streambank protection design of riprap protection stephen t. maynord

Streambank ProtectionStreambank Protection

Design of Riprap ProtectionDesign of Riprap Protection

Stephen T. Maynord

Objectives:Following this lecture, the students will be able to:

1)Use riprap in different ways on streambank protection projects.

2)List significant riprap design factors common to most of the different ways of using riprap.

3)Describe significant design features associated with toe protection.

4)Determine riprap size, gabion size, and estimate scour depth in bends using PC program “Chanlpro”

Objective 1: Use riprap in different Objective 1: Use riprap in different ways on streambank protection ways on streambank protection

projects.projects.

Goal: Use minimum amount of structural protection required to accomplish project objectives. Achieving this goal could result in the following ways to use riprap:

A. Standard revetment constructed over the entire bank

B. Upper bank protection

C. Lower bank protection

D. Toe protection

E. Launchable stone protection such as windrow, trench-fill, or weighted riprap toe

F. Indirect protection- dikes, hardpoints, bendway weirs- to be covered by others

G. Environmental benefits

TRENCHFILL REVETMENT


OBJECTIVE 2:OBJECTIVE 2:DESIGN FACTORS & DESIGN FACTORS & FAILURE CAUSESFAILURE CAUSES

Design & failure can be scary!!!!

A.) RIPRAP CHARACTERISTICSA.) RIPRAP CHARACTERISTICS

• UNIT WEIGHT - >150 LBS/FT3

• SHAPE – BLOCKY RATHER THAN ELONGATED

• ANGULARITY – ANGULAR BEST ROUNDED = 1.25* ANGULAR

• SOURCES – ROCK QUARRIES, BROKEN CONCRETE, STREAM ROUNDED STONE ? HAS YOUR OFFICE USED ANYTHING OTHER THAN CRUSHED ROCK FOR RIPRAP?

C.) LAYER THICKNESSC.) LAYER THICKNESS

• SIGNIFICANT IMPACT ON STABILITY• NOT LESS THAN d100(MAX) OR 1.5 d50(MAX)• THICKNESS > 1 d100(MAX) ALLOWED

REDUCTION IN STONE SIZE• UNDERWATER PLACEMENT REQUIRES 50%

INCREASE

D.) SIDE SLOPE INCLINATIOND.) SIDE SLOPE INCLINATION• RARELY STEEPER THAN 1V:1.5H• 1V:2H TO 1V:3H PREFERRED• STONE SIZE LARGE WHEN BANK ANGLE

APPROACHES REPOSE ANGLE• REPOSE ANGLE VARIES WITH SLOPE HEIGHT• SLIDING PROBLEMS ON FILTER FABRIC LIMIT

TO 1V:2H• GEOTECHINICAL STABILITY OFTEN DEFINES

LIMITING SLOPE ?WHAT SIDE SLOPES ARE USED IN YOUR AREA?

E.) FILTER REQUIREMENTSE.) FILTER REQUIREMENTS (PRIMARILY A GEOTECH RESPONSIBILITY)(PRIMARILY A GEOTECH RESPONSIBILITY)

FILTER PURPOSES:FILTER PURPOSES:• PREVENT STREAM TURBULENCE FROM

REMOVING BANK MATERIAL• PREVENT GROUNDWATER FROM MOVING

BANK MATERIAL THROUGH RIPRAP• SERVE AS FOUNDATION

SUCCESSFUL REVETMENTS HAVE BEEN CONSTRUCTED WITHOUT A FILTER ? DOES YOUR OFFICE REQUIRE A FILTER?

F.) REVETMENT HEIGHTF.) REVETMENT HEIGHT

• TOTAL BANK PROTECTION• PARTIAL BANK PROTECTION

-REDUCED STONE VOLUME -PROVIDES ENVIRONMENTAL BEEFITS -DEPENDS ON: -HYDRAULIC FORCES -BANK MATERIAL STRENGTH -VEGETATION -HYDROGRAPH -SUCCESSFUL IN SECTION 32

G.) VEGETATION IN RIPRAPG.) VEGETATION IN RIPRAP• ADVANTAGES

-LESS MAINTENANCE -ENVIRONMENTAL BENEFITS

• DISADVANTAGES -DIFFICULT TO INSPECT -INCREASED WATER LEVELS -TURBULENCE INCREASE -LARGE TREE REMOVAL

•

• ? WHAT ARE DISTRICT VEGETATION PRACTICES?

H.) TRANSPORT AND PLACEMENTH.) TRANSPORT AND PLACEMENT

• TRANSPORT OFTEN MAJOR PART OF COST

• TRUCK $ = 10 * BARGE $• DUMPING AND SPREADING

PROMOTES SIZE SEGREGATION AND BREAKAGE

• RELEASE NEAR FINAL POSITION ? COMMENTS ON TRANSPORT AND PLACEMENT


OBJECTIVE 3:OBJECTIVE 3:TOE PROTECTIONTOE PROTECTION

TOE TOE SCOURSCOUR DESIGNDESIGN

• ESTIMATE MAXIMUM SCOUR

• PROTECT AGAINST MAXIMUM SCOUR

SCOUR DEPTH DEPENDS ON:SCOUR DEPTH DEPENDS ON:

• CHANNEL PLANFORM• CROSS-SECTION• VELOCITY, SHEAR STRESS• WATER AND SEDIMENT HYDROGRAPH• BED MATERIAL SIZE AND GRADATION• BANK ERODIBILITY COMPLEX PROBLEM. THE FOLLOWING

TECHNIQUES ARE AVAILABLE FOR SCOUR DEPTH ESTIMATION

TOE SCOUR TOE SCOUR ESTIMATIONSESTIMATIONS

• EXPERIENCE AND “RULES OF THUMB” (MOST WIDELY USED METHOD)

-MAXIMUM SCOUR WILL BE A CERTAIN DISTANCE BELOW THE DEEPEST POINT IN THE EXISTING CROSS-SECTION

Riprap Test Facility

MODEL TESTS

• WELL-GRADED, EVEN QUARRY-RUN IS USED INSTEAD OF UNIFORM GRADATION, D85/D15>2

• LAUNCHABLE STONE TECHNIQUES INCLUDE -WEIGHTED TOE-TOE OF BANK -TRENCH-FILL REVETMENT – MID BANK -WINDOW REVETMENT – TOP OF BANK

• WIDELY USED ON SAND BED STREAMS• SOME FAILURES IN GRAVEL-BED STREAMS

WINDROW REVETMENTSWINDROW REVETMENTS

Defined: A line of stone placed along the top of an eroding bank, either on ground surface or partially buried.

WINDROW REVETMENTSWINDROW REVETMENTSAdvantages:1. Ease of construction- Minimal

disturbance and site prep2. Stone manipulation minimized3. Excess stone can be later salvaged4. Vegetation will invade5. Can be constructed from land or

floating plant

WINDROW REVETMENTSWINDROW REVETMENTS

Windrow requirements:1. Cleared, relatively flat upper bank

areas2. Non-or weakly cohesive bank

material in the protected zone

TRENCH-FILL REVETMENTSTRENCH-FILL REVETMENTSDefined: Upper bank graded and

protected, usually with riprap. Large mass of stone placed in trench along the riverward edge of the upper bank protection. As erosion occurs on the lower bank, rock launches out of the trench. Protecting the lower bank. Bottom of trench is 7-8 ft. below mean-low water on Arkansas river.

TRENCH-FILL REVETMENTSTRENCH-FILL REVETMENTSAdvantages:1. Ease of construction- Eliminates most of

the underwater bank grading and stone placement.

2. Stone can be added to trench if depleted.3. Used on Mississippi River for large

launch depths.4. Widely used on Arkansas and Red

Rivers.


