Riprap Revetment Ahn Polvi Trabant

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HYDRAULIC AND SEDIMENT

TRANSPORT MODELING OF SAN

JOAQUIN RIVER, FRIANT DAM TO

MENDOTA DAM

Riprap RevetmentRiprap Revetment

Jungkyu AhnLina Polvi

Stuart Trabant

OutlineOutline

• Introduction

• Where riprap is used

• Riprap design

• Example design

• Types of riprap failure

• Design considerations: How to preventriprap failure

• Examples of riprap failure & success






MENDOTA DAM

IntroductionIntroduction

Riprap Revetment:Strengthen riverbanks against erosion

Factors that affect riprap resistance to erosiveforces

Materials : natural stones

▪ construction quality

▪ channel alignment

▪ cross section

▪ bank slope

▪

velocity distribution▪ ground water

▪ stone size

▪ shape

▪ weight

▪ durability

▪

gradation▪ layer thickness

Where Riprap Revetment is Useful:Where Riprap Revetment is Useful:

• On the outside ofbends

• At hydraulicstructures

• At flow contractions

• In man-made channels

• In widening channels• At locations with high

turbulence

Outside Bend Riprap, Rapid Crk, SD (MEI)






MENDOTA DAM

1. Stone size

There are two methods to determine stone size.(1) Shear-stress method

where dm : effective rock size (usually, dm ≈d65 ≈ 1.25d50)

τ 0 : applied shear stress

τ *c : critical value of Shields number γ : specific weight of water

G : specific gravity of riprap

θ 1 : side slope angle

φ : angle of repose of riprap

Riprap DesignRiprap Design

⎥⎥⎦

⎤

⎢⎢⎣

⎡−−

=

φ

θ τ

τ

2

1

2

*

0

sin

sin1)1(G

d

c

m

φ

(2) Velocity method

where ds ≈d65 ≈ 1.25d50

Vc

: critical mean flow velocity


φ

⎥⎦

⎤⎢⎣

⎡

−

−−≅

φsin1

θsin11)gd2(GK V

2

2

scc

φ tan4

log ⎟⎟ ⎠

⎞⎜⎜⎝

⎛ =

s

cd

hK






MENDOTA DAM

Standard Riprap Sizing MethodsStandard Riprap Sizing Methods(Velocity(Velocity--Based)Based)

• U.S. Corps of Engineers, EM 1110-2-1601

• American Society of Civil Engineers

• US Bureau of Reclamation Method

• U.S. Geological Survey Method

• Isbash Method

• California Bank and Shore ProtectionMethod

• Federal Highways HEC-11


2. Stone shape (should satisfy the following)

i) Stone shall be predominantly angular

ii) Not more than 30% of stones a/c >2.5

iii) Not more than 15% of stones a/c >3.0

iv) No stone a/c > 3.5

where a : maximum length of long axis

c : maximum length of short axis

3. Stone weight

where D% : equivalent-volume spherical stone diameter

W% : weight of individual stone having diameter of D%


φ φ φ

3/1%

%

6⎟⎟ ⎠

⎞⎜⎜⎝

⎛ =s

W D

πγ






MENDOTA DAM

4. Filter i)

ii)

iii)

5. Riprap layer thicknessi) Riprap layer thickness >d

100& >1.5×d

50ii) Increase thickness by 50% when placing under water

Riprap DesignRiprap Designφ φ

40)(

)(

50

50 <based

filter d

40)(

)(5

15

15 <<based

filter d

5)(

)(

85

15 <based

filter d

6. Revetment toe protection

Toe protection may be provided by two methods

(1) Extend to maximum scour depth

▪ place end of toe below the expected scour depth or

▪ ground it on nonerodible material

(2) Place launchable stone

▪ Windrow : riprap placed at top of bank

▪ Trench-fill : riprap placed at low water level

▪ Weighted riprap toes : riprap placed at intersection of channel bottom and side slope

▪ Windrow ▪ Trench-fill ▪ Weighted riprap toes


φ φ φ






MENDOTA DAM

Example Design:Example Design:

