Bridge Unit Property Visualisation

Bridge Unit Property Visualisation

This functionality enables the modeller to view processed properties of bridge units, such as area and conveyance, either in tabulated or graphical form. These can be viewed independently of a simulation; the results displayed are properties of the bridge at various intermediate water levels between bed level and maximum elevation.

The main purpose of this functionality is twofold - to give the user a better insight into the values used within the bridge calculations, and to identify any anomalies in the data, such as decreasing conveyance.

What does this function show?

Tabulated properties

This function allows the user to tabulate the following bridge properties: elevation (above datum), depth, wetted area (unobstructed and obstructed), conveyance (unobstructed and obstructed), kinetic energy coefficient α (unobstructed and obstructed), blockage ratio and channel eccentricity for one or more bridge units (ARCH, USBPR, PIERLOSS). It also enables the user to plot any two of these variables against one another for one or more units. Note: Tabulated river section properties can also be calculated at the same time if selected, although will be shown in a different table/graph.

Section plot

This function enables the conveyance of a bridge unit section to be plotted on the same vertical (elevation) scale as the section points. This enables the user to view the conveyance with respect to section data, and potentially identify any locations within the cross-section which cause anomalies (e.g. decreasing conveyance).

How to access the Bridge Properties function

Tabulated properties

Select one or more bridge sections from the Map view or Network Panel and from the Toolbox Panel, select Model Review Tools > 1D Models > Tabulate Cross Section Properties.

Section plot

Bring up a cross section plot of the bridge unit. Select Edit Series from the context (right-click) menu, or alternatively Ctrl-D. To overlay the conveyance plot on the cross section, check the box marked Conveyance. Please note that in the case of PIERLOSS Bridge, if both faces of the bridge are provided, only the downstream cross-section is displayed. This is because the downstream face is used in the calculations.

How is conveyance calculated in a bridge unit?

For a Bridge unit, there are two conveyance calculations performed - the conveyance for the unobstructed section (i.e. without the bridge data), and that for the obstructed section (i.e. that including the bridge data), as well as the equivalent areas. For these calculations, only the data in the bridge unit is used, irrespective of the upstream and/or downstream River section.

However, due to their nature, the conveyance in each section is calculated slightly differently, with respect to panels. For a complete explanation of the use of panel markers, please see the River Section topic. The following sections describe where panels are imposed within the bridge unit.

Unobstructed section

For the unobstructed section, panel markers are added internally at the "LEFT" and "RIGHT" embankment markers. The principle of panel markers is the same as river sections - it effectively calculates the conveyance in each individual panel, which are then summed to form the composite conveyance for the section.

Obstructed section

For the obstructed section, panel markers are added internally at each bridge opening, so each aperture is effectively considered as a separate panel. Furthermore, horizontal panels are added at the springing level so the area underneath each arch above the springing level is a separate panel from that beneath the springing level. NB The "LEFT" and "RIGHT" embankment markers have no effect on the obstructed section calculations.

When is Bridge cross-section data used and when is River cross-section data used?

The section data in the bridge unit is used ultimately in calculating the blockage ratio. For the USBPR unit, it is also used to calculate the skew and eccentricity factors. The afflux is generally a function of the blockage ratio and the downstream and/or upstream river section properties.

For the ARCH bridge unit, afflux is calculated as a function of the mean depth and Froude number, both measured at the downstream RIVER (or CONDUIT) section, and the blockage ratio.

For the USBPR bridge unit, afflux is calculated as a function of the velocity heads at the upstream and downstream RIVER (or CONDUIT) sections, and the blockage ratio and velocity head in the obstruction, with corrections for skewness, alpha, pier effects and eccentricity, all calculated at the BRIDGE section.

For the PIERLOSS bridge unit, afflux is calculated as a function of the velocity head at the downstream face, the blockage ratio and the shape of the piers.

How is blockage ratio defined?

For ARCH and PIERLOSS bridge units, the blockage ratio is defined as one minus the ratio of obstructed area to unobstructed area .

For a USBPR bridge unit, the blockage ratio is defined as the ratio of obstructed conveyance to unobstructed conveyance .

Thus, for an ARCH or PIERLOSS bridge, a blockage ratio of zero indicates no obstruction (flow is completely contained within the opening), and a value tending towards one would indicate full obstruction.

However, for an USBPR bridge, a blockage ratio of unity indicates no obstruction (flow is completely contained within the opening), and a value tending towards zero would indicate full obstruction.

What are alpha and eccentricity?

Eccentricity is a measure of relative conveyance in the left and right portions of the section data, i.e. those outside of the embankment markers. If the flow is beyond only one panel, the eccentricity is 1; if the conveyances outside each panel are equal (including both zero, i.e. no panels or the flow is in-bank), the eccentricity is zero. The eccentricity can never exceed 1, and is a property of the section data only in the bridge unit (i.e. not the arch data).

Alpha is the kinetic energy correction coefficient, and is dependent on relative areas and conveyances in each of the panels.

Both alpha and eccentricity are used only in the USBPR unit.

What can I do if this reveals errors?

A significant cause of concern would be if the conveyance decreases with increasing water level. This could lead to inconsistent afflux calculations and 'hunting' between alternate solutions. This can usually be resolved by addition of effective panel markers. Although not as flexible with respect to panels as the RIVER section, panel markers can be set in the bridge units by imposing LEFT and RIGHT embankment markers at the appropriate location. Note that the conveyance in a BRIDGE unit only affects the USBPR unit, and not the ARCH or PIERLOSS units.