Swiss experience in monitoring sediment and wood fluxes

Transcription

1 Sedalp Final Conference Bolzano, 9 June 2015 Swiss experience in monitoring sediment and wood fluxes Dieter Rickenmann WSL - Swiss Federal Research Institute Mountain Hydrology and Mass Movements Birmensdorf, Switzerland with contributions by: Carlos Wyss, Jens Turowski, Bruno Fritschi, Peter Waldner, Tilo Usbeck, Flurin Sutter, Christian Rickli, Daniel Köchli (all WSL); Volker Weitbrecht, Robert Boes, Daniela Lange, Lukas Schmocker (all VAW) Content 1) Flood event in August 2005 in CH 2) Bedload monitoring with geophones 3) Large wood during the 2005 flood 4) Conclusions D. Rickenmann, WSL p. 1

2 Floods of August 2005 in Switzerland: Rainfall sum 20 to 22 August 2005 Floods of August 2005 in Switzerland: Debris flows, sediment transport and large woody debris high and long lasting flood flows in mountain rivers, many channel avulsions debris flows and intense sediment in torrent catchments insufficient discharge capacity and blockage of flow cross-sections large amounts of woody debris entrained considerable damage to infrastructure (e.g. traffic routes) and settlements D. Rickenmann, WSL p. 2

3 Floods of August 2005: documentation of channel processes Mountain rivers Torrents discharge capacity exceeded bedload transport lateral erosion discharge capacity exceeded and bedload ransport discharge capacity exceeded and lateral erosion bedload ransport and lateral erosion debris flows fluvial sediment transport 5000 to 25,000 m 3 25,000 to 100,000 m 3 > 100,000 m to 25,000 m 3 25,000 to 100,000 m 3 > 100,000 m Events: Observations on transported sediment volumes for - 34 debris flow events - 39 flood events with fluvial bedload transport (Rickenmann & Koschni 2010 HP) Floods 2005 and bedload transport formulae bedload volume / runoff volume Conventional bedload transport equations applied to steep slopes (S GF / V wt debris flow steeper than about 5% ) tend to over-predict sediment loads of fluvial bedload transport. 1 fluvial transport Rotlauibach Riera de les Arenas Haldibach Rickenmann, eq. (3) Glyssibach Acherlibach MPM-upper, eq. (7) 0.1 MPM-lower, eq. (7) Recking-upper, eq. (8) Recking-lower, eq. (8) ca. 5 % (Rickenmann & Koschni 2010 HP; Nitsche et al WRR) channel S slope [%] [%] Rickenmann & Koschni (2010) D. Rickenmann, WSL p. 3

4 Content 1) Flood event in August 2005 in CH 2) Bedload monitoring with geophones 3) Large wood during the 2005 flood 4) Conclusions The standard design of the WSL system Steel plate 36 cm x 50 cm acoustically isolated Geophone sensor fixed in the center records vibrations Old version of the aluminum housing for the geophone sensors Old piezoelectric bedload impact sensor New version of the aluminum housing which is water-tight New geophone sensor D. Rickenmann, WSL p. 4

5 Amplitude [v] Spannung [V] Probability of triggering Amplitude [v] Signal and recorded geophone summary values Impulses are recorded whenever the signal exceeds a predefined threshold value (on positive amplitude range), typically A min = 0.1 V At most measuring sites, we record the following geophone summary values: Impulses Integrals / Squared integrals Max. Amplitude t [s] Time [s] Raw signal is recorded only at a few sites during the calibration measurements. (Rickenmann et al ESPL) Time [s] Triggering of impulses Code mass b-axis g pebbles mm g pebbles mm rounded flat single grain Number of grains multiple grains Triggering of impulses and number of impulses depend on: grain size grain shape type of movement number of grains grain velocity limiting grain size is about 20 mm (b-axis) (Böckli 2011; Rickenmann et al. 2012, 2014) D. Rickenmann, WSL p. 5

6 The Erlenbach bedload observatory small catchment (0.7 km 2 ) long-term sediment transport observations (>25 years) Bedload transport rates are measured in various ways step-pool morphology geophone sensors Swiss plate geophone measurements at various sites Fischbach, Austria (glacial melt) Rofenache, Austria (glacial melt) Rofenache, Austria (glacial melt) Fischbach, Austria (glacial melt) Ruetz, Austria (glacial melt) D. Rickenmann, WSL p. 6

