Debris flows + sediment transport in torrents. Debris flows and sediment. transport in steep catchments

Transcription

1 International Workshop "Erosion, Transport Debris flows and sediment transport in steep catchments Dieter Rickenmann WSL - Swiss Federal Research Institute, Mountain Hydrology and Torrents, Birmensdorf, Switzerland Debris flows + sediment transport in torrents p. 1

2 Sediment transport processes in torrent channels Sediment transport Debris flow Structure of content 1) Introduction 2) Debris flows: hazard and sediment delivery 3) Bedload transport in torrents and mountain rivers 4) Example of floods 2005, Swiss Alps 5) Conclusions p. 2

3 Fast mass movements: variability of material composition rock avalanche Debris flow initiation by landsliding p. 3

4 Erosion by debris flows: events of 18.7./ , Val Varuna,, GR/CH (1520 m a.s.l.) (a) before 1987 (b) after first event (c) after second event Example application: Val Varuna (canton Grisons, CH), Catchment area: 6.5 km 2 Altitude: m a.s.l. Mean slope angle (torrent): 40% Mean slope angle (fan): 15% Event volume: m 3 Peak discharge: 600 m 3 /s p. 4

5 Flooding due to aggradation (Poschiavino/Val Varuna,, CH, ) debris flow Debris flow experiments: Zailysky Alatau mountains (Kazakhstan) Chemolgan test site p. 5

6 Transition sediment transport - debris flows Takahashi (1991), equilibrium sediment concentration in debris flows (theory + experiments): C s / C* = (s 1) tan θ / [(tan φ tan θ) / C*] (Rickenmann 1990), bedload transport experiments: Q b = 6.8 Q m S 2.1 ==> C s / C* = 6.8 S 2.1 / (6.8 S ) Tognacca (1999), laboratory experiments on debris flows + bedload transport experiments of Smart/Jäggi: C s / C* = [tanh(9.0 S e ) / 2.3] C s : volume sediment concentration of the flow C* : maximum packing density of the bed material S e S: energy slope or bedslope (tanθ sinθ) Transition sediment transport - debris flows Comparison of equations for sediment concentration estimates, with data from Chemolganexperiments Rickenmann (2005) C S / C* Rickenmann (1990) Takahashi (1991) Tognacca (1999) S = tan θ p. 6

7 Headwater catchments: interaction of runoff processes, shallow landslides, sediment transport and debris flows Location: channel bed, small to medium gradients (<= %) channel bed, medium to high gradients (>= %) sideslopes Trigger: critical discharge (min. shear stress) critical discharge or runoff blockage (of water and/ or sediment) soil saturation (reduction of shear strength) Process: sediment transport debris flows landslide Structure of content 1) Introduction 2) Debris flows: hazard and sediment delivery 3) Bedload transport in torrents and mountain rivers 4) Example of floods 2005, Swiss Alps 5) Conclusions p. 7

8 Sediment transport: equation of Meyer-Peter & Müller (1948) For gravel bed rivers, based on laboratory experiments with channel slopes S in the range: S exponent 2 is more likely for steep slopes φb = 8 ( m' θ θc) 1.5 later corrections (e.g. Wong & Parker, 2006) strong reduction for pronounced bed forms in torrent channels φ b = dimensionless sediment transport rate θ = dimensionless bed shear stress θ c = critical dimensionless bed shear stress at initiation of transport m' = losses due to form resistance q φ b b = ( s 1) g d 3 m hs θ = ( s 1) d m Sediment transport in steep channels: equations Basis: laboratory experiments at ETH Zürich of Meyer-Peter & Müller (1948), Smart & Jäggi (1983) and Rickenmann (1990). Here: simplified equations are shown only, for s = 2.68 and (d 90 /d 30 ) 0.2 = Steep channel slopes, 0.03 S 0.20 q b = 5.8 S 2 (q - q c ) (1) All channel slopes, S 0.20 φ b = 2.5 θ 0.5 (θ - θ c ) Fr θ 0.5 (θ - θ c ) Fr (2) q b = 1.5 S 1.5 (q - q c ) (3) Remark: (3) is exactly equivalent with (2) only for small sediment concentrations and approximative similarity between q c and θ c. q b = transport rate (per m ); q = discharge (per m ); q c = critical q at initiation of transport; S = channel slope; φ b = dimensionless transport rate; θ = dimensionless bed shear stress; Fr = Froude number p. 8

9 Comparison of sediment transport in torrents and gravel bed streams Rickenmann (2001) No. Stream Slope, S EG [km2] Qc [m3/s] Qp [m3/s] Observation period Measuring method 1 Erlenbach SRB, Sensors 2 Melera SRB 3 Rio Cordon Metall grill, ST 4 Rappengraben ST 5 Sperbelgraben ST 6 Pitzbach /1995 SB 7 Bas Arolla SB 8 Bridge Creek BLS 9 Rappengraben ST 10 Torlesse Stream , 1980 VTS 11 Schwändlibach , SRB 12 Rotenbach , SRB 13 Ashiraidani CVC 14 Jordan (Kinneret) , 1974, 1975 Dep 15 Sagehen Creek BLS(HS) 16 Oak Creek VTS 17 Turkey Brook (7) BLP 18 Virginio Stream , 1988 VTS 19 Aare (Brienz) BLS 20 Inn (Tirol) BLS 21 Rhein (Brugg) (1400) 1936 BLS 1E+02 1E+01 1E+00 1E-01 1E-02 1E-03 1E-04 Comparison of sediment transport in torrents and gravel bed streams A = Q b /S 1.5 Q/Q m -Q c bzw. G E /V re /S 1.5 b = A S 1.5 (Q - Q c ) Erlen, winter Erlen, summer Rio Cordon Pitzbach Bridge Creek I Bridge Creek II Roaring river, us Roaring river, ds Torlesse Ashiaraidani Jordan Sagehen Oak, winter 69/70 Oak, winter 71 Turkey Virginio Aare Inn (Tirol) MPM SJ RI Rhein (Brugg) Swiss torrents Rappen 1+2, Sperbel Rickenmann (2001) Rickenmann Schoklitsch h/d Relative flow depth p. 9

