Method for delaying run-off of flash-storm water or ordinary rainwater from roofs and other surfaces with water-retention capability

A method for the delayed run-off of flash-storm water or ordinary rainwater from roofs and other surfaces, the method providing for a water-retention capability for sporadic or permanent retention through a drain pipe into a sewage system, the method comprising steps of:

• providing a throttle element, and a drain pipe having a roof-side inlet in fluid communication with the throttle element;
• constructing said throttle element with an outer wall encircling an axis of said drain inlet for guiding incoming water into a vortex flow pattern about said axis;
• forming in said throttle element an inlet port disposed in said outer wall and being oriented relative to said outer wall for directing the incoming water against an inner surface of said outer wall for development of said vortex flow pattern;
• and
• spacing said outer wall apart from said pipe in a radial direction from said axis to provide a vortex diameter of said flow pattern which is larger than a cross sectional dimension of said pipe, and enabling a vortex of said flow pattern to perform a throttling function to limit a rate of flow of said incoming water into said pipe at a maximum vortex flow rate, said maximum vortex flow rate being less than a laminar rate of flow of the rainwater into said pipe.