Modeling Reference - Headwalls

Product(s): CivilStorm, SewerCAD, SewerGEMS, StormCAD
Version(s): 08.11.04.XX and later (including CONNECT Ed.)
Area:  Modeling

 

Introduction

This Technote explains how to use the Headwall node element. 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.

Calculations

Hydraulically a culvert can be under inlet control or outlet control conditions. The computational procedures for these conditions are very different. 

A culvert is under inlet control if the culvert barrel hydraulic capacity is higher than that of the inlet (entrance) and there is no backwater from downstream. In this condition, the relationship of flow and headwater is mainly dependent on the inlet configurations. A culvert is under outlet control when the culvert barrel is not capable of conveying as much flow as the inlet opening will accept. When the culvert is under outlet control, the flow will depend not only on the headwater but also the tailwater. Dynamic culvert conditions are complicated in that the flow can change from inlet control to outlet control or vice versa. As a result of this complexity, the computation of culverts can be tedious. A sophisticated procedure has been developed to build up a comprehensive EQT data set for any culvert configuration. The EQT represents the headwater (E), flow (Q), and tailwater (T) tabular curves in the way it covers all possible operating ranges of the headwater and tailwater so that any hydraulic conditions are accounted for by the EQT. The numerical solver builds the EQT for every culvert and uses the EQT for culvert computation dynamically at any time step. Additional details are different depending on the product and numerical solver used:

  • GVF-Rational (StormCAD) and GVF-Convex (SewerCAD) solvers - The GVF solvers in StormCAD (GVF-Rational, GVF-Convex) use procedures that are similar to HDS-5 methods, an energy-equation-based backwater calculation is carried out from the downstream end-wall-node up to the upstream inlet node and the depth from the backwater profile calculation is compared with the culvert inlet control depth and the largest depth is used, and the control status (inlet control or outlet control) is determined accordingly based on the depth being used at the inlet-head-wall-node.
  • Implicit dynamic (DW) solver - Dynamic culvert conditions are complicated in that the flow can change from inlet control to outlet control or vice versa. As a result of this complexity, the computation of culverts can be tedious. In StormCAD DW solver , a sophisticated procedure has been developed to build up a comprehensive EQT data set for any culvert configuration. The EQT represents the headwater (E), flow (Q), and tailwater (T) tabular curves in the way it covers all possible operating ranges of the headwater and tailwater so that any hydraulic conditions are accounted for by the EQT. The StormCAD DW dynamic engine builds the EQT for every culvert and uses the EQT for culvert computation dynamically at any time step.
  • Explicit dynamic (SWMM) solver - The explicit solver in StormCAD (SWMM) uses culvert code developed in EPA SWMM to carry out the culvert calculations.
  • Broken-Back-Culvert: see further below.

Layout and Configuration

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.

 

The conduit link element adjacent to a headwall should also be configured to represent a culvert, by selecting "True" for the "Is Culvert?" property. Select "Use Start Node" and/or "Use Stop Node" for the additional fields that appears when designating a conduit as a culvert, and the conduit will automatically use the inlet description (HDS-5 coefficients) that you already selected in the adjacent headwall, to model the headloss across the headwall.

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.


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.

 

Headwall Boundary Condition Types

The "Boundary Condition" field in the headwall properties refers to how the headwall element is connected to other elements (the way in which it acts as the "boundary" between two hydraulic conditions). The Boundary Condition Type will change automatically based on the network connectivity:

  • Inlet (upstream-most element)
  • Intermediate (between elements)
  • Outlet (terminating point of network)
  • Pond Outlet (connected to upstream pond)
  • Discharge to Pond (connected to downstream pond)

Inlet (headwall with no upstream elements)

A headwall node can be the most upstream node in a gravity sub-network, similar to other gravity network nodes. In this case, the 'Network Boundary Type' will automatically show as 'Inlet' when it detects that there is no upstream element.

Note that in this case, the 'Cross Section' related attributes are visible in the property grid but they will not be used. You will receive a user notification "Cross section dimensions are ignored. Conduit attributes are used instead."

Intermediate (headwall between elements)

If the headwall connects between two link elements (for example a conduit discharging into a channel), the "Network Boundary Type" will automatically show "Intermediate" when it detects that the headwall connects between two link elements.

Outlet (headwall as terminating point of the network)

A headwall node can be the terminal point of a gravity sub-network, in place of an outfall element. In this case, the 'Network Boundary Type' will automatically show as 'Outlet' when it detects that there is no downstream element. You will also see the same options in the "boundary condition type" field that you would get for the outfall node element. For example you can choose the free outfall option, user defined tailwater, and others.

