Linear drainage systems accept runoff along their whole length rather than just at specific points. This gradual inflow along the length of the drain disrupts the flow of the water below. For this reason linear drains do not achieve the flow rates you would expect from an equivalent pipe. The following equation has been developed to estimate the maximum flow through a linear drain with gradually varied flow;

Q = Flow (m³/s)

Note that this formula includes a factor of safety of 0.85.

This spreadsheet calculates the design runoff flow for a site in accordance with the Flood Estimation Handbook. It then calculates the maximum flow from a specified linear drainage channel and checks whether the channel is sufficient for the specified site and storm conditions. It includes the following functions;

• Calculates the design runoff flow for any site in the UK using actual rainfall data
• Non UK users can specify a storm duration and intensity to calculate the design runoff
• User can alter all runoff parameters including;
• Different surfacing conditions can be modelled by altering the runoff coefficient
• Output
• Gives a design runoff volume for the specified site and storm conditions
• Then the program calculates the flow capacity of a specified linear drainage channel
• User can alter all linear drainage parameters including;
• Ground Slope
• Length of channel
• Single size or stepped channel
• The user can specify any one of 38 different standard channel sizes over 8 standards shapes
• Or a custom unit can be specified using only the maximum water height and cross sectional area of the channel
• Output
• Plots a chart showing the chosen channel dimensions making it easy for the designer to visualise or even specify the correct channel cross section
• Calculates the maximum flow from the specified channel
• Compares this with the runoff volume calculated above
• Plots a chart of flow capacity over ground slope making it easy for the designer to see how the ground slope affects the channel capacity
• For standard channel shapes a stepped channel can be designed
• The program calculates the increased flow capacity of the stepped channel
• Then the program calculates the lengths at which the channel section must be stepped up  