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 and the headwater elevation due to outlet control is shown below from the HY-8 help file.

STEP

1. Compute critical depth (dco)
2. Compute normal depth (dno)
3. Compute fullflow if nomograph solution assumed "6-FFt or FFc".
4. If dco > rise, assume fullflow "6-FFc".

   If oh < ih, compute S2 curve, "5-S2n" if submerged, else "1-S2n"

5. If dco < dno, assume mild slope.

   A. If tailwater (twh) >= So(L) + rise, compute fullflow "4-FFt".

   B. If twh >= rise, outlet submerged, assume inlet unsubmerged

   i. Pipe flows partially full, compute M1 curve "3-M1f".

   Length of curve = L - (twh - rise) / So

   Outlet DEPTH = rise.

   ii. If inlet submerged, full flow "6-FFt".

   C. If twh >= dco, outlet depth = twh.

   i. If twh is >= dno, calculate M1 curve "3-M1t"

   If depth = rise, fullflow from that point. "7-M1t"

   ii. If twh is < dno, calculate M2 curve "3-M2t"

   If depth = rise, fullflow from that point. "7-M2t"

   D. If twh is < dco, outlet depth = dco.

   i. Calculate M2 curve "2-M2c".

   If depth = rise, fullflow from that point. "7-M2c"

6. If dco >= dno, assume steep slope.

   A. If twh is >= So(L) + rise, assume fullflow "4-FFt".

   B. If twh is >= rise, outlet submerged, assume inlet unsubmerged.

   i. Steep channel-hydraulic jump forms

   ii. Calculate fullflow headwater (oh) "6-FFt or FFc"

   iii. If oh < rise, then inlet is unsubmerged.

   a. Length full = (twh - rise) / So

   b. Calculate S1 curve "1-S1f".

   If oh >= rise, inlet submerged "5-S1f".

   c. If oh is < twh, tailwater drowns out jump.

   Calculate M1 curve "3-M1f".

   If oh >= rise, inlet submerged "5-M1f".

   C. If twh is < rise, outlet is unsubmerged, inlet can be either.

   i. Headwater (oh) = fullflow headwater

   Calculate S2 curve "1-S2n" for outlet depth.

   If oh >= rise, inlet submerged "5-S2n"

   ii. If twh > headwater, tailwater drowns out jump.

   Calculate M1 curve "3-M1t".

   If culvert flows part full, "7-Mit".

OUTLET CONTROL 'FIXES' FOLLOWING HY-8 6.1

KE = KE(IC)

A7 = 64.4: A8 = 29: A1 = 1: A2 = .25: A3 = 2: A4 = 4 / 3: A5 = 3.14159: A6 = 4

SELECT CASE PSHAPE 'CRITICAL DEPTH

CASE 2

GOSUB 6210

CASE 1, 3 TO 10

GOSUB 6220

END SELECT

GOSUB 6280 'NORMAL DEPTH

SELECT CASE WSP$ '***added 3.1, 4.4 changed wsp CASE "F"

GOSUB fullflow

CASE ELSE

SELECT CASE DCO 'changed to DCO from DNO 4.5

CASE IS >= RISE'If Yn = to RISE, assume full. ***added >.95 3.3, revised 4.5

WSPTYPE$ = "FFc"

GOSUB fullflow

IF (IH + I1E) > (OH + I1E) THEN

WSPTYPE$ = "S2n": GOSUB S2CURVE

IF IH > RISE THEN FLTYPE = 5 ELSE FLTYPE = 1 'added 3/05 to correct FLTYPE=6

END IF

CASE IS < DNO 'Mild slope

SELECT CASE TWH

CASE IS >= S0 * L + RISE 'Pipe flows full

GOSUB fullflow

CASE IS >= RISE 'Outlet submerged, Inlet unsubmerged

'Pipe flows partially full, M1 Curve

D0 = RISE

LCURVE = L - (TWH - RISE) / S0: LADJ = LCURVE

GOSUB M1CURVE 'M1 Profile

DL = RISE: GOSUB SectionProp: VOUT = Q / AC: DOUT = RISE '***added 3.3

FLTYPE = 3: WSPTYPE$ = "M1f"

'Inlet unsubmerged, outlet unsubmerged

'If Inlet is submerged FLTYPE = 7 is returned

CASE IS >= DCO

SELECT CASE TWH

CASE IS >= DNO

FLTYPE = 3: WSPTYPE$ = "M1t"

D0 = TWH: LADJ = L

GOSUB M1CURVE: DL = TWH: GOSUB SectionProp: VOUT = Q / AC: DOUT = TWH

CASE IS < DNO

FLTYPE = 3: WSPTYPE$ = "M2t"

D0 = TWH '**3.3 changed from DO=DCO

GOSUB M2CURVE: DL = TWH: GOSUB SectionProp: VOUT = Q / AC: DOUT = TWH

END SELECT

CASE IS < DCO

FLTYPE = 2: WSPTYPE$ = "M2c"

D0 = DCO: GOSUB M2CURVE: DL = DCO: GOSUB SectionProp: VOUT = Q / AC: DOUT = DCO

END SELECT 'TWH for mild slope

CASE IS >= DNO 'Steep slope

SELECT CASE TWH

CASE IS >= S0 * L + RISE

GOSUB fullflow

CASE IS >= RISE 'Outlet submerged, Inlet unsubmerged

'STEEP CHANNEL-HYDRAULIC JUMP FORMS

IF TWH < DNO THEN TWH = DNO '***3.3 moved

GOSUB fullflow

IF OH < RISE THEN

LFULL = (TWH - RISE) / S0

D0 = RISE: LCURVE = L - LFULL

GOSUB S1CURVE

IF OH + I1E > TWH + I2E THEN

FLTYPE = 1: WSPTYPE$ = "S1f"

ELSE

FLTYPE = 3: WSPTYPE$ = "M1t"

D0 = RISE: LADJ = L 'Tailwater drowns out jump

GOSUB M1CURVE

END IF

END IF

CASE IS < RISE 'Outlet unsubmerged, Inlet unsubmerged & submerged

'OH = IH deleted 12/05 (should check OH from M1 with IH

'IF OH + I1E > TWH + I2E THEN

' 'GOSUB fullflow 'added 45 to revise plot, deleted 61

' FLTYPE = 1: WSPTYPE$ = "S2n"

' GOSUB S2CURVE

'ELSE 'Tailwater drowns out jump

' FLTYPE = 3: WSPTYPE$ = "M1t" 'M1 profile

' D0 = TWH: LADJ = L

' GOSUB M1CURVE: DL = TWH: GOSUB SectionProp

' VOUT = Q / AC: DOUT = TWH

' 'If culvert flows part full FLTYPE = 7 is returned

'END IF

IF TWH <= DNO THEN

FLTYPE = 1: WSPTYPE$ = "S2n"

GOSUB S2CURVE

ELSE 'assume Tailwater forces jump calculate M1

FLTYPE = 3: WSPTYPE$ = "M1t" 'M1 profile

D0 = TWH: LADJ = L

GOSUB M1CURVE: DL = TWH: GOSUB SectionProp

VOUT = Q / AC: DOUT = TWH

'If culvert flows part full FLTYPE = 7 is returned

IF IH > OH THEN 'M1 does not control calculate S2

FLTYPE = 1: WSPTYPE$ = "S2n"

GOSUB S2CURVE

END IF

END IF

IF OH >= RISE THEN FLTYPE = 5

END SELECT 'TW for steep

END SELECT 'DNO

END SELECT 'HDS5

WSPTYPE$(0, 0) = WSPTYPE$

RETURN