Pipe network geometries are defined by Nodes (represented by a points layer) and Conduits (represented by a polyline layer), and their associated attributes. Nodes provide the connections between multiple Conduits at junctions, provide connections from the pipe network to 1D and 2D surface elements (via drop inlets or daylighting pipes such as culverts), and are where external boundary conditions can be introduced to the system.

HEC-RAS Pipe Networks are Geospatial

Unlike many other stormwater modeling software which rely on stick model representations of the geometry, HEC-RAS Pipe Network geometries are fully geospatial. That means when adding pipe network nodes and conduits in HEC-RAS, they should reflect the true georeferenced locations and dimensions.

This chapter starts by describing how to enter, edit, and import pipe network geometry in HEC-RAS Mapper. Then, details about what the Node and Conduit attributes represent and how they are used to create Pipe Networks is covered.

Creating Pipe Network Geometry

Pipe network geometries can be created by drawing Nodes, drawing connecting Conduits and defining the attributes of each in their respective attribute tables. All geometry creation and editing for pipe networks is done from HEC-RAS Mapper.

Creating pipe network geometry can also be done via a shapefile importer that allows users to import georeferenced features and map the corresponding attributes (inverts, conduit shape, size, roughness, etc.) to the appropriate HEC-RAS geometry fields. In addition to the shapefile importer, an importer is available to assist modelers in converting existing US Environmental Protection Agency's Storm Water Management Model (EPA-SWMM) elements from the *.inp file into HEC-RAS pipe networks.

Drawing Nodes and Conduits

Both Nodes and Conduits can be added to the model geometry using the standard editing tools within HEC-RAS Mapper. The preferred sequence for creating a new pipe network in HEC-RAS is as follows:

1. Add Nodes to the Pipe Network geometry:  Nodes should be added at the junctions of conduits (where two or more conduits meet), at locations where the pipe network connects to surface elements (drop inlets or culvert openings), and where boundary conditions (inflow hydrographs, downstream stage or normal depth) will be applied. While it is possible to create conduits prior to creating endpoint nodes, it is preferable to add nodes first and then add conduits. Added in this order, the conduits will snap directly to the nodes and populate the connectivity attributes for both nodes and conduits.
   
   
2. Add Conduits to the Pipe Network geometry:  Conduits should be added, connecting all the nodes to create a pipe network. Note that every Conduit must have a Node on both ends. 
   
   
3. Populate Node and Conduit Attributes:  Once the features of the pipe network are added, the required attributes for Nodes and Conduits then should be populated by editing their respective Attribute Tables in HEC-RAS Mapper.

The process for drawing Nodes and Conduits is very similar. In HEC-RAS Mapper, begin an editing sessions, select the desired layer to edit in the tree view, and select the Add New tool . Nodes can then be drawn on the map with a single click as shown below:

When a node is added, a default name is provided that can be changed later from the node attribute table. After nodes have been added, their location can be moved using the Select/Edit Tool .

After nodes are placed, conduits should be drawn from upstream node to downstream node.

Draw Conduits Upstream to Downstream

 Though not a computational requirement, drawing conduits upstream to downstream can make for easier interpretation of results and model review. The direction of the conduit lines impact where entrance and exit loss coefficients are applied, and what Node Type is assigned to the connected nodes.

The direction of the conduits can be shown by turning on the Directional Arrows plot option as shown below:

With nodes already drawn, drawing conduits is simple. To start drawing a conduit, single-click in the map window to place the upstream vertex and consecutive vertices, and double-click to finish. If cursor is inside of the snapping distance, the conduit vertex will automatically snap to the node as shown below.

A handy tool for editing conduits that have already been drawn, but not snapped to nodes, is the Snap Conduits to Nodes tool. This will snap the conduit ends to the closest node if any exists within a search radius around the first and last points of a conduit. 

 This tool can be accessed from right-clicking the conduits layer in the tree view as shown below. Accessing the option here will attempt to snap all nodes to conduits.

Individual conduits, or a selection of conduits can be snapped by right-clicking the conduit feature in the map view:

Conduit Speed Draw Mode

In order to help draw pipe networks more quickly, the Speed Draw mode automatically places nodes at the ends of newly finished conduits. To turn on Speed Draw mode check the box in the conduit editing tool bar, then nodes will be placed at the ends of each newly completed conduits if one does not exist there already.

Use 'Split Selected Feature' To Add Nodes to Existing Conduit

An existing conduit can be split using the Split Selected + Add Node tool. This tool is handy when additional drop inlets or manholes are desired to be placed on an existing conduit. 
When Split the Selected tool is used on a conduit:

1) Two conduits will be created and attributed from the existing conduit's information
2) A node will be placed where the conduits are split and the elevation of the new node will be populated to maintain the slope of the existing conduit.

