Welcome to the RAS Solution › Forums › HEC-RAS Help › Question regarding Bridge cross sections 1-4 placement
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August 23, 2019 at 6:59 pm #7410AnonymousGuest
All the resources I have found suggest placing cross sections 2 and 3 just outside of the limits of the bridges influence (ie. just outside the embankment). Additionally the guidance suggests placing cross sections 1 and 4 based on the length of flow blocked by the embankments as well as the estimated contraction and expansion ratios (generally of 1:1 and 2:1).
I have a couple questions related to the placements of cross sections 1 and 4.
1. If I am modeling multiple return period at once which each have different lengths of blocked flow do I then need to also place multiple approach cross sections based on the respective blocked flow distance and contraction ratios? The guidance doesn’t address this concept to my knowledge and I am left quite confused.
2. The stream I am currently modeling has a situation which the guidance doesn’t address. The stream does not significantly contract for the flood return period I am looking at (50 year storm for scour purposes). For this case there is no contraction nor expansion distance that can be estimated due to the lack of obstructed flow. As a result there is no clear way to decide which upstream cross section is designated as cross section 4. Which is a problem for me because the NCHRP 24-20 method of calculating scour requires an identified “approach cross section” (cross section 4) and uses its width and depth as part of the scour calculation. The width of my upstream cross sections vary a bit and depending on which cross section I select there is either >5 feet of scour when the width is slightly wider than the bridge opening or zero feet of scour when the width is slightly narrower than the bridge opening. I have in the past been told to use a cross section that is one hydraulic opening upstream of the bridge, but this is completely arbitrary and i don’t trust that it is the right answer in all cases.
Any discussion/help would be greatly appreciated. I am actually surprised this topic hasn’t been asked about before. I looked around for a long time and have found no answers. I really hope that one of you who know what you are talking about can clear this up for me.
August 24, 2019 at 12:24 am #12464AnonymousGuest1) No. One RAS geometry would work fine for the analysis of a bridge; typically. What i have found is that only when and for cross sections for which levees are used rather than ineffective flow areas where the WSEL changes, sometimes i will need an independent geometry. I believe this blog has a write up directyl about the 4 cross sections (6 really but you know what i mean im sure)
The below PDF should help you in your quest for those answers. What i have notices is EVERYONE models differently especially in unsteady state because we all have our tricks. The most important thing to understand is that RAS creates 2 internal cross sections based on the US and DS cross section of the bridge. If this is not representative of what the internal cross section should look like, then the internal cross section should be altered to represent an actual representative cross section of the inside of the bridge.
Typically as a rule of thumb I wont even stress on the cross sections distance to the existing or proposed bridge within about 20-ft US and DS of the face bridge. I make adjustments accordingly after an initial run if needed. The definition of your inneffective flow areas also directly correlates to the distance of these cross section locations. When does contraction occur? Expansion? Sometimes you can see within the terrain or even the vegetation as to when this occurs. Hope this helps
https://etd.ohiolink.edu/!etd.send_file?accession=dayton1462356833&disposition=inline
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