Tentative Design Procedure for Riprap-Lined Channels: Field Evaluation

(1)

ST. ANTHONY FALLS HYDRAULIC LABORATORY

PROJECT

REPORT

NO. 146

Tentative Design Procedure for

Riprap- Lined

Channels-Field Evaluation

by

AL VIN G. ANDERSON

NationaI Cooperative Highway Research Program

Project 15-2

Prepared for

IDGHWAY RESEARCH BOARD

National Cooperative Highway Research Program

National Academy of Sciences

June 1973

(2)

Acknowledgment

This work was sponsored by the American Association

of State Highway Officials, in cooperation with the

Federal Highway Administration,

and was conducted in

the National Cooperative Highway Research Program

wbich is administered by the Highway Research Board

of the National Academy of Sciences--National

Research

Council.

• Disclaimer

Tbe opanaons and conclusions expressed or implied in

the report are tbose of the research agency.

Tbey are

net necessarily

those of the Highway Research Board, the

National Academy of Sciences, the Federal Highway

Admin-istration,

the American Association of State Highway

Officials,

or the individual states participating

in

the National Cooperative Highway Research Program.

(3)

ST. AN

THO

N

Y

F

ALLS HYDR

AU

LIC

LABORATORY

PROJECT

REPORT

NO. 146

Tentative Design Procedure for

Riprap- Lined

Channels-Field Evaluation

by

ALVIN G. ANDERSON

National Cooperative Highway R

e

search Program

Project 15-2

Prepared for

HIGHWAY RESEARCH

BOARD

National Coop

e

rati

ve

Hi

g

hw

a

y Re

s

earch Program

National Acad

e

my of Scienc

e

s

June 1973

(4)

(5)

Individual Aclalowledgments •••••••••••••••••••••••••••.••••

iv

•

S~y ••••••••••••••••••••••••••••••••••••••••••••••••••• v

Chapter 1. INTRODUCTION

AND RESEARCH APPROA

C

H

•.••••••••••

1

Chapter 2. FINDIN"GS. . . . . 26

Chapter

3. INTERPRETATIONS

•••••••••••••••••••••••••••••••

3

1 Chapter

4. REGOMMENDATIONS

FOR FURTHER STITDY •••••••••••••

32 ~c~s

33 Appendix A.

Appendix B.

Appendix C.

Appendix D.

Appendix E.

MANCHESTER,

CONN

g

CTICUT,

CHANNEL •••••••••••••

₃

₅

4

0

45

4

8

51 MOOSE LAKE, MIN"NESOTA, CHA.N1ffiL

KLAMATH FALLS, OREGON, CHANNEL •••••••••••••••

CHIPPEWA COiJNTY, WIS

C

ONSIN", CHANNEL ••.•••••••

BILLINGS, MONTANA, CHAN3EL ••.••••••••••••••••

(6)

-iii-INDIVIDUAL ACKNOWLEDGMENTS

This phase of NCHRP Project 15-2 was under the direction of Alvin G. Anderson, Professor of Civil Engineering, who served as principal investigator. The field evaluation was greatly aided by the cooperation of highway engineers in the states in which the channels are located. These include Dr. Robert A. Norton, Connecticut Highway Department; Mr. Paul G. Velz, Minnesota Highway Department; Mr. Louie Schmidt, Wisconsin Highway Depart-ment; Mr.

Robert

M.

Hudnall and

Mr.

Gene Larson, Montana Highway

Department; and Mr. Richard L. Lenz, Oregon Highway Department.

•

(7)

-iv-The objective of NCHRP Project 15-2 has been to establish criteria and develop procedures for the design of armored channels. The first phase of the project resulted in NCHRP Report 108, "Tentative Design Procedure for Riprap-Lined Channels"

(1),

which describes procedures for designing such channels and proportioning the riprap so as to mlnlmlZe erosion. The second phase has been a field evaluation of

channels designed in accordance with these procedures. Since this report was completed, five such channels have been proposed, of which four have been conat ruc ted and one is in the planning stage. Two of the four completed channels have been subjected to discharges that approached the design discharges and henca provided reasonably defi ni-tive tests. Both channels appeared to be stabIe and in good condition af ter the floods. Although these results are somewhat sparse, it

appears that drainage channels designed according to the proposed procedures will convey design discharges without significant erosion.

(8)

v-TENTATIVE DESIGN PROCEDURE FOR RIPRAP-LINED CRANNELS -- FIELD EVALUATION Chapter 1. INTRODUCTION AND RESEARCH APPROACH

As a result of the basic study outlined in the previous report

(1)*,

a field evaluation of riprap-lined channels designed in accordance with the procedures outlined therein was recommended to determine the effective-ness of the procedures at scales considerably larger than any that can be produced in the laboratory •

Because drainage channels are constructed as the need arises, and the necessity for a riprap lining depends on the local circumstances, consider-able time may elapse before a substantial number of channels is available for examination. In addition, hydrologic events of the magnitude necessary to provide an effective test of the riprap's stability are relative i nfre-quent. These conditions have militated against collection of sufficient data to thoroughly test the procedures developed in the study. Specifically, five channels have been proposed since the completion of the report outlin-ing these design procedures; four of these have been constructed and one is still in the planning stage. Of the four completed projects, two were in-tended for stream relocations involving relatively large discharges and two are roadside drainage ditches. Some data have been obtained for these four channels, but in only two cases has the discharge been large enough to ap-proach the design discharge and provide a reasonably definitive test. A detailed description of each of these channels and the available data on the results of the discharges which have occurred are given in the appendices.

Po Ll.owf.ngcompletion of the previous report a letter of inquiry was sent to each of the regional hydraulic engineers of the Federal Highway Ad-ministration and to all members of the Highway Research Board Committee on

Surface Drainage of Highways to alert these individuals to the completion of the report and suggest that representative future drainage channels be designed in accordance with these procedures. It was hoped that field ob-servation of these channels could then be undertaken to evaluate the appro-priateness of the proposed design techniques. A form (Figure 1) that could be used to record pertinent field data for any hydrological event that might occur during the life of a particular channel was also sent.

With the passage of time it is expected that additional channels will be designed on the basis of the proposed procedures, but because of imminent project termination, the evaluation study was limited to the channels de-scribed in the following.

The reasons for the proposed design proGedures and the basis for their development can be summarized as follows:

Whenever highway construction interferes with the natural flow of water, erosion-resistant drainage channels must be designed and built to redirect the water to a natural waterway. A protective lining is usually required to prevent damage to the channel. The most extensively used pro -tective linings are turf cover, by sodding or other methods, and various types of pavement. These linings are quite effective for a wide variety of conditions, but have certain limitations; for example, (1) turf cover is difficult to establish in arid areas and over sandy soil; (2) turf

(9)

FIELD EVALUATION

-

R

IPRAP-LINED DRAINAGE CHANNELS

NCHRP Proje

c

t 15-

2 :

Des

i

gn to

C

ontrol Erosion in

R

oadside Drainage Channels

ORIGIN

AL DESI

GN

1. Design discharge cfs ~

___

2 .

Des

i

gn slope ft per ft

~ __~~

~

___

3 .

Plan and sections as designed (drawings if available)

---4

.

Longitudinal profile (drawings if available)

---5

.

Si

z

e of riprap

(50

per cent size) ft

__

6. Grading specification (if

any)

~~

___

7. Contr

i

butory drainage area, sq miles

...,.--~---8

.

Character of drainage area (cover

,

shape, etc.)

---9

.

Soil type under riprap

__

10. Alternatives considered

__

C

ONSTRUCTION

1. Cross sections as constructed

_

2 .

Size of riprap as constructed

~--~--...,.--~---...,.__~---3

.

Construction

procedures for riprap,

placing,

grading,

compacting,

etc.

4. Contract or force account

__

5. Cost of material

-:--

___

6 .

Cost of construction

__

7. Comments

__

FI

E

L

D

DATA (fo

r

each storm)

1. Peak discharge (if available) cfs

_

2 .

M

aximum depth ft

3 .

Rainfall (amount

-an~d-l.':"'""·

n~t-e-n-s-:-i-:-ty""""")

---4.

Stability of channel

r

iprap (photographs)

---5.

Photographs and other desc

r

iptive matte

r

6 .

Diary of events and comments

(10)

3

cover is effective only with relatively low flow velocities;

(3)

paved ditch linings are usually difficult to construct and rather costly; and

(4)

paved ditch linings require extensive maintenance at times due to unde rcutt ing. As aresult there has been a need for a type of economical protective lining for roadside channels suitable for conditions inter

-mediate between those for which turf cover performs satisfactorily and those for which paved channels are more economical. The objective of the study mentioned previously was the development of criteria and design procedures for the use of aggregate or riprap linings for this inter

-mediate category.

This was accomplished by synthesizing the principles of open-channel flow with the results of experiment al data on the critical boundary shear and resistance due to the flow on a bed of discrete particles. By use of the information that had been reported in the literature and the inter-relationships between the discharge, slope, size, and shape of the chan-nel and the size of the riprap material, relationships were derived for the size of riprap lining necessary to provide an erosion-resistant surface.

For the purposes of such design the following conditions were assumed to apply:

1. The drainage channel will be essentially straight.

2. The flow will be essentially uniform and can be described by the Manning formula:

v

= ~

R2/3 S 1/2

n b

(1)

in which

V

is the mean velocity, n is the roughness coeffici-ent, R denotes the hydraulic radius, and Sb represents the longitudinal bed slope.

3.

The roughness coefficient will depend on th~ effective size of the riprap and can be expressed as

1/6

n

=

0 .

04

d

50 (2)

in which d

50

represents the particle size than which

50

percent is finer by weight.

4 .

The critical boundary shear stress is directly proportional to the effective size of the riprap and can be expressed as

(3)

5 .

The ratio of the maximum shear stress to the mean shear stress is taken to be

1.5

for trapezoidal channels and

2

for wide triangular channels with very mild side slopes; that is,

• (

) =

1 .

5 Y RS

b (trapezoidal)

(11)

and

~o(max) = 2 y RSb (triangular)

(5)

For regular trapezoidal ehannels these assumptions give rise to the

following equations relating the discharge, veloeity, and hydraulic radius

to the longitudinal slope, the size of the riprap, and the shape of the

ehannel: d

5/2

1 ..:2.Q__ P Q

=

118 S 13/6

R

b

(

6)

(7)

and d R

=

0.0428

lO

b

(

8 )

Eq.

6

shows that for a given discharge and slope the minimum size of riprap needed to protect the ehannel depends only on the channel's shape as pr

e-seribed by P/R. Once the size of riprap is determined, the veloeity and

hydraulie radius can be eomputed from Eqs.

7

and 8. From these two eq

ua-tions and the diseharge, the required eross-seetional area and wetted perim

-eter ean be obtained. These, in turn, provide the basis for eomputing the

bottom width of the channel and the water depth. To faeilitate sueh eom

pu-tations a set of eharts prepared for the original report is ineluded here

for referenee. Figures 2 and

3

represent Eq.

6

for P/R

=

13 .

3

anti

P/R

=

30, respeetively. These values of P/R represent the range of

ehannel shapes likely to be eneountered. The smaller value represents a

relatively deep and narrow ehannel; the larger, a wide and relatively

shallow channel. The respeetive values of riprap size obtained from these

two figures represent the maximum and minimum sizes that will be just stabIe

in their respective channels. Any intermediate size would result in an

intermediate value of P/R, and hence an intermediate ehannel shape. Once

the size of riprap is chosen, the veloeity and hydraulie radius are deter

-mined from Figures

4

and

5

and the cross-sectional area is obtained from

Figure

6.

The side slope required for stability is then obtained from

FigQres

7

and 8. By use of this side slope and the calculated cross-sec

-tional area and hydraulie radius, the channel geometry can be obtained

directly from the appropriate seetion of Figure

9 .

