HY8:Outlet Control Computations

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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.


File:HY8image38.gif


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


OutletControlFlowchart.png


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