Culvert Outlet
• 23 Oct 2022

# Culvert Outlet

Article summary

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

KO

Reverse Flow Mode

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

rfmode

## 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:

1. a Culvert Inlet
2. two or more Conduit type or River Section nodes forming a reach
3. 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

Vb ³ Vdc

Equation

 (1)

where:

KO = Outlet head loss coefficient

Vb = Culvert barrel flow velocity (m/s)

Vdc = Downstream channel flow velocity (m/s)

g = Acceleration due to gravity (m/s2)

#### Mode 2 - Reverse Flow, Outlet Type Loss

 Condition Q < 0Vdc ³ Vb 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 h1 = h2

#### Mode 3 - Positive Flow, Inlet Type Loss

Condition

Q ³ 0

Vb < Vdc

Equation

 (2)

where:

KO = Outlet head loss coefficient

Vb = Culvert barrel flow velocity (m/s)

Vdc = Downstream channel flow velocity (m/s)

g = Acceleration due to gravity (m/s2)

#### Mode 4 - Reverse Flow, Inlet Type Loss

Condition

Q < 0

Vdc < Vb

Equation

if Reverse Flow Mode = Zero then h1 = h2

if Reverse Flow Mode = Calculated then:

 (3)

where:

KO = Outlet head loss coefficient

Vb = Culvert barrel flow velocity (m/s)

Vdc = Downstream channel flow velocity (m/s)

g = Acceleration due to gravity (m/s2)

### 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 - KO, rfmode, headwd