The Advanced Hydrologic Prediction Service for Chehalis Washington

A great example of where flood inundation maps and accessible flood warning systems can help a community prepare for floods.

 

Contact:               Brent Bower                                                                      FOR IMMEDIATE RELEASE 

206-526-6095 x228                                                           December 15, 2015

                               

Flood Inundation Maps to Enhance Flood Forecasts for the Chehalis River

People living along the Chehalis River have a new tool from the National Weather Service to help them understand their risk during floods. A large section of the river is now depicted on flood maps that show people where the water will flow and what it will look like in their community when the river crests beyond its banks. The new flood inundation maps will also help local officials reduce flood impacts to communities by giving them more advanced information for planning.

HEC-RAS 5.0 versus TUFLOW versus MIKE21 – HEC’s official response

In case you haven’t seen it, there has been a very popular LinkedIn discussion on the Hydraulic/Hydrologic Modeler’s Forum debating the advantages, disadvantages, and merits of HEC-RAS 5.0, TUFLOW, and DHI’s MIKE21.  There was a lot of great information (and some misinformation) and insight provided in that thread.  It is well worth the read.  Some of the misinformation was directed at the beta version of HEC-RAS Version 5.0.  Enough that HEC decided to publish a response to clear up any confusion.  

At play is a result of HEC-RAS’s 2D solution scheme using the full shallow water equation where for highly dynamic events where flows severely contract, using too small of a time step could lead to a divergence from the true solution, rather than a convergence.  This is not an issue when a computation interval is selected within the guidance presented by HEC in their user’s manual.  While HEC disagrees that this is a necessarily a “problem” with its software, as some on the discussion claim,  HEC has elected to make this a non-issue by “improving the portion of the full shallow water equation formulation, such that user’s will be able to use very small time steps without the results changing significantly.”   I’ve included HEC’s response, written by Gary W. Brunner, to the LinkedIn discussion below, but I highly recommend you read the LinkedIn discussion first by clicking here: 

I’m posting this not just to allow HEC to reach a larger audience with their rebuttal, but also because there is a LOT of great information in this document about how HEC-RAS 2D works and how we, as HEC-RAS 2D modelers should use it.  Please enjoy!  A downloadable pdf is available here.

The following text is copyrighted by the Hydrologic Engineering Center and Gary W. Brunner:

Pressure and Weir Flow

Here’s a video of pressure and weir flow yesterday at a bridge near my house.  This was brought on by some very intense rainfall over a short period of time.  What do you think the weir coefficient would be?

‘Tis the season in many parts of the world for heavy rains and high water.  This is a good reminder to always try to get out and view your rivers and streams when they are flooding.  There is nothing like a first-hand view of a flood to understand how water interacts around our infrastructure.  This is invaluable information for setting up and calibrating your HEC-RAS models.