What is Surface Drainage?
Surface drainage is the orderly removal of excess water from the surface of land through improved natural channels or constructed ditches via shaping of the land's surface.

Surface drainage applies primarily on flat land where slow infiltration, low permeability, restricting layers in the soil profile, or shallowness of soil over rock/deep clays prevents ready percolation of rainfall, runoff, seepage from uplands, or overflow from streams through the soil to a deep stratum. The land surface to be drained should have a continuous fall to a field ditch and the field ditch should have a continuous grade to the field lateral. The water surface in the field lateral at design depth should be low enough to drain the field.

Surface drainage systems, when properly planned, eliminate ponding, prevent prolonged saturation, and accelerate flow to an outlet without siltation or erosion of soil. Combinations of both surface and subsurface drainage, such as land grading and smoothing over subsurface drains, often provide better and more economical results.
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​For surface drainage, a coefficient is usually expressed in terms of flow rate-per-unit of the area, which varies with the size of the area. Where the drainage system or parts of the system primarily convey discharge from one or more fields, farms or large land areas to an outlet, the depth, capacity, and hydraulic grade line are added factors to consider. Wide variations in climate, topography, soils, and crops and farming practices between regions of the United States alter surface-drainage requirements. Therefore, when planning and designing surface-drainage systems, reference should be made to state handbooks and local drainage guides.