes in Hydro cad.
I have a basin which is 129 ft wide and 355 ft long with 3:1 side slopes. The basin depth is 4.83 ft. Based on these dimensions I would expect the total basin storage to be around 266,000 CF. When I enter the same dimensions in Hydro cad the total storage volume is reported around 256,000 cf. Not sure why there is a discrepancy. Our routings are scrutinize by the reviewing municipal engineer and I am not sure how to explain this off as just rounding errors. Any insight? I would think the total volume should be the same, nothing irregular about the basin shape.

The exact volume will depend on the specific storage method you select in HydroCAD. For custom storage, it will also depend on the geometry, which can be prismatic, conic, or irregular. in order to match your hand calculations you need to be sure you're using the exact same procedure. All the details are provided in the help system and HydroCAD manual.

Peter Smart

For a prismatoid with a rectangular base of area A1, a half-height area of A2, a top area of A3, and a height of Y, the exact volume is given by:

V = Y/6 (A1 + 4A2 + A3)

where:

A1 = L1 W1 (area of base)
A2 = L2 W2 (area at half-height Y/2)
A3 = L3 W3 (area of top surface)

L2 = ½ (L1+L3) (length at half-height Y/2)
W2 = ½ (W1+W3) (width at half-height Y/2)

L3 = L1 + 2 Y Z (length of top surface)
W3 = W1 + 2 Y Z (width of top surface)

Peter Smart

Using these equations I get the same result as HydroCAD: 256,416 CF

Peter Smart

(OP)
Peter,
I was basing my volume on 1/2(a+b)x h x l (top length), where a and b are the bottom and top width.
My approximation is accordingly different.

How many slopped sides do you have? Two or four?

The HydroCAD procedure uses four equally sloping sides. Your equation (which is basically the average-area method) would apply if you have two opposite sloping sides.

Peter Smart

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