Yuba River, California Yuba River, California

• Purpose: Protect

outside of bend at

fish ladder exit

• Stone sizing

method: Velocity

Method (after Julien,

2002)

• Stone Sizing

Equation:

⎥⎦

⎤⎢⎣

⎡

−

−−≅

φsin1

θsin11)gd2(GK V

2

2

scc

Proposed Riprap

Fish Ladder

Exit

Q

Example Design (cont):Example Design (cont):

Yuba River, California Yuba River, California

• From hydraulicmodel, Vc~8 ft/s

• Use particle stabilitydiagram (stone sizevs velocity curves)

• For bank slope of2H:1V, ds ~ 0.6 feet

• Use D50 = 9” (fromds ~ 1.25 D50)

• Therefore, D10 ~3”and D90 ~16”






MENDOTA DAM

Example Design (cont):Example Design (cont):

ApplicationApplication

• Design Flow Depth=22’

• Freeboard=2’

• Est. Scour depth=6.6’

• Thickness=2* D50=18”

• Length=500’

• Factor of Safety=2 (for

self-launching toe

riprap)

• Total Volume=H*T*L*FS*sin ~760yd3

= 205 tons

θ

θ

110

120

130

140

150

160

170

0 10 20 30 40 50 60 70 80 90 100

Station (ft)

E l e v a t i o n

( f t )

Left Bank at Proposed Riprap

Riprap as constructed

Riprap After Scour and Launch*

*Assuming riprap lost during launch

Riprap Height=24 ft

Scour Depth=6.6 ft

Slope =1V:2H

Design WSE

Schematic of riprap design for YubaRiver with launchable toe (MEI)

Types of Riprap FailureTypes of Riprap FailureParticle Erosion:•Causes: Stone size too small, stones removed by

impact, bank sideslope greater than angle of repose

for riprap material, riprap gradation too uniform

•Solution: Coarser riprap, reevaluate riprap gradation

& angularity

Slide: Downslope mass movement of stones•Causes: Bank sideslope too steep, excess

hydrostatic pore pressure, loss of material at toe of

riprap blanket

•Solution: Strengthen toe of riprap blanket, use larger

stones near channel bed






MENDOTA DAM

Types of Riprap FailureTypes of Riprap Failure

(cont.)(cont.)Slump: Mass movement within riprap

blanket•Causes: Bank sideslope too steep, lack of toe slope

support

•Solution: Add coarser material at toes of

embankment, reduce sideslope angle at top of

embankment

Sideslope failure: rotation/gravitational

movement along surface•Causes (related to shear failure of underlying base

material): Excess pore pressure in base material,

sideslopes too steep

•Solution: Reduce embankment slope, drain base

material

Design Considerations:Design Considerations:

Preventing Riprap FailurePreventing Riprap Failure• Tieback

– Tie upstream and

downstream ends of

blanket into bank

• Launching Apron

– Horizontal riprap on

foot of revetment

• Overlap revetment

– Overlap revetment

on opposite banks

Overlapping riprap on Rapid Creek, SD






MENDOTA DAM

General Design ConsiderationsGeneral Design Considerations

Alt ernating Riprap Design,

Puerco River, NM (MEI)

Riprap with d rainage,

Minera Yanachocha, Peru (MEI)

• Overlap riprap whenalternating left and

right banks

• Use proper drainage

for saturated soils

General Design ConsiderationsGeneral Design Considerations

(cont.)(cont.)

• Extend revetment

up and downstream

of active erosion

• Ensure riprap is

well-graded and

angular

• Use tie-back toprevent flanking Tie-back on Merrill Creek, TX (MEI)






MENDOTA DAM

Examples of Riprap FailureExamples of Riprap Failure

Failed riprap on

Tijeras Arroyo, NM

due to poorgradation

Failed riprap on

Tijeras Arroyo, NM

due to slide orslump and further

bank erosion

Failed riprap

flanking on

Whitnall Park

Creek, WI

Examples of Riprap SuccessExamples of Riprap Success

Revegetation of riprap

along Rapid Creek, SD

Riprap along alternating bends

on Whitnall Park Creek, WI

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