Presentation on theme: "Streambank Protection Design of Riprap Protection Stephen T. Maynord."— Presentation transcript:
Streambank Protection Design 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 ways on streambank protection 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
Streambank Protection OBJECTIVE 2: DESIGN FACTORS & FAILURE CAUSES Design & failure can be scary!!!!
A.) RIPRAP CHARACTERISTICS UNIT WEIGHT - >150 LBS/FT 3 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 THICKNESS SIGNIFICANT IMPACT ON STABILITY NOT LESS THAN d 100 (MAX) OR 1.5 d 50 (MAX) THICKNESS > 1 d 100 (MAX) ALLOWED REDUCTION IN STONE SIZE UNDERWATER PLACEMENT REQUIRES 50% INCREASE
D.) 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 REQUIREMENTS (PRIMARILY A GEOTECH RESPONSIBILITY) 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 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 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 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
Streambank Protection OBJECTIVE 3: TOE PROTECTION
TOE SCOUR DESIGN ESTIMATE MAXIMUM SCOUR PROTECT AGAINST MAXIMUM SCOUR
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 ESTIMATIONS 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
WELL-GRADED, EVEN QUARRY-RUN IS USED INSTEAD OF UNIFORM GRADATION, D 85 /D 15 >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 REVETMENTS Defined: A line of stone placed along the top of an eroding bank, either on ground surface or partially buried.
WINDROW REVETMENTS Advantages: 1.Ease of construction- Minimal disturbance and site prep 2.Stone manipulation minimized 3.Excess stone can be later salvaged 4.Vegetation will invade 5.Can be constructed from land or floating plant
WINDROW REVETMENTS Windrow requirements: 1.Cleared, relatively flat upper bank areas 2.Non-or weakly cohesive bank material in the protected zone
TRENCH-FILL REVETMENTS Defined: 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 REVETMENTS Advantages: 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.
Streambank Protection OBJECTIVE 4: STONE 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 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 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 TWO PROBLEMS DETERMINE IMPOSED FORCE (VELOCITY) DETERMINE RESISTING FORCE (RIPRAP SIZE VERSUS VELOCITY) WHICH IS MORE DIFFCULT?
STONE SIZE WORKSHOP Problem No. 1 Subject: Natural channel bend with riprap on outer bank only Given: Unit weight of stone = 165 #/ft 3 Riprap blanket thickness = 1.0 D 100 (max) Local depth of toe of outer bank = 25 ft Local depth at 20% upslope from toe = 20 ft (use in chanlpro) Channel side slope = 1V:2H Use average channel velocity option “A” Minimum centerline bend radius = 1700 ft Natural channel Average velocity = 7.2 ft/sec Water-surface width = 500ft Use standard safety factor = 1.1 Use ETL gradation
Problem No. 1 Required: Find computed d 30, thickness for ETL gradation and d 30 (min) for the following: (a)Determine stable riprap gradation for outer bank of channel bend (b)Change unit stone weight γ s = 155 #/ft 3 (c)With γ s = 155 #/ft 3, change average velocity to 6.1 ft/sec (d)With γ s = 155, v=6.1, change side slope to 1V:1.5H
Problem No. 2 Subject: Riprap downstream of concrete channel Given: Unit weight of stone = 165 #/ft 3 Subcritical flow in concrete channel shown in Figure Thickness = 1 D 100 (max) Depth at end of concrete = 15 ft Average velocity (Q/A) at end of concrete = 8 ft/sec Top of riprap and concrete at same elevation Due 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) = (see p. 3- 8, EM ) Use ETL gradation (Table 3-1, EM ) Input Cotan of side slope = 4 to specify bottom riprap
Problem No. 2 Required: 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 d 30 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”