sheet flow
n.— «Norfolk Public Works spokeswoman Sylvia Ballinger said Little Bay depends on a process called “sheet flow” for drainage. That means water flows off the streets into a grassy era. It’s a drainage method also referred to as “no drainage method.”» —“Little Bay. Midsize rains. One real big puddle” by John Warren Virginia-Pilot (Hampton Roads) June 3, 2007. (source: Double-Tongued Dictionary)
Further reading
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Actually, “sheet flow” is not as specific a term as noted here. It does not specifically refer to water flowing into a grassy area. It is a term used in the hydrology and civil engineerng fields that simply refers to water flow or drainage over surfaces (either paved or unpaved, planted or unplanted), rather than being diverted into channels, culverts, or pipes. Where the water ends up — a drainage pit, a pipe, the ocean, or a grassy field — doesn’t matter when you are referring to a “sheet flow.”