HY8:Outlet Control Computations

Outlet Control Flow Types

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 as are shown below and as described in HDS-5. HY-8 flow types 2, 3, 4, 6, and 7 are all outlet control flow types and are shown in the figure below. The various flow type properties may be found in HY-8 by selecting the Flow Types button from the Culvert Summary Table and are shown below.

Outlet Control Computations

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

Inlet control means that the amount of water the culvert barrel can carry is limited by the culvert entrance. Flow passes through critical depth at the culvert entrance and is supercritical in the barrel. There are several flow profiles possible, HY-8 simulates so-called Type A, B, C, and D conditions as shown below and as described in HDS-5. These profiles are known as Type 1 (A, C) and Type 5 (B, D) within HY-8. The various flow type properties may be found in HY-8 by selecting the Flow Types button from the Culvert Summary Table and are shown below. Because the flow in the barrel is supercritical, outlet losses and friction losses are not reflected in the headwater elevation. The headwater elevation is a function of the entrance size, shape, slope, and culvert type. The computed inlet control headwater elevation is found by accessing the results of scaled physical model tests. The logic for determining what inlet flow control type prevails is shown below (from the original HY-8 help file).

Inlet Control Logic

Determine Applicable Inlet Control Equation

1. IF circle or box with IMPROVED INLETS then use INLET equations.
2. For Straight (previously called conventional) INLETS
A. If Q is < Q at .5D, then assume LOW FLOW INLET CONTROL:
i. calculate CRITICAL DEPTH (DCO)
ii. calculate Section Properties
iii. VH = (Q / AC)^2 / 64.4
iv. IH = DCO * LMULT + (1 + KELOW) * VH * VHCOEF
IF no Depression THEN IHI = IH + I1E
For Depression, HF = IH and check head on CREST.
B. If Q > Q at .5D, but < Q at 3D, then use INLET REGRESSION EQUATIONS.
C. If Q > Q at 3D, then assume HIGH FLOW INLET CONTROL.
i. IH = (Q / CDAHI)^2 + .5 * RISE
ii. IF no Depression THEN IHI = IH + I1E
For Depression, HF = IH and check head on CREST.

Inlet Regression Equations (Q between Q at 0.5D and Q at 3D)

1. CIRCULAR
A. See Straight inlet equations
B. SIDE TAPERED ELLIPTICAL TRANSITION, THROAT CONTROL
ZZ = Q / SQR(RISE ^ 5), Y = LOG(ZZ) / 2.30258
i. IF n < .015 THEN SMOOTH PIPE IMPROVRD INLET.
ii. If n >=.015 then ROUGH PIPE IMPROVED INLET.
iii. Calculate THROAT CONTROL
iv. Calculate FACE CONTROL
v. IF Depression Then CW = CWF, calculate CREST control.
C. SIDE TAPERED RECTANGULAR TRANSITION or SLOPE TAPERED
i. Calculate THROAT CONTROL
ii. Calculate FACE CONTROL
iii. IF Depression Then CW = CWF, calculate CREST control.
2. BOX CULVERTS
A. See Straight inlet equations
B. SIDE TAPERED RECTANGULAR TRANSITION or SLOPE TAPERED
i. Calculate THROAT CONTROL
ii. Calculate FACE CONTROL
iii. IF Depression Then CW = CWF, calculate CREST control.
3. PIPE ARCHES AND ELLIPSES
A. See Straight inlet equations
4. IRREGULAR SHAPE
A. See Straight inlet equations

Straight Inlet Equations

1. For IRREGULAR shape, X = Q / (AC * SQR(RISE))
IF X <= .5 THEN IH = (A(1) * (X / .5)) * RISE
ELSE IH = (A(J - 1) + (A(J) - A(J - 1)) * ((X - J + 2) / INC)) * RISE
2. For all others shapes, X = Q / (SPAN * SQR(RISE^3)): SR = SR(IC)
IH = (A + (B + (C + (D + (E + F * X) * X) * X) * X) * X - SR * S0) * RISE
3. Headwater elevation (IHI) = IH + I1E if no Depression.
4. For Depression, CREST headwater is checked.

Throat Control Tapered Inlet

1. X = Q / (SPAN * SQR(RISE^3))
2. HT=RISE*(.1295033+(.3789944+(-.0437778+(4.26329E-03-1.06358E-04*X)*X)*X)*X)

Face Control-Side Tapered Inlet

1. ZZ = Q / (BF * SQR(RISE^3))
2. Calculate UNSUBMERGED: HF1 = (.56 * RISE) * (ZZ ^ .66667)
3. Calculate SUBMERGED
A. For bevels: HF3 = (.0378 * (ZZ * ZZ) + .86) * RISE
IF HF1 > RISE THEN HF = HF3
IF HF1 < RISE THEN HF = HF1
IF HF1 >= HF3 THEN HF = HF1
B. For other edges: HF2 = (.0446 * (ZZ * ZZ) + .84) * RISE
IF HF1 > RISE THEN HF = HF2
IF HF1 < RISE THEN HF = HF1
IF HF1 >= HF2 THEN HF = HF1

Face Control For Slope Tapered Inlet

1. ZZ = Q / (BF * SQR(RISE^3))
2. Calculate UNSUBMERGED: HF1 = (.5 * RISE) * (ZZ ^ .66667)
A. For bevels: HF3 = (.0378 * (ZZ * ZZ) + .7) * RISE
IF HF1 > RISE THEN HF = HF3
IF HF1 < RISE THEN HF = HF1
IF HF1 > HF3 THEN HF = HF1
B. For other edges: HF2 = (.0446 * (ZZ * ZZ) + .64) * RISE
IF HF1 > RISE THEN HF = HF2
IF HF1 < RISE THEN HF = HF1
IF HF1 > HF2 THEN HF = HF1

Crest ControlL

1. HC = .5 * (Q / CW) ^ .66667

Outlet Control Procedures That Produce an Inlet Control Profile

STEP

1. Compute critical depth (dco)
2. Compute normal depth (dno)
3. Compute fullflow if nomograph solution assumed "6-FFt or FFc".
4. If dno > .95(rise), assume fullflow "6-FFn".
5. If dno > dco, assume mild slope (SEE OUTLET.DAT).
6. If dno <= dco, assume steep slope.
A. If twh is >= So(L) + rise, assume fullflow "4-FFt".\
B. If twh is >= rise, outlet submerged, assume inlet unsubmerged.
C. If twh is < rise, outlet is unsubmerged, assume inlet unsubmerged. 