Question 1

What is an ogee spillway?

Accepted Answer

An ogee spillway is a concrete overflow weir whose downstream face is shaped to match the lower nappe trajectory of a sharp-crested weir at the design head Hd. Pressure on the crest equals atmospheric at design head, so there is no separation and the discharge coefficient is at maximum. Ogees pass roughly 18% more flow per unit length than a broad-crested weir of the same head.

Question 2

What is the design head Hd?

Accepted Answer

Hd is the head selected by the designer when shaping the crest profile. Choosing Hd close to the routed peak head He (so He/Hd ≈ 1) maximizes discharge but pushes pressures toward atmospheric — Hd is often picked at about 0.75·He_max to keep crest pressures slightly above atmospheric and avoid cavitation at high heads.

Question 3

What happens if He > Hd?

Accepted Answer

At He > Hd, the nappe lifts off the crest, sub-atmospheric pressures develop on the downstream face, and the discharge coefficient increases above C_design. USACE EM 1110-2-1603 Plate 3-2 gives C/C_design ≈ 1.04 at He/Hd = 1.33 and ≈ 1.06 at He/Hd = 1.5. Sub-atmospheric pressures introduce cavitation risk — current practice keeps He/Hd ≤ 1.33.

Question 4

What is C_design for a vertical-faced ogee?

Accepted Answer

For a high (P/Hd ≥ 1.33) ogee with a vertical upstream face, C_design = 4.03 in US customary units (Q in cfs, L and He in ft) or 2.225 in SI (Q in m³/s, L and He in m). Lower approach depths reduce C through approach-velocity and pier-contraction effects; see USACE EM 1110-2-1603 Plate 3-1.

Question 5

How do piers and abutments affect discharge?

Accepted Answer

Piers and abutments reduce effective crest length: L_eff = L − 2·(N·Kp + Ka)·He, where N is the number of piers, Kp is the pier contraction coefficient (0.01 round-nosed, 0.02 square), and Ka is the abutment coefficient (0 rounded, 0.10 square). For a screening calc with no piers and rounded abutments, L_eff ≈ L.

Question 6

When should I use a broad-crested instead of an ogee?

Accepted Answer

Use a broad-crested weir for emergency / auxiliary spillways cut into earth, vegetation-lined chutes, or any case where the simpler geometry, lower cost, and acceptance of a lower coefficient are worth the trade-off. Use an ogee for service spillways on concrete dams where you need the discharge coefficient and where cost of the shaped crest is justified by the flood it has to pass.

P / H_d	C_design (US)	C_design (SI)
≥ 1.33 (high approach, vertical U/S face)	4.03	2.225
1.00	3.95	2.18
0.50	3.78	2.09
0.20	3.55	1.96
≤ 0.10 (very low approach)	3.20	1.77

H_e/H_d	C / C_design	Crest pressure
0.20	0.83	positive (separation)
0.40	0.89	positive
0.60	0.93	positive
0.80	0.97	near atmospheric
1.00	1.00	atmospheric (design)
1.20	1.03	slightly sub-atmospheric
1.33	1.04	typical upper design limit
1.50	1.06	cavitation risk

Ogee Spillway Discharge Calculator

C_design for ogee crests at design head

Off-design correction C / C_design

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Example — vertical-face ogee at 25% above design head

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Cdesign for ogee crests at design head

Off-design correction C / Cdesign

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Example — vertical-face ogee at 25% above design head

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C_design for ogee crests at design head

Off-design correction C / C_design