 22 Sep 2022
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Culvert Inlet
 Updated on 22 Sep 2022
 8 Minutes to read

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The Culvert Inlet models the inlet of a culvert (with an optional trash screen). The unit must be placed immediately upstream of a properly formed conduit or river reach (two or more consecutive Conduit or River Section nodes).
Data
Field in Data Entry Form  Description  Name in Datafile  

Upstream Node  Upstream node label  Label1 

Downstream Node  Downstream node label  Label2 

Upstream Remote Node  Upstream remote node label  Label3 

Downstream Remote Node  Downstream remote node label  Label4 

K  Unsubmerged inlet control loss coefficient  K  
M  Exponent of Flow Intensity for inlet control  M  
C  Submerged inlet control loss coefficient  c  
Y  Submerged inlet control adjustment factor  Y  
Conduit Type Code  Conduit type code; Type A (ctype='A'),for Type B (ctype='B')  ctype  
Ki  Outlet control loss coefficient  Ki 

Screen Width  Trash screen width (m)  WS 

Bar Proportion  Proportion of trash screen area occupied by bars (0 to 1.0)  r 

Debris Proportion  Blockage ratio (proportion of trash screen area occupied by debris) (0 to 1.0)  b 

Max Trash Screen Height  Max. Trash Screen Height *  height 

Loss Coefficient  Trash screen head loss coefficient, typical value is 1.5  KS 

Reverse Flow Mode  Reverse Flow Mode; keyword ZERO (for zero headloss in reverse flow) or CALCULATED (for calculated head loss in reverse flow)  rfmode 

Headloss Type  Keyword TOTAL to denote headloss based on total head, otherwise (keyword STATIC or blank) headloss is based on static head  headwd 

 variables are required for inlet control calculations only
*  The Max trash screen height is the height of the trash screen from bed level (specified in metres). It is used as the maximum height for the calculation of cross sectional area of flow (and hence velocity through the trash screen). The area calculation uses the piezometric head while water levels are below the specified trash screen height. When water level exceeds this height then it is capped to the trash screen height in the area calculation. It should be noted that piezometric head (and therefore ultimately the headloss) can differ significantly from true water level if the culvert is pressurised. However, the piezometric head is the default behaviour to preserve backwards compatibility.
Theory and Guidance
The Culvert Inlet models the inlet of a culvert (with an optional trash screen) based initially on the methodology contained within the Culvert Design Manual (1997) and subsequently updated to reflect the contents of the Culvert design and operation guide (2010).
The Culvert Inlet must be placed immediately upstream of a properly formed conduit or river reach (two or more consecutive Conduit type or River Section nodes). These nodes will determine the hydraulic properties and variables used in the flow calculations.
To model a culvert in Flood Modeller, the sequence of units used would normally be:
 a Culvert Inlet
 two or more Conduit type or River Section nodes forming a reach
 a Culvert Outlet.
The Culvert Inlet will model the losses at the culvert's entrance, while the Culvert Outlet will model the culvert's exit losses. Friction losses associated with the main body of the culvert are modelled by the sequence of Conduit type or River Section nodes. Any losses due to bends in the culvert can be modelled using a Culvert Bend.
The Culvert Inlet can operate either under inlet control or outlet control, and can switch freely between these modes depending on the hydraulic conditions.
When the Culvert Inlet is in outlet control, the governing equations are based on a simple velocity head multiplier and the head downstream of the Culvert Inlet will affect the calculation of the upstream head. By default, the head loss in Outlet Control mode (and of the Trash Screen) is applied to the static head, although there is an option to calculate the head loss based on the total head. The two will be appreciably different if there is a significant difference in the upstream and downstream velocity heads.
The Reverse Flow Mode parameter determines which equation is used in reverse flow. If it set to Calculated, the Culvert Inlet will use the same equation as for mode 5, but with h1 and h2 reversed, and with VB obtained from the upstream river section. If it is set to Zero then the head loss imposed under reverse flow conditions will be zero. You will always be informed at the start and finish of reverse flow conditions.
Under inlet control the upstream water level is solely dependent on the culvert flow and the properties of the culvert entrance. The inlet head is thus independent of the head downstream of the unit in inlet control.
The Culvert Inlet can model the losses of a simple rectangular, barred trash screen, which is regarded as placed in front of the culvert's entrance. You specify the geometry and proportion of the screen blocked by debris. The Culvert Inlet will then apply the head loss equations associated with a typical trash screen. These equations derive from the Culvert Design Manual (1997) .
Equations
Mode 1  Unsubmerged Inlet (Type A Conduits)
Condition 
 
Equation 
where: h_{I}_{ }= Inlet depth (post trash screen  m) H_{C} = Total head at critical flow D = Height of culvert barrel (m) K = Unsubmerged inlet control loss coefficient A = Total culvert crosssectional area (m^{2}) S = Culvert slope M = Exponent of flow intensity for inlet control Q = Culvert Flow (m^{3}/s) 
Mode 2  Unsubmerged Inlet (Type B Conduits)
Condition 
 
Equation 
where: Q = Culvert flow (m^{3}/s) h_{I} = Inlet depth (post trash screen  m) D = Height of culvert barrel (m) K = Unsubmerged inlet control loss coefficient A = Total culvert crosssectional area (m^{2}) M = Exponent of flow intensity for inlet control 
Mode 3  Submerged Inlet
Condition 
 
Equation 
where: h_{I} = Inlet level (post trash screen) D = Height of culvert barrel c = Submerged inlet control loss coefficient A = Total culvert crosssectional area Y = Submerged inlet control adjustment factor S = Culvert slope 
Mode 4  Drowning Transition
Condition 
 
Equation  Mode1, 2, or 3 equations, depending on culvert type (A or B), and which flow mode (submerged or unsubmerged inlet) gives higher inlet head. 
Mode 5  Outlet Control

where: h_{1} = Upstream water level (or total head if TOTAL head loss selected) (m AD) h_{2} = Downstream water level (or total head if TOTAL head loss selected) (m AD) K_{I}_{ }= Outlet control loss coefficient V_{B} = Flow velocity of culvert barrel g = Acceleration due to gravity 
Modes 1 to 4 are regarded as inlet control modes and mode 5 is the outlet control mode. The Culvert Inlet switches between inlet and outlet control based on which control mode gives the higher inlet head.
Mode 6  Reverse Flow
Condition 
 
Equation  if Reverse Flow Mode = Calculated then use the equation for Mode 5 with direction of head loss reversed. if Reverse Flow Mode = Zero then h1 = h2 
Trash Screen Head Loss

where: h_{1} = Inlet water level pre trash screen (or total head if TOTAL head loss selected) (mAD) h_{I} = Inlet level post trash screen (or total head if TOTAL head loss selected) (mAD) Q_{1} = Inlet flow (m3/s) K_{S} = Trash screen loss coefficient W_{S} = Trash screen width y_{I} = Inlet depth (pre trash screen  m) g = Acceleration due to gravity s = Ratio of area of screen gaps to total screen area Notes = (1  r)(1  b) where: r = Proportion of screen occupied by bars b = Proportion of screen blocked by trash 
General
The remote node Upstream Control Node will only be required when there is no channel node (Conduit type or River Section node) directly upstream of the Culvert Inlet. Similarly, a node name should only be entered in the Downstream Control Node field if a channel node is not connected directly downstream of the Culvert Inlet. If you use these fields, the Culvert Inlet will obtain the upstream and downstream flow velocities it requires from the nodes specified in Upstream Control Node and Downstream Control Node respectively.
The equations governing inlet control include several empirical variables which may be entered or edited by the user; however, a "wizard" is also available (appearing by default on adding a culvert inlet), which populates the requisite fields with appropriate parameters. These parameters derive from the 2010 CIRIA report (and earlier research), and depend on the culverts shape, material and inlet and edge type (see figure below). The table from the report showing typical values for the various inlet control parameters is reproduced below as a guide.
Inlet Control Parameters
Circular Conduits
Material  Inlet Type / Edge Type  Parameters  

Type  K  M  c  Y  Ki  
Concrete  Headwall / square edge  A  0.0098  2.0  0.0398  0.67  0.5 
Headwall/socket end of pipe  A  0.0078  2.0  0.0292  0.74  0.3  
Projecting/socket end of pipe  A  0.0045  2.0  0.0317  0.69  0.3  
Corrugated Metal  Headwall / square edge  A  0.0078  2.0  0.0379  0.69  0.5 
Mitred to slope  A  0.021  1.33  0.0463  0.75  0.7  
Projecting  A  0.034  1.5  0.0553  0.54  0.9 
Rectangular Conduits
Material  Inlet Type / Edge Type  Parameters  

Type  K  M  c  Y  Ki  
Concrete  Headwall / 20mm chamfers  B  0.5150  0.667  0.0375  0.79  0.5 
Headwall / 45° bevels  B  0.4950  0.667  0.0314  0.82  0.5  
Headwall / 33.7° bevels  B  0.486  0.667  0.0252  0.865  
30° to 75° flared wingwalls/single barrel  B  0.469  0.696  0.033  0.751 
 
