New results from sediment transport measurements in two Alpine torrents

Transcription

1 Hydrology, Water Resources and Ecology in Headwaters (Proceedings of the HeadWater'98 Conference held at Meran/Merano, Italy, April 1998). IAHS Publ. no. 248, New results from sediment transport measurements in two Alpine torrents INTRODUCTION DIETER RICKENMANN Swiss Federal Institute for Forest, Snow and Landscape Research, CH-8903 Birmensdorf, Switzerland VINCENZO D'AGOSTINO, GIANCARLO DALLA FONTANA, MARIO LENZI Department of Land and Agro-Forest Environments, University ofpadova, Agripolis, Legnaro (Padova), Italy LORENZO MARCHI Research Institute for Hydrological and Geological Hazard Prevention, National Research Council, Corso Stati Uniti 4, Padova, Italy Abstract In the Erlenbach Torrent (Swiss Alps, 0.70 km 2 ), the intensity of bed-load transport is recorded indirectly and continuously by special sensors installed in the bottom of the channel. At the observation station in the Rio Cordon Torrent (eastern Italian Alps, 5 km 2 ) coarse bed load is separated from water and fine sediment, and the volume of both deposits is recorded. The Erlenbach shows a much more pronounced response to similar rainfall conditions. The geological conditions lead to a lower slope and channel stability in the Erlenbach than in the Rio Cordon. In both streams a relationship exists between the sediment load of a flood event and the corresponding runoff volume. A trend towards "equal mobility" is observed for higher discharges. Peak bed-load transport may occur before or after the flood peak. Small, steep headwater basins are of importance in the context of mountain watershed management, as most sediment transfer from slopes to the stream network takes place here. Nevertheless, a lack of experimental data still exists on hydrological and sediment transport processes in small mountain streams. In recent years new or improved approaches for bed-load measurement have been developed in some small instrumented watershed of the Alps. Such new techniques include the indirect measurement of bed-load rate by means of acoustic or seismic sensors (Bànziger & Burch, 1990; Govi et al, 1993) and the complete measurement of bed load and suspended transport in especially designed gauging stations (Fattorelli et al., 1988). The aim of this paper is to analyse results from bed-load measurements in two small Alpine basins of comparable size and equipped with different facilities for sediment transport monitoring. EXPERIMENTAL CATCHMENTS The Erlenbach stream is situated in central Switzerland, some 10 km south of Einsiedeln. The Rio Cordon lies in the eastern Italian Alps (Dolomites), some 35 km

2 284 Dieter Rickenmann et al. Fig. 1 Location of the experimental catchments. northwest of Belluno. The main characteristics of the studied basins (Fig. 1) are shown in Table 1. Regarding the geological characteristics, the Erlenbach basin is located in a flysch zone, whereas the Rio Cordon basin displays more complex settings: dolomites make up the highest parts of the catchment, volcaniclastic conglomerates, tuffs, sandstones and marly-calcareous rocks outcrop are found in its middle and lower parts; scree and morainic deposits are also widespread. In the Erlenbach stream, a debris basin allows the measurement of total sediment load transported during floods. Moreover, a system devised for continuously measuring the intensity of bed-load transport has been in operation since 1986 (Bànziger & Burch, 1990). It consists of a number of "hydrophones" installed at the bottom of the inlet channel to the sediment retention basin. The vibrations caused by coarse particles passing over the sensor are recorded in a simplified manner as the number of impulses per minute. The minimum grain size to produce an impulse is estimated to be about 1 cm. The hydrophone impulses have been found to correlate with the volume of the material deposited in the retention basin (Rickenmann, 1997). The system of measuring bed-load transport in the Rio Cordon has been in operation since 1986 (Fattorelli et al, 1988). It is based on the separation of coarse bed load (minimum size >20 mm) from water and fine sediment. Coarse bed load slides over an inclined grid and accumulates in a storage area. The volume of coarse bed load deposited is measured by means of ultrasonic sensors installed over the storage area. This recording system makes it possible to measure bed load at time intervals of minutes. Water and fine sediment pass through the grid and are directed to an outlet channel. Fine gravel (<20 mm) and coarse sand fractions have Table 1 The experimental catchments Erlenbach (Switzerland) and Rio Cordon (Italy). Erlenbach Rio Cordon Basin area (km 2 ) Maximum elevation (m) Minimum elevation (m) Average gradient of main stream (%) Mean annual precipitation (mm) Mean annual temperature ( C) Forest cover (%) 40 7 Wetland + grassland (%) Bare land (%) - 14

