Culvert Bend
• 23 Oct 2022
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# Culvert Bend

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Article Summary

The Culvert Bend models bends within a culvert. It can operate either in upstream mode (where flow velocities are obtained from the unit upstream of the culvert bend) or in downstream mode (where flow velocities are obtained from the downstream connected unit).

## Data

Field in Data Entry Form

Description

Name in Datafile

Upstream Node

Upstream node label

Label1

Downstream Node

Downstream node label

Label2

Remote Node

Remote node label

Label3

Loss Coefficient

KB

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 Bend models bends within a culvert based on the methodology contained within the Culvert Design Guide (1997).

The Culvert Bend can operate either in upstream mode (where flow velocities are obtained from the unit upstream of the culvert bend) or in downstream mode (where flow velocities are obtained from the downstream connected unit). When in upstream control the Culvert Bend must be placed immediately downstream of a channel section (for instance a River Section or Conduit type unit). Similarly, when in downstream control, the unit must be placed immediately upstream of a River Section or Conduit type unit. If the appropriate condition is not met, a remote node should be used to specify a channel node from which to obtain the flow area (and hence velocity).

To model a culvert with a bend in Flood Modeller, the sequence of units would typically be:

1. a Culvert Inlet
2. two or more Conduit type units forming a conduit reach
3. a Culvert Bend
4. at least two more Conduit type units forming another conduit reach
5. a Culvert Outlet

River Sections could be used instead of conduit type nodes.

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 units. Any losses due to bends in the culvert can be modelled using the Culvert Bend.

The Reverse Flow Mode parameter determines which equation is used in reverse flow. If it set to Calculated, the Culvert Bend will use the same equation as for mode 5, but with h1 and h2 reversed. If, however, 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 in the Culvert Bend.

The Culvert Bend is identical to the General Headloss.

### Equations

#### Mode 1 - Positive Flow

Condition

Q ³ 0

Equation (1)

where:

KB = Bend head loss coefficient

VB = Bend flow velocity (derived from the upstream channel in upstream mode, and the downstream channel in downstream mode)

g = Acceleration due to gravity

#### Mode 2 - Reverse Flow

 Condition Q < 0 Equation if Reverse Flow Mode = Calculated then use the equation from Mode 1if Reverse Flow Mode = Zero then h1 = h2.

### General

The Remote Node will only be used if the connected node in the direction indicated by the Flow Velocity Node field is not a River Section or Conduit type node. In this case the required flow velocity will be obtained from the Remote Node.

The following chart presents bend coefficient (Kb) values as a function of the angle of the bend, and of the ratio of the radius of curvature of the bend to the culvert width (R/B). Please refer to the CIRIA Report for further details. The unit state for this unit is the current bend head loss coefficient.

## Datafile Format

Line 1 - keyword `CULVERT' [comment]

Line 2 - keyword `BEND'

Line 3 - Label1, Label2, [Label3]

Line 4 - Keyword `UPSTREAM' or `DOWNSTREAM'

Line 5 - KB, rfmode 