7 Impulses Swiss plate geophone (calibration) measurements at various sites Stream Location Drainage area Operation period Calibration [km 2 ] (sensor type) Erlenbach (basin) Alptal, Schwyz, CH (PBIS), (GS) yes Erlenbach (bridge) Alptal, Schwyz, CH (PBIS), (GS) Vogelbach Alptal, Schwyz, CH (GS) Pitzbach Pitztal, Tyrol, AT (PBIS) yes Spissibach Leissigen, Berne, CH (PBIS) yes Rofenache Vent, Tyrol, AT (GS) yes Drau Lienz, Tyrol, AT (GS) Drau Dellach, Carynthia, AT (GS) yes Isel Lienz, Tyrol, AT (GS) Schweibbach Eisten, Valais, CH (GS) Fischbach Mühlau, Tyrol, AT (GS) yes Ruetz Mutterbergalm, Tyrol, AT (GS) yes Riedbach Grächen, Valais, CH (GS) yes Nahal Eshtemoa Negev Desert, Israel (GS) yes Navisence Zinal, Valais, CH (GS) yes Ötztaler Aache Sölden, AT (GS) yes Urslau Maria Alm, Salzburg, AT (GS) yes Elwha River Washington, USA (GS) yes Suggadinbach St. Gallenkirch, AT (yes) Solda River Valle Venosta, I 130 (?) 2014 (?) (yes) Albula River Tiefencastel, CH (yes) GS = geophone sensor, PBIS = piezoelectric bedload impact sensor (Rickenmann et al. 2014) Calibration of the geophon system at the Erlenbach 10'000'000 1'000' '000 10'000 Erlenbach basket sampler 1' D 3 D 2 D '000 10' '000 1'000'000 10'000'000 Bedload mass (kg) Erlenbach retention basin Linear calibration from basket samplers D 1 : many particle sizes in criticial detection range D 2 : impulses not well constrained D 3 : short sampling time Linear calibration between impulses and bedload mass works well at Erlenbach (Rickenmann et al ESPL) D. Rickenmann, WSL p. 7

8 Number of impulses Transported mass K b = linear calibration coefficient k b [Impulses/kg] number of Impulses, Sum IMP Comparison of geophone calibrations at different sites Erlenbach Fischbach Ruetz Rofenache Eshtemoa Fischbach: y = 22.9x r² = 0.97 Ruetz: y = 16.3x r² = 0.85 Rofenache: y = 3.87x r² = 0.78 Eshtemoa y = 0.419x r² = 0.99 Erlenbach: y = 5.45 x r² = bedload mass, M [kg] Calibration relations differ for various streams; Larger scatter at some sites Local hydraulics and particle (transport) characteristics are probably important (Rickenmann et al. 2014) Comparison of geophone calibrations at different sites [ 1 [ kg Erlenbach (D > 20mm), Vw with slope correction Fischbach (D > 20mm) Ruetz (D > 20mm) Rofenache (D > 20mm) Lab, quartz sphere (40mm) Lab, particles 45.3mm < D < 63mm Lab, particles 63mm < D < 80mm Lab, particles 80mm < D < 112mm Mean water flow velocity V 10 w [m/s] Flow or particle velocity likely explains part of differences in mean calibration coefficient (Rickenmann et al. 2014) D. Rickenmann, WSL p. 8

9 Dmax, b-axis [mm] 1000 Comparison of geophone calibrations at different sites Erlenbach Fischbach Ruetz Ru: y = 100 x 0.41 R² = 0.52 Fb: y = 94.5 x 0.44 R² = Eb: y = 99.1 x 0.38 R² = MaxMaxA [V] Maximum grain size correlates with signal strength, almost independent of site (Rickenmann et al. 2014) Grain size estimation? Hypothesis: Amplitude Histogram method displacement of the plate proportional to the size of the transported particle every particles collides only once against the steel plate amplitude thresholds can discriminate between grain-size classes (Wyss et al. 2015, in revision) D. Rickenmann, WSL p. 9

10 plate s displacement noise pack 1 pack 2 pack 3 Grain size estimation: Amplitude Histogram method th 5 th 4 th 3 th 2 th 1 th 0 time Analysis of moving basket bedload samples at Erlenbach (Wyss et al. 2015, in revision) Grain size estimation at the Erlenbach PbAH = Pack based Amplitude Histogram method r = ratio of calculated to observed bedload mass D m,g = mean diameter of grain-size class IbAH = Impulse based Amplitude Histogram method (Wyss et al. 2015, in revision) D. Rickenmann, WSL p. 10

11 Erlenbach: Comparison of calibration approaches C IP Total impulses and total mass IbAH, PbAH Amplitude Histogram method (Wyss et al. 2015, in revision) Content 1) Flood event in August 2005 in CH 2) Bedload monitoring with geophones 3) Large wood during the 2005 flood 4) Conclusions D. Rickenmann, WSL p. 11

