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Fluvial Geomorphologic Analysis CE154 Hydraulic Design Lecture 4

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Fluvial Geomorphologic Analysis CE154 Hydraulic Design Lecture 4 * Fall 2009 CE154 * * * * * * * * * * * * * * * * * * * * * As driving forces (flow and sediment ... – PowerPoint PPT presentation

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Title: Fluvial Geomorphologic Analysis CE154 Hydraulic Design Lecture 4


1
Fluvial Geomorphologic Analysis CE154
Hydraulic DesignLecture 4
2
Fluvial Geomorphology
  • Fluvial water
  • Ge earth, land
  • Morph - form
  • Ology knowledge
  • Fluvial Geomorphology the study of river forms
    and the processes that shape them

3
Why study geomorphology?
  • Natural rivers
  • Purposes- Flood protection- Navigation-
    Recreation- Habitat preservation
  • Yesterdays practice hardscape, straightening
    at our will, our way
  • Todays practice natural material, natures way

4
Objectives
  • Learn
  • - Fundamental geomorphic concepts - how a river
    functions
  • - Relevant geomorphic parameters how to
    simulate natures way
  • - Natural channel design

5
Important Fluvial Geomorphic Concept I
  • 1. Flow and Sediment combine to shape a channel

6
Sediment Transport Mechanisms
7
Calabazas Creek at Pruneridge 1999
8
Calabazas Creek at Pruneridge 2005
9
Flow and Sediment shape a channel
  • Optimum channel cross-section and slope
  • most efficient in transporting flow and sediment
    of the watershed

10
Important Fluvial Geomorphic Concept II
  • 2. Channel evolves toward an equilibrium
    condition- dynamic equilibrium process
  • Example
  • b. No lasting disturbance
  • c. Increased flow

11
Lanes Equilibrium Scale
12
Dynamic Equilibrium
  • Channel geometry and slope remain relatively
    constant with time, given flow and sediment
    fluctuations
  • A long-term stable condition

13
Important Fluvial Geomorphic Concept III
  • 3. Changes take time time scale

14
Comer Debris Dam
15
Time Scale
  • Geologic, Modern and Present time scales
  • Project focus Present time scale tens of
    years

16
Important Fluvial Geomorphic Concept IV
  • 4. Geomorphic responses are not continuous
    geomorphic threshold exists

17
Geomorphic thresholds
  • Shear stress threshold
  • Stability threshold - degradation
  • Planform threshold - channel avulsion creation
    of a new channel

18
Summary Geomorphic Concepts
  • Flow and sediment equally important
  • Dynamic equilibrium
  • Time scale
  • Thresholds

19
Geomorphic Parameters
  • Planform- meander- channel length- valley
    length- sinuosity

20
Geomorphic Parameters
  • Braided stream- steep slope- high sediment
    load- erodable banks
  • Meandering stream
  • Straight stream

21
Geomorphic Parameters
  • Riffle
  • Pool
  • Point bar

22
Point bar
23
Geomorphic Parameters
  • Low flow channel
  • Bankfull Channel- bankfull width- bankfull
    depth- floodplain- terrace
  • Large flow channel

24
Typical bankfull channel
25
Incised channel
26
Entrenchment Ratio
  • An index of channel incision

2Dbf
Dbf
27
Analysis of Geomorphic Properties
Property Physical Significance Application in Urban Streams
Bankfull Channel Optimal flow sediment Transport Basis of stable channel geometry
Equilibrium Slope Sediment load balance Basis of stable channel profile
Riffle/Pool/ Sinuosity Energy dissipation Measures to achieve energy balance
28
Analysis of Geomorphic Parameters
  • Approach to develop generalized empirical
    relationships (e.g., bankfull width vs. drainage
    area, bankfull dimension vs. flow, slope vs.
    drainage area) is not recommended for rivers in
    urbanized areas
  • Relationships should include Q, Qs, S, and ds
    plus effect of artificial hardpoints

29
Stable Channel Design Procedure
  1. Determine bankfull cross-sectional geometry
  2. Develop equilibrium longitudinal slope via.
    sediment transport modeling
  3. Identify hardpoints in project reach
  4. Layout cross-section and profile and, if
    necessary, locations of grade control structures
  5. Design surface protection, if necessary

30
1. Design Cross-Sectional Geometry
  • Determine low flow channel geometryLow flow
    base flow or min 7-day average flow
  • Determine bankfull channel geometryBankfull flow
    ? see next slide
  • Design floodplain within real estate constraints

31
Bankfull Concepts
  • Bankfull channel is hypothesized to be the
    cross-section shaped by nature to most
    efficiently transport flow and sediment loads
  • Bankfull depth is determined where the
    width/depth ratio is a minimum, or where
    vegetation changes to perennial trees

32
Bankfull Channel Indicators
  • Change in bank slope
  • Tops of Point bars formed on bends
  • Change in vegetation
  • - Change from shrubs/grass to woody trees
  • Change in particle size
  • Finer material on flood plain
  • 5. Analytically moving the most sediment
    Effective Flow (see Slide 39)

33
Shear Stress
?o
34
Shear stress Initiation of motion
?o/(?s-?w)ds
Vds/?
35
Shear Stress
  • V shear velocity
  • Critical sediment diameter subject to motion

36
Why bankfull channel?
37
Why bankfull channel?
38
Effective Discharge
  • To determine the most efficient cross-section in
    carrying flow and sediment- Effective flow
    calculation compute sediment frequency curve
    using flow data from gauge station and sediment
    rating curve for the reach

39
Effective Discharge
Effective discharge
Guadalupe River near Almaden Expressway
40
Effective Discharge verified using HEC-RAS
Cross section of Guadalupe River near Almaden Exp.
41
Results of effective discharge calculation for
Guadalupe River near Almaden Expressway
  • Observed bankfull flow 900 cfs
  • Calculated effective flow 800 - 1000 cfs
  • Corresponding recurrence interval 1.2 year

42
1. Design Cross-Section Geometry
  • Calculation shows Bankfull flow flow that
    carries most sediment effective flow
    channel-forming flow dominant flow
  • Preserve bankfull channel characteristics through
    modification
  • Superimpose large flow area allowed by right of
    way

43
2. Determine equilibrium slope
  • Lanes qualitative relationship Qsd50 ? QS
  • Quantitatively, use momentum and energy
    balance? Sediment in sediment out

44
2. Determine equilibrium slope
  • Determine sediment size distribution for project
    reach
  • Use SAM to calculate sediment transport capacity
    in project reach
  • Do the same for upstream and downstream reaches
  • Compare sediment capacity (15) and adjust slope
    as necessary
  • Extend bankfull flow to cover other return-period
    flows to estimate sediment yield and verify
    equilibrium (30)
  • Details described in Chapter 6 of Hydraulic
    Design Manual
  • Sediment transport workshop in February will
    cover application of SAM and HEC-6

45
Transport Capacity Calculation
Example of SAM sediment transport model results
for Calabasas Creek
46
Transport Modeling
measured elevation in 2004
HEC-6 sediment transport model results for
Calabasas Creek
47
3. Identify hardpoints in project reach
  • Hardpoints hard surface cover that prevents
    degradation and controls grade

48
4. Layout x-section profile
  • Determine need for grade control
  • Design grade control structures (chapter 8,
    hydraulic design manual)
  • Layout plan and profile (workshop in Apr 07)

49
5. Design surface protection
  • Although equilibrium slope provides sediment
    balance, there is continuous sediment deposition
    and erosion taking place
  • Surface armor concept armor will occur if
    Dcritical D90-95 and the D90-95 materials will
    cover bed surface (pavement)
  • Surface cover should be compatible with flow
    regime

50
Armoring Example
  • Bankfull channel Velocity 2.8 ft/secDepth
    2.5 ftSlope 0.003Mannings n 0.03Hydraulic
    Radius 1.875Bed Materials ?s 165 lb/ft3
    d50 6 mm d90 15 mm d95 25.4 mm
  • Will the bed armor?

51
Armoring Example
  • Shields ?c 0.047 (165-62.4) dc 4.8
    dc ?o 62.4?RS 0.351 lb/ft2 when ?c
    ?o, dc 0.351/ 4.8 0.073 ft 22 mm d90lt dc
    lt d95The bed will armor with 22 mm stones

52
Example
  • A rectangular channel with a bottom width of 20
    ft and a longitudinal slope of 0.8 is conveying
    2000 cfs during a storm. The channel is covered
    with sediment of a mean diameter of 5 mm.
    Determine if this sediment material will be moved
    under this flow condition.

53
Example Solution
  • Given the sediment diameter, we can use the
    Strickler formula to estimate the Mannings
    roughness coefficient
  • es 5 mm 0.0164 ft
  • n 0.031 (0.0164)1/6
  • n 0.0155 (this is low, accounting for grain
    roughness only, not including vegetation or form.
    Lets nevertheless use it in the Mannings
    equation to compute flow velocity in the channel)

54
Example solution
  • Q 1.486AR2/3S1/2/n
  • For rectangular channels, AYW, RA/PYW/(2YW)
  • Substituting W20, S0.008, n0.0155, and Q2000
    cfs into the equation to compute Y
  • Y 5.15 ft, A 103 ft2, R 3.4 V19.4 ft/sec
  • ? ?RS 62.43.40.008 1.7 lb/ft2?/(?s-?)ds
    1.7/((165-62.4)0.0164) 1.01
  • V 0.936 ft/sec, Vds/? 0.9360.0164/10-5
    1535
  • From the critical shear stress chart, it will be
    moved

55
In Conclusion
  • Understand meaning of important geomorphic
    parameters such as bankfull flow
  • Focus on applicable parameters and incorporate
    into design
  • Include flow and sediment transport processes
  • Determine long term stability
  • Adopt bioengineering methods sustainable
    (green) design
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