Multiple Opening Analysis

Written by Chris Goodell | May 24, 2024


Chris Goodell

Kleinschmidt Associates

Copywrite © 2024

Did you know that if you have a bridge opening and one or more culverts at a single river crossing a Multiple Opening Analysis is required?  The Multiple Opening Analysis has been an option in HEC-RAS for 1D modeling as long as I can remember.  It’s a great way to let HEC-RAS figure out flow distributions between bridge openings, culverts, and conveyance zones, as well as the resulting headwater elevation upstream of the crossing.  It requires the users to enter “stagnation limits” to define zones of influence for the different openings.  You can even provide what’s called a “floating” stagnation limit, effectively giving HEC-RAS a range of stagnation point locations, allowing HEC-RAS to figure out the stagnation limits through an optimization scheme.  Figure 1 shows the upstream face of a crossing with a bridge opening and some piers (middle of the crossing), as well as a culvert group of five barrels (on the left side) and a secondary bridge opening (right side).  This comes from HEC-RAS Example Data Set “Multiple Openings – Example 5”, MULTOPEN.prj from the HEC-RAS Applications Guide.  

Figure 1. Multiple Opening Analysis

The red vertical dashed lines represent the stagnation limits that the user provides, breaking the crossing into three flow zones for HEC-RAS to work with, one for each opening group.    Notice zones 1 and 2 and zones 2 and 3 overlap, using the “floating stagnation limits” technique.  This communicates to HEC-RAS that you think the stagnation point is somewhere in the “overlap” zone, but you’ll let HEC-RAS use its optimization scheme to figure out the best place for it.

That’s about as far as I want to go with the multiple opening analysis procedure for this post.  To learn more about this approach, read up in Chapter 6 of the HEC-RAS User’s Manual and Chapter 9 of the HEC-RAS Hydraulic Reference Manual.  However, I want to bring up a very important and often overlooked requirement.

If you have a 1D model with a bridge opening and culverts, using the multiple opening analysis is REQUIRED!  While the manual states that if you have a bridge opening and culverts in the same crossing you can use the multiple opening analysis, it doesn’t explicitly say you have to use the multiple opening analysis.  And in the software itself, if you build a crossing with a bridge opening and a culvert, it won’t provide you with a warning if you don’t use the multiple opening analysis.  It will run as if nothing’s wrong, give you some output and off you’ll go, never the wiser. But here is what happens.  If you build a bridge in HEC-RAS, and then you add a culvert, HEC-RAS will change that crossing from a bridge crossing to a culvert crossing.  As soon as that happens, the bridge opening is no longer recognized by HEC-RAS.  Even though it is visibly still there, RAS will not consider the bridge opening at all in the computations.

Let’s take the Single Bridge – Example 2 (BEAVCREK.prj) example data set from the Applications guide.  I’ve removed the bridge piers for this demonstration (Figure 2).  Notice in the Title, over each cross section it’s labeled “Bridge” in the parentheses.

Figure 2. Single Bridge Opening

Now as soon as I add in a 5 foot by 10 foot box culvert, the label in the parentheses changes to “Culvert” in this zoomed in view (Figure 3).  Notice I had to switch my ineffective flow areas to “multiple blocks” to provide conveyance for the culvert.

Figure 3. Bridge Opening with 5-foot by 10-foot Relief Culvert

So HEC-RAS now considers this a culvert, not a bridge.  That bridge opening might as well be filled in, because HEC-RAS won’t consider it at all in the computations.  Not sure you believe me?  Let me show you the water surface profile results of these two geometry setups (Figure 4).  Notice that the geometry with the relief culvert added in resulted in a headwater almost 5 feet higher than without the relief culvert (i.e. the bridge on its own).  The relief culvert should reduce the headwater elevation, not increase it by several feet.

Figure 4. Comparison Water Surface Elevation Results with and without Relief Culvert

Another clue can be found in the detailed output table (Figure 5).  Notice first, there is no longer a Bridge output table type.

Figure 5. Detailed Output Table – No Bridge Type

And if you check the flow distribution in the culvert detailed table, you can see for the 10,000 cfs total flow, 9,508.63 cfs is weir flow and 491.37 is culvert flow (Figure 6).  There is none left for the bridge opening.

Figure 6. Detailed Output Table showing Flow Distribution

Again, HEC-RAS is not considering the bridge opening at all, even though it is clearly included in the geometry (see figure 2).  There were no warning messages during run-time and nothing in the Summary of Warnings Errors and Notes to let us know, “Hey, head’s up, RAS is not considering that big bridge opening you have at River Station 5.4”.

So, for a crossing with a bridge opening and at least one culvert, it is REQUIRED to use the multiple opening analysis.  Why did I emphasize the word REQUIRED?  Quite simply I don’t think a lot of RAS modelers know that you cannot model a bridge opening and a culvert in a single 1D crossing without it.  In fact, I recently stumbled across a FEMA model that was used in the development of a Flood Insurance Study that has been around for decades.  It had this exact same problem and the water surface profile that was published was overestimating the headwater elevation above the crossing by over three feet!  Bottom line, if you have multiple openings and you forego the multiple opening analysis, the answer will be wrong.

So let’s rerun the model using the multiple opening analysis and see what happens.  In Figure 7, you can see I’ve allowed the stagnation point to “float” so that HEC-RAS can use its iterative approach to balance energies for determining the best location.  Alternatively, you could set the stagnation limits by avoiding an overlap.  That way the non-overlapped line between the established flow zones becomes the final stagnation point and HEC-RAS doesn’t not need to iterate to determine it computationally.

Figure 7. Multiple Opening Analysis

And the results prove to be much more realistic as shown in the water surface profile plot (Figure 8) where the headwater drops slightly with the multiple opening approach geometry compared to the bridge only geometry (as it should).  The highest water surface profile in this plot represents the bridge opening and culvert without the multiple opening analysis and demonstrates the problem with foregoing the multiple opening analysis when you have both a bridge opening and one or more culverts in the same crossing.

Figure 8. Comparison Water Surface Elevation Results with and without Relief Culvert (Multiple Opening Analysis)

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