10 Form resistance in torrent channels - Total resistance k st from Rickenmann (1996), based on field data - Grain roughness k r according to Wong & Parker (2006): n r /n tot = k st /k r (d 90 ) Gl. (A) Gl. (B) Palt 2001 Palt (2001): n r /n tot = 0.1 S kr = d 90 (A): n r /n tot = Q 0.19 / (S 0.19 d g ) (B): n r /n tot = S (h/d 90 ) 0.55 Regression for data calculated with eq. (A): n r /n tot = S R 2 = S red n r = S n d q = 12.6 b s d ( q q ) S ( ) 90 c 1 30 a S Rickenmann et al. (2006) A = Q b /[S 1.5 red,k ( Q m -Q c )] bzw. G E /[V re S 1.5 red,k ] Sred = f(j, h/d90), Sk = slope correction Abrahams (2003) 1E+03 Comparison of sediment transport in torrents and gravel bed streams 1E+02 1E+01 1E+00 1E-01 Erlen, winter Erlen, summer Rio Cordon Pitzbach Bridge Creek I Bridge Creek II Roaring river, us Roaring river, ds Torlesse Ashiaraidani Jordan Sagehen Oak, winter 69/70 Oak, winter 71 Turkey Virginio Aare Inn (Tirol) MPM SJ RI Rhein (Brugg) Swiss torrents Rappen 1+2, Sperbel Rickenmann Schoklitsch 1E-02 h/d 90 1E Rickenmann (2005) Relative flow depth p. 10

11 Sediment transport simulation model: : SETRAC SETRAC = Sediment Transport Model for Alpine Catchments [developed at BOKU Univ. in Vienna] Accounting for form flow resistance losses in steep channels Inclusion of fractional transport (multiple grain sizes) and bed level changes Testing of the simulation model for flood events in torrents and mountain rivers Rickenmann et al. (2006) Example: Sediment transport in Luetschine Flood event of 20 to 23 August 2005, Lütschine mountain river, canton Berne, Switzerland Erosion along a steep river reach (Stalden - Buechholz) Deposition along a flat river reach (Buechholz Baumgarten) p. 11

12 Example: Sediment transport in Luetschine Flood event of 20 to 23 August 2005, Lütschine mountain river, canton Berne, Switzerland Sept 2005 before 2005 Flow is from right to left on this map Sediment transport simulation with SETRAC Flood event of 20 to 23 August 2005, Lütschine mountain river, canton Berne, Switzerland confluence of Flow is from right to left on this diagram p. 12

13 Structure of content 1) Introduction 2) Debris flows: hazard and sediment delivery 3) Bedload transport in torrents and mountain rivers 4) Example of floods 2005, Swiss Alps 5) Conclusions Floods 2005: main processes in torrents not extremely high flows ( water flow excluding sediment") in small torrent catchments for some events large landslides were important triggers partly very high channel erosion rates insufficient discharge capacity and blockage of flow cross-sections p. 13

14 Floods 2005: main processes in mountain rivers high sediment (bedload) transport importance of lateral erosion and channel migration blockage of flow cross-sections high damage to infrastructure (e.g. traffic routes) discharge capacity exceeded bedload transport lateral erosion discharge capacity exceeded and bedload ransport discharge capacity exceeded and lateral erosion bedload ransport and lateral erosion single process Combination of processes Example of overload situation Sediment transporting flood: overloading of retention basin Buoholzbach (NW), 70'000 m3 p. 14

15 Floods 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 2005 Events: Observations on transported sediment volumes for - 34 debris flow events - 39 flood events with fluvial bedload transport Analysis with simple bedload transport equation Integration over duration of flood event (or observation period): Q b = bedload transport capacity S = channel slope Q b = 1.5 ( Q Q ) S Q = discharge Q cr = threshold discharge at beginning of bedload transport cr 1.5 bedload volume rainfall-volume (ψ = 1) runoff volume (ψ = ) reduced runoff volume (main rainfall period) (neglecting effect of Q cr ) p. 15

16 Floods 2005: comparison of bedload transport GF / V wt bedload volume / runoff volume 1 Haldibach Rotlauibach 0.1 debris flow fluvial transport Riera de les Arenas Glyssibach Acherlibach 0.01 GF = 1.95 V re S channels slope [%] [%] Rickenmann et al. (2008) Structure of content 1) Introduction 2) Debris flows: hazard and sediment delivery 3) Bedload transport in torrents and mountain rivers 4) Example of floods 2005, Swiss Alps 5) Conclusions p. 16

17 Conclusions 1. Debris flows may supply large sediment loads to receiving streams 2. Fluvial sediment transport during floods 2005 is in agreement with simple transport equation but at channel slopes above ~ 5% transport is reduced (form resistance losses / limited sediment availability) 3. Sediment volume of debris flows 2005: - sediment load is affected by channel slope and runoff volume (similar to fluvial b.l.t.) - some very large events due to landslide input 4. Continuous transition from debris flows to bedload transport p. 17