A retaining wall at the outlet of a drain or culvert.(noun)
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Use "headwall" in a sentence
"Thence along the face of the main headwall south 27°27\'09\ east 62.95 feet to a point where the south wingwall joins."
"In mountaineering terms the headwall usually refers to the rock immediately below the summit."
"Then I dropped down the headwall and kept working on that move on the yellow route on the same rope that still defeats me ... and was defeated by it."