Hydraulic design can be quite complex depending on how accurate you need the calculations. The most accurate method is to utilize differential equations to calculate the input along the trench run length and properly verify the flow rates at all depths of the trench. This is very difficult by hand, but we do offer this type of calculation done on a special computer program for a small fee.
For most projects it is sufficient to utilize Mannings equation for open channel flow to determine the required flow and size the trench. You must first know the flow rate required. For indoor applications this is generally determined by the equipment flow rates, wash down hoses, dumping containers, etc. For exterior applications it is generally calculated as follows:
Qr = C*I*A
Where: C is a constant that relates to the surface the drain is installed in (ex. for rigid pavement use 0.9)
I is the rainfall intensity of the design storm (ex. in the south this can be as much as 8 or 9 inches per hour, in the northern united states it is closer to 4 or 5 inches per hour)
A is the drainage area in acres (ex. take the length in feet times the width in feet and then divide by 43560 to convert square feet into acres)
When you multiply these factors together you get the design flowrate (Qr = Q required) of the drain in CFS (cubic feet per second)
**Note: There are other methods of calculating design flows and these calculations are intended to get you in the ball park. Consult a licensed civil engineer for expert advice and in applications where more accurate measures may be required.
Now it is time to calculate the channel flow capacity to determine what size trench may be required for your application.
Qc = A*(1.49/n)*(R)^2/3*(S)^1/2
Where: A is the cross sectional area of the trench drain in square feet
n is the coefficient of friction of the trench drain body (concrete forming systems = 0.013, fiberglass and polymer concrete systems 0.010)
R is the hydraulic radius which is calculated by A / P where A is the cross sectional area found above and P is the perrimeter surface of the trench
S is the slope of the bottom of the trench (be sure to add the site slope and the built in slope of the trench drain system together to get the total effective slope - ex. site slope is 0.5% and trench slope is 1% therefore the total slope is 1.5% or 0.015)
**Note: For help with sizing the trench please contact our technical department and they will be happy to assist you with your application.