OBJECTIVE 4:OBJECTIVE 4:STONE SIZINGSTONE SIZING

• AVAILABILITY AND EXPERIENCE OFTEN DETERMINE ROCK SIZE RATHER THAN DESIGN GUIDANCE

• EVEN WITH DESIGN GUIDANCE YOU ARE OFTEN CHOOSING FROM A LIMITED SET OF GRADATIONS THAT ARE AVAILABLE IN YOUR AREA

DESIGN CONDITIONSDESIGN CONDITIONS SINGLE CHANNELS –

BANKFULL DISCHARGE OR HIGHER IS GENERALLY MOST SEVERE DESIGN FOR LOCATION HAVING MAXIMUM VELOCITY, NORMALLY USE SAME SIZE FOR ENTIRE REACH OR BEND

DESIGN CONDITIONSDESIGN CONDITIONS

BRAIDED CHANNELS – INTERMEDIATE FLOW CAN BE

MOST SEVERE BECAUSE DIVIDED FLOW TENDS TO “IMPINGE” ON LEVEE OR BANKLINES AT SHARP ANGLES

RIPRAP DESIGN HAS RIPRAP DESIGN HAS TWO PROBLEMSTWO PROBLEMS

• DETERMINE IMPOSED FORCE (VELOCITY)

• DETERMINE RESISTING FORCE (RIPRAP SIZE VERSUS VELOCITY)

WHICH IS MORE DIFFCULT?

VELOCITY ESTIMATIONVELOCITY ESTIMATION

• NUMERICAL MODELS

• PHYSICAL MODELS

• ANALYTICAL MODELS

• EMPIRICAL METHODS

• PROTOTYPE DATA

STONE SIZE WORKSHOPSTONE SIZE WORKSHOP

Problem No. 1Subject: Natural channel bend with riprap on outer bank onlyGiven:

Unit weight of stone = 165 #/ft3

Riprap blanket thickness = 1.0 D100 (max)Local depth of toe of outer bank = 25 ftLocal depth at 20% upslope from toe = 20 ft (use in chanlpro)Channel side slope = 1V:2HUse average channel velocity option “A”Minimum centerline bend radius = 1700 ftNatural channelAverage velocity = 7.2 ft/secWater-surface width = 500ftUse standard safety factor = 1.1Use ETL 1110-2-120 gradation

Problem No. 1Required: Find computed d30, thickness for ETL

gradation and d30 (min) for the following:

(a)Determine stable riprap gradation for outer bank of channel bend

(b)Change unit stone weight γs = 155 #/ft3

(c) With γs = 155 #/ft3, change average velocity

to 6.1 ft/sec

(d)With γs = 155, v=6.1, change side slope to

1V:1.5H

Problem No. 2Subject: Riprap downstream of concrete channelGiven:

Unit weight of stone = 165 #/ft3

Subcritical flow in concrete channel shown in Figure

Thickness = 1 D100 (max)Depth at end of concrete = 15 ftAverage velocity (Q/A) at end of concrete = 8 ft/sec Top of riprap and concrete at same elevationDue to expansion, an eddy forms at downstream end of concrete channel causing a flow concentration along right bank. Observers report that the left 1/3 of the channel is an eddy with flow in an upstream direction.Consider difference in roughness of concrete and riprap by increasing safety factor to 1.25 (1.1) = 1.375 (see p. 3- 8, EM 1110-2-1601)Use ETL 1110-2-120 gradation (Table 3-1, EM 1110-2-1601)Input Cotan of side slope = 4 to specify bottom riprap

Problem No. 2Required: Determine stable riprap size downstream of concrete channel.

Specify local velocity option (L) instead of average channel velocity.Procedure:

(a) Estimate local depth-averaged velocity at point A. Consider influence of eddy and flow concentration.

(b) Determine d30 and ETL gradation using CHANLPRO.(c) Can you think of other things to do to improve the problem of the difference in

boundary roughness?(d) Estimate distance downstream for large riprap?

Objectives (review):Following this lecture, the students will be able to:

1)Use riprap in different ways on streambank protection projects.

2)List significant riprap design factors common to most of the different ways of using riprap.

3)Describe significant design features associated with toe protection.

4)Determine riprap size, gabion size, and estimate scour depth in bends using PC program “Chanlpro”

QUESTIONS?

streambank protection design of riprap protection stephen t. maynord

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