HY8:Outlet Control Computations

Outlet control means that the amount of water the culvert barrel can carry is limited by the barrel and/or tailwater conditions downstream. As a result, the flow in the barrel is subcritical, and the energy equation may be used to find the upstream headwater depth. Several flow profiles are possible and are shown below and on page 32 of HDS-5. The following correlations apply to HY-8: Type A in HDS-5 is Type 4 in HY-8, Type B in HDS-5 is Type 3 in HY-8; Type C in HDS-5 is Type 6 in HY-8, Type D in HDS-5 is Type 7 in HY-8, and Type E in HDS-5 is Type 2 in HY-8. Descriptions of these flow types may be found in HY-8 by selecting the “Flow Types” button from the Culvert Summary Table.

The logic for determining flow type due to outlet control is shown below:

This flowchart uses the following terms:

HJ = Check for Hydraulic Jumps

Full flow = Check if the culvert is flowing full

TWH = Depth of the tailwater from the invert of the tailwater channel at the culvert outlet

twOutletDepth = Depth of the tailwater from the invert of the culvert at the culvert outlet. If the culvert is buried, this value is taken from the top of the embedment material.

IH = Inlet control headwater depth measured at the inlet invert of the culvert

OH = Outlet control headwater depth measured at the inlet invert of the culvert

RISE = Height of the culvert. If the culvert is buried, this value is taken from the top of the embedment material.

Inlet Depth = The depth computed at the entrance to the culvert using the direct step profile computation method

Critical = The critical depth in the culvert

Normal = The normal depth in the culvert

The following table describes the various flow types defined in the above flow chart:

Flow Type	Flow Control	Submerged Inlet	Submerged Outlet	Length Full	Flow Regime	Outlet Depth
1	Inlet	No	No	NONE	S2n	Normal
1	Inlet	No	No	NONE	S1t	Tailwater
1	Inlet	No	Yes	Part	S1f	Full
1	Inlet	No	No	NONE	JS1t	Jump to Tailwater
1	Inlet	No	Yes	Most	JS1f	Jump to Full
5	Inlet	Yes	No	NONE	S2n	Normal
5	Inlet	Yes	No	NONE	S1t	Tailwater
5	Inlet	Yes	Yes	Part	S1f	Full
5	Inlet	Yes	No	NONE	JS1t	Jump to Tailwater
5	Inlet	Yes	Yes	Part	JS1f	Jump to Full
2	Outlet	No	No	NONE	M2c	Critical
3	Outlet	No	No	NONE	M1t	Tailwater
3	Outlet	No	No	NONE	M2t	Tailwater
3	Outlet	No	Yes	Part	M1f	Full
4	Outlet	Yes	Yes	All	FFf	Full
6	Outlet	Yes	No	Most	FFt	Tailwater
6	Outlet	Yes	No	Most	FFc	Critical
7	Outlet	Yes	No	Part	M1t	Tailwater
7	Outlet	Yes	No	Part	M2t	Tailwater
7	Outlet	Yes	No	Part	M2c	Critical