8/12/2019 tr25Appx

1/10

TECHNICAL RELEASENUMBER 25 A

DESIGN OF OPEN CHANNELS

CHAPTER 1.CHAPTER 2.CHAPTER 2.

CHAPTER 3CHAPTER 3

CHAPTER 4CHAPTER 5.CHAPTER 5.CHAPTER 5.

CHAPTER 6CHAPTER 6CHAPTER 7.

CHAPTER 7CHAPTER 7CHAPTER 7CHAPTER 7

CHAPTER 8

CONTENTSGENERAL CONSIDERATIONSFIELD SURVEYS PLAN LAYOUTAPPENDIX A. LANDSCAPE ARCHITECTURE SITESURVEY AND ANALYSISSITE INVESTIGATIONSAPPENDIX. OUTLINE TO PLAN SITE INVESTIGATIONS AND PREPAREREPORTS FOR CHANNEL IMPROVEMENTDETERMINING DESIGN DISCHARGECHANNEL LOCATION, ALIGNMENT, HYDRAULIC DESIGNAPPENDIX I. TRANSITIONSAPPENDIX 11. MOMENTUM METHOD OF DETERMINING BRIDGE PIER

LOSSSTABILITY DESIGNAPPENDIX A. STREAM ARMOR DESIGN CONCEPTSENVIRONMENTAL CONSIDERATIONS IN CHANNEL DESIGN, INSTALLATION,AND MAINTENANCEAPPENDIX A. EVALUATING CHANNELS FOR RECREATION DEVELOPMENTSAPPENDIX B. FISH STREAM INVESTIGATION GUIDE SAMPLE)APPENDIX C. POOR QUALITY RECOGNITION GUIDEAPPENDIX D. HABITAT REQUIREMENTS

LANDSCAPE ARCHITECTURE DESIGNthis chapter to be added in near future)

8/12/2019 tr25Appx

2/10

8/12/2019 tr25Appx

3/10

Pageesign Fea ture s Rela ted t o Maintenance 6 84

dded Depth o r Capacity f o r Dep osit ion 6 84Rel at io nsh ip of Side Slopes t o Maintenance Methods 6 85

erms 6 85aintenanceRoadways 6 85poil 6 85ntrance of Sid e Surf ace Water t o Channel 6 85

Seedin g 6 86i lo t Channels 6 86

Glos sar y of Symbols 6 87Referen ces 6 90Appendix Stre am Armor Design Concep ts 6 94

8/12/2019 tr25Appx

4/10

8/12/2019 tr25Appx

5/10

6-Appendix A

TECHNICAL RELEASE No. 25 CHAPTER 6. APPENDIX A.Stream Armor Design Concepts

PurposeThis appendix 1) explains the underlying physical processes affectingarmoring, 2) describes different SCS-approved math models available,and 3) presents an example illustrating one way to estimate armoring.

The various math models for critical and recommended allowable tractivestress discussed in this appendix are accepted in the engineering profes-sion; they differ mainly in choice of a safety factor, scope of applica-tion, or both. TWO different math models for recommended allowable trac-tive stress are used in SCS. They differ solely in their safety factors.The armor designer is free to select the most applicable model.

Actual transverse tractive stress of each situation must be determinedthrough a hydraulic analysis. The example in this appendix uses a sim-plistic model to determine the hydraulic radius. In real situations,actual cross-sectional geometry and, possibly, precise water surfaceprofile calculations are required. ow ever this requirement does notinvalidate the concepts illustrated by the example.

Physical ProcessesArmoring is a well-known natural phenomenon. Furthermore, its importantfeatures already are used in some engineering structures, for example,riprap. Armoring is sometimes called hydraulic sorting. It is a limit-ing or special case of sediment transport. It has been studied by vari-ous scientists over the years, e.g., A. Shields, A Strickler, E. Lane,I{ Einstein, and others). Understanding the primary principles of armor-ing is still developing and is leading to various math models and pro-cedures for field application.

Armoring is the result of the dynamic interaction of unsteady fluid flowand a mobile bed composed generally of a broad range of discrete parti-cles.

8/12/2019 tr25Appx

6/10

6 9Appendix A

At low flows, the boundary is stationary; as the flow increases, how-ever, the smallest particles begin to move. As the flow increases fur-ther, larger particles also begin to move but at a lower velocity. Fi-nally, the discharge can increase to a point where the entire boundaryis moving, although the larger particles move more slowly than thesmaller. As the flow decreases, the process reverses itself; but ifthe smaller particles are not replaced, the bed is left degraded andcoarser.

Armoring occurs when smaller particles are transported from the bound-ary but not replaced and coarser particles are exposed but not trans-ported. Whether true armoring occurs depends on whether the exposedcoarser particles originated at their present position or upstream. Ifthey originated upstream, what has occurred is not armoring but sedimenttransport by unsteady flow.

A design of a stable channel that depends upon armoring for stabilitycan be a contradiction unless the armor surface has already been estab-lished and will not be disturbed during construction. Otherwise, degra-dation must occur before a complete armor surface can exist, and result-ing eroding bed material contributes a downstream sediment load to the sys-tem. Furthermore, this degradation causes undercutting of the toes of thebank, which can lead to bank sloughing. Ultimate design value of armoringmay be that it is the last line of defense against the more extreme eventsthat otherwise may completely unravel a channel and possibly lead to eco-logical disaster or catastrophic failure of important cultural features.

