Aug 09 2011

Which Way Does the Water Go?

When setting up a 1-D hydraulic model, you are basically trying to figure out which way the water is flowing and layout your cross sections accordingly, as I discussed earlier.  This is not that difficult when the flow for the reach of river you are modeling is primarily 1-D in nature.  The difficulty comes when it is not readily apparent which way the water will flow.  Even if you can accurately estimate how the water flows, if a significant amount of flow is transverse to the channel a 1-D model is just not able to accurately calculate the complex hydraulics involved.  So, let’s take a look at another hydraulically interesting highway crossing to learn a little more about where water goes, and what to be aware of when using 1-D models, and why 2-D models are a better tool for determining complex hydraulics.

100yr_Velocity_Bridge Close up


Highway 1 crossing over the Lynches River near Bethune, South Carolina

Map picture



The original modeling was for a bridge replacement.  The use of the modeling for this post is to highlight specific hydraulic features and some limitations and/or assumptions of 1-D models.

Site Specifics:

Before we look at some results, lets walk through the site and get familiar with some hydraulically significant features of this site.

The approximate 100-yr flood extents are shown in yellow on the USGS topo image.  The 100-yr discharge is 14,801 cfs.  The floodplain in this reach varies from 0.75 – 1.5 miles wide.  The Lynches River in this reach flows roughly north to south.  The Seaboard Coast Line RR runs roughly parallel to, and about 1,000 ft upstream of, Hwy 1.


The RR grade has a 600 ft main span and two relief bridges that are 100 ft and 200 ft openings. Hwy 1 has one 1200 foot main span bridge. The Lynches River makes a complete “S” shaped meander back and forth under this bridge.  All the Bridge openings are highlighted in purple in the image below.


The floodplain is heavily vegetated, except for near the RR bridge, Hwy 1 bridge, a few utility crossings, and some clearings.  Below is the 2005 aerial image of the model.


Below is a close up of the same image near the bridge crossings.


The image below shows the topography of the 2-D mesh.


And a little more detail near the bridges.


So, to sum up here is what we have;

  • Meandering stream
  • Wide floodplain
  • Multiple RR/Road crossings with some skew to flow
  • Multiple openings in RR crossing
  • Large Hwy 1 bridge with meander

What do you think the water will do through this reach?  How will the meander effect flow under the Hwy 1 bridge?  What are some inherent weaknesses of a 1-D model for this scenario?

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