Presentation on theme: "Streambank Protection Design of Riprap Protection"— Presentation transcript:
1 Streambank Protection Design of Riprap Protection Stephen T. Maynord
2 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”
3 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:Objective 1 is to present a list of ways of using riprap without getting into specifics.
4 Standard revetment constructed over the entire bank Upper bank protectionLower bank protectionToe protectionLaunchable stone protection such as windrow, trench-fill, or weighted riprap toeIndirect protection- dikes, hardpoints, bendway weirs- to be covered by othersEnvironmental benefitsThis is list of ways to use riprap to be followed by pictures
5 Before showing the list, this slide demos a bank that is eroding Before showing the list, this slide demos a bank that is eroding. It was actually protected by a used tire mattress that failed with the remnants of the tire mattress at the bottom of the slide.
6 This is another example of an eroding bank that we might choose to use some form of riprap protection to address erosion problem.
8 This is example of riprap used in traditional manner to protect the entire bank. Often used when consequence of failure may be high.
9 This is example of a widely used method on the Mississippi River, riprap on top bank and another protection method, articulated concrete mattress in this case, on the lower bank.ASK: Why was mattress used instead of riprap?ANSWER: One reason is the difficulty of placing rock and getting good coverage in deep depths and velocities found in Mississippi River?ASK: Why was mattress laid out along channel bottom?ANSWER: to prevent scour at the toe from undermining the protection.
10 This is artist’s rendering of Mississippi River with riprap and ACM. This is the clearest the MR has ever been.
11 This is example of riprap on lower bank only in zone where attack is most frequent and most intense. Rock toe is intended to “launch” or fall down the slope as scour occurs on the channelward side of the riprap.ASK: What is purpose of periodic tiebacks above riprap on lower bank?ANSWER: To prevent flanking of structure.
12 Another example of lower bank protection, also in Northern Mississippi as in previous slide.
13 Same concept but minimal amount of riprap used with no bank shaping to see if stabilizing the toe only would be effective. Working last time seen.
14 Another use is riprap used to provide launching toe protection for another bank protection method. Although difficult to see, this is a double row of fencing filled with used tires.
15 This is the bank shown in the first few slides that failed the tire mattress. The failed protection was replaced with another tire mattress but with the addition of a launching riprap toe protection. Note that willows have been planted in each tire and tires are banded together. Working last time seen after about 10 years.
16 Another example toe protection only with a comprehensive vegetation scheme on the upper bank. I believe this was a site designed by a restoration consultant Robin Sotir.
17 Another form of launchable riprap is called a “windrow revetment” that is placed on top of bank along the desired alignment of the bank. Riprap falls from top of bank to toe of slope. Significant use on Missouri River.
18 Although dark, this is a windrow revetment on Missouri River Although dark, this is a windrow revetment on Missouri River. Lauch slope is steep because bank material has cohesive properties. In non-cohesive material such as sand, launch slope is about 1V:2H.
19 In addition to launchable riprap placed at the toe or at top of bank, a trench fill revetment is used where the riprap section is placed at mid-bank near the low water line. Widely used on Red and Arkansas Rivers. One site on Mississippi River has launch depth approaching 60 ft.TRENCHFILL REVETMENT
20 Indirect protection such as dikes and hardpoints will be covered by others.
23 This in a river in the northwest in which these riprap dikes were placed for environmental purposes to provide low velocity refuge areas for spawning fish.
24 End of objective 1 showing different ways of using riprap.
25 Streambank Protection OBJECTIVE 2: DESIGN FACTORS & FAILURE CAUSES Dave Derrick decided my slides needed some humor.Design & failure can be scary!!!!
26 A.) RIPRAP CHARACTERISTICS UNIT WEIGHT - >150 LBS/FT3SHAPE – BLOCKY RATHER THAN ELONGATEDANGULARITY – ANGULAR BEST ROUNDED = 1.25* ANGULARSOURCES – ROCK QUARRIES, BROKEN CONCRETE, STREAM ROUNDED STONE ? HAS YOUR OFFICE USED ANYTHING OTHER THAN CRUSHED ROCK FOR RIPRAP?After reading bullets, ask question on screen and see if class will discuss use of broken up concrete.
27 Simply note the importance of rock quality Simply note the importance of rock quality. This rock was on Snake River and had suffered massive reduction in size due to poor quality.
28 Note that riprap gradation is important in stability of riprap from being moved by hydraulic forces and stability of material beneath riprap being moved by turbulence in stream and groundwater. Explain diameter and weight on above curve and concept of percent finer by weight %. Also explain two limiting curves concept given to quarry to produce and that if curves are too close together, it will be difficult to produce.
