Monday, January 22, 2018

Bridge Momentum: Understanding the Weight Component

Written by James Woidt, P.E.  |  Woidt Engineering and Consulting
Copyright © The RAS Solution 2018.  All rights reserved

Take a look at the water surface profile in Figure 1 below. The two profiles are for Class A low flow (subcritical throughout) with identical boundary conditions, flow conditions, and geometry. The only difference is the bridge modeling approach: the blue line is calculated using the energy method; the red line is calculated using the momentum method. For this example, which is a simple rectangular channel, the difference in computed water surface elevation and energy grade line from the downstream cross-section to the upstream cross-section is 0.10 feet and 0.16 feet for the energy method, respectively; those differences increase to 1.64 feet and 1.64 feet, respectively, for the momentum method. Those of you well-versed in the applicability of the various bridge modeling approaches (see page 181 of the HEC-RAS River Analysis System, version 5.0, Hydraulic Reference Manual [Brunner, 2016] for a great discussion on this topic), will answer: “the momentum method accounts for the drag imparted by bridge piers on the flow and should be used when pier losses are significant; that’s why.” But check out Figure 2. There are no piers. The bridge doesn’t even touch the water surface!

Figure 1: Comparison of bridge modeling results between energy and momentum methods

Figure 2: Representative cross-section of modeled bridge

From a hydraulic perspective, the bridge is doing nothing in this model. We could ignore it, stick in two cross-sections, press “run”, use the tried-and-trusted energy equation, and be on our way. However, “momentum” has been as familiar and comfortable a term as “energy” ever since we first smashed two model cars in physics class. We know the one-dimensional form of the conservation of momentum equation backwards-and-forwards, have solved the above problem by hand in our open channel hydraulics class, and know that the momentum equation should give us an answer similar, if not identical, to the energy equation. Even though the title of this post gives away the reason for the difference, I’m going to let the suspense build and provide some background to understand the implementation of the momentum equation in HEC-RAS at bridges for steady discharge.

Tuesday, January 2, 2018

Considerations of River Ice Breakup Uncertainties in HEC-RAS

A fantastic blog post by Mr. Juha Aaltonen of the Freshwater Centre of Finnish Environment Institute SYKE.  In his post, Mr. Aaltonen discusses river ice breakups and the advantages of incorporating uncertainty into river ice modeling in HEC-RAS.  He was able to do this by harnessing the power of the HECRASController as detailed in "Breaking the HEC-RAS Code".

While his post is specific to river ice breakup modeling, uncertainty can be used for just about any HEC-RAS application.  Give Mr. Aaltonen's post a read here: