Chapter 1 – Introduction
The purpose of this HEC-25 document is to provide guidance for the analysis, planning, design and operation of highways in the coastal environment (HICE). The focus is on roads and bridges (highways)near the coast that are always, or occasionally during storms, influenced by coastal tides and waves.
This document is intended to be a reference guidance document for Federal Highway Administration (FHWA), State Departments of Transportation (SDOT), the American Association of State Highway and Transportation Officials (AASHTO), consultants to these organizations, and others.
This is nominally the second edition of HEC-25. The first edition was entitled “Tidal Hydrology,Hydraulics and Scour at Bridges” and reflected results of a SDOT pooled fund study investigating coastal scour. This second edition is a completely new document and incorporates and presents more comprehensive discussions of the coastal environment.
Nationally, there are few transportation (and specifically highway related) documents that focus on the coastal environment. The existing guidance most similar to this documents a Chapter of the “Highway Drainage Guidelines” published by AASHTO. This HEC-25 HICE document provides additional details on many of the topics discussed in those AASHTO guidelines.
The target audience for HEC-25 is civil engineers, coastal engineers, hydraulic engineers, roadway designers, field inspectors, construction supervisors, planners, and other technical personnel involved with transportation systems in the coastal environment.
This HEC-25 document should assist persons with little experience in coastal engineering to understand; and as appropriate, apply; scientific methods and engineering approaches that are unique to the coast. For experienced coastal engineers, HEC-25 should also serve as a reference document in providing specific highway-oriented assistance and consultation for FHWA and State DOT projects.
The State-of-Practice in the coastal environment is complex,with many major constituents and principles not well understood by a typical FHWA or State DOT hydraulic engineering unit. Some areas related to highways in the coastal environment are still undergoing research to determine appropriate practices.
The document does not attempt to “simplify” this complex practice into mechanistic, “one-size-fits-all” approaches. Rather the document provides the highway hydraulic community with an overview and awareness of constituents of good coastal hydraulic analysis and design. The result of this awareness will allow practitioners to seek appropriate technical documentation and expertise for specific projects.
This HICE document is organized into three major parts:
- Part 1 (Chapters 1 and 2) discusses the background and context of highways in the coastal environment.
- Part 2 (Chapters 3 through 5) presents some of the principles of coastal science and engineering.
- Part 3 (Chapters 6 through 12) presents some of the issues and applications of coastal engineering and science in highway planning and design.
Of interest within Part 1 of this document:
- Chapter 2 outlines what is meant by coastal highways and provides an estimate of the extent of them in the United States.
- Chapter 2 also briefly discusses some of the societal and natural changes that likely will make the planning, design and operation of coastal highways even more challenging in the future.
- Chapter 2 concludes with a description and explanation of the field of coastal engineering and some brief discussion of how coastal engineering can be better integrated into the highway engineering process.
Part 2 very briefly summarizes some of the science that is unique to the coast. This includes water levels, waves and sand movement with a focus on coastal highways. The planning,design and construction of highways in the coastal environment can require an understanding of some unique aspects of that environment. These are parts of the sciences of coastal oceanography, coastal geology, and coastal meteorology. Each of these earth sciences has extensive bodies of knowledge focused specifically on the coastal areas. For example, near shore physical oceanography is the sub discipline of oceanography that focuses on the edge of the oceans where deep water waves, currents, and tides interact with the land. Coastal geomorphology is the sub discipline of geology that focuses on the resulting changes in coastal land forms. These aspects of this unique design environment are not important in the design of highways not located near the coast.
- Chapter 3 discusses tides and water levels including tidal datums, storm surge, and sea level rise. Tides and other water level fluctuations control the location of wave attack on the shoreline.
- Chapter 4 discusses water waves and engineering models of water waves. Waves are often the primary hydraulic force of interest in coastal engineering.
- Waves and tides generate currents in the coastal zone including those that can move sand near the coast and cause important changes in shorelines and inlets. Chapter 5 provides a broad introduction to some of the coastal sediment processes including an overview of coastal geology, coastal sediment characteristics and transport, tidal inlet dynamics,and the role of physical models in coastal engineering.
Note that Part 2 summarizes a relatively small subset of the coastal engineering sciences with specific emphasis on areas with applications to the engineering of coastal highways. Other references; including summary manuals, textbooks, and original sources in the coastal sciences and engineering fields; are cited for further details.
This HICE documents not meant as a substitute for more in-depth study of these fields but rather as a very basic, entry-level primer for someone with a general civil engineering background. Part 2 of this document introduces some of the terminology and concepts used in the engineering tools discussed in Part 3.
Part 3 addresses several of the highway and bridge planning and design issues that are unique to the coastal environment including coastal revetment design, planning and alternatives for highways that are threatened by coastal erosion, roads hatteras in storms, and wave loads on bridge decks.
- Chapter 6 addresses one of the most common coastal highway issues – the design of revetments or sea walls to resist wave attack.
- Chapter 7 describes broader issues of what can be done with highways that are threatened by coastal shoreline recession.
- Chapter 8 presents some engineering strategies for some coastal roads that, because of their location and elevation, are occasionally over washed by storms.
- Chapters 9 through 12 discuss issues related to bridges near the coast including wave loads on bridge decks. By discussing example situations, these chapters outline some of the available engineering and analysis tools for addressing the issues and cite references in the coastal and transportation engineering literature for further details. A qualified, experienced coastal engineer should be an integral part of the analysis and related design team for most of the issues outlined here.
Other materials in this document include references cited (Chapter 13), a glossary of terms (following the references), and several appendices:
- Appendix A – Units and conversions.
- Appendix B – An estimate of the extent of coastal highways in the U.S.
- Appendix C – Equations for estimating fetch-limited waves in shallow water.
- Appendix D – A method for estimating wave loadson bridge decks.
- Appendix E – Examples of wave load methods applied to different SDOT bridges
To the extent possible, this document avoids specifying units in most equations and examples. When needed, the document provides only a single set (either SI or CU). Appendix A provides information on units and unit conversions.