Monday, December 31, 2018

Using a HEC-RAS Storage Area and Lateral Structures to Replace Standard Reach Junctions

Written by Lonnie Anderson, P.E., CFM  |  Pape-Dawson Engineers, Inc.
Copyright © The RAS Solution 2018.  All rights reserved.


Advances in HEC-RAS now allow for improved simulation of overbanks using 2D Flow Areas. Coupled 1D/2D models where 1D cross-sections represent the bank-to-bank cross-section data and 2D flow areas represent overbank areas has greatly improved the accuracy and robustness of HEC-RAS models. This is particularly the case in flat, urban areas with significant overbank flow paths.

These coupled 1D/2D models have highlighted several simplifications and shortcomings of the traditional HEC-RAS Junction methodology. Note that the standard method is still required in 1D steady flow modeling.  The following points suggest an alternative Junction method is worth considering when building a coupled 1D/2D model.

 Figure 1 - Traditional HEC-RAS Junction Methodology with a Coupled 1D/2D Unsteady Model - Simple Junction

  • 1D to 2D offline flow transfer over the junction length is not possible in HEC-RAS.  In other words, a lateral structure cannot span across a junction. In complex confluences, this transfer region may be critical. Simply reducing the distance between bounding cross-sections to minimize this region may not be an option depending on the channel and bank alignments (see Figure 1). 

  • The volume of water within the bounding cross-sections of junctions is not accounted for, whether forcing the water surface elevation to match the downstream bounding cross-section or using the Standard Step one dimensional Energy equation. Both solution techniques simplify the hydraulics of the region, and in particular, the main reach which conveys the greatest volume (see Figure 1).

Simplification of Junction hydraulics has been “accepted” as reasonable, as                          demonstrated by the following guidance from the USACE-HEC: 

Friday, December 7, 2018

Free Webinar on Dam Breach Modeling

Webinar: Dam Breach Modelling


I'm honored to join Krey Price and Bill Syme in presenting a brief webinar on dam breach modeling hosted by ICEWaRM and the Australian Water School.  Half hour presentation followed by discussion and a Q&A session.  Please join if you can.

A vigorous discussion with a group of highly regarded dam breach modellers
Approaches to dam breach modelling vary greatly when estimating the discharge through the breach and potential downstream inundation impacts.

This webinar will address 3 key issues:
1) upstream of the dam (reservoir) Krey Price 5 min
2) the dam itself (breach parameters) Chris Goodell 10-12 min
3) downstream of the dam (flood wave routing) Bill Syme 10-12 min

The different approaches to generating the breach hydrograph, the numerical modelling of downstream flood inundation, benchmarking of solution schemes, and uncertainties associated with the modelling will be presented and explored.
Click the link below for more information and to register.