Jun 23 2011

Assume No More

In my last post I asked how to best lay out cross sections for a HEC-RAS model for a section of the Little River in Georgia.  Remember that cross sections in a 1-D model represent locations of equal water surface elevation.  Let’s look at some 2-D results for this section of river.  (Note: the large fan shaped portions of the model are to help buffer any adverse effects from the boundary conditions, similar to how a physical model is setup).

Water Surface Elevation100yr_WSE





Based on the water surface elevation contours it is easy to see where this section of river transitions from generally 1-D flow, to more complex 2-D flow patterns, and then back to 1-D.  The extreme skew of this roadway compared to the floodplain, as well as the multiple bridge openings along the roadway, create a hydraulic condition that HEC-RAS can not adequately represent.  The lateral flow both before and after the bridge will not be accounted for in a 1-D model.

Areas of 1-D and 2-D Flow100yr_WSE_1-D-2_D

The original question I asked was tricky.  In order to accurately model this reach of river a 2-D model is a better tool because of the inherent limitations that exist with how a 1-D model functions. 

I should also share that the context of this model was for a redesign of the bridge crossings to better convey flood flows.  For this type of project more accurate hydraulics are needed to determine appropriate bridge locations, sizes, skews, and associated scour and countermeasures.  If you just need to know the general water surface elevations, then a well thought out 1-D model might be a good enough tool for this location.  We have also used 2-D model results to inform 1-D model studies.  While this works, the final 1-D model is still approximate.  The purpose of the project should help in understanding the correct tool to use.

Ineffective Flow Areas

Another area that 2-D models are helpful is in understanding flow direction.  One of the comments from the previous post asked how 2-D results can help in determining appropriate expansion/contraction coefficients.  Here are two images showing the velocity vectors near the two bridge openings.

Velocity Contours and Vectors100yr_Velocity_closeup


Water Surface Contours and Velocity Vectors100yr_WSE_closeup

The velocity vectors are also helpful in determining angle of attack on piers/abutments for scour calculations, as well as helping determine appropriate bridge/pier skew to flow for bridge designs or replacements.

Where Does the Flow Go

One last 2-D model output I want to highlight is the flow line visualization.  These can be helpful when trying to understand complex flow patterns.


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  1. Matt Simpson

    What software package are you using for the 2-D modeling? Was that software able to directly produce those graphics, or did you have to use a separate piece of software to create the graphics from the model output?

    Matt Simpson

    PS. Tell Anthony “Hi”.

    1. Dusty Robinson

      We use SMS (Surface Water Modeling) by Aqueveo. SMS is a pre and post processor for 2-D modeling. There are several models supported by SMS that you can choose from. Because of our clients we typically use RMA2 (U.S. Army Corps model) and FESWMS (FHWA model).

      Almost all of what you see is from SMS, though I use other software packages to clean things up some since SMS does not have the best layout tools.

  2. Cameron Jenkins

    One small correction, SMS stands for Surface-water Modeling System not Surface Water Modeling.

    1. Dusty Robinson

      Aqueveo actually changed it to Surface-water Modeling “Solution” on the website. Solution or System, it is their surface water modeling package.

  3. Cameron Jenkins

    You mentioned in the last post that the 100yr and 500yr event went over the road, what flow event is in the figures above? Did you model the larger flood events for this model?



    1. Dusty Robinson

      All of the plots are for the 100yr event. There is no data shown on top of the roadway because of how we modeled the weir flow over the road. Both RMA2 and FESWMS have an option for weir flow, but both are a little buggy in their own way. In this model we used node strings at the location of the roadway over topping and did the weir calculation outside of SMS then hardwired the weir flows with the node strings. The roadway over topping at this event is not significant.

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