#INFOSWMM

How to use Transects in InfoSWMM and InfoSWMM SA


How to use Transects in InfoSWMM and InfoSWMM SA

Transects refer to the geometric data that describe how bottom elevation varies with horizontal distance over the cross section of a natural channel or irregular-shaped conduit. The following figure displays an example of a transect for a natural channel.

Each transect must be given a unique name. Conduits refer to that name to represent their shape. A special Transect Editor is available for editing the station-elevation data of a transect. InfoSWMM H2OMap SWMM InfoSWMM SA internally converts these data into tables of area, top width, and hydraulic radius versus channel depth. In addition, as shown in the diagram above, each transect can have a left and right overbank section whose Manning’s roughness can be different from that of the main channel. This feature can provide more realistic estimates of channel conveyance under high flow conditions.

Transects Extractor

InfoSWMM H2OMap SWMM InfoSWMM SA can create transects from DEM or contour data. With contour data, the stations are taken at the contour lines. For DEM data, the total number of stations may be specified or else stations will be taken at a change in slope.

To extract a transect;

  • Select InfoSWMM H2OMap SWMM InfoSWMM SA-> Tools -> Transect Extractor

The following is a description of the part and use of the Transect Extractor:

Name Description
Source Data Format Choose between contour and raster elevation data.
Source Layer Specify the layer containing elevation data.
Elevation Field Choose the field containing elevation values
Transect ID Choose the ID of the transect receiving the station / elevation information.
Total Station Specify the total number of stations for which to extract elevations.
Add station when slope changed Sample stations at each change in slope.
Draw Line Draw a line on the map from which to extract elevations.

Transects Editor

Transects refer to the geometric data that describe how bottom elevation varies with horizontal distance over the cross section of a natural channel or irregular-shaped conduit. The Transect Editor is invoked whenever a new Transect object is created or an existing Transect is selected for editing.

To edit a transect;

  • Select Operation from InfoSWMM H2OMap SWMM InfoSWMM SA Browser.
  • Expand Hydraulics by clicking on the sign, click on transects and create one. This initiates the transect editor shown below. Description of each of the transect data inputs defined using this editor is presented below.

It contains the following data entry fields:

Name Description
Name The name assigned to the transect.
Description An optional comment or description of the transect.
Station/Elevation Data Grid Values of distance from the left side of the channel along with the corresponding elevation of the channel bottom as one moves across the channel from left to right, looking in the downstream direction.
Manning’s N Values of Manning’s roughness for the left overbank, right overbank, and main channel portion of the transect. The overbank roughness values can be zero if no overbank exists. For recommended values, refer to Manning’s N for Open Channels.
Bank Stations The distance values appearing in the Station/Elevation grid that mark the end of the left overbank and the start of the right overbank. Use 0 to denote the absence of an overbank.
Modifiers
  • Stations Multiplier modifier is a factor by which the distance between each station will be multiplied when the transect data is processed by InfoSWMM . Use a value of 0 if no such factor is needed.
  • Elevations Offset modifier is a constant value that will be added to each elevation value.
  • Meander modifier is the ratio of the length of a meandering main channel to the length of the overbank area that surrounds it. This modifier is applied to all conduits that use this particular transect for their cross section. It assumes that the length supplied for these conduits is that of the longer main channel. InfoSWMM will use the shorter overbank length in its calculations while internally increasing the main channel roughness to account for its longer length. A value larger than 1.0 will decrease overall flow in the channel and a value smaller than 1.0 will increase overall flow in the channel.
Maximum Depth The maximum flow depth of the transect. This is automatically calculated from the elevations entered in the transect table.

Note: Clicking the button will bring up a window that illustrates the shape of the transect cross section.

Each transect must be given a unique name. Conduits refer to that name to represent their shape. The maximum depth of the transect is calculated automatically and displayed in the transect editor. A special Transect Editor is available for editing the station-elevation data of a transect. Transects may be extracted from a digital elevation surface as well. InfoSWMM H2OMap SWMM InfoSWMM SA internally converts these data into tables of area, top width, and hydraulic radius versus channel depth. In addition, as shown in the diagram above, each transect can have a left and right overbank section whose Manning’s roughness can be different from that of the main channel. This feature can provide more realistic estimates of channel conveyance under high flow conditions.

Note: Since this data is internally converted to area, top width, and hydraulic radius versus channel depth, only the relative elevations are important. This feature allows a transection to be extracted at any representative section along the channel reach without concern for absolute elevation.

Excerpt from the EPA manual Storm Water Management Model Reference Manual Volume II – Hydraulics (PDF)

 

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