Aug 11 2011

The Water Goes That Way

Part 1 of 4

Let’s continue our review of the hydraulics at the Hwy 1 crossing over the Lynches River in SC.   There are a lot of unique hydraulic aspects of this crossing, so it will take a few posts to cover everything.  If you have a specific question about this crossing that I do not cover feel free to post a question in the comments.

Throughout the discussion of results I will be referring to a few different flows that were modeled.

  • 2-yr = 4,655 cfs
  • 10-yr = 8,778 cfs
  • 25-yr = 10,835 cfs
  • 50-yr = 12,864 cfs
  • 100-yr = 14,801 cfs
  • 500-yr = 19,188 cfs

First, I want to take a look at the signature hydraulic feature of this bridge, the meander under the main span.  A range of flows were ran to better understand how the meander would impact the hydraulics at the bridge.  Following are velocity contours and vector plots of all the flow ranges listed above.

2-yr Velocity 2-yr_velocity

10-yr Velocity10-yr_velocity

25-yr Velocity25-yr_velocity

50-yr Velocity50-yr_velocity

100-yr Velocity100-yr_velocity

500-yr Velocity500-yr_velocity

At the 2-yr flow the channel is still the primary conveyance path.  The flow still follows the direction of the channel through the meander.  There is some overbank flow crossing the channel at multiple locations, and flow is exiting the channel where it makes a 180° bend just south of the main opening.

At the 10-yr flow the lateral portion of the meander is no longer conveying flow through it.  Instead, the flood flow is now perpendicular to this section of channel.  Most of the flow is being conveyed through the main RR and Hwy 1 openings, but the eastern RR opening and eastern reach of the main channel are starting to convey more flow as well.

As flow increases the middle channel in the meander starts conveying flow in reverse of it’s normal direction.  At the 500-yr flow the velocity in this middle channel is about 2 ft/s flowing south where it would normally flow north.

Throughout all of the flows modeled there is significant interaction between the flow in the channel and the overbanks.  A water surface plot at the 100-yr flow helps show how complex the hydraulics get around the two crossings.

100-yr Water Surface Elevation100-yr_WSE

Remember that flow direction is generally perpendicular to lines of equal water surface elevation.  The RR embankment upstream definitely complicates the hydraulics by splitting the flow into three, unequal, sections.   The return flow from the RR bridge on the left is perpendicular to the overall floodplain until it reaches the main channel again.  The meander under the main span bridge further complicates things by maintaining two primary flow paths, and forcing most of the flow into the floodplain after the 180° bend.  Here is the flow trace for this area.

Flow line visualization–100-yr

I will share some more results and highlight a few things to think about from a 1-D model perspective.  Here are some questions to ponder before the next few posts.

  1. What is the typical location of ineffective flow areas at a bridge for a HEC-RAS model?
  2. How does HEC-RAS handle lateral flow?
  3. How can exposure to 2-D models make you a better 1-D modeler?


1 ping

  1. kenneth

    Hey guys, i know this is an old post but I have a question on ineffective areas for a stream that meandersimmediately upstream or downstream of a culvert. If you draw a standard 2:1 or 1:1 expansion or contraction line, it intersects the stream which is obviously wrong. How do you define the ineffective areas in this case.

    1. Dusty Robinson

      Ineffective flow areas are always located based on assumptions made by the modeler. There are several things that might impact the best location to place ineffective flow areas including, topography, the potential for overtopping, amount of discharge, and cross section location and alignment.

      The best guidance is available in:
      HDS-7 – http://www.fhwa.dot.gov/engineering/hydraulics/pubs/hif12018.pdf
      HEC-RAS reference manual – http://www.hec.usace.army.mil/software/hec-ras/documentation/HEC-RAS_4.1_Reference_Manual.pdf

      As the modeler, you have to visualize how water will flow through your project area and place ineffective flow areas wherever flow would not be conveyed in the downstream direction (ie, should not be calculated as the conveyance by HEC-RAS). Note that a 2D model does not require the need to make this assumption. If visualizing how water will be conveyed through a project area is difficult to determine, that is a good indication that a 2D model should be used.

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