12 Clogging by large wood at many locations Grosse Melchaa (Obwalden, CH) Clogging by large wood at many locations Reuss (weir at Perlen, canton Luzern, CH) Lake of Thun (canton Berne, CH) D. Rickenmann, WSL p. 12

13 Analysis of large wood (LW) of 2005 flood events Sources of woody debris Case studies to construct LW budgets Composition of transported woody debris Recommendations for future stream management Channel Debris flow Landslide Lateral erosion Clogging Wood deposition Geschiebe Driftwood torrent mountain river lake river Study catchments for LW analysis Analysis of size in LWD depositions: Visual Transect by number Study areas Reconstruction: Sources of WD Budget of WD fresh disturbed D. Rickenmann, WSL p. 13

14 Volume fraction Large wood characteristics Determination of characteristics of "deposited" woody debris: Size of peaces (diameter, length) tree type degree of weathering identifaction of fresh, greenish bark Characteristics of deposited LW: length of pieces Length [m] Degree of weathering Deposition type 4 (strong) all 3 (medium) disturbed 2 (little) undisturbed 1 (almost none) D. Rickenmann, WSL p. 14

15 Volume fraction Sedalp Final Conference Bolzano, 9 June 2015 Characteristics of deposited LW: diameter of pieces Diameter [cm] Degree of weathering 4 (strong) 3 (medium) 2 (little) 1 (almost none) Processes of LW entrainment Kleine Emme Gerinne Totholz Seitenerosion Totholz Seitenerosion Rutschung Totholz Rutschung Murgang Totholz Murgang Bauholz Lanquart Gerinne Totholz Seitenerosion Totholz Seitenerosion Rutschung Totholz Rutschung Murgang Totholz Murgang Bauholz Grosse Melchaa Kander Gerinne Totholz Seitenerosion Totholz Seitenerosion Rutschung Totholz Rutschung Murgang Totholz Murgang Bauholz Gerinne Totholz Seitenerosion Totholz Seitenerosion Rutschung Totholz Rutschung Murgang Totholz Murgang Bauholz fresh not fresh debris flow landslide bank erosion channel deposition D. Rickenmann, WSL p. 15

16 Wildbäche torrent Mittellauf upper part Unterlauf lower part Wildbäche torrent Mittellauf upper part Unterlauf lower part Wildbäche torrent Mittellauf upper part Unterlauf lower part Wildbäche torrent Mittellauf upper part Unterlauf lower part Processes of LW entrainment 100% 80% 60% 40% 20% 0% Kander Kleine Emme Grosse Melchaa Gerinne Totholz Seitenerosion Totholz Seitenerosion debris flow Rutschung Totholz Rutschung landslide Unklar Totholz Murgang bank Totholz erosion Unklar channel Murgang Bauholz fresh not fresh Lanquart (Klosters) construction wood Example of LW budget: Kander debris flow landslide bank erosion deposition D. Rickenmann, WSL p. 16

17 Example of LW budget: Melchaa debris flow landslide bank erosion deposition Summary of LW budgets Summary of estimation of LW input or entrainment and LW deposition or extraction. (*) The numbers indicate total volumes of large wood without pore spaces [m 3 ] Catchment Input [m 3 ] (*) Deposition [m 3 ] (*) best estimate minimum/maximum best estimate minimum/maximum D. Rickenmann, WSL p. 17

18 Content 1) Flood event in August 2005 in CH 2) Bedload monitoring with geophones 3) Large wood during the 2005 flood 4) Conclusions Conclusions: Large wood study LW entrainment and deposition Floods 2005: long duration, high discharge and landslides enhanced problems with LW Main sources: debris-flow like processes, large landslides, lateral bank erosion Clogging at enginnering structures: main cause for large damage Transport and deposition of LW did not cause damage everywhere Protection measures and emergency intervention Forest management can reduce (but not avoid) entrainment Rapid emergency intervention can reduce clogging risk (requires good communication and emergency management) To prevent clogging at engineering structures: free board, horizontal clearing, smooth upstream cover; LW dimensions relative to clearing Retention structures: if possible separate bedload and LW deposition D. Rickenmann, WSL p. 18

19 Conclusions: Swiss plate geophone system Advantages of the system relatively cheap low maintenance continuous measurements at high temporal resolution robust, even for high-energy environments reasonable accuracy for total bedload mass estimates grain-size estimation of bedload for Erlenbach Disadvantages of the system needs a fixed (stable) cross section (e.g. check dam) needs independent calibration lower limit of particle size (~2 cm) the effects of hydraulic conditions, particle shapes, g.s.d. and type of movement are all important Thank you for your attention D. Rickenmann, WSL p. 19