Multi-scale survey of suspended sediment concentration in the Bléone river basin (Southern French Alps) O. Navratil (1), M. Esteves (1), J. Némery (1), C. Legout (1), A. Poirel (2), N. Gratiot (1) and P. Belleudy (1)
The Bléone river basin (905 km²) is located in the Southern French Alps (Fig. 1). The Mediterranean and mountainous climate, with frost in winter and high intensity rainfall in summer, leads to “badlands” topography, gully development and high solid transport (see pictures at Fig. 1). The basin was instrumented with six hydrometric stations with high frequency water discharge and SSC acquisition. The catchments areas range from 20 km² up to 870 km², with different geology (sedimentary marl and limestone), and land use (forests, cultivated areas, grassland). They are characterised by a low human impact (no dam, very low urbanisation). Upper Bléone river bassin (limestone geology)
Hydrometric stations were equipped with: (1) a 24 GHz radar (Paratronic Crusoe) which measures the water level at four sites. The flow discharges at the Bès at Péroure and the Bléone at Malijai are respectively monitored by the French Environment Department (DIREN Paca) and EDF-DTG; (2) a turbidity sensor (WTW Visolid 700 IQ sensor) which measures the turbidity into the water flow by infra-red light retro-diffusion; (3) an automatic sampler (ISCO 3700) which collects one litre water sample above a fixed turbidity threshold (Fig. 3), in order to calibrate the turbidity sensor. The turbidity threshold and the sample frequency for each site were chosen according to SSC dynamics; (4) a data logger (Campbell CR800) which records the water level and the turbidity every 10 minutes and transmits the data via GSM or phone modems. These sites are easy of access and are located in the vicinity of bridges (Photo 1).
Discharge and SSC data are available since the 5th October 2007. As an illustration, we show here the 22-23 November 2007 flood (Fig. 3). The maximum flow discharge (15 m3/s) occurs later than the peak of SSC (36 g/l).
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automatic water samples
16 14
50
12
Radar for water level survey
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4
20
5
3 2
30
10 8 6
1
11 7 8
910
6 12
4
10
2
0
0
20:00
00:48
05:36
10:24
15:12
20:00
Time (hh:mm)
Data recorder with GSM transmitters
Figure 3: SSC and discharge during a flood event (photo: flood at the same period, source: Diren)
Bès at Péroure 165 km² Marl, limestone, gypse Forest, grassland
This shift could be attributed to 1) large volume of sediment stored in the riverbed provided by the last flood, 2) the spatial distribution of precipitations and sediment sources. So no relation exists between discharge and SSC. Water sampler
Turbidity Probes
Duyes at Mallemoisson 124 km² Limestone Cultivated area, forest
The rainfall variability is estimated using 24 rain gauges located in the vicinity of the Bléone river basin. Discharges are measured (1) with salt (NaCl) dilution method and current meter (low flows), and (2) with Rhodamine WT dilution method (high flows). A 1-D hydraulic model (HEC-RAS) also provides, on each site, high flow discharge validation as few discharge measurements are generally available. Modele hydraulique Galabre@Robine
Plan: Plan 01
05/03/2008
1301 1268.*
20
30 20
b.
10 0
1 10 100 Particule size (µm)
1000
0
10 20 Turbidity (%o)
30
Suspended sediment size does not vary significantly during this event (Fig. 4a); D50 is 11.2 µm on average, with roughly the same fraction of clay (7%), silt (86%) and sand (7%). It could explain the good statistical relation (power law) we obtained between the turbidity value and SSC (Fig. 4b). At other station (e.g. Galabre at Robine), no relation can be estimated as the turbidity-SSC relationship exhibits an hysteretic pattern that could be attributed to a change in sediment size, mineralogy or colour.
1235.*
b.
1202 1152.5 * 1103 105 3.5* 1004 954.5* 905 806 707
CONCLUSION AND PERSPECTIVES
608 410 360 .5* 311 278.* 2 45.* 212 162.5* 113
Acknowledgement: This research is funded by the Agence Nationale pour la Recherche Contacts:
[email protected];
[email protected] (1) LTHE, Grenoble, France (www.lthe.hmg.inpg.fr) ; (2) EDF-DTG, Grenoble, France (www.edf.fr)
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8 4 2 11
1.78
Turb = 0.11*SSC R² = 0.95
Figure 4: a) Distribution of particle sizes (µm) for samples 2, 4, 8, 11 (see Fig. 3), and b) SSC-turbidity relationship
a. Figure 1: Location of the Bléone river basin and the six hydrologic stations These sites complete five gauged stations monitored by the Research Observatory of Draix (Cemagref) which mainly focuses on small drainage basins (less than 1 km²).
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0.1
Photo 1: Gauged station on Galabre creek Erosion area of marl
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0
Bléone at Prads 65 km² Limestone, marl Forest, grassland Bouinenc at Draix 22 km² Marl, limestone Forest, grassland
a.
40 SSC (g/l)
Fraction (%)
100
Galabre at Robine 20 km² Gypse, limestone, marl Forest
Bléone at Malijai 870 km² Marl, limestone, gypse Forest, cultivated area
SSC threshold for the automatic sampler
Discharge (m /s)
STUDIED CATCHMENTS
PRELIMINARY RESULTS AT THE BES AT PEROURE IN FALL 2007
3
STREAMS project – Sediment TRansport and Erosion Across MountainS – aims to improve the knowledge of suspended sediment dynamics using a nested basins approach. Indeed, in mountainous areas, especially in the French Alps, a reduced number of Suspended Sediment Concentration (SSC) data are available to evaluate properly annual flows and spatiotemporal variability. This lack of high temporal resolution data is very restrictive for scientific studies as well as for river management such as hydraulics works or water resources and ecological requirements. This poster describes the studied sites at the Bléone river basin, the instrumentation and the preliminary results obtained from the 2007 fall events.
HIGH FREQUENCY DATA ACQUISITION
SSC (g/l)
INTRODUCTION
Figure 2: (a) Topographical survey with a total station and DGPS, and (b) 1-D hydraulic model of Galabre at Robine
This work corresponds to the first step of a larger study covering (1) trace element geochemistry of sediments – chemical and isotopic analysis –, (2) sediment source mapping and (3) suspended sediment transport modelling. All these data will provide an original new data set to analyse temporal and spatial variability of suspended sediment transport for increasing river basin sizes.
