CALCULATE THE SIZE OF AN OPEN CHANNEL OVERLAND FLOW PATH
NOTE: Complies with AS 3500.3.2:2003 "Stormwater Drainage Acceptable Solutions"

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LOAD AND RUN CALCULATOR

Instructions & Notes:
The Flow must be calculated elsewhere. Advise on this can be found in the plumbing code, and in text books on stormwater, also in the Institution of Engineers "Australian Rainfall and Runoff".
However as a general principle you must:-
1. Determine the Average recurrance Interval (ARI).
This is governed by the likely damage, and ranges from 1 year for small impact low density areas, to 20 years for ponding against buildings etc.
2. Determine the time of concentration.
This is the time it takes for water to reach the point under consideration, from the furtherest point in the catchment.
3. From these two figures, Charts (click here) are available for all areas, from which you can find the rainfall intensity in mm/hr.
4. Now you must determine the coefficient of runoff.
This is the fraction of the total rainfall that actually makes it to the overland flow path, eg a coeficient of 0.6 means 60% runoff.
This depends on the soil type, eg sand has not much runoff, whereas rock or clay has a large runoff. New Zealand also uses a slope factor ie the steeper the slope, the less ponding, therefore the more runoff.
For the majority of small building projects with no external cachment, if you use ARI 10 years for surface water, time of concentration of 5 mins, and coefficient of runoff as 0.8 you will be well and truely covered.
However this will be a massive overdesign for large projects, or projects with an external catchment entering the site.

Finally the plumbing code states that your lowest building floor level or damp course, must be at least 300mm above the water level in an adjacent overland flow path.

The equation used for all of this, is an equation derived by Mr Manning, and is convienently known as the Mannings Formula. He also determined that all surfaces have a certain roughness which impeades the flow. He gave this roughness a value which he called "N". We now call it "Mannings N"
You can find Mannings formula, and the "N" values all over the internet.
For building sites, there is usually only a few surfaces to use, and the mannings N are shown below:-
Grass N = 0.025
Concrete N = 0.012
Asphalt N = 0.015

Start with a guess of dimensions A,B,& C or use dimensions to fit the site, 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)


DISCLAIMER: It is the users responsibility to ensure input data is calculated and entered correctly, and that the program is suitable for the situation required.
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