Example sizing information for riprap energy dissipators at culvert outlets

Example sizing information for riprap energy dissipators at culvert outlets. Source: Kentucky Division of Water
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Culvert size	Avg. rock diameter	Apron width1	Apron length2	Apron length3
8"	3"	2–3 ft.	3–5 ft.	5–7 ft.
12"	5"	3–4 ft	4–6 ft.	8–12 ft.
18"	8"	4–6 ft.	6–8 ft.	12–18 ft.
24"	10"	6–8 ft.	8–12 ft.	18–22 ft.
30"	12"	8–10 ft.	12–14 ft.	22–28 ft.
36"	14"	10–12 ft.	14–16 ft.	28–32 ft.
42"	16"	12–14 ft.	16–18 ft.	32–38 ft.
48"	20"	14–16 ft.	18–25 ft.	38–44 ft.

¹Apron width at the narrow end (pipe or channel outlet)
²Apron length for slow-flow (no pressure head) culverts
³Apron length for high flow (pressure head) culverts