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1-1. Purpose

EM1110-2-1601

1Jul91

Chapter1

Introduction

1-1.Purpose

This manual presents procedures for the design analysis and criteria of design for improved channels that carry rapid and/or tranquil flows.

1-2.Scope

Procedures are presented without details of the theory of the hydraulics involved since these details can be found in any of various hydraulic textbooks and publications avail-able to the design engineer.Theories and procedures in design,such as flow in curved channels,flow at bridge piers,flow at confluences,and side drainage inlet struc-tures,that are not covered fully in textbooks are discussed in detail with the aid of Hydraulic Design Criteria(HDC) charts published by the US Army Engineer Waterways Experiment Station(USAEWES).The charts and other illustrations are included in Appendix B to aid the desig-ner.References to HDC are by HDC chart number.The use of models to develop and verify design details is discussed briefly.Typical calculations are presented to illustrate the principles of design for channels under vari-ous conditions of flow.Electronic computer program-ming techniques are not treated in this manual.However, most of the basic hydraulics presented herein can be adapted for computer use as illustrated in Appendix D.

1-3.References

References are listed in Appendix A.

1-4.Explanation of Terms

Abbreviations used in this manual are explained in the Notation(Appendix I).The symbols employed herein conform to the American Standard Letter Symbols for Hydraulics(American Society of Mechanical Engineers 1958)with only minor exceptions.

1-5.Channel Classification

In this manual,flood control channels are considered under two broad classifications:rapid-and tranquil-flow channels.The most important characteristics that apply to rapid and tranquil flows are listed below:

a.Velocities.Rapid flows have supercritical velocities with Froude numbers greater than1(F>1),

and tranquil flows have subcritical velocities with Froude numbers less than1(F<1).

b.Slopes.Invert slopes in general are greater than critical slopes(S

o

>S

c

)for rapid flow and less than criti-

cal slopes(S

o

c

)for tranquil flow.

c.Channel storage.Channel storage is usually negligible in rapid flow,whereas it may be appreciable in natural rivers with tranquil flow.

d.Discharg

e.All discharges are normally confined within the channel for rapid flow(no overbank flow). Other characteristics such as standing waves,surges,and bed configuration that differ under the influence of rapid-or tranquil-flow conditions should be recognized and considered as the occasion demands.Rapid and tranquil flows can occur within a longitudinal reach of a channel with changes in discharge,roughness,cross section,or slope.Channel improvements may bring about changes in flow characteristics.

1-6.Preliminary Investigations for Selection of Type of Improvement

The investigation required in selecting the type of channel improvement to be adopted involves three considerations: physical features of the area,hydraulic and hydrologic aspects,and economy.

a.Physical features.The topography of the area controls in a general way the channel alignment and invert grades.Of prime importance,also,are width of available right-of-way;location of existing channel;and adjacent existing structures,such as bridges,buildings, transportation facilities,utility structures,and outlets for local drainage and tributaries.Invert slopes may be con-trolled by elevations of existing structures as well as by general topography,elevations at ends of improvements, and hydraulic features.

b.Historical and observed elements.The flow char-acteristics noted in historical records and indicated from detailed observation of existing conditions will usually be basic to the selection of type of improvement or design. With the flood discharges determined,the interdependent factors that determine improvement methods and general channel alignment are slope of invert,width and depth of flow,roughness coefficient,the presence or nature of aggradation and degradation processes,debris transporta-tion,bank erosion,cutoffs,and bar formations.

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1-1. Purpose

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