I am an architect and I am wanting to create a water feature where the rain water falls directly off of the sloped roof on to the ground 31 feet below. Similar to a waterfall. The problem is that I need to have a hard surface or some type of trench for the water to hit on the ground. This way it can be collected. How can I calculate all of this? I understand that this trench will most likely be large in size due to width of the roof and varying rain amounts. A small rain will fall directly off of the roof where a heavy rain may shoot some distance off of the roof as it falls to the ground. The roof is a rectangular with no cuts in or out and is 222’ x 74’ giving it a surface area of 16,428 sqft. The roof slopes in only one direction (222’ in length) and the slope of the roof is ½” per 1’. I believe this comes to .041667. The project location is in Dallas, TX where there is a 7” per hour precipitation rate. The roofing material is called a single ply membrane which a rolled on sheet and is not very rough. The roof is 31’ above the ground, the location where the trench will be. I have looked into the Manning Equation but this seems to only calculate water in a channel at a specific water level. Does anyone have any thoughts?