30° flared wingwalls /top edge bevelled 45° /single barrel (Form A)  A  0.005  1.05  0.04  0.48  0.26  
30° flared wingwalls /top edge bevelled 45° /single barrel (Form B)  B  0.44  0.74  0.04  0.48  0.26  
30° flared wingwalls /top edge bevelled 45° /single barrel/span to rise 2:1 to 4:1
 B  0.48  0.65  0.041  0.57  0.2  
30° flared wingwalls /top edge bevelled 45° /multiple barrels (24)
 B  0.47  0.68  0.04  0.62  0.32  
30° flared wingwalls /top edge bevelled 45° /multiple barrels and 15° skew
 B  0.69  0.49  0.029  0.95  0.36  
30° flared wingwalls /top edge bevelled 45° /multiple barrels and 30° to 45° skew
 B  0.69  0.49  0.027  1.02  0.45  
0° flared wingwalls /top edge square /single barrel (Form A)
 A  0.005  0.68  0.047  0.55  0.79  
0° flared wingwalls /top edge square /single barrel (Form B)
 B  0.55  0.64  0.047  0.55  0.79  
0° flared wingwalls /top edge bevelled 45° /150mm corner fillets  B  0.56  0.62  0.045  0.55  0.48  
0° flared wingwalls /top edge bevelled 45° /multiple barrels (24)  B  0.55  0.59  0.038  0.69  0.52  
0° flared wingwalls /top edge bevelled 45° /span to rise 2:1 to 4:1  B  0.61  0.57  0.041  0.67  0.37  
0° flared wingwalls /crown 200mm radius /150mm corner fillets  B  0.56  0.62  0.038  0.67  0.24  
0° flared wingwalls /crown 200mm radius /300mm corner fillets  B  0.56  0.62  0.038  0.67  0.3  
0° flared wingwalls /crown 200mm radius /300mm corner fillets /multiple barrels (24)  B  0.55  0.6  0.023  0.96  0.54  
0° flared wingwalls /crown 200mm radius /no corner fillets/ span to rise 2:1 to 4:1  B  0.61  0.57  0.033  0.79  0.3 
Vertical Eclipse
Material  Inlet Type / Edge Type  Parameters  

Type  K  M  c  Y  Ki  
Concrete  Headwall / square edge  A  0.01  2.0  0.0398  0.67 
 
Headwall / socket end of pipe  A  0.0018  2.5  0.0292  0.74 
 
Projecting / socket end of pipe  A  0.0095  2  0.0317  0.69 

Pipe Arch Conduits (ARMCO)  450 mm corner radius
Material  Inlet Type / Edge Type  Parameters  

Type  K  M  c  Y  Ki  
Corrugated metal  90° Headwall  A  0.0083  2.0  0.0379  0.69  0.5  
Mitred to slope  A  0.0300  1.0  0.0463  0.75  0.7  
Projecting  A  0.0340  1.5  0.0496  0.57  0.9 
Pipe Arch Conduits (ARMCO)  750 mm corner radius
Material  Inlet Type / Edge Type  Parameters  

Type  K  M  c  Y  Ki  
Corrugated metal  Headwall / square edge  A  0.0087  2.0  0.0361  0.66  0.5 
Headwall/ 33.7° bevels  A  0.003  2  0.0264  0.75  0.25  
Projecting  A  0.296  1.5  0.0487  0.55  0.9 
Full Arch Conduits
Material  Inlet Type / Edge Type  Parameters  

Type  K  M  C  Y  Ki  
Corrugated metal  90° Headwall  A  0.0083  2.0  0.0379  0.69  0.5 
Mitred to slope  A  0.0300  1.0  0.0463  0.75  0.7  
Thin wall projecting  A  0.0340  1.5  0.0496  0.57  0.6 
Control Regime
When the Culvert Inlet operates under inlet control, this effectively means that the whole culvert is operating under this regime. Thus, the hydraulic conditions in or downstream of the culvert will not affect the flow through the culvert inlet. This usually means that the water passes through the critical depth at or near the culvert's inlet.
Similarly, when the Culvert Inlet is in outlet control, the whole culvert's flow is also outlet controlled. In this case downstream hydraulic conditions can affect flow conditions at the inlet.
The CIRIA methodology is based on single barrelled culverts, and is not necessarily fully applicable to multibarrelled culverts. If multibarrelled culverts are to be modelled, each barrel should have its own Culvert Inlet and Culvert Outlet. The barrels should be joined to the connecting upstream and downstream channels using Junctions upstream and downstream of the Culvert Inlet and Culvert Outlet respectively. You can then use the remote node capability to tell the Culvert Inlet the location of the upstream and/or downstream flow velocities.
The diagrams and table used within the Culvert Inlet are reproduced from the Culvert Design Manual (1997) with kind permission of CIRIA.
Datafile Format
Line 1  keyword 'CULVERT' [comment]
Line 2  keyword 'INLET'
Line 3  Label1, Label2, [Label3, Label4]
Line 4  K, M, c, Y, KI , e
Line 5  W_{S}, r, b, K_{S}, rfmode, headwd, height