Tip

If a conduit has been drawn or otherwise entered with the wrong flow direction, it can be reversed by selecting it with the editing tools and right-clicking on the conduit.  Then select Reverse Selected and all of the selected conduits will have their drawn direction reversed.

Importing Nodes and Conduits from GIS data

If existing GIS data is available for a stormwater system, the data can easily be imported to create a Pipe Network in HEC-RAS. This shapefile importer affords the flexibility to import geometry information from various sources such as utility maintenance files, design files, or exports from other pipe modeling software

To import a shapefile or GeoJSON file begin, editing the geometry by right-clicking on the geometry or selecting the edit icon. Then right-click the layer in the tree view and select Import Features .

When the Feature Importer opens, Select the shapefile or GeoJSON file containing the node or conduit data in the Filename box. The features available for import will display in left panel and in the map panel of the dialog. The user can decide which features to import by selecting or unselecting them in the left panel. 

The bottom panel allows for mapping the shapefile fields (show in the drop downs) to the required HEC-RAS conduit or node fields (shown in column headers). If the HEC-RAS required fields are not available from the imported shapefile, further populating of attribute tables will be required after import. 

Importing Nodes and Conduits from EPA-SWMM Models

The U.S. Environmental Protection Agency's Storm Water Management Model (EPA-SWMM) is the most widely used stormwater software in world, and is also the backbone of many other popular stormwater modeling software suites. If a modeler has in existing EPA-SWMM based model, it's geometry can be brought into HEC-RAS to help create pipe networks.

In order to import EPA-SWMM geometries to HEC-RAS, the EPA-SWMM elements must be georeferenced in a known coordinate reference system.

One way to bring in EPA-SWMM geometry is to first export the SWMM geometry to a shapefile or GeoJSON file, and then import to HEC-RAS using the Feature Import tool discussed in the section above. This method give the user the opportunity to visualize and manipulate data in a GIS before importing it into an HEC-RAS pipe network.  Many SWMM based models have an export capability built in, or GIS plugins are freely available in Quantum GIS to do perform the same task for a given *.inp file. 

The other option for importing EPA-SWMM geometry is to use the Import SWMM Geometry tool from HEC-RAS Mapper to import the elements directly to Nodes and Conduits in HEC-RAS Mapper. This option requires fewer steps and is good for getting started with simple SWMM models that won't require much GIS editing. 

Note that both import options are valuable for providing a good starting place to build an HEC-RAS pipe network, but some further editing and modifications will be required to create a representative geometry from a SWMM model. The primary reason for this is that HEC-RAS pipe networks are fully geospatial and the computations rely on the spatial locations and dimensions of the geometry. This is in contrast with SWMM based stick models in which locations and dimensions of geometry elements can be arbitrary, even of the model is georeferenced.

To import EPA-SWMM geometry directly to HEC-RAS nodes and conduits in RAS Mapper: Start an edit session on a new or existing geometry, Right-click the geometry and select Import SMM Geometry . 

When Import SWMM Project Dialog launches, select the SWMM *.inp file you wish to import. Then select from the following options:

Merge Subcatchments Into Single 2D Area: When ON this option will import the SWMM subcatchments and merge them to a single HEC-RAS 2D Area. This option is useful when the SWMM subcatchments if the watershed are well delineated and when using their extents to model surface flooding is desired. THe accompanying Buffer Smooth Distance: is the buffer distance that will be applied to the subcatchment polygons to ensure they are overlapping, such that they can be merged into a single 2D Area.

Use Catchment Boundaries As Breaklines: When ON This option will import the SWMM subcatchment boundaries as breaklines for the 2D mesh in the RAS Model.

Remove Unconnected Nodes: When ON this option will omit SWMM nodes that are not connected the the network by an HEC-RAS compatible SWMM link such as a SWMM orifice or weir link.

Attributing Nodes and Conduits

Once the nodes and conduits are drawn, properly attributing them is the next step in characterizing the pipe network system. This section describes tools for viewing and editing pipe network attributes. For Details about what the Node and Conduit Attributes represent is covered in the Node Attributes and Conduit Attributes Sections. 

The attributes for the nodes and conduits are viewed and edited exclusively in HEC-RAS Mapper by right-clicking the layer in the tree and and selecting Open Attribute Table  .

When an edit session is initiated on the layer and the attribute table is open, attribute values can be entered by hand, copy/pasting, or bulk edited by column using the column sort and edit tools. The column sort and edit tools are located at the top of each column and can be used to edit the selected values globally by setting them directly performing math on the values in the column as shown in the figure below. 

