Drainage Swale Calculator

Manning's n represents how much friction the channel surface creates against flowing water -- rip-rap and grass resist flow more than smooth bare soil does. To carry the same Design Flow Rate against more friction, the channel needs a larger Cross-Section Area, which is why raising Manning's n increases the calculated cross-section.

Smaller -- a steeper Channel Slope means gravity accelerates the water more, so the same Design Flow Rate can pass through a smaller Cross-Section Area at a steeper slope than it would need at a gentler one. This is why increasing slope reduces the calculated cross-section for a fixed flow rate.

Sizing a channel for capacity (carrying the Design Flow Rate) and sizing it for erosion resistance are two different questions. A channel that is technically large enough to carry the design flow can still convey it at a velocity too high for the channel lining to withstand -- grass-lined swales are generally limited to under about 6 ft/s and bare soil to under about 3 ft/s to avoid scour.

Side Slope primarily reshapes the channel -- flatter side slopes (like 4:1, often used for mowing safety) spread the same cross-sectional area over a wider, shallower profile, while steeper side slopes concentrate it into a narrower, deeper one. It has a smaller effect on the total capacity required than Design Flow Rate, Manning's n, or Channel Slope do.