9/14/2018 11:37:05 AM
9/14/2018 11:33:02 AM
WESTERN DAM ENGINEERING NEWSLETTER, VOLUME 6, ISSUE 2, AUGUST 2018
HEC-HMS VERSUS HEC-RAS, HUMAN FACTORS IN DAM INCIDENTS, LOW LEVEL OUTLET CONDUITS
Document Type - Reference Library
Research, Thesis, Technical Publications
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<br /> <br /> <br /> <br />2 <br />What the HEC? – Selecting the <br />Best HEC model for the Job <br />By: Chris Shrimpton, PE, and Chad Vensel, PE <br />Introduction <br />You may be familiar with some of the U.S. Army Corps <br />of Engineers (USACE) Hydrologic Engineering Center <br />(HEC) software, like the Hydrologic Modeling System <br />(HMS) and River Analysis System (RAS). These <br />software packages, in particular, are widely used <br />within the water engineering community both <br />domestically and internationally, but what the HEC do <br />they actually do, and when should you use them? <br />In general, HEC-HMS is used to estimate precipitation <br />runoff rates within a study watershed, whereas HEC- <br />RAS is generally used to simulate the hydraulic <br />interactions between runoff and defined watercourses, <br />floodplains, and hydraulic structures. As such, HEC- <br />HMS and HEC-RAS are often utilized as complementary <br />software packages – that is, surface water engineering <br />studies frequently utilize HEC-HMS to estimate peak <br />runoff rates or runoff hydrographs, which are <br />subsequently input to HEC-RAS to estimate hydraulic <br />characteristics at specific downstream locations. <br />The primary focus of this article is to present: <br />• The functionality of HEC-HMS and HEC-RAS; <br />• Tips and guidance on how to decide when it is <br />appropriate to use each software; and <br />• Tips and guidance on how to apply each software <br />package to meet study objectives. <br />Back to Basics <br />Before discussing the functionality and advantages/ <br />disadvantages of HEC-HMS and HEC-RAS, let’s review <br />some of the governing principles and equations on <br />which these software programs are based. <br />Hydrology vs. Hydraulics <br />While there are many similarities and overlap between <br />hydrology and hydraulics, there are also distinct <br />differences that are important to understand as they <br />relate to surface water engineering studies. <br />Hydrology “is concerned with the circulation of water <br />and its constituents through the hydrologic cycle. It <br />deals with precipitation, evaporation, infiltration, <br />groundwater flow, surface runoff, streamflow, and the <br />transport of substances dissolved or suspended in <br />flowing water”. <br />Hydraulics, on the other hand, is “the study of practical <br />laws of fluid flow and resistance in pipes and open <br />channels”. More simply, hydraulics focuses on the <br />characteristics of surface runoff and flow, particularly <br />within defined watercourses and hydraulic structures, <br />like bridges, culverts, diversions, etc. <br />Hydrologic analyses are generally focused on a macro- <br />scale (refer to Figure 1) and are typically the first step <br />in analyzing a water system in order to determine the <br />quantity of water and rate at which it reports to a <br />particular location. Hydrologic flood routing is typically <br />lumped, which means it is calculated as a function of <br />time based on the continuity equation (i.e., <br />conservation of mass) without accounting for spatial <br />variability. <br />The results of a hydrologic analysis can then be used in <br />a hydraulic analysis to determine how that water <br />interacts with a channel or hydraulic structure to <br />determine specific hydraulic characteristics such as <br />flow depth, velocity, shear stress, etc. Hydraulic flood <br />routing is typically distributed, meaning it is a function <br />of time and space, relying on conservation of <br />momentum as well as mass.
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