Modeling Reference - Headwalls

Product(s): CivilStorm, SewerCAD, SewerGEMS, StormCAD
Version(s): 10.00.xx.xx, 08.11.04.xx and later
Area:  Modeling

 

Introduction

This technote explains about the use of the Headwall element, which was introduced starting with the V8i SELECTseries 4 release.

Headwalls are generally concrete or masonry retaining walls, which are placed at the outlet side of a drain or culvert. Headwalls should be designed as such by considering factors such as earth pressure, loading and soil properties.

The Headwall element enables you to directly analyze and design these structural elements without having to insert inlet and/or cross-section elements as proxy hydraulic elements. Headwalls use the standard HDS-5 methodology for computing headloss for a given flow and as such, they support Circular, Box, Ellipse and Arch (Pipe-Arch) cross sectional shapes.

The Headwall element is available for modeling in the layout toolbar as shown below for V8i, or within the Layout toolbar for CONNECT Edition.

                                

Headwall node within the Standalone Layout Toolbar

A headwall node that is the Start Node of a conduit will be laid out as a mirror image of a headwall node that is a stop node. A headwall can only be used with a conduit, and only with closed section shapes. It is not possible to connect two conduit links with a headwall node during layout.

By hydraulic convention, culvert entrances usually have end-treatments that correspond to headwalls at the entrance and endwalls at the outlet. We will refer to all end treatments, regardless of upstream or downstream location or structural aspect as Headwalls.

While setting up the headwall in the model you will need to define the referenced culvert in the properties of the headwall as seen below.

 

A conduit that is declared to be a culvert link may have a headwall node at either end. Conduits can be set as culverts by changing "Is Culvert?" property to 'True' in the properties of the conduit.

You may model projected or mitered end treatments on a culvert link with connection to a cross section or outfall node. Different possible combinations of headwalls can be seen below. If you attempt a configuration that is not permissible, you will get user notification for that.

You may model projected or mitered end treatments on a culvert link with a connection to a cross section or  an outfall node. Different possible combinations of headwalls can be seen below. If you attempt a configuration that is not permissible, you will get user notification for that.

More Valid Configurations: Culvert with channel links

Some Invalid Configurations, yet possible to layout. User notifications expected

 

Headwall Property Grid: Basic Attributes

The following attributes of a headwall node are visible in the headwall property grid. 

Headwall Property Grid

Culverts in older versions that did not have the Headwall element

Culverts in V8i SELECTseries 3 and below were modeled using a cross section node at the end. Culvert inlet coefficients could be specified through conduit properties. With the release of V8i SELECTseries 4, headwall elements can now be used to specify different characteristics for the headwall vs. the endwall, in addition to being able to see them visually in the model. 

Culverts in SS3 & Culverts in SS4

The headwalls are the end treatments of the culverts. If you need to model a headwall there has to be an adjacent culvert for that. Set the 'Is Culvert?' to 'True' in the properties of the conduit and then upstream and downstream headwalls in the physical properties of culvert (see screenshot above).

 

Culvert inlet Coefficients:

For headwalls, the 'Culvert Inlet Coefficients' manager is accessible from the Components menu, appearing just after Conduit Catalog.  This manager dialog is similar to Conduit Catalog manager, including sync capabilities to the Culvert Inlet Coefficients type of engineering library data. The headwall section properties can be set through the headwall property grid & as well as the conduit property grid.

The Culvert Inlet Coefficients manager dialog has two label columns: Label and Barrel Shape.


Mock-up of Culvert Inlet Coefficients dialog

All the attributes of culvert inlet coefficients engineering library data are mirrored and made available for editing in the right side of the dialog.

From a headwall property grid, you can reference from the culvert inlet coefficient manager by selecting […] from inlet description.  This function is similar to selecting a conduit catalog reference.  A non-editable preview of referenced support element data values is displayed in the corresponding fields in the headwall property grid.

Conduit Property Grid & Headwall Property Grid

The culvert barrel shape field reflects as a read-only value of the downstream conduit’s shape if the headwall node is a start node.  It reflects the value of the upstream conduit shape if it is a stop node.

The read-only 'Is Inlet?' attribute is 'True' if the headwall node is a start node.  It will be set to 'False' if the headwall node is the connected downstream node of the culvert link.

 

Boundary types of Headwall

  • Inlet
  • Intermediate
  • Free outfall
  • Discharge to Pond
  • Pond Outlet

Headwall with no upstream connections- Inlet

A headwall node can be the most upstream node in a gravity sub-network, similar to other gravity network nodes.  In this case, a derived result field 'Boundary Type' will have value of 'Inlet'. This value is presented in headwall property grid when it detects there is no incoming link.

The default value of this attribute is 'Intermediate', if headwall is an intermediate element in the model.

When the node has a state of 'Inlet' or 'Outlet', the 'Cross Section' related attributes are visible in the property grid but they will not be validated, you will get a user notification – Cross section dimensions are ignored. Conduit attributes are used instead. 

 

Headwall with no downstream connections – Free Outfall

A headwall node can be the terminal point of a gravity sub-network, in place of an outfall element.  In this case, a derived result field "Network Boundary Type" has value of "Outlet".  This value is presented in headwall property grid when it detects there is no outflow link.

 

Headwalls as end nodes for a channel

By setting the headwall property field "Has Cross Section?" to True, you can place a channel between two headwalls. When you set this to True, you will be able to enter the start and/or stop properties for an irregular or trapezoidal channel. 

Headwall as an alternative to a Pond Outlet Structure

In previous releases, only the Pond Outlet Structure node modeled the downstream connections in the network from a pond.  But you may now model the Pond outflow through a headwall node and culvert link. 

When headwall node is not a stop node (the headwall node has no incoming channel or conduit link),

When a headwall node is at an upstream end of the network with no upstream elements, then the upstream pond can be connected to headwall, which will act as pond outlet structure, if required.

A dashed line connected to pond will be drawn, with the same behaviour as a pond outlet structure node.  In this configuration, the 'Boundary Type' is set to 'Pond Outlet'.

Valid configuration: Headwall instead of a Pond Outlet Structure.

 

Headwall empties to a Pond 

If the headwall node has no outgoing link, then user can select the 'Boundary Element' option, so the Boundary Element field can be used from the headwall property grid to select a neighboring downstream pond. This mimics the function of an outfall to pond connection. In this configuration, the 'Boundary Type' is set to 'Pond Inlet'.


Property grid state for Headwall when selecting a Pond as a Boundary Element.

 


 Valid configuration: Headwall empties to a Pond.

 Headwall orientation:

The orientation of the headwall depends on the referenced culvert in the headwall properties. As shown in the below screenshot headwall can be referenced to different conduits when they are adjoining, based on that it will change its orientation/direction

Referenced  culvert - CO-5

Referenced culvert - CO-4

Profile View:

The headwall element can be seen in the profile view as below, wherein the drop in the HGL is seen in the conduit. 


Limitations of Headwall:

At a headwall node you cannot have:

  • Junction headloss
  • Locally injected sanitary flows
  • Incoming conduit (closed Pipe shape) links
  • Outgoing conduit (closed Pipe shape) links
  • Headwall elevations are not modified during Design run
  • Headwalls cannot be designed using Explicit SWMM solver
  • Velocity Head (In-Governing), because multiple incoming pipes are not permitted
  • Elevation (Invert in 1, 2, 3…), because multiple incoming pipes are not permitted

 

Sample File

You can download the sample file of Headwall model from the below mentioned link.

http://communities.bentley.com/products/hydraulics___hydrology/m/hydraulics_and_hydrology_gallery/270016.aspx 

 

Broken Back Culverts

Broken Back Culverts are supported starting with the CONNECT Edition Update 1 release (Build 10.01.01.04). A "broken back culvert" is composed of a series of intermediate inflow points, bends and slope changes between a head-wall and end-wall. In this case the conduits are connected by transition nodes and only the most upstream culvert conduit is specified as a culvert.

The principle behind this is to break the flow by changing the slope so that flow transitions from super-critical (high velocity) to sub-critical (low velocity) by means of hydraulic jump formed at the point of slope change. To model a broken pipe case, one can introduce transition elements on a culvert to locate points of slope change. Through these transitions inflows from other points can be let into the system.

An example of this configuration is a culvert crossing a highway with an inflow point at the median.

Prior to the release of the CONNECT Edition Update 1 (Build 10.01.01.XX) Storm and Sewer products, this configuration was not usable and would require that the culvert conduits have both a headwall and an endwall defined.

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