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<br /> <br /> <br /> <br />9 <br />complex and robust model. Therefore, a more complex <br />and robust model could be a better option. <br />When do I need to model bridges/culverts? <br />Bridges and culverts often produce backwater effects <br />that impact flow immediately upstream, although it is <br />not always necessary to model them explicitly. <br />If the capacity of a culvert or bridge is significantly less <br />than the simulated discharge (e.g., the probable <br />maximum flood is being simulated and the culvert is <br />designed for a 1 in 50 AEP flood event), it may not be <br />prudent to model flow through the culvert and instead <br />assume that the culvert is blocked or washed out <br />(depending upon potential impacts to upstream <br />areas/structures). <br />However, it would be prudent to include a bridge if <br />mapping a 1 in 100 AEP flood event in an urban area <br />with structures located immediately upstream. <br />When do I need to model dams in series? <br />Occasionally, when performing a dam breach analysis, <br />it is necessary to model cascading failures of <br />downstream dams. This applies when the storage <br />capacity of the downstream dam is large relative to the <br />breach outflow of the dam being analyzed. However, it <br />is often safe to assume that smaller dams will be <br />washed out by the breach outflow and can be ignored. <br />Conclusion <br />The choice between using HEC-HMS or HEC-RAS comes <br />down to the objectives of the study. HEC-HMS is <br />generally intended for hydrologic modeling (i.e., <br />converting precipitation into discharge, reservoir <br />routing, routing flow through watershed networks, <br />basic channel routing, etc.). HEC-RAS is generally <br />intended for hydraulic modeling (i.e., routing discharge <br />through channels, floodplains, hydraulic structures, <br />etc.). For applications in which either software <br />program could be used, the choice often depends on <br />the degree to which backwater effects are anticipated. <br />For simple, uniform reaches with negligible backwater <br />effects, HEC-HMS is typically appropriate. For flat <br />watercourses where convergence, divergence, or <br />backwater effects are present, HEC-RAS is likely to be <br />more appropriate. <br />Within HEC-RAS, the choice between steady/unsteady <br />and 1D/2D generally depends on the complexity of the <br />study area, the desired level of accuracy, and <br />time/budget constraints. For reaches where flow does <br />not spread, a steady 1D model could be appropriate. In <br />a flat urban area with bridges and culverts, a 2D <br />unsteady dynamic wave model could be required. <br />For the evaluation of multiple events, it could be <br />prudent to develop both a simple 1D model and a <br />detailed 2D model and compare the results for a single <br />simulation to understand the sensitivity of the <br />modeling approach. The remaining simulations can be <br />modeled with the approach that is most appropriate <br />given the constraints of the study. <br />In short, there is not necessarily a right answer as it <br />pertains to modeling hydrologic and hydraulic systems, <br />but it is best to use the simplest/easiest model that <br />meets the needs of the study. Just remember, simple <br />doesn’t necessarily mean easy. <br />References <br />[1] United States Army Corps of Engineers (USACE), Hydrologic <br />Engineering Center. http://www.hec.usace.army.mil/ <br />[2] Maidment, David R. Handbook of Hydrology. February 1, 1993. <br />[3] Lindberg, Michael R. Civil Engineering Reference Manual for the PE Exam. Fourteenth Edition, 2014. <br />[4] United States Army Corps of Engineers (USACE). River Analysis <br />System: HEC-RAS - Hydraulic Reference Manual. Version 5.0, <br />February 2016. <br />[5] United States Army Corps of Engineers (USACE). Hydrologic Modeling System: HEC-HMS - Technical Reference Manual. 2000. <br />[6] Western Dam Engineering Technical Note. Turning Rainfall to <br />Runoff: Estimating Flood Inflows. Volume 2, Issue 1, April 2014. <br />[7] Western Dam Engineering Technical Note. Calibration, Validation, and Verification: Bringing More Certainty to the Uncertainty of Hydrologic Modeling Results. Volume 5, Issue 1. <br />May 2017. <br />[8] Price, K. The RAS Solution. http://hecrasmodel. <br />blogspot.com <br />[9] Faulkner, Ellen B. You are Smarter than your Model. Really. <br />Annual Conference Proceedings – Association of State Dam <br />Safety Officials. September 26, 2011. <br />[10] http://www.geol.ucsb.edu/faculty/sylvester/Teton_Dam/Teton <br />%20Dam.html