In these design eharts

the side slope is established so that the riprap on the side is as stabIe

as that on the bottom, which in turn is the minimum size that will be

stabIe for the given discharge and channel slope.

For triangular channels, whieh for safety reasons are often neees

-sary in median strips, the caleulations are somewhat simpIer beeause the

shape of the triangular channel represented by P/R depends only on the

side slope. In addition, the riprap size needed for sueh channels would

usually be somewhat smaller than that neeessary for larger trapezoidal

(12)

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(14)

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(15)

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2 3456789

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(16)

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40

30

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51 Area - sq ft

Figure

6. Area of a trapezoidal

channel

in terms of discharge

and maximum mean v

e

locity

(1, Fig.

2 3).

(17)

-I

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24).

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(18)

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(19)

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(20)

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channels with 2.5:1 side slopes (1, Fig. 26c).

(21)

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(23)

channels, the riprap sizes were chosen from standard sizes of coarse aggre-gate such as those listed in AASHO designation

M43-54,

"Standard Sizes of Coarse Aggregate for Highway Construction. " The gradations of eight of

these standard sizes having a reasonably systematic change in mean diam-eter are given in Table 1. It is thought that local aggregates having

Table 1. SIZES ANTI MEAN DIAMETERS OF COARSE AGGREGATES* Percent by Weight Passing AASHO Size(a)

Sieve

Size No. 1 No. 2 No. 24 No. 4 No.357 No.467 No. 57 No. 68 4 In. 100 3 1/2 In. 90-100 3 In. 100 100 2 1/2 In. 25-60 90-100 90-100 100 2 In. 35-70 100 95-100 100 1 1/2 In. 0-15 0-15 25-60 90-100 95-100 100 1 In. 20-55 35-70 95-100 100 3/4 In. 0--5 0-10 0-15 35-70 90-100 1/2 In. 0-5 10-30 25-60 3/8 In. 0-5 10-30 30-65 No. 4 0-5 0-5 0-10 5-25 No. 8 0-5 0-10 No. 16 0-5 d (b) O.185 O.149 0.lO9 0.080 0.059 0.044 0.034 0.024 50

.

'

(a) Adapted from AASHO Standard Specification M43-54.

(b) Mean partic1e size diameter (in feet) at which 50 percent is finer by weight.

(24)

17

approximate1y the same gradation and mean diameter eould be used. The equation re1ating the diseharge size, longitudinal slope, and side slopes ean then be written as

d

1/2 2

1 ~Z +1

Q

=

64

·

4

8

13/6

z

b

(9 )

in whieh Z is the side slope, and the depth, y, is obtained from

d

(2

+ 1)1/2

Y

=

0 .

064

850 ....lo.:Z=--..:...;:.__

b

Z

(10)

Inas~Qeh as the depth of flow is also a funetion of the side slope, it ean be determined direetly from Eq. 10 onee the side slope has been established. This has been done in Figures 10 thro~gh 17, from whieh the size of riprap and the depth of flow ean be determined for any given diseharge, longitud-inal slope, and side slope.

(25)

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.

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c

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for channel

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CD

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channel

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slope 3:1

(1,

Fig. 27) .

• o

0\

• o

0

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8-•

Discharge

- cfs

(26)

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11 Depth of flow and size of standard

aggregate

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10

20

60

80

1

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30

40

3

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(27)

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o

2

3

4

6

8

10

20

30

40

60

80

100 Discharge

- cfs

(28)

4 ~O'

Figure

13. _{Depth of flow and size of standard}

_a

_ggr

for e

h

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stabil ity

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•

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₆₀

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Dise

h

arge

- e

fs

(29)

2

•

0 for channel

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fr

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Figure

15. _{Depth of flow and size of standard}

_aggregate

for channel

stability

Triangular

channel

Side slope

8:1

(1, F

i

g.

32).

2 10-2

/

1

2 ₃

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100 Discharge

- cfs