Channel Design.ppt

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1、Channel Design,River Engineering Stream Restoration Canals,References,Chapter 12 Stable Channel Design Functions in the HEC-RAS Hydraulic Reference FISRWG (10/1998). Stream Corridor Restoration: Principles, Processes, and Practices. By the Federal Interagency Stream Restoration Working Group (FISRWG

2、) Chapter 4 in Water Resources Engineering by David Chin (2000),Outline,Sediment transport Effects Suspended and Bed load Stable unlined channel design Tractive Force method Bed forms Channel forms River Training Stream Restoration Principles,Problems of Sediment Transport,Impingement of Sediment Pa

3、rticles damage to bridge abutments by boulders huge boulders (up to several tons) can be set in motion by torrential flood flows in mountain streams sand-sized particles damage turbines and pumps Sediment in Suspension fish dont like muddy water municipal water treatment costs are related to amount

4、of sediment in the water,Problems of Sediment Deposition,Flood Plain Deposits may bury crops deposition of infertile material (like sand) may reduce fertility Urban areas may receive deposition on streets, railroads, and in buildings,irrigation ditches reduce carrying capacity require extensive main

5、tenance drainage ditches raise the water table fine sediments are usually fertile - increase vegetation growth - increase Manning n,Problems of Sediment Deposition,channels, waterways, and harbors requires extensive dredging to maintain navigation decrease carrying capacity and thus increase floodin

6、g lakes and reservoirs in lakes with no outlets all of the incoming sediment is deposited converts beaches to mud flats fine sediment can encourage prolific plan growth storage capacity is lost by 1973 10% of reservoirs built prior to 1935 in the Great Plain states and the Southeast had lost all usa

7、ble storage!,Sediment Load,Mass of sediment carried per unit time by a channel Sediment load is carried by two mechanisms Bed load: grains roll along the bed with occasional jumps primarily course material Suspended load: material maintained in suspension by the _ of flowing water primarily fine mat

8、erial,turbulence,Suspended Load,Sediment suspended by fluid turbulence Concentration can be substantial in cases of high flows and fine sediment (up to 60% by weight!) Vertical distribution higher concentration near bottom coarse fractions - concentration decreases rapidly above bed fine fractions -

9、 concentration may be nearly uniform no theory for concentration at the interface with the bed given sediment concentration at one elevation above the bed it is possible to derive sediment concentration as a function of depth (compare local fall velocity with local turbulent transport),Suspended Sed

10、iment Upward Transport,upward transport is due to diffusion flux (Ficks first law),The diffusion coefficient is a function of depth!,D,Dt,z,k = von Krmns universal constant k = 0.4 for clear fluids,D = Velocity * Distance,Suspended Sediment Concentration Profile,at steady state we have: upward trans

11、port = downward transport,Result after integration,boundary condition: c = ca z = a by convention: a = 0.05h,where,sedimentation velocity,Suspended Sediment Equilibrium Profile,0,5,10,15,20,0,0.2,0.4,0.6,0.8,1,sediment concentration,Depth/D,D,z,a,v,Dt,Why?,Bed Load,Dependent on sediment size distrib

12、ution bed shape (ripples, dunes, etc.) sediment density shear stress at the bed Bed Load Equations many researchers have proposed equations each equation only applies to the data that was used to obtain the equation!,Total Sediment Carrying Capacity,Power law relations between sediment flux (Js) and

13、 specific discharge (q) fit the data when the exponent (n) is between 2 and 3 Consequences: as q decreases Js decreases abstraction of flow from a river for irrigation, water supply or flood relief sediment carrying capacity decreases river channel tends to clog with sediment to reach new equilibriu

14、m greatest transport of sediment occurs during floods rivers below reservoirs tend to erode,Sediment Rating Curve:,10Q yields 100Js,Causes of Stream Erosion,What can increase the rate of erosion? Increased stream flow Increased runoff Decreased flood plain storage Decrease in sediment from upstream,

15、Channel Design: Identify the Parameters,Channel Geometry Channel Slope Cross section Roughness Meander Soil Grain size Cohesive/uncohesive,Lining type Lined Unlined Grass Design Flow Bank full Or based on a recurrence interval,Stable Unlined Channel Design,Threshold of movement Will determine minimu

16、m size of sediment that will be at rest Can be used as basis for stable bed design Based on Shields diagram Modified to include the effect of side slope,Basic Mechanism of Bed Load Sediment Transport,drag force exerted by fluid flow on individual grains retarding force exerted by the bed on grains a

17、t the interface particle moves when resultant passes through (or above) point of support,Grains: usually we mean incoherent sands, gravels, and silt, but also sometimes we include cohesive soils (clays) that form larger particles (aggregates),Fd,h,force of drag will vary with time,V,Fg,point of supp

18、ort,Threshold of Movement,Force on particle due to gravity,Force on particle due to shear stress,We expect movement when,dimensionless parameter,Force balance,Shields Diagram (1936),0.01,0.1,1,1,10,100,1000,Threshold of movement,Turbulent flow of bed,Laminar flow of bed,Suspension,Saltation,No movem

19、ent,0.056,Re* _ =,Shear Reynolds,inertial,viscous,at the bed!,d = particle diameter,Shear Velocity,Bottom shear,u* = shear velocity =,From force balance,Shear velocity is related to _ velocity,turbulent,Manning Eq. (SI) units,assume n of 0.03,Velocity fluctuations in rivers are typically _,Magnitude

20、 of Shear Velocity in a River,Example: moderately sloped river Susquehanna at Binghamton S = 10-4 d =Rh= 1 m,0.1V,Application of Shields Diagram,Often bed is turbulent,Find minimum particle size that will be at rest,Example (Susquehanna River at Binghamton) 1 m deep, S = 10-4 Therefore 1.1 mm diamet