This is appropriate in cases where the downstream end of your system (the point after which you do not want to model, and the flow leaves the system) is an endwall structure. The outfall node element can be used to terminate the downstream end of the network as well, but the headwall element enables you to visualize the system more accurately since you can see the endpoint as the endwall symbol and be able to list it along with other actual headwall/endwall elements (for example in the flextables).

Note that in this case, the 'Cross Section' related attributes are visible in the property grid but they will not be used. You will receive a user notification "Cross section dimensions are ignored. Conduit attributes are used instead."

 

Pond Outlet (Headwall as a Pond Outlet Structure)

The headwall element can be used as the outlet of a pond, instead of using the pond outlet node element with the culvert modeled in the composite outlet structure. In this case, the 'Network Boundary Type' will automatically show as 'Pond Outlet' when it detects that there is an upstream pond. The pond outflow will then be based on the hydraulics of the headwall element.

A dashed line connected to pond will be drawn, with the same behavior as a pond outlet structure node.

Valid configuration: Headwall instead of a Pond Outlet Structure.

 

Discharge To Pond (Headwall that empties into a Pond)

If the headwall node discharges into a pond, you can select the 'Boundary Element' option for the "Boundary Condition Type", then select the pond as the "boundary element" (similar to using the outfall element) In this case, the 'Network Boundary Type' will automatically show as 'Discharge to Pond' when it detects that there is a downstream pond. 


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

 

 Valid configuration: Headwall empties to a Pond.

Headwall on both sides of a channel

By setting the headwall property field "Has Cross Section?" to True, you can place a channel between two headwalls. The channel link element needs to define its cross sectional area at the end nodes (normally done with the cross section node element) so this option in the headwall properties facilitates that. 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 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. Note that a small, non-zero width is used to display the headwall in profile view, and this width cannot currently be changed. (reference # 64687)


Limitations of the Headwall Element

  • Junction structure headlosses are not supported
  • Locally injected sanitary loads are not supported
  • Incoming conduit (closed Pipe shape) links are not supported
  • Outgoing conduit (closed Pipe shape) links are not supported
  • Headwall elevations and inlet descriptions are not modified during automated Design runs (GVF-Convex and GVF-Rational solvers)
  • The result field Velocity Head (In-Governing) is not supported, because multiple incoming pipes are not permitted
  • The fields Elevation (Invert in 1, 2, 3…) are not supported, 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. Only the most-upstream culvert conduit is specified as a culvert (is culvert = true). The culvert data can be defined either from the head-wall and end-wall data or by user-defined culvert data in the conduit. 

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.

HY-8 style HDS-5 Culvert Reports

To create a culvert report similar to HY-8, create a new project-level conduit flextable and add the respective fields, renaming the column headers if needed. Below are the HY-8 results with their equivalent results field in the OpenFlows product. More information on customizing Flextables can be found here.

  • Culvert discharge: see "Flow"
  • Total discharge: This would be the same as "culvert discharge". If this is only different in cases where roadway overtopping flow occurs, that is functionality that is only available with the Implicit dynamic or Explicit (SWMM) dynamic solvers available with a CivilStorm or SewerGEMS license activated. In such cases, roadway overtopping flow would be seen in the "Flow (roadwater overtopping)" result, culvert flow would be seen in the "Flow (middle)" result, and the total flow combining both the culvert and roadway flow would be seen in the "Flow" Result.
  • Headwater elevation: see "Upstream structure hydraulic grade line"
  • Inlet control / Outlet control depth: although the comparison is performed, only the controlling depth is used. To see the headwater depth you would either need to look at the "Depth (in)" for the upstream headwall element, or create a conduit formula-derived User Data Extension that subtracts the conduit start invert from the "Upstream structure hydraulic grade line" result field. See this.
  • Flow Type: see "Profile Description". More information here.
  • Normal depth and critical depth: see "Depth (Normal)" and "Depth (Critical)"
  • Outlet depth and tailwater depth: see "Depth (out)". In StormCAD, CivilStorm and SewerGEMS in most cases these would be equal since the network is solved as a whole. If there are significant losses across the downstream node, see "Depth (in)" in the downstream node.
  • Outlet Velocity: see "Velocity (out)"
  • Tailwater velocity: If this is different than the velocity on the downstream side of the culvert, you would look at the "Velocity (In)" for the next-downstream conduit or channel.

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