*Sheet Width: Manufacturers cut the Valley Gutter sheet out of a 1200 wide roll.
Therefore to save wastage, it is best to make sheet sizes 400 500 or 600 Then multiples of 100.
You can use any sheet width larger than the required sheet width if you so choose.
If nothing is entered for the chosen sheet width, this line won't appear in the print out.
The Method
This programme has been derived from hundreds of calculations with a computational fluid dynamics (CFD) programme.
The programme used was FLOW-3D Hydro.
A range of flows from 1 L/s to 100 L/s was used.
And a range of slopes from 5 degrees to 35 degrees was also used.
This program will not calculate anything outside this range.
The slopes used were 5, 10, 12.5, 15, 20, 23, 23.5, 23.7, 25, 25.1, 27.5, 30, 35 degrees
The flows used for each slope were 1, 2, 5, 10, 15, 20, 25, 30, 40, 50, 100 L/s
3D CAD dwgs were produced for each slope.
This gave a total of 143 separate CFD calculations.
This was sufficient to plot charts for each slope Flow/Depth combination. A typical chart looks like fig1.
An equation was then found that fitted the curve as in fig 2.
The equation results were then checked for accuracy against the CFD results, as shown in fig3
This huge number of CFD results was sufficient to plot charts and calculate equations for over a 143 different cases.
The programme uses the resultant equations to calculate valley gutter sizes for anything within the above range of flow and roof slopes.
If the desired roof slope does not fall on a calculated case the programme will interpolate.
NOTE: For roof slopes greater than 25deg, the roof sheeting protrudes into the running water.
To prevent this obstructing the flow, an extra free board of 15mm has been applied to all roof slopes steeper than 25 degrees.
This has the effect of increasing the sheet width.
BUT ARE THE ANSWERS CORRECT
As a check of the results, the model was calibrated with Table 3.6.2 in AS/NZS 3500.3-2025 where possible.
Table 3.6.2 relates to only one roof slope, stated as 23.5 deg, and one side slope of 16.5 deg, and one catchment area of 20 sqm.
However to get a side slope of 16.5deg requires a roof slope of 23.7deg.
This figure has been taken as more accurate as it can be verified using trigonometry.
However it makes no difference to the results.
So you can check it out yourself:-
Enter 20 as the catchment area, select 23.7 as the roof slope, and for rainfall, set the dropdown list to
"I prefer to enter a known intensity"
Then check with the larger of the two intensities shown in the intensity column in table 3.6.2
A PERFORMANCE SOLUTION
According to the latest Australian standard AS/NZS 3500.3-2025 A Computational Fluid Dynamics (CFD) program is acceptable as a performance solution.
One More Check
Check with Mannings equation.
Mannings equation is used to find the "normal" depth of flow in an open channel.
However Manning's equation requires the flow to be constant along the channel.
Whereas the flow in a valley gutter is increasing all the way along the channel.
But as a quick check, Manning's depth at the end of the channel at the maximum flow,
was checked with the maximum depth given by CFD.
As can that be seen from the chart below, there is good agreement of the depth at the lower flow range.
The lower flow has less wave motion, hence a reasonable agreement with Manning's equation,
which has no allowance for wave motion.
As the flow increases, so does the depth, and so does the wave motion.
This accounts for the divergence between the two calculations at the larger flows.
The similarity of the results at the lower flows gives us more confidence in the CFD result.