21、er sand will be at rest.,Result is “armoring” of river bed with large gravel as smaller sediment is flushed out.,quartz sediment,Application to Channel Stability,Assumed uniform shear stress distribution, = max angle of repose 35,max,river,to prevent erosion of bottom,Channel Side Slope Stability,Ta

22、kes into account the shear stress, force of gravity and coefficient of frictionMeandering (sinuous) canals scour more easily than straight canals (see Table 4.15 in Chin),Ch 12 in HEC-RAS Hydraulic Reference,Critical shear stress on the side slope,Critical shear stress on the bed,Side slope angle,An

23、gle of repose,Tractive force ratio,HEC-RAS Hydraulic Design: Stable Channel Design,Copeland* Regime* Tractive Force Doesnt account for input sediment Utilizes critical shear stress to determine when bed motion begins Particle size (d) Depth (D) Bottom Width (B) Slope (S) Uses shear stress and Mannin

24、g equations,*Require input sediment discharge,Given any two can solve for the other two,Implications,How could you reduce erosion in a stream? Are we managing causes or treating symptoms?,Decrease slope,Decrease depth (increase width or decrease flow),Increase particle size,Vertical Stabilizing Tech

25、niques,stabilizing eroding channels upstream controlling erosion on the watershed installing sediment traps, ponds, or debris basins narrowing the channel, although a narrower channel might require more bank stabilization,flow modification grade control measures other approaches that dissipate the e

26、nergy,Aggradation,Degradation,meanders,boulders,Bank Stabilizing Techniques,Indirect methods extend into the stream channel and redirect the flow so that hydraulic forces at the channel boundary are reduced to a nonerosive level dikes (permeable and impermeable) flow deflectors such as bendway weirs

27、, stream “barbs,” and Iowa vanes,Surface armor Armor is a protective material in direct contact with the streambank Stone and other self-adjusting armor (sacks, blocks, rubble, etc.) Rigid armor (concrete, soil cement, grouted riprap, etc.) Flexible mattress (gabions, concrete blocks, etc.),Vegetati

28、ve can function as either armor or indirect protection and in some applications can function as both simultaneously.,Bed Formation,Variety of bed forms are possible may be 3 dimensional may vary greatly across a river or in the direction of flow Bed forms depend on Froude number and affect _ Bed for

29、ms result from scour and deposition deposition occurs over the crests and scour occurs in the trough Bed forms are the consequence of instability a small disturbance on an initially flat bed can result in formation of crests and troughs,roughness,Bed Forms,Ripples, Fr 1,Dunes with superposed ripples

30、, Fr 1,Dunes, Fr 1,boil,weak boil,larger and more rounded than ripples,intermediate between ripples and dunes,low velocity, fine sediment sand wave moves down stream wavelength less than 15 cm,Bed Forms (2),Flat bed, Fr = 1,Standing waves, Fr 1,Antidunes, Fr 1,incipient breaking and moving upstream,

31、Standing waves in phase with water waves,Sand waves move upstream wavelength is,Dunes are eroded at Froude number close to 1 Note reduction in friction factor or Manning n!,River Channels,Alluvial soils river can form its own bed river will meander in time and space steep slopes braided channel inte

32、rmediate slopes riffle pool formation mild slopes meandering channel,Meandering Channel,L,B,flow centerline,rc,scour,surprisingly small variation!,Bed Forms in Meandering Channels,Channel is deepest on the outside of the curves,River Training,Prevent shifting of river bed! navigation want the docks

33、to be on the river! flood control want river to be between the levees! bridges want bridges to cross the river! Canalize - straighten out meanders cutoff meander - increases slope increases erosion deposition further downstream,Changes to Mississippi River,Arkansas,Mississippi,Consequences?,River Tr

34、aining,Current practice - “Stabilize” in natural form bank protection rip-rap (armoring) Groins (indirect),Stream Corridor Condition Continuum,At one end of this continuum, conditions may be categorized as being natural, pristine, or unimpaired by human activities At the other end of the continuum,

35、stream corridor conditions may be considered severely altered or impaired,Common Impaired or Degraded Stream Corridor Conditions,Stream aggradationfilling (rise in bed elevation over time) Stream degradationincision (drop in bed elevation over time) Streambank erosion Impaired aquatic, riparian, and

36、 terrestrial habitat,Increased peak flood elevation Increased bank failure Lower water table levels Increase of fine sediment in the corridor Decrease of species diversity Impaired water quality Altered hydrology,Stream Corridor Restoration: Principles, Processes, Practices p 227,Design of Open Chan

37、nels,The objective is to determine channel shape that will carry the design flow Reasonable cost Limit erosion Limit deposition Efficient Hydraulic Section Freeboard to prevent overtopping Return to “natural state”,Most Efficient Hydraulic Sections,A section that gives maximum discharge for a specif

38、ied flow area Minimum perimeter per area No frictional losses on the free surface Analogy to pipe flow Best hydraulic shapes best best with 2 sides best with 3 sides,Why isnt the most efficient hydraulic section the best design?,Minimum area = least excavation only if top of channel is at grade,Cost

39、 of liner,Complexity of form work,Erosion constraint - stability of side walls,Freeboard is also required,Freeboard and Superelevation,Freeboard: vertical distance between the water surface at the design flow and the top of channel Rational design could be based on wave height, risk of flows greater than design flow, and potential damage from overtopping Empirical design 0.5 m to 0.9 m Superelevation at bends T is top width rc is radius of curvature of the centerline Valid for rc 3T,