 23 Oct 2022
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Culvert Outlet
 Updated on 23 Oct 2022
 4 Minutes to read

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The Culvert Outlet models the outlet of a culvert. It must be placed immediately downstream 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 Control Node  Upstream remote node label  Label3 
Downstream Control Node  Downstream remote node label  Label4 
Loss Coefficient  Outlet head loss coefficient  K_{O} 
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 
Theory and Guidance
The Culvert Outlet models the outlet of a culvert based on the methodology contained within the Culvert Design Manual (1997) and has now been updated to reflect the contents of the Culvert design and operation guide (2010).
The Culvert Outlet must be placed immediately downstream 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.
So that the outlet head loss be modelled correctly, the downstream flow area is required to calculate the flow velocity. This will be obtained from the connected channel node (a Conduit type or River Section node) if this is present. Otherwise, remote nodes should be used to specify an appropriate channel section form which to derive this (usually the next section downstream [or upstream in the case of the upstream remote node]).
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 Outlet is primarily designed to model the losses associated with the expansion in flow area that is normally found at a transition between a culvert and an open channel. If the flow area reduces in the direction of flow, the Culvert Outlet will apply an inlet type loss equation (modes 3 and 4) using the given k value. When reverse flow occurs, equations similar to those used for forward flow are applied. You will be given an additional warning that reverse flow is occurring.
The Reverse Flow Mode parameter determines which equation is used in reverse flow. If it set to Calculated, the Culvert Outlet will use the equations shown below. Generally these are the same equations as for forward flow, but with directional variables changed. If, however, Reverse Flow Mode 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. By default, the head loss 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.
Equations
Mode 1  Positive Flow, Outlet Type Loss
Condition  Q ³ 0 V_{b} ³ V_{dc}  
Equation 
where: h_{1} = Upstream water level (or total head h_{2} = Downstream water level (or total head K_{O} = Outlet head loss coefficient V_{b} = Culvert barrel flow velocity (m/s) V_{dc} = Downstream channel flow velocity (m/s) g = Acceleration due to gravity (m/s^{2}) 
Mode 2  Reverse Flow, Outlet Type Loss
Condition  Q < 0 V_{dc} ³ V_{b} 
Equation  if Reverse Flow Mode = Calculated then use the equation for Mode 1 with direction of head loss reversed. if Reverse Flow Mode = Zero then h_{1} = h_{2} 
Mode 3  Positive Flow, Inlet Type Loss
Condition  Q ³ 0 V_{b} < V_{dc}  
Equation 
where: h_{1}_{ }= Upstream water level (or total head if TOTAL head loss selected) (mAD) h_{2}_{ }= Downstream water level (or total head if TOTAL head loss selected) (mAD) K_{O} = Outlet head loss coefficient V_{b} = Culvert barrel flow velocity (m/s) V_{dc} = Downstream channel flow velocity (m/s) g = Acceleration due to gravity (m/s^{2}) 
Mode 4  Reverse Flow, Inlet Type Loss
Condition  Q < 0 V_{dc} < V_{b}  
Equation  if Reverse Flow Mode = Zero then h_{1} = h_{2} if Reverse Flow Mode = Calculated then:
where: h_{1} = Upstream level (mAD) h_{2} = downstream level (mAD) K_{O}_{ }= Outlet head loss coefficient V_{b} = Culvert barrel flow velocity (m/s) V_{dc} = Downstream channel flow velocity (m/s) g = Acceleration due to gravity (m/s^{2}) 
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 Outlet. 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 Outlet. If you use these fields, the Culvert Outlet will obtain the upstream and downstream flow velocities it requires from the nodes specified in Upstream Control Node and Downstream Control Node respectively.
The value of the head loss coefficient is usually taken to be 1.0, although the provision of outlet structures and/or wingwalls can reduce this to between 0.7 and 0.8.
The unit state for this unit is the outlet head loss coefficient.
Datafile Format
Line 1  keyword `CULVERT' [comment]
Line 2  keyword `OUTLET'
Line 3  Label1, Label2, [Label3, Label4]
Line 4  K_{O}, rfmode, headwd