CALCULATE THE SIZE OF AN IRREGULAR SHAPED BOX GUTTER
NOTE: AS 3500.3.2:2003 "Stormwater Drainage Acceptable Solutions"
DOES NOT COVER THIS SITUATION

Instructions & Notes:
Use one of the other calculators to determine the Flow, (and downpipe size if necessary, however guessing the DP dia is ok).
Note: The rain water head depth is dependant only on the size of the downpipe chosen. (which is dependant only on the flow). So the shape of the box gutter is irrelevant.
However, the length of the rain water head is dependant on the velocity and trajectory of water leaving the box gutter, so the shape of the gutter is important. For this reason, the length of the rain water head is calculated.

Start with a guess of dimensions A,B,& C or use dimensions to fit the roof, these dimensions can be fine tuned later after the program calculates the water depth.
Note that the slope on the left hand side 'S1', and right hand side 'S2', is in the form vertical:horizontal ie a slope of 1:2 means 1 (vertical) to 2 (horizontal), 'S1' would be entered as '2' in this example. eg if dimension A was 200 then the rise of the sloping bit would be 100.

Entering a zero for 'S1 or 'S2' will give a vertical slope, in which case A or C would be eliminated and automatically set to zero.
Similarly entering a zero for A or B will automatically set the corresponding slope to zero. eg if you entered zero for S1 and 300 for A, the program will set A to zero. (and visa versa)

The program must use a different algorithm than the one used to fit a rectangular box gutter to the code, this one is based on Manning, Bernoulli, and the standard backwater curve equation, and as such it is a trial solution involving thousands of iterations. The figures you can see ticking over are the areas, and wetted perimeters, of the sections used.
However, the results are surprisingly consistent with the Code when checked for a rectangular section, and differ only by -3mm and +5mm when used over flows ranging from 1 to 15 litres/sec, grades 1:50 to 1:500, and widths 300 to 400.
Given a freeboard of approx 60mm, the results are still within a suitable factor of safety.
I found that the momentum based equations for spatially varied flow are not as consistant, and pose problems when passing through the critical slope etc.