Improving sediment management in the reservoirs of the Lower Isère: a modelling-based approach

Transcription

1 ICSE6-169 Improving sediment management in the reservoirs of the Lower Isère: a modelling-based approach Jean-Claude CARRE 1, Pierre NEGRELLO 2, Sébastien MENU 2 1 SOGREAH Groupe ARTELIA BP Grenoble Cedex 9 - Jean-Claude.CARRE@arteliagroup.com 2 EDF-CIH Service Environnement Savoie Technolac The Bourget du Lac - pierre.negrello@edf.fr - sebastien.menu@edf.fr Sediment transport on the river Isere is among the highest of all French rivers (about 2 million tonnes per year). These large flows of sediment arriving in the five EDF reservoirs are managed by performing regular flushing operations. Flushing consists in lowering the reservoir level during flooding. Velocities are then sufficient to carry the incoming sediment through, but also to erode and remobilize sediments that had settled in the reservoir between two flushing operations. On the river Isère, these operations involve significant volumes of sediment, around several hundred thousand cubic metres. They are the subject of feedback and successive improvements with an aim to reduce sediment deposition and flow concentration during flushing. In this context, EDF and Sogreah-Artelia have conducted studies to improve their knowledge of the influence of the Isere reservoirs on fine sediment transit, and are testing different ways to manage sediment (e.g. increasing the frequency of flushing, lowering the reservoir level for small floods). The spatial scale of this study is large (80 km). It covers the five reservoirs in the downstream part of the river Isere. In order to integrate a representative hydrological and sedimentary time series, the time scale is also very large (20 years). Because of the large size of the study area (temporal and geographical), the aim of the study was not to represent the local phenomena with accuracy, but to compute volume and sediment flows consistent with reality at the reservoir scale. Testing different scenarios made it possible both: (i) to understand the role of each reservoir in the overall functioning of deposition / erosion; and (ii) to compare different management scenarios modelled. In the end, the results obtained with these models provided decision-making tools that will be useful for improving sediment management on the Isere reservoirs. Key words Modelling, Suspension, Dam, Lower Isère. I INTRODUCTION Five hydropower schemes (Beauvoir, St Hilaire, Pizançon, La Vanelle, Beaumont Monteux) have been built on the Lower Isère, harnessing the motive force provided by the altitude difference of approx. 55 m over a 60 km stretch of the river (cf. figure 1). The plants are run-of-river, with the possibility of daily releases. The dams form reservoirs with capacities of a few millions cubic metres forming a direct stepped configuration. The tail ends of the reservoirs stretch almost as far back as the outlets of the preceding dams. 1289

2 Figure 1: location of the reservoirs on the Lower Isère Sediment transport on the river Isère is among the highest of all French rivers (approx. 2 million tonnes per year). These large flows of sediment into the five reservoirs run by EDF on the Lower Isère are managed mainly by performing regular flushing operations. Flushing consists in lowering the reservoir level during flooding. Velocities are then sufficient to carry the incoming sediment through, but also to erode and remobilize sediments that had settled in the reservoir between two flushing operations. On the river Isère these operations generate significant volumes of sediment, around several hundred thousand cubic metres. They are the subject of feedback and successive improvements with an aim to reduce sediment deposition and flow concentration during flushing. In this context, EDF and Sogreah-Artelia have conducted studies to improve their knowledge of the influence of the Isere reservoirs on fine sediment transit, and are testing different ways to manage sediment (e.g. increasing the frequency of flushing, lowering the reservoir level for small floods). The approach they have adopted is large-scale hydrosedimentary modelling. II MODEL PRINCIPLE This is a conceptual model intended to provide a means of assessing the management of large quantities of fine sediment passing through the reservoirs in relation to the level and flushing management rules applied, over a period that is sufficiently long (20 years) to be representative of all the phenomena and all the ranges of values encountered on the Isère. Its aim is to produce an aid for the comparison of various scenarios from a long-term perspective, i.e. by reproducing flows as a whole on the time scale of each episode. The goal is not to study a given event in detail, but to analyse the long-term variations brought about by a given management method. Given the sediment yield of the river Isère (a few million tonnes per year) and the size of the reservoirs (a few million cubic metres), highly accurate calibration is not necessary. On the other hand, it is important to model the processes in a robust, physically acceptable manner in order to obtain plausible differences between the various scenarios. One-dimensional river modelling was therefore adopted, incorporating fine sediment deposition/ consolidation and erosion phenomena: - suitable for the time scale: time series covering 18 years to be simulated ( ) - suitable for the spatial scale: distance modelled = 80 km - suitable for the constraints of study deadlines - suitable for the existing measurements. This choice of modelling technique introduces a number of limits. The model provides a good representation of phenomena concerning longitudinal sections, but not necessarily of those concerning cross 1290

