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The entered weir width that is reflected in the plan view schematic represents the width of the roadway or bridge deck, and the hydraulics are computed using the entered culvert length (also reflected in the schematic plan) however, the water surface elevations used to balance weir flow and pressure flow are not taken at the inlet and outlet locations shown in the schematic view. Keep in mind that the weir/culvert in the SA/2D Area Connection model is connected at adjacent grid cells located on either side of the bridge centreline (as required by the documented limitations of HEC-RAS 5.0.3), with the bridge deck removed from the 2D terrain data and entered as a 1D weir embankment. Here is the embankment profile showing the location of the openings: Here is the cross section of the bridge/culvert:ĢD structure modelled as a standard SA/2D Area Connectionįor the SA/2D Area Connection, I entered the roadway alignment as an enforced breakline and used the “Weir and Culverts” configuration to enter the embankment profile and culvert configuration (matching the geometry in the 1D bridge and culvert models). I then entered two openings under the roadway, using vertical abutments, piers, and upstream and downstream faces in order to allow the same geometry to be entered as either a bridge or a culvert. I then set up the following basic geometries in the Geometric Data Editor (click to enlarge):įor the 1D and coupled 1D-2D models, I used the bridge/culvert editor to place a roadway deck between the bounding cross sections. I then merged that terrain with the 5-metre grid resolution DEM that I downloaded from Geoscience Australia (using “Create New Terrain” in RAS Mapper). Here’s how the merged terrain appears in RAS Mapper and in the empty geometry viewer, with the Harbour Bridge on the upper left of the image and the Opera House on the right:įor this example, I generated a 0.5-metre by 0.5-metre DEM grid from the 1D trapezoidal cross sections (using “Export Layer | Create Terrain GeoTiffs from XS” in RAS Mapper). With the 1D geometry in place, I then used the cross sections to cut a channel through a 2D terrain surface. Here is the oblique view of the 2D geometry, which I’ve cut right through the middle of Sydney just for kicks: Here is the wireframe schematic for a 1D model that I set up to represent a uniform, trapezoidal channel with a single bridge crossing:
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I’ll go ahead and use one of these more commonly applied roadway examples for this post, comparing the results between various model runs with the opening modelled as a 1D culvert, a 1D bridge, a SA/2D Area Connection, and a wormhole culvert. A much more common application of wormhole culverts – or “Katsoulverts”, if you please – is to simply connect the channel on the upstream side of a roadway with the channel on the downstream side in order to represent a bridge or culvert with the deck surface modelled as 2D terrain. I then used a similar example (described in Part 2) in order to introduce an internal boundary condition and get around another current limitation in HEC-RAS. The example in Part 1 was for a diversion pipe that takes flow from a reservoir to a downstream outlet, with a dry stretch of riverbed between the inlet and outlet.
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Quite a few users have told me they now apply the method regularly in their models this makes me a bit nervous: I have confirmed that wormhole culverts work in principle, but how valid are the hydraulics over the full range of open channel flow, pressure flow, and weir flow? Given the amount of use they are getting, I think it’s high time to put wormhole culverts to the test!
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Since SMEC’s Con Katsoulas first told me about his creative workaround for the long culvert limitations in the current version of HEC-RAS, I have used this approach on hundreds of culverts in my models, and the online post ( Part 1 of this series of articles) has received thousands of hits.
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