When values are missing or there is an issue with the input data, the attribute table will highlight in red the problem cells. The figure below shows an example of a Node attribute table where an Invert Elevation value is missing for the first row. In addition to the red Invert Elevation cell, the Node Type shows Error since the invert elevation is required to determine the Node Type. Finally the Node Status also provide an indication of the error: *Missing Invert Elevation*. 

The second row has an Drop Inlet Elevation indicating a Drop Inlet is intended, but is missing the Drop Inlet Weir Length which is highlighted in red. The Node Status also indicates that there is an issue with the input data: *Missing drop inlet attributes*.

The Error Layer  which appears at the bottom of the geometry while in an Edit Session shows which conduits and nodes have errors in the map window and provides details in the Error Layer attribute table.

Note

Some of the error checking in Pipe Networks is done on the fly during an edit session. However, other error checks are completed on the closure of an edit session since the Pipe Network connectivity must be recomputed. That said, if error messages still appear after input data was modified, try to cycle through an edit session.

Use Profile Plot to Verify Geometry Attributes

Another useful tool when building and checking Pipe Network geometry is to plot the profile of the geometry. To do this, select the desired conduits to plot in the map window, then right-click and select Plot Conduit Profile for the selected conduits. As an alternative to selecting individual conduits, the video below shows some additional selection tools 

Pipe Network Mesh

Once Nodes and Conduits are drawn and attributed appropriately, a Pipe Network mesh will be created automatically and cell and face property tables will be computed when the edit session is closed. An algorithm places mesh faces placed based on the Cell Mesh Size attribute of the conduits, the Length of the conduits, and the the Base Area of the Node cells. Depending on the junction Base Area relative to the size of the incoming conduits, the junction cell will borrow portions of the connected conduits. This borrowing is intended to prevent the creation of very small cells at the junctions so the computation times are not limited by the small cells.

The pipe network mesh will be displayed in the map window of RAS mapper by default. Property tables for the cells and faces can be accessed by selecting the Pipe Networks Layer in the tree and right-clicking cells or faces in the map window.  Below is an example of what the mesh cells look like where three conduits come together at a junction node. This node has a non-zero Base Area defined, giving it the circular shape seen. Also note that the node cell here is borrowing portions of the connected conduits. 

Nodes

The pipe Nodes in HEC-RAS provide connections between conduits and other elements in the model. Nodes can represent a variety of physical structures (e.g., manholes, drop inlets, junction boxes, etc.) or they can represent no structure at all (e.g., daylighting pipes such as culverts). What a node represents is reflected in its attributes, its conduit connections, and its connection to 1D or 2D surface element. The combination of these conditions results in an automatically assigned Node Type which will be discussed further in the Node Attributes section.

Nodes should be placed at the end of all conduits and are used to model:

• Junction boxes or manholes between multiple conduits
• Locations where conduits change in size or slope
• Wet wells or other subsurface storage
• Drop inlets connecting surface elements to the pipe network
• Locations of inflow boundary conditions to the pipe network
• Outfalls from conduits to surface elements or outfalls leaving the model

Node Types

When nodes are added in HEC-RAS, they are automatically assigned a Node Type based on a variety of their parameters and attributes: the number of conduit connections, the direction of conduit connections, the depth of the connected conduits relative to the terrain (or terrain override elevation) and whether a surface geometry exists at the node.

The Node Type dictates how flow into and out of the node will be computed, what boundary condition types are allowed at a node, and how flow to and from the node will interact with the surface geometry, if at all. This section describes the different types of nodes, how they are determined, and how they can be used in the model.

Node Attributes

Provided that the drawn or imported Nodes and Conduits are connected and an associated terrain exists, the minimum required attributes for a valid Node geometry layer are Invert Elevation, System Name, Terran Elevation, and Base Area. 

Conduits

Conduit Attributes

Provided that the drawn or imported Nodes and Conduits are connected and an associated terrain exists, valid Conduit geometry requires at a minimum,  System Name, Modeling Approach, Mesh Cell Length, Rise/Span, Manning's n US/DS Offsets, and Entrance/Exit Losses.

Conduit Shapes:

Several different conduit cross-sectional shapes are available to use in pipe networks. These shapes are analogous to the ones used in the HEC-RAS culvert editor for inline, lateral, and SA/2D structures. These shapes include: circular; box (rectangular); arch; low profile arch; high profile arch; ellipse (horizontal and vertical); semi-circular, and Con/Span culverts (see figure below).

As described in the conduits attribute section the size of each shape is determined by a Rise and Span parameter which represent the maximum height and width inside the conduit. Note, for circular and semi-circular conduits, the rise and span should bet set to the same value.