Math ModelsThe math models developed by Shields and Strickler provide the basis forthe armoring design procedure. The procedure was verified by Lane s fieldwork. The designer must analyze (1) the active or driving forces and (2)the passive or resisting forces. The analysis of active forces consistsof determining the hydraulics or depth of flow and determining the bound-ary roughness shear or tractive stress. The latter determination is nec-essary because not all energy loss is due to boundary roughness. Bends orchanges in cross-sectional area cause energy loss through internal fluidshear.

8/12/2019 tr25Appx

7/10

AppendixSCS has adopted Manning's equation to estimate the rate of total energyloss (Se) ; i.e.

S = [(Q ne)/(l.486 AR~/~)]'where

n E retardance coefficient for total energy loss.eFurthermore, SCS has adopted the Manning-Strickler equation to estimatethe energy loss due to boundary roughness, (St); i.e.,

St [(Q nt)/(l.486 A R ~ / ~ ) ] ~where

n K d the Strickler equation - - retardance coefficientt m mdue to boundary roughness only.d a characteristic boundary particle size.mm empirical coefficient relating dm to nt*Units for K must be consistent with units chosen for d .m m

Report 108 of the National Cooperative Highway Research Program recom-mends using K 0.0395, with dm = d,, expressed in feet. The Km valuemis the same as the default value for C in Eq. 2 of TR-59, HydraulicnDesign of Riprap Gradient Control Structures.This leads to the following formula.for actual average transverse stress

T YR Stac t .where

R hydraulic radius, ft.St (n /n 2 Se; Eq. 6-3, TR-25.t e

Shield's work establishes the critical relationship between the activeand passive forces; i.e., it relates the critical fluid tractive stressT for incipient motion to the gravitational resisting force. It wasCverified for coarse grained materials (d 6 mm by Lane's study of'

prototype field canals and for discrete particle material (dm .1 mmby Report 108.

8/12/2019 tr25Appx

8/10

6 97Appendix

Lane reports:Critical tractive stress, r 6 dm; where dm is in feet andCis in psf.CThis critical tractive stress is nearly identical with Shield s work for

dm inch.

Lane recommends:Allowable tractive stress, all. 4.8 dm; dm rc same units as above.This allowable tractive stress is conservative with respect to Shield s

work for d 4 and gives results identical to those from Eq. 6-5 ofmTR- 5

Report 108 reportsCritical tractive stress, = 5 dm; dm, T~ same units as above.CThis critical tractive stress is conservative with respect to Shield s

work for dm mm.

Report 108 recommends:Allowable tractive stress, r -all. - dm; dm, r same units as above.

rhis allowable tractive stress is conservative with respect to Shield swork for d 2 mm and gives results identical to those from Eq. 24 ofmTR-59, setting the FS value equal to 1 and using the default value for50

For armoring design analysis, the characteristic armor particle size(dm) is chosen from the coarser portion of the original material sincemost of the fine material will be hydraulically removed. Usually d =d,,;mtherefore, m = 90. Furthermore, for design purposes, all materialsmaller than the d is assumed to be sorted out. Therefore, the depthmof degradation (D isdDd = dm/[(lOO m)/100] = 10 d,, (see page 6-31).This assumption has a physical interpretation. The d,, size of theoriginal bed material (before armoring) will become the d5, of the finalexposed surface bed material -(after armoring).

8/12/2019 tr25Appx

9/10

6-98AppendixExamp 1This example illustrates the armoring design concept. The uniform flow-unit slice assumption was made for convenience in computing the depth offlow; it may not be valid for most field applications. Furthermore, itis not a conservative assumption, from a stability viewpoint, for sub-critical but supernormal flows. Also, the choice of the numeric valuefor the modifying value no), which accounts for energy loss due to fac-tors other than boundary roughness, should be determined reach-by-reachfor each application. See NEH-5, Supplement y for guidance). Thesmaller the no value, the more conservative the design from a srabilityviewpoint.

Problem: concrete emergency spillway is planned to discharge onto analluvial valley floor of at least 6 feet of homogeneous material. Whatmaximum steady-state unit discharge would limit scour by permittingarmoring to the d,, size material? What would be the expected depth ofscour? The valley slope So) is 0.00520 ft/ft, the d is 110 milli-meters, and the modifying value no) is assumed to be 0.005. Assumeuniform flow-unit slice principles are applicable; therefore, the hy-draulic radius is equal to the depth of flow y R), the rate of totalenergy loss is equal to the valley slope Se So), and the actual trans-verse tractive stress is uniformly distributed 7 T Use theact. all.recommended allowable tractive stress formula from Report 108 that iscompatible with TR-59. Use Km 0.0395.

Given: So 0.00520m 90d d,, 110 0.3609 ft.mn 0.005

Required qmax for 7act. all.b) D for m 90d

Solution: a) nt = K d I6m m0.0395 0.3609)~/~0.0333

8/12/2019 tr25Appx

10/10

6- 9Appendix An see 7th step, page B.6, Supplement B,e t oNEH- )

all. = 4 dm4 0.036091.444 pounds per sq. ft.

= Tact. all.YR St 4 dmR 4 dm) t

1.444/ 62.4 0.00393)

Therefore, the maximum steady-state unit discharge that would limit scourby permitting armoring is approximately 54 cfslft. The expected depth ofdegradation before complete armoring one layer) is almost 4 feet.