Channel Design River Engineering Stream Restoration Canals Monroe L. Weber-Shirk School of Civil and Environmental Engineering 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) 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 Particles 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 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 maintenance 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 flooding 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 usable 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 turbulence _________ of flowing water primarily fine material 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 - 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 Sediment Upward Transport upward transport is due to diffusion flux (Ficks first law) c J =Dt z The diffusion coefficient is a function of depth!
z Dt =ku* z 1 D z D D = Velocity * Distance u* o k = von Krmns universal constant k = 0.4 for clear fluids Dt
Suspended Sediment Concentration Profile at steady state we have: upward transport = downward transport Dt dc vc z where Dt =ku* z 1 - D dz boundary condition: c = ca @ z = a by convention: a = 0.05h v sedimentation velocity
ku* Result after integration c a ( D - z ) = ca z ( D - a ) Suspended Sediment Equilibrium Profile Why? 1 z 0.8 D
0.6 Depth/D 0.4 v 0.2 0 0 5 10 15
sediment concentration 20 Dt a Bed Load Dependent on sediment size distribution 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 specific discharge (q) fit the data when the exponent (n) is between 2 and 3 n Consequences: J s Bq 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 equilibrium 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 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 minimum 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 at the interface particle moves when resultant passes through (or above) point of support
V h force of drag will vary with time Grains: usually we mean incoherent sands, gravels, and silt, but also sometimes we include cohesive soils (clays) that form larger particles (aggregates) Fd Fg point of support Threshold of Movement Force on particle due to gravity Force on particle due to shear stress We expect movement when
o g o 2d 3 tan 2 tan gd 3 dimensionless parameter 4
Fg g r 3 3 Fshear or 2 or 2 tan 4 g r 3 3 4 Fg g r 3 3 o gRh S Force balance Fshear or 2
Shields Diagram (1936) inertial Re* _____________ Shear Reynolds = at the bed! viscous d = particle diameter Suspension t cr qcr = Dr gd t cr = Dg d Saltation
0.056 Threshold of movement No movement u* = gRh S f u*d Re* = n
Laminar flow of bed Turbulent flow of bed Shear Velocity Bottom shear u* = shear velocity = t o =r gRh S f to r From force balance
u* = gRh S f turbulent velocity Shear velocity is related to _________ Magnitude of Shear Velocity in a River Example: moderately sloped river Susquehanna at Binghamton S = 10-4 d =Rh= 1 m u* gRh S f u* ( 9.8 m/s ) ( 1 m) ( 1 10 ) =0.03 m/s 1 2/3 1/2 V = R h So
n 1 V = (1m) 2 / 3 (1 10 - 4 )1/ 2 =0.33m / s 0.03 2 -4 Manning Eq. (SI) units assume n of 0.03 Velocity fluctuations in rivers 0.1V are typically _____ Application of Shields Diagram Find minimum particle size that will be at rest Often bed is turbulent
t cr t cr =r gRh S f =0.056 Dr gd r Rh S f 1650 kg/m 3 quartz sediment d= 0.056 Dr d @11Rh S f Example (Susquehanna River at Binghamton) 1 m deep, S = 10-4 Therefore 1.1 mm diameter sand will be at rest. Result is armoring of river bed with large gravel as smaller sediment is flushed out. Application to Channel Stability
d 11Rh S Assumed uniform shear stress distribution river max d 20 Rh S to prevent erosion of bottom = max angle of repose 35 Channel Side Slope Stability Takes into account the shear stress, force of gravity and coefficient of friction
Critical shear stress on the side slope t cr , s =t cr ka Critical shear stress on the bed Side slope angle Tractive tan 2 a ka =cos a 1 Angle of repose tan 2 f force ratio Meandering (sinuous) canals scour more easily than straight canals (see Table 4.15 in Chin) Ch 12 in HEC-RAS Hydraulic Reference 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) Given any two can solve for the other two Uses shear stress and Manning equations *Require input sediment discharge
Implications d @11Rh S f How could you reduce erosion in a stream? Decrease slope Decrease depth (increase width or decrease flow) Increase particle size Are we managing causes or treating symptoms? Vertical Stabilizing Techniques Aggradation 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 Degradation flow modification grade control measures other approaches that dissipate the energy 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, stream barbs, and Iowa vanes Vegetative can 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.) 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 forms result from scour and deposition roughness
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 Fr V gy Bed Forms low velocity, fine sediment sand wave moves down stream wavelength less than 15 cm intermediate between ripples and dunes
Ripples, Fr << 1 weak boil Dunes with superposed ripples, Fr < 1 larger and more rounded than ripples boil Dunes, Fr < 1 Bed Forms (2) Dunes are eroded at Froude number close to 1 Note reduction in friction factor or Manning n!
Flat bed, Fr = 1 Standing waves in phase with water waves Standing waves, Fr > 1 Sand waves move upstream wavelength is 2V 2 incipient breaking and moving upstream g Antidunes, Fr >> 1 River Channels Alluvial soils river can form its own bed
river will meander in time and space steep slopes braided channel intermediate slopes riffle pool formation mild slopes meandering channel Meandering Channel L rc B L B
7 to 10 flow centerline scour rc 2 to 3 surprisingly small variation! B 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 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 Braided channel Arkansas Former Oxbow
Mississippi Consequences? River Training 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, stream
corridor conditions may be considered severely altered or impaired Common Impaired or Degraded Stream Corridor Conditions Stream aggradation filling (rise in bed elevation over time) Stream degradation incision (drop in bed elevation over time) Streambank erosion Impaired aquatic, riparian, and 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 Channels 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 specified 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 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
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