3 New results from sediment transport measurements in two Alpine torrents 285 Flow rate Q [m3/s] Rainfall R [mm/min.] m :_. i'rl 11 i \ /jk_ _ J i 7 ^^ m Rainfall R Flow rate Q -1! hi Time [min.] Fig. 2 Erlenbach: Flood event of 14 July 1995 (start time 15:10 h), the second largest event during the measuring period not been measured in the period In order to properly measure also fine bed load, a settling basin for fine material (fine gravel, sand and silt) was constructed in 1994 at the end of the outlet channel; the basin can trap a maximum sediment volume of about 500 m 3. ANALYSIS OF LARGE FLOODS Discharge measurements in the Erlenbach stream have been made since The second largest flood event occurred on 14 July Figure 2 shows the corresponding hydrograph recorded after an intense rainfall of short duration. The peak discharge of 10 m 3 s" 1 occurred about 40 min after the onset of the intense rainfall. Within the 40 min, the cumulative rainfall amounted to about 40 mm. The rapid response of the stream is mainly due to a high drainage density of 27 km km" 2 and to relatively shallow soils with clay-rich layers of very low infiltration permeability. The largest recorded flood event in the Rio Cordon is shown in Fig. 3. It presents two peaks caused by two bursts of high intensity rainfall which occurred during a storm event of 7 h. The first burst (3 mm per minute) shows the highest intensity, the second, less intense but more prolonged burst occurred over saturated soil and produced the higher peak of 10 m 3 s" 1. If a flood frequency analysis is performed for the Erlenbach data, the best Flow rate Q [m3/s] Rainfall R [mm/min.] Time [min.] Fig. 3 Rio Cordon: Flood event of 14 September 1994 (start time 11:00 h), the largest event during the measuring period

4 286 Dieter Rickenmann et al. Bedload [m 3 ] _ m Erlenbach, summer A Erlenbach, winter x Rio Cordon XS A JE i!w J W^ A, ni» «r A <i@ 'TU o,, > Effective runoff volume [m 3 ] Fig. 4 Bed load per flood event (G E ) in relation to effective runoff volume (VJ during flood events in the Erlenbach and Rio Cordon. representation is found using a second extremal distribution (Gumbel distribution). The data for the two streams show a similar trend in the flood frequency diagram, although the unit discharge for the same recurrence interval is about 5 times smaller in the Rio Cordon than in the Erlenbach. It is estimated that the 100 year peak discharge is about m 3 s" 1 in the Erlenbach, and about m 3 s" 1 in the Rio Cordon. SEDIMENT TRANSPORT AND FLOOD PARAMETERS The analysis of about 150 flood events with bed-load transport in the Erlenbach shows that the total sediment load per event, G E, clearly depends on the runoff volume (Rickenmann, 1994). A further improvement of the correlation is obtained if the sediment load G E is related to the effective runoff volume (above the threshold for beginning of transport), V re, to the peak discharge, Q p, of the flood event, and to the threshold discharge for beginning of bed-load transport, Q c (Rickenmann & Dupasquier, 1994; Rickenmann, 1997). The effective runoff volume is the hydrograph volume computed above the threshold discharge for the beginning of transport. These findings are also supported by the analysis of the flood events in the Rio Cordon (Billi et al, 1994; D'Agostino & Lenzi, 1996). A comparison of the relationship between bed-load volumes and effective runoff volumes is shown on Fig. 4 for the Erlenbach and Rio Cordon field data. It is observed that the data groups show a similar slope on the diagram. The fewer data points for the Rio Cordon define a range similar to the scatter of the Erlenbach data points. Effect of increasing peak discharge In Fig. 5, the bed load is shown in relation to the peak discharge of each flood event. The data from the Erlenbach show a bend at a Q p value of about 1 m 3 s 1, the influence of Q p being smaller above this threshold value. This bend may be related to the range of fluctuations of the threshold discharge, between 0.4 and 0.8 m 3 s" 1 for flood events in summer (Rickenmann, 1997). It is assumed that within this range a large part of sediment grains are set in motion, considering the sizes which are

5 New results from sediment transport measurements in two Alpine torrents 287 Bedload [m 3 ] A». 4 + ' m % "W DCJ^E 4 O J»",t* * Tfl«Erienbach, summer A Erienbach, winter x flood x Rio Cordon + Missiaga 1 1 II HIM Peak discharge [m 3 /s] Fig. 5 Bed load per flood event (G E ) in relation peak discharge (Q p ) for flood events in the Erienbach, Rio Cordon and Missiaga Creek. transported at all during flood events. Consequently, up to a discharge of about 1 m 3 s" 1 the proportion of grains in motion contributing to the total sediment transport increases rapidly. When the discharge exceeds this level, larger sediment grains may be set in motion but may not contribute so much to the total sediment volume. A moderate increase of total transported sediment with increasing peak discharge is observed for the Missiaga Torrent in Fig. 5, an instrumented stream of the eastern Italian Alps that covers an area of 4.4 km 2 (Anselmo et al, 1989). Discharge thresholds for sediment transport in the Missiaga Creek are lower than in the Erienbach and Cordon and high sediment loads are transported during floods with peak discharges of about m 3 s" 1. For the highest peak discharges, differences among the considered creeks decrease and a sediment load of about 1000 m 3 is observed for the largest floods, with flood peaks of about 10 m 3 s' 1. (Measured sediment loads in the Missiaga Torrent have been divided by a factor of 2, in order to discount the fine material which is also deposited in the debris basin.) This general conclusion is in agreement with more detailed observations on grain sizes in transport that are available for the Rio Cordon (D'Agostino & Lenzi, 1996). In Fig. 6 the ratio of grain sizes in transport to those in the bed surface material are shown as a function of the peak discharge. It is seen that increasing particle sizes out of the bed are transported if the peak discharge is about twice or three times the critical discharge for beginning of bed-load motion, Q c. In other words, at the higher discharges the conditions of "equal mobility" are approached. Similarly, Wilcock m - ^ «" è"~~~~~~ ~ ~~ : w -0-- \ A i = 50% = 84% = 90% Qp/Qc Fig. 6 Rio Cordon: Ratio of transported to bed material grain size in relation to normalized peak discharge.

6 288 Dieter Rickenmann et al. Hydrophone H3 [1000 impvmin.] 25 x event of July 2, event of July 14, Flow rate [m3/s] Fig. 7 Erlenbach: Time sequence of the variation of bed-load transport intensity (hydrophone impulses H3) with flow rate, shown for two flood events. Bedload transport rate [kg/s] I I Sep. 14, 1994 flood I x "ordinary" floods X X ^ /f- t - ^ p X Flow rate [m3/s] Fig. 8 Rio Cordon: Average hourly values of bed-load transport rate vs flow rate. (1992) found in flume experiments that "equal mobility" is reached when the shear stress is about times the critical shear stress for initiation of motion. Based on other field observations in gravel-bed streams it can be concluded that "equal mobility" is reached at discharges which are about 2-5 times the critical discharge for beginning of motion (e.g. Komar & Shih, 1992). The pulsing nature of bed-load transport in the Erlenbach stream is evident from the continuous hydrophone measurements since 1986 (Rickenmann, 1994). In Fig. 7, the time sequence of the variation of transport intensity (expressed by hydrophone impulses per minute) with flow rate is shown for two flood events. A trend for a hysteresis behaviour can be observed. When considering all flood events, no systematic pattern is found; the peak transport intensity may occur either before or after the flood peak. It may be noted that the total sediment volume deposited in the retention basin was 590 m 3 for the flood event of 14 July, and 170 m 3 for the flood of 2 July A scatterplot of average hourly values of bed-load transport rate vs water discharge (Fig. 8) compares data from some floods which occurred in the Rio Cordon from 1987 to 1992 and includes the largest recorded event of 14 September Bed-load transport rates range from 0.1 to 6 kg s" 1 during "ordinary" floods (discharge up to 5 m 3 s" 1 ). The mobilization of large amounts of bed material during the flood of 14 September 1994 caused sediment concentration in this flood to be

7 New results from sediment transport measurements in two Alpine torrents 289 higher than in other recorded floods: the observed intensity of bed-load transport exceeds by more than one order of magnitude those of "ordinary" floods, while peak discharge is only approximately twice. FINAL REMARKS The two experimental hydrological catchments Erlenbach and Rio Cordon are compared with respect to flood generation and sediment transport. The following conclusions can be drawn: (a) the Erlenbach shows a more pronounced response to intense rainfall due to a denser drainage network and soils of lower permeability; (b) the sediment production is stronger in the Erlenbach due to slope movements; (c) for similar flood parameters, bed-load transport is comparable in both streams; (d) a trend towards "equal mobility" is observed for higher discharges in both streams. Acknowledgement The research was partially supported by the European Commission, DGXII, Environment and Climate Programme, Climatology and Natural Hazards Unit, in the framework of Contract ENV4-CT REFERENCES Anselmo, V., Marchi, L., Tecca, P. R. & Villi, V. (1989) Sediment transport in a small watershed of Dolomite mountains (NE Italy). Quaderni di Idronomia Montana, no. 9, Bânziger, R. & Burch, H. (1990) Acoustic sensors as indicators for bed load transport in a mountain torrent. In: Hydrology in Mountainous Regions. I Hydrological Measurements; Tlie Water Cycle (ed. by H. Lang & A. Musy) (Proc. Lausanne Symp., August 1990), IAHS Publ. no Billi, P., D'Agostino, V., Lenzi, M. A. & Marchi, L. (1994) Geomorphological processes and sediment yield in a small basin of the Dolomites (north-eastern Italy). In: Proc. Int. Symp. on Forest Hydrology (Tokyo, lapan), D'Agostino, V. & Lenzi, M. A. (1996) La valutazione del trasporto solido di fondo nel bacino attrezzato del Rio Cordon (The validation of bed-load transport in the Rio Cordon catchment). L'Acqua 4, Fattorelli, S., Keller, H. M., Lenzi, M. & Marchi, L. (1988) An experimental station for the automatic recording of water and sediment discharge in a small alpine watershed. Hydrol. Sci. J. 33(6), Govi, M., Malaga, F. & Moia, F. (1993) Seismic detectors fo continous bed load monitoring in a gravel stream. Hydrol. Sci. J. 38(4), Komar, P. D. & Shih, S. M. (1992) Equal mobility versus changing bed load grain sizes in gravel bed streams. In: Dynamics of Gravel-Bed Rivers (ed. by P. Billi, R. D. Hey, C. R. Thorne & P. Tacconi), John Wiley, Chichester, UK. Rickenmann, D. (1994) Sediment load and discharge in the Erlenbach Stream. In: Dynamics and Geomorphology of Mountain Rivers (ed. by P. Ergenzinger & K.-H. Schmidt), Lecture Notes in Earth Sciences 52, Rickenmann, D. (1997) Sediment transport in Swiss torrents. Earth Surf. Processes andlandforms 22, Rickenmann, D. & Dupasquier, P. (1994) Messung des Feststofftransportes im Erlenbach (Measurement of sediment transport in the Erlenbach). Beitr. zur Hydrol. der Schweiz 35, Wilcock, P. (1992) Experimental investigation on the effect of mixture properties on transport dynamics. In: Dynamics of Gravel-Bed Rivers (ed. by P. Billi, R. D. Hey, C. R. Thorne & P. Tacconi), John Wiley, Chichester, UK.