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Jul 19 2012

Hydraulic Analysis Considerations

Continuing with our coverage of the new HDS 7 manual from FHWA.

I have posted previously about assumptions that are made with 1-D models, differences between 1-D and 2-D modeling, and some other posts showing some benefits of using 2-D models in complex flow situations.  While the whole manual is a great reference, I think Chapter 4 is an important one to spotlight since it dives into all of these topics in more detail.

From; FHWA, HDS 7, April 2012

“Any hydraulic model, whether it is numerical or physical, has assumptions and requirements. It is important for the hydraulic engineer to be aware of and understand the assumptions because they form the limitations of that approach.”

INTRODUCTION

Chapter 3 provides background on the fundamental open channel flow concepts that comprise the basis for the majority of the numerical hydraulic modeling and calculations encountered in open channel flow and bridge hydraulic analysis. The calculations are often complex and tedious, and many require iterative solution techniques due to interaction between variables. Therefore, computer programs have been the primary tool for hydraulic engineers ever since computers have become widely available. As computer technology has advanced, so has numerical hydraulic modeling. The primary analysis approach for bridge hydraulics is one-dimensional modeling, although two-dimensional modeling is becoming common and three-dimensional modeling is used to analyze complex flow fields. Chapters 5 and 6 provide information and guidance on the use of one- and two-dimensional numeric models for bridge hydraulic analysis. This chapter includes information on selecting the most appropriate approach whether it is one-, two-, or three-dimensional numerical modeling, steady or unsteady modeling, or physical hydraulic modeling. This chapter also provides background on developing input data and other considerations that are common to all bridge hydraulic problems regardless of the specific approach.

HYDRAULIC MODELING CRITERIA AND SELECTION

Any hydraulic model, whether it is numerical or physical, has assumptions and requirements. It is important for the hydraulic engineer to be aware of and understand the assumptions because they form the limitations of that approach. It is the goal of any hydraulic model study to accurately simulate the actual flow condition. Violating the assumptions and ignoring the limitations will result in a poor representation of the actual hydraulic condition. Treating the model as a black box will often produce inaccurate results. This is not acceptable given the cost of bridges and the potential consequences of failure. Therefore, the approach should be selected based primarily on its advantages and limitations, though also considering the importance of the structure, potential project impacts, cost, and schedule.

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