The Level Gutter Is a Sediment Trap: Minimum Fall for Self-Cleansing in a 125 mm Half-Round Gutter

Builders frequently lay eaves gutters dead level because a level gutter is easier to set out and looks neater, and a persistent trade belief holds that the fall makes no practical difference because rain will find the outlet anyway. This paper tests that belief with open-channel arithmetic for a standard 125 mm half-round gutter flowing half full. At a fall of 0.5 percent the flow reaches 0.638 m/s and the bed shear of 1.53 Pa moves grit up to 1.89 mm across. Reaching the commonly used self-cleansing velocity of 0.6 m/s requires a fall of 0.443 percent, while moving 2 mm grit requires 0.528 percent. At zero fall the velocity and shear are both zero, so any sediment that enters a level gutter stays there until it is removed by hand. The belief that fall is optional does not survive the calculation. All results are simulated, not measured.