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Jul 17 2012

Hydraulic Analysis Overview

From; FHWA, HDS 7, April 2012

The hydraulic analysis of a bridge opening is a complicated undertaking. Decisions must be made regarding the type of model computational methods, model extent, and amount of topographic data that needs to be collected. An assessment of flow resistance caused by channel and floodplain conditions needs to be made and the impacts on flow due to different seasonal conditions also needs to be evaluated. An understanding of flow type, historic flow conditions, and flooding at the site also provides valuable insight into the approaches that need to be employed.

Once the preliminary data has been collected and an understanding of the flow complexity at the bridge opening is obtained, a decision must be made regarding the type of hydraulic model that should be used at the hydraulic crossing. Some situations call for a one-dimensional gradually-varied steady-state flow model while others require the use of unsteady flow models, or two-dimensional steady or unsteady flow models to more fully understand the flow conditions at the hydraulic crossing. Some situations call for a more sophisticated modeling approach because of other factors. These can include the need for a more complete understanding of the flow conditions because of bridge scour or bank stabilization.

There are also regulatory requirements that must be adhered to. The FHWA, USACE, FEMA, EPA, state and local agencies, and others have requirements that must be considered when determining the best overall approach for evaluating the flow through a bridge opening and its impact on adjacent land owners, the environment, and economic concerns. These types of issues must be considered when developing the best approach for analyzing the flow through a bridge opening or reach of river.

HDS 7 MANUAL ORGANIZATION

[HDS 7] is intended to be a general resource for bridge hydraulic design. The concepts are valid for a range of one- and two-dimensional numeric models, not just for the specific models that are mentioned.

Chapter 1 – Introduction

The purpose of HDS 7 is to provide FHWA guidance on hydraulic analysis and design of safe bridges. Significant aspects of bridge hydraulic design are discussed, which include regulatory topics, specific approaches for bridge hydraulic modeling, model selection, scour and stream instability, and sediment transport.

Chapter 2 – Design Considerations and Regulatory Requirements

Chapter 2 provides information and discussion on the range of design considerations, environmental considerations, and regulatory requirements that may be encountered during bridge design and construction. Topics include FHWA guidance, AASHTO Specifications, freeboard, setback and road grade requirements, design considerations, scour and channel instability concerns, Federal regulations, navigation permits, and environmental permits.

Chapter 3 – Governing Equations and Flow Classification

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Chapter 3 provides background on fundamental open channel flow concepts. Although this is not a hydraulic engineering textbook, there is sufficient information to serve as a reference source on the equations used in open channel and bridge hydraulics.

Chapter 4 – Hydraulic Analysis Considerations

Chapter 4 builds on the background from Chapter 3 to discuss hydraulic modeling. One- and two-dimensional modeling are compared and contrasted as well as steady versus unsteady flow modeling. Criteria for selecting a modeling approach are identified. Topics that are relevant to the range of hydraulic modeling approaches are also included in this chapter. These include model extent, boundary conditions, hydrology, and model calibration.

Chapter 5 – One-Dimensional Bridge Hydraulic Analysis

Chapter 5 provides information and guidance on the use of one-dimensional models for bridge hydraulic analysis. Information focuses on the use of HEC-RAS, but the guidance is applicable to a wide range of one-dimensional models. The chapter covers standard applications as well as special cases. The chapter also provides a discussion of the assumptions that the one dimensional approach requires and how violating these assumptions leads to error and uncertainty in the modeling results.

Chapter 6 – Two-Dimensional Bridge Hydraulic Analysis

Chapter 6 provides information and guidance on the use of two-dimensional models for bridge hydraulic analysis. Information focuses on the use of FST2DH, but the guidance is also applicable to other finite element and finite difference models. The chapter covers standard applications as well as special cases, although finite element models are primarily used for complex cases.

Chapter 7 – Unsteady Flow Analysis

Chapter 7 discusses modeling unsteady flow with one- and two-dimensional models. Topics discussed in this chapter include the basic equations that define unsteady flow, upstream and downstream model extents, floodplain storage and connections, and boundary conditions. River and tidal applications are included.

Chapter 8 – Bridge Scour Considerations and Scour Countermeasure Hydraulic Analysis

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Chapter 8 discusses an extremely important aspect of bridge safety. Scour during floods is a significant part of bridge design and is a primary contribution of the hydraulic engineer to the bridge structural design. This topic is covered in detail in HEC-20 and HEC-18. A general discussion of the types of scour and information on obtaining appropriate hydraulic variables from one- and two-dimensional models are the focus of this chapter. The importance of considering future channel change (width adjustments, changes in channel alignment and channel migration, and long-term aggradation and degradation) are addressed.

Chapter 9 – Sediment Transport and Alluvial Channel Concepts

Chapter 9 provides an overview of sediment transport concepts, which are covered thoroughly in HDS 6, River Engineering for Highway Encroachments.

Chapter 10 – Other Considerations

Chapter 10 is a resource for hydraulic engineers to identify additional factors that may impact bridge design and structure safety. These topics include bridge deck drainage, hydraulic forces on bridge decks, piers and pile groups, coincident flows at confluences of rivers, physical modeling, and computational fluid dynamics.

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Chapter 11 – Literature Cited

Chapter 11 provides all the references for the document. For references that are produced by government agencies (FHWA, USGS, NCHRP etc.) the agency is indicated as the author. This format was selected to group all agency documents together in the reference list. The authors are listed within the reference.

While the entire manual is a helpful resource, we will only be posting some sections of Chapter 4 over the next few weeks.

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