29 Explain this is a “grizzly” that is used to separate riprap sizes Explain this is a “grizzly” that is used to separate riprap sizes. Many are better constructed than this one.
30 C.) LAYER THICKNESS SIGNIFICANT IMPACT ON STABILITY NOT LESS THAN d100(MAX) OR 1.5 d50(MAX)THICKNESS > 1 d100(MAX) ALLOWED REDUCTION IN STONE SIZEUNDERWATER PLACEMENT REQUIRES 50% INCREASEPresent bullets. 3rd bullet is based on testing of riprap stability we have done in our flumes which showed that depending on gradation, a larger thickness of a smaller riprap could provide the same protection as a smaller thickness of a larger riprap when measured relative to the D100.
31 D.) SIDE SLOPE INCLINATION RARELY STEEPER THAN 1V:1.5H1V:2H TO 1V:3H PREFERREDSTONE SIZE LARGE WHEN BANK ANGLE APPROACHES REPOSE ANGLEREPOSE ANGLE VARIES WITH SLOPE HEIGHTSLIDING PROBLEMS ON FILTER FABRIC LIMIT TO 1V:2HGEOTECHINICAL STABILITY OFTEN DEFINES LIMITING SLOPE ?WHAT SIDE SLOPES ARE USED IN YOUR AREA?Note this is generally a geotech concern but side slope affect stability of riprap. To address exit quiz, tell class that a 1V:1.75H side slope tends to use the least volume of riprap, all other factors being equal. Flatter slopes use more volume because length of slope increase. Steeper slopes use more rock because rock size increases because of decreasing stability from trying to roll down the slope.
32 E.) FILTER REQUIREMENTS (PRIMARILY A GEOTECH RESPONSIBILITY) FILTER PURPOSES: PREVENT STREAM TURBULENCE FROM REMOVING BANK MATERIALPREVENT GROUNDWATER FROM MOVING BANK MATERIAL THROUGH RIPRAPSERVE AS FOUNDATION SUCCESSFUL REVETMENTS HAVE BEEN CONSTRUCTED WITHOUT A FILTER ? DOES YOUR OFFICE REQUIRE A FILTER?Emphasize this is a geotech concern. Ask question on screen.
33 Some riprap is widely graded such that some believe it has a built in filter. Quarry run riprap which has little processing at the quarry is an example. This riprap is the 10” thick riprap used on the upper bank of the Mississippi River over hundreds of miles. It has no separate filter.
34 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 32Note that many form of partial bank protection have been used successfully but its use depends on the above factors.
36 Another shot of partial bank protection that did not work well. ASK: What are possible reasons it did not work here?ANSWER: Alignment, Timing of completion of construction, height if riprap, severity of event
37 Another example of a distressed partial bank protection project.
38 I believe this is a derrick site with riprap toe protection and willow posts driven into the bank above riprap. Advantages are the posts provide roughness, slow velocity above riprap, promote deposition, and encourage vegetation.
39 G.) VEGETATION IN RIPRAP ADVANTAGES LESS MAINTENANCE ENVIRONMENTAL BENEFITSDISADVANTAGES DIFFICULT TO INSPECT INCREASED WATER LEVELS TURBULENCE INCREASE LARGE TREE REMOVAL? WHAT ARE DISTRICT VEGETATION PRACTICES?Read and ask screen question.
40 Shot of Snake River riprap with vegetation showing difficulty of inspection.
41 H.) TRANSPORT AND PLACEMENT TRANSPORT OFTEN MAJOR PART OF COSTTRUCK $ = 10 * BARGE $DUMPING AND SPREADING PROMOTES SIZE SEGREGATION AND BREAKAGERELEASE NEAR FINAL POSITION ? COMMENTS ON TRANSPORT AND PLACEMENTRead and ask screen question.
42 Streambank Protection OBJECTIVE 3: TOE PROTECTION
43 Read first line and say that if not true it is certainly near the top Read first line and say that if not true it is certainly near the top. This slide shows what happens to a channel cross section in a bend after we harden the bank. State if we place the riprap only down to the bottom w/o protection, we risk undermining when the section narrows and deepens after protection.
44 A second mechanism that is difficult to distinguish from the first process is the change of the bed throughout a flood event. Note that crossings, or the straight connections between bendways, do the opposite during floods.
45 TOE SCOUR DESIGN ESTIMATE MAXIMUM SCOUR PROTECT AGAINST MAXIMUM SCOUR Note toe scour protectioon is two problems.ASK: Which is most difficult?ANSWER: Estimating because so many factors affect scour.
46 Example of local failure of riprap at toe of slope
47 SCOUR DEPTH DEPENDS ON: CHANNEL PLANFORMCROSS-SECTIONVELOCITY, SHEAR STRESSWATER AND SEDIMENT HYDROGRAPHBED MATERIAL SIZE AND GRADATIONBANK ERODIBILITYCOMPLEX PROBLEM. THE FOLLOWING TECHNIQUES ARE AVAILABLE FOR SCOUR DEPTH ESTIMATIONRead
48 TOE SCOUR ESTIMATIONSEXPERIENCE AND “RULES OF THUMB” (MOST WIDELY USED METHOD)-MAXIMUM SCOUR WILL BE A CERTAIN DISTANCE BELOW THE DEEPEST POINT IN THE EXISTING CROSS-SECTIONThis is one method. The sentence “Maximum…” is an example of a rule of thumb used. For example, on a small stream we might use 2-3 ft below existing bed whereas on the mississippi river we might use ft below existing bed depending on location.
49 This is a second method when we do not have experience in a particular stream. These lines are based on data where we have observed the maximum scour as a function of R/W, depth upstream, and aspect ratio Width/Depth. Note that R/W is a common parameter to describe the sharpness of a bend. Sharp bends have low R/W. Note that this curve does not include total bend angle because we do not have data to distinguish different bend angles. The equation used to develop this curve is in the EM and is programmed into the PC program we will use later.
50 Note two methods. Note that finding that volume of rock is most important because it is difficult to configure a rock section underwater.
51 This is a facility we used to have at ERDC in which we tested various launchable riprap sections. Riprap Test Facility
54 Equation used to define rock volume per unit length of bank.
55 Uncertainty factor simply opinion, not based on testing.
56 Even though configuration is secondary, the best shape is to have a “H” as shown and “L” based on required volume. If H is too small coverage will be sparse as the section launches. If H is too large too much rock will launch as scour occurs. Note that 1V:2H launch slope in sandy material.
57 WIDELY USED ON SAND BED STREAMS SOME FAILURES IN GRAVEL-BED STREAMS WELL-GRADED, EVEN QUARRY-RUN IS USED INSTEAD OF UNIFORM GRADATION, D85/D15>2LAUNCHABLE STONE TECHNIQUES INCLUDE WEIGHTED TOE-TOE OF BANK TRENCH-FILL REVETMENT – MID BANK WINDOW REVETMENT – TOP OF BANKWIDELY USED ON SAND BED STREAMSSOME FAILURES IN GRAVEL-BED STREAMSMainly note we don’t use uniform riprap in launchable sections.
58 WINDROW REVETMENTSDefined: A line of stone placed along the top of an eroding bank, either on ground surface or partially buried.Read
59 WINDROW REVETMENTS Advantages: Ease of construction- Minimal disturbance and site prepStone manipulation minimizedExcess stone can be later salvagedVegetation will invadeCan be constructed from land or floating plantRead
60 WINDROW REVETMENTS Windrow requirements: Cleared, relatively flat upper bank areasNon-or weakly cohesive bank material in the protected zoneRead
61 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.Read
62 TRENCH-FILL REVETMENTS Advantages:Ease of construction- Eliminates most of the underwater bank grading and stone placement.Stone can be added to trench if depleted.Used on Mississippi River for large launch depths.Widely used on Arkansas and Red Rivers.Read
63 When we terminate riprap revetments, we often have scour at the downstream end that can be a maintenance problem but rarely results in a failure of the project unless completely ignored over a significant time. We ran tests to try an find a better way to terminate a revetment. Scour was reduced when we slope the termination point down over some distance along the channel. This likely helped because the eddy that tends to form below revetment was spread out over a greater length rather than concentrated in one area.
64 Streambank Protection OBJECTIVE 4: STONE SIZING Take a break while set up computer demo of CHANLPRO
65 Simply note the guidance to be presented is not applicable to hydraulic structures or steep slopes, both of which are covered in EM’s.
66 This is example of hydraulic structure which is old river control structure
68 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 AREANote that the years of experience your district has accumulated on specific project will likely not change what is used on that project. Also note, you don’t specify a custom gradation for each project unless you are proposing to use large quantities.
69 DESIGN CONDITIONSSINGLE CHANNELS –BANKFULL DISCHARGE OR HIGHER IS GENERALLY MOST SEVERE DESIGN FOR LOCATION HAVING MAXIMUM VELOCITY, NORMALLY USE SAME SIZE FOR ENTIRE REACH OR BENDDesign conditions differ in different types of channels. Meandering rivers generally have a single channel that is being protected. Also emphasize we rarely use a different size along a bend because the savings in cost in using smaller rock where allowed is offset by construction difficulties.
70 Example of meandering channel bend to be protected.
71 DESIGN CONDITIONSBRAIDED CHANNELS –INTERMEDIATE FLOW CAN BE MOST SEVERE BECAUSE DIVIDED FLOW TENDS TO “IMPINGE” ON LEVEE OR BANKLINES AT SHARP ANGLESAnother channel type is braided which has multiple channels.
72 Snake river is a braided channel in some reaches that has severe flow impingement. This is a low flow and impingement is not severe.
73 This is a higher flow on Snake and we measured surface velocities up to 14 ft/sec.
74 RIPRAP DESIGN HAS TWO PROBLEMS DETERMINE IMPOSED FORCE (VELOCITY)DETERMINE RESISTING FORCE (RIPRAP SIZE VERSUS VELOCITY)Just like scour, riprap design is two problems. We have determined the second bullet. The first is the most difficult because of diverse configurations.WHICH IS MORE DIFFCULT?
75 VELOCITY ESTIMATION NUMERICAL MODELS PHYSICAL MODELS ANALYTICAL MODELS EMPIRICAL METHODSPROTOTYPE DATAThese are techniques that can be used to estimate velocity to use in riprap design. For streambank protection it is rarely justified to use numerical or physical models to determine design velocity. Prototype velocity is rarely available at design conditions. Analytical methods are a possibility because of low effort required. Empirical methods are generally easiest to use and are presented herein. Note that empirical denotes based on observed data.
76 This is empirical curve for natural channels based on lots of observed data. Curve is drawn on the conservative side of the cloud of data. Note that like scour, we are using the parameter R/W. Vavg is the average channel velocity Q/A that is generally known. Vss is the velocity on the side slope, specifically 20% upslope from the toe. Vss is used in riprap design equations and is what we are after. This curve is in PC program CHANLPRO.
77 This is curve for trapezoidal channels that have been protected on bottom and sides and do not have sediment moving thru that causes a point bar to form and concentrate velocity along the outer bank. These curves are based on a numerical model and include the effects of bend angle and aspect ratio. These curves are programmed into the PC program CHANLPRO.
78 This slide is intended to help class understand flow in bends and is one of the quiz questions. Flow is from right to left. The channel cross section is trapezoidal. The upstream has a bend in the opposite direction so flow approaching this bend is predominantly on the left side (looking downstream). QUESTION: Where in this bend did we measure the highest velocity along the outside of the channel bend? ANSWER. Near the downstream end of the bend. Helical or secondary currents occur in bendways because the surface velocities are larger than the bottom velocities. It takes a while for these helical flows to move the highest velocity over to the outside bank of the bend. In a natural channel that can erode and the entire bend move down the valley, what does the direction of bend migration downvalley tell you about the location of maximum attack along the bend?
79 STONE SIZE WORKSHOP Problem No. 1 Subject: Natural channel bend with riprap on outer bank onlyGiven:Unit weight of stone = 165 #/ft3Riprap 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 gradationStop and let me stretch at their tables. Start chanlpro and run this problem.
80 Problem No. 1Required: Find computed d30, thickness for ETL gradation and d30 (min) for the following:Determine stable riprap gradation for outer bank of channel bendChange unit stone weight γs = 155 #/ft3With γs = 155 #/ft3, change average velocity to 6.1 ft/secWith γs = 155, v=6.1, change side slope to 1V:1.5HUse rerun option to modify input for b, c, and d to show class rerun option.
81 Problem No. 2 Subject: Riprap downstream of concrete channel Given: Unit weight of stone = 165 #/ft3Subcritical flow in concrete channel shown in FigureThickness = 1 D100 (max)Depth at end of concrete = 15 ftAverage velocity (Q/A) at end of concrete = 8 ft/secTop 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) = (see p. 3- 8, EM )Use ETL gradation (Table 3-1, EM )Input Cotan of side slope = 4 to specify bottom riprapSkip 2 slides to show channel configuration. The intent of this example is to show that determining the velocity often requires judgement and the standard bend curves for Vss can not be used. You don’t have to run the problem on chanlpro. Just describe process of determining velocity to use in chanlpro.
82 Problem No. 2Required: Determine stable riprap size downstream of concrete channel.Specify local velocity option (L) instead of average channel velocity.Procedure:Estimate local depth-averaged velocity at point A. Consider influence of eddy and flow concentration.Determine d30 and ETL gradation using CHANLPRO.Can you think of other things to do to improve the problem of the difference in boundary roughness?Estimate distance downstream for large riprap?Do a)Forget b.c) Roughen concrete, put in something to break up eddy.d) Return of lateral velocity distribution to normal shape generally scales on width of channel and requires about 5-10 channel widths depending on degree of lateral asymmetry.
84 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”