Small difference in gutter worksheet result between US and SI units

  Applies To 
  Product(s): Bentley FlowMaster
  Area:  Modeling
  Original Author: Jesse Dringoli, Bentley Technical Support Group


When computing a gutter worksheet with metric units, why does the results appear to be slightly different than hand calculations (usually less than 1%)?


This small difference is likely due to the constant used in the equation. In SI units, the constant is 0.376 and in US units, the constant is 0.56 (see help documentation, equation 6.2). FlowMaster internally computes with US units and converts back to the selected display units (in this case metric, L/s for Q), so it uses the 0.56 constant. Here is what's going on behind the scenes in an example case:

Channel Slope: 0.01 m/m
Road cross slope: 0.03 m/m
Mannings n: 0.016
Gutter Width: 0 m
Spread: 5 m
Solver for: Discharge

Kc = 0.56
n = 0.016
Sx = 0.03 ft/ft
Sl = 0.01 ft/ft
T = 16.4041994750 ft (5 m)

Q = (0.56/0.016)*0.03^(5/3)*0.01^(0.5)*16.4041994750^(8/3)

Q = 17.6123340206 ft^3/s

Now, 17.61 ft^3/s = 498.726 L/s, which is what FlowMaster displays. A hand calculation using the SI constant of 0.376 will yield a discharge in L/s of 497.573.

So, why doesn't it use the SI units constant? FlowMaster allows the user to select any unit they'd like for any field. So, you could potentially have half of the fields with a US unit selected and the other half with a metric unit. Therefore, FlowMaster can't really know whether a worksheet is in SI or US units. So it always converts the entered data into US units, calculates with that, then converts the result back to the display units (which could be metric).

This assumption should be fine, even in the extremes like this where a constant results in a tiny difference. The difference is relatively insignificant, especially when you consider how accurate the drainage system will actually be constructed.