3 sections. Once calibration has been completed on the basis of real events, the results obtained from running the model must be compared with each other and not with measurements taken during previous flushing episodes. The length of the time series required to assess the differences between the scenarios is also a major constraint, since it means long input data series must be available (which is easy in relation to river discharge but very difficult in relation to sediment transport). The hydro-sedimentary model was built according to the simplified diagram below (figure 2). The input data are those relating to hydraulics (discharge, roughness, reservoir water level management rules, bottom surveys) and to fine sediment flow (concentration, sediment load, particle size). These data are processed by functions representing the major physical phenomena driving hydraulic variations (laws of hydraulics) and sedimentary variations (deposition, consolidation, erosion, propagation). All these functions must be: - defined on the basis of information drawn from the existing literature (physical laws, calculation formula, lessons learned from previous models, measurements, etc.); - adapted (or calibrated) to the Lower Isère on the basis of measurements taken in the past both during and outside flushing periods (discharges, bed changes, level measurements, SS [suspended solids] concentration measurements, etc.). The model incorporates these different functions, reservoir by reservoir and then for the entire study area. It then delivers output data depending on the management rule adopted: outgoing discharges, levels reached, sediment loads, volumes of sediment deposited and eroded, SS concentrations, etc. III INPUT DATA Figure 2: schematic diagram of the model All hydraulic, bottom level and management rule data are available for the study area. As regards measurements of sediment load on the Isère, data on SS concentrations are only partial for the period , and non-existent for the period Measurements with an hourly time step are only available as of July 2002 and the measurements over this period contain numerous gaps. The missing data therefore had to be completed. This was done in two stages and in two different ways: - the first method provided a means of completing the period : it consisted in adopting sediment load values from periods of comparable hydraulic conditions for the periods contain gaps; - the second method was used to complete the period : the initial plan was to either set a linear log relation between discharge and wash load (SS carried from the upstream reaches). The resulting discharge/wash load relation is either too perfect or too random and risks compromising the pertinence of the model input data. Another approach based on a neural network was hence adopted. The neural network model provided a means of obtaining a long series that was statistically plausible with regard to known wash load values while reproducing the statistical links between discharge and solid flow. 1291

4 IV FUNCTIONS IV.1 Hydraulic The MASCARET software was used to calculate river flows. It solves the Barré de Saint-Venant onedimensional equations, making a distinction between different flow conditions (torrential, critical, subcritical). In the framework of this study, given the hydraulic constraints, the unsteady subcritical computation module was used as a priority. The steady computation module was also used, but only sporadically. The supercritical module was not used due to the fact that it does not allow a computation time step greater than 2 seconds; the computation times generated would have been too long. The Mascaret flow models comprise a set of geometrically defined cross sections with which a roughness is associated. IV.2 Sedimentary The nature of the phenomena (deposition, erosion or simple suspended sediment transit) is determined on the basis of knowledge of the shear stress associated with the friction forces [1], [2]. This stress is expressed as follows: water g Rh J With Rh = hydraulic radius (= cross-section of stream / wetted perimeter) J = head slope = Q² / D², D being the discharge capacity Thus: - if < cd deposition takes place - if cd < < ce simple transit takes place - if > ce erosion takes place with: cd = critical shear stress for deposition ce = critical shear stress for erosion The deposition rate D (kg/m 2 /s) is expressed as follows: D=W.C.(1- / cd ) with: W = sediment settling velocity (m/s) C = concentration of suspended sediment transported in the water The numerous calibration tests performed revealed the need to introduce the notion of sediment consolidation. This is characterised by a consolidation time (Tc) on the one hand, and a specific shear stress for erosion, which is constant and greater than that of fresh sediment, on the other hand. This phenomenon is taken into account by considering that the mean height of fresh sediment deposited between t and t+tc becomes consolidated sediment at t+tc. A distinction is hence made between the critical shear stress for erosion of fresh sediment ( ef ) and the critical shear stress for erosion of consolidated sediment ( ec ). Erosion is expressed as follows: E=M ( - ce ) E = eroded sediment load in kg/m 2 /s. M = coefficient having the inverse function of the velocity (s/m). The critical shear stress for erosion varies as follows:. If there is a fresh sediment layer: throughout the fresh sediment thickness ce. If there is no fresh sediment layer: ce throughout the consolidated sediment e C thickness. Since the erosion shear stresses are constant, an extreme bottom and not an erodible bottom must be used. e F 1292

5 IV.3 Integration of the various model components The chain of reservoirs on the river Isère is mainly linear. The following diagram sums up the sequence. Beaumont St-Egrève Isère Beauvoir Isère St Hilaire Pizançon La Venelle Monteux (reservoir) (river) (reservoir) (river) (reservoir) (reservoir) (reservoir) (reservoir) (St Gervais) Furon Bourne Figure 4: sequence of reservoirs For each reach or stretch modelled, the model operates in three phases: 1. It launches execution of the flow calculations using Mascaret over a time D1 with potentially variable computation time steps dt1, 2. It rereads the hydraulic calculation results and determines the changes to the bottom for each cross section (erosion, deposition) at a potentially variable time step dt2, 3. It consequently modifies the geometry of the Mascaret model cross sections. After step 3, it starts again at step 1 and performs this cycle throughout the duration of the simulation. V SEDIMENTARY CALIBRATION The purpose of sedimentary calibration is to represent the following, as accurately as possible and in order of importance: 1. sediment volumes transiting through the reservoirs during flushing, 2. sediment load variations over time, 3. changes reservoir bathymetry. On completion of calibration, the following parameters were eventually adopted: cd = 3 Pa; W = m/s ce = 5 Pa; ce_consolidated = 11 Pa; M= s/m, T consolidation = 3 months. The calibration results were satisfactory insofar as the differences between measurements and calculations were much lower than one might have expected (differences in the order of 50% are frequent) with this type of conceptual modelling (it is true that the present case can be considered similar to simplified deterministic modelling). Generally speaking, however, the model tends to overestimate erosion at the start of flushing. This can lead to concentrations that are higher than in reality. On the other hand, it does not take into account two- or even three-dimensional phenomena that can arise, nor the bank collapse that is observed in the wake of sediment erosion. As a result, the sediment loads at the end of flushing are underestimated. This does not affect the risks of deposition in the river Rhône, because these sediment loads at the end of flushing remain low. As regards the general balance between volumes, calibration demonstrated that the underestimated sediment loads at the end of flushing more than compensate for the overestimated erosion at the start of flushing. In view of the complex nature of hydraulic models and, especially, of sedimentary models with bed modification, the sequence of reservoirs, and the duration of the initial time series, the calculation times were very long: it took 3 hours to calibrate a reservoir, and 20 hours to calibrate a scenario. These are the present limits of these modelling techniques: introducing more detailed or more complex processes or increasing the interleaving between hydraulic and sedimentary models would have led to calculation times that are incompatible with the duration of this study and the implementation of a large number of scenarios. For example, figure 5 illustrates the variation in the volumes calculated in Beauvoir reservoir (red curve) and the target volume obtained from analysing the 1999, 2005 and 2008 bathymetric data (blue square): 1293

6 Cumulative deposited volume - Beauvoir 1999 / 2008 Measured volume at Beauvoir Consolidation 3 months Td3-Te5-Tec1-MO 's flushing Date Volume's variation observed (target) Volume's variation modelled difference (%) 's drawdown Balance / bathymétries: +1.4Mm3 Balance /bathymétries: Mm /08/ /08/ /08/ /08/ /08/ /08/ /08/ /08/ /08/ 's flushing Figure 5: Cumulative deposited volume modelled and balance in volume between different bathymetric data Between 1999 and 2005 the volume variation calculated on the basis of the bathymetric data is reconstituted correctly. The same applies for the period , even though the deposited volume obtained in 2008 is slightly positive in comparison with the 1999 state, whereas it should be negative according to the records. Nevertheless, the orders of magnitude are acceptable: the relative difference in terms of volume variations is no greater than 26% over the period compared with 6% over the period Figure 6 shows the representation of sediment loads at St Hilaire during the flushing that took place in October The sediment load peaks and the shape are represented correctly in spite of a time lag on the first point Output Solid Discharge - St. Hilaire - October 2000 Flushing Solid Discharge at St. Gervais Output solid discharge at Beauvoir - Modelled Output solid discharge at St. Hilaire - Measured at Pont des Fauries Output solid discharge at St. Hilaire - modelled Drawdown's beginning Solid discharge (kg/s) Gates fully opened Filling's beginning /10/ /10/2000 Date 18/10/ /10/2000 Figure 6: Sediment load - calibration 1294

7 VI MODELLING VI.1 Scenario Six management scenarios to be simulated were defined for all the reservoirs on the Lower Isère and on the reach of the Rhône. Scenarios 1 and 2 are scenarios without flushing operations and without dams, defining the boundaries of the system. The other four simulate different flushing instructions. The management rules tested recreate the criteria of the present instructions unless a change was specifically sought. It was decided to only vary one parameter per scenario with respect to the present flushing instructions, in order to pinpoint the changes precisely. The scenarios are summarised below. - SCENARIO 1: WITHOUT FLUSHING This scenario simulates operation of the Lower Isère reservoirs without any flushing. Only the management rules, especially the laws governing flood drawdown, are applied. The dams are operated with reservoirs having relatively high sediment levels and transit during floods. - SCENARIO 2: WITHOUT DAMS This scenario simulates operation as it would be in the absence of the dams. The chain of dams on the Lower Isère is transparent. The flow entering the river Rhône corresponds to the hydraulic transfer of the natural flow entering the Lower Isère system. - SCENARIO 3: PRESENT The modelling is carried out using the present flushing instructions. - SCENARIO 4: FREQUENT FLUSHING The thresholds at which flushing operations are triggered are lowered substantially (400 m 3 /s instead of 500 m 3 /s, the current instruction) in order to make them occur more frequently. Two flushing operations are added to the eight triggered in the framework of scenario 3. - SCENARIO 5: PARTIAL DRAWDOWNS This scenario corresponds to the application of scenario 3 completed by partial drawdowns in order to allow the sediment flow to transit when the ingