The Importance of Riparian Vegetation in Channel Restoration: Moving Towards Quantification in Design

Transcription

1 The Importance of Riparian Vegetation in Channel Restoration: Moving Towards Quantification in Design Rob Millar Department of Civil Engineering The University of British Columbia "Nothing is as practical as a good theory Kurt Lewin 1

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6 Channel Metamorphosis: Slesse Creek MEANDERING BRAIDING Flow 200 m Role of vegetation: Two Issues Bank Strength: How to quantify or parameterize vegetation effects (e.g. c, φ, τ crit ) 6

7 Role of vegetation: Two Issues How bank strength influences hydraulic geometry (W, D, S) φ =40 7

8 Bank Stability photos φ =70 Relative Bank Strength (Dimensionless) = µ τ crit (bank) τ crit (bed) 8

9 Relative Bank Strength (Dimensionless) µ = τ τ crit (bank) crit (bed) = 40 tan φ tan µ =1 9

10 Bank Stability photos µ =3.3 Other Dimensionless Variables Q* = Q g d s ( ) W* = W / d 50 D* = D / d 50 C* = -log C 10

11 Empirical Regime Equations (Data from Hey and Thorne, 1986) Channel Width (m) Bankfull Discharge (m3/s) W = 3.0 Q 0.5 Empirical Regime Equations W = aq 0.5 D = bq S = cq Coefficients a, b and c are functions of sediment size, sediment load, bank strength 11

12 Lane s Balance Experimental Basis (Gilbert, 1914) Slope Width 12

13 Gilbert (1914) Gilbert (1914) 13

14 Experimental Basis (Gilbert, 1914) Slope P Width bed Numerical Solution Curve Slope P Width bed Regime or Equilibrium Solution Corresponds to the Optimum or Minimum Slope 14

15 1980 s Minimum Stream Power Extremal Hypothesis Minimum Unit Stream Power An Illusion of Progress?? Numerical Solution Curve Application to Natural Rivers Slope? Width 15

16 Numerical Solution Curve Application to Natural Rivers Slope P Width bed Numerical Solution Curve Application to Natural Rivers Slope Bed Width 16

17 Numerical Solution Curves Range of Relative Bank Strengths Slope Width (m) Bed Width Rational Regime Solution Curves Range of Relative Bank Strengths Slope Width (m) 17

18 Range of Optimum Solutions W/D µ Theoretical Regime Equations (Dimensionless) W* = 28.1Q * C * µ D * * * µ = Q C S = 1.98Q * C * µ

19 Comparison with Empirical Equations: Consistent with the Empirically Derived W D S Theoretical Q 0.50 d Q 0.37 d Q d Hey and Thorne (1986) Q 0.50 Q 0.37 d Q d Andrews (1984) Q 0.48 d Q 0.37 d Q d Theoretical Regime Equations (Dimensionless) W* = µ 16.5Q * S D * = 0.125Q S µ W / D = µ 155Q * S 19

20 Verification (Assume µ = 1.0) (A) (C) Sparse Vegetation Dense Vegetation Perfect Agreement 100 Theoretical W* Theoretical W/D Observed W* SparseVegetation Dense Vegetation Perfect Agreement Observed W/D Verification (Assume µ = 1.0) (A) (C) Sparse Vegetation Dense Vegetation Perfect Agreement 100 Theoretical W* Theoretical W/D Observed W* SparseVegetation Dense Vegetation Perfect Agreement Observed W/D 20

21 Verification (Calibrated µ ) (A) (C) Sparse Vegetation Dense Vegetation Perfect Agreement 100 Sparse Vegetation Dense Vegetation Perfect Agreement Theoretical W* Theoretical W/D Observed W* Observed W/D Variation of µ with Vegetation Class Data source Bank vegetation Class I II III IV Thin Thick Hey and Thorne (1986) Andrews (1984)

22 Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry W * = µ 16.5 Q * S D * Q S µ = W / D = 155 Q * S µ 22

23 Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) 23

24 Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) 24

25 Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) Grain Size (d 50 ) Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters 100 Discharge (Q) % Finer Grain Size (d 50 ) 50 0 Grain Diameter d 50 25

26 Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) Grain Size (d 50 ) Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) Grain Size (d 50 ) Slope (S) 26

27 Slope Valley Slope (S v ) is Imposed Relationship between Average S and Sinuosity (S v /S) Longitudinal Slope Variation Sinuosity (S v /S) Increasing Sinuosity Decreasing Average S 27

28 Long Profile Long Profile 28

29 Long Profile Long Profile Equilibrium Slope 29

30 Long Profile Equilibrium Slope Higher Sediment Load Steeper Equilibrium Slope Wider Riffle Spacing Long Profile Reach Average Slope, S Local Equilibrium Slope 30

31 Design Channel Slope, S Constrained by Valley Slope (S S v ) Can reduce average S by increasing sinuosity Use Reach-Average S with Regime Equations Use riffles to control sediment transporting capacity and local slope Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) Grain Size (d 50 ) Slope (S) 31

32 Where are we now Set of Design Equations to Determine the Reach-Averaged Hydraulic Geometry Design Parameters Discharge (Q) Grain Size (d 50 ) Slope (S) Bank Strength (µ ) Bank Strength Models (include vegetation effects) φ model (Millar and Quick, 1993) Gravel Banks Unable to measure directly Estimate or obtain by calibration (like Manning s n) Scale effects 32

33 Bank Strength Models (include vegetation effects) τ crit model Cohesive silt/sand banks Can (potentially) be measured directly insitu (e.g. Hanson and Simon, 2001) Difficult to measure and account for vegetation Bank Strength Models (include vegetation effects) Eaton (2006) model Composite bank of gravel and overlying silt Easily measure H max in the field Initial results appear very promising Yet to be field tested H max Gravel 33

34 Bank Strength Models (include vegetation effects) Mass Failure models (c, φ) Cohesive silt/sand banks Apply slope-stability models Can measure soil properties directly insitu or in laboratory Difficult to measure vegetation components Secondary failure mechanism Conclusion "In theory, there is no difference between theory and practice. 34

35 Conclusion "In theory, there is no difference between theory and practice. But, in practice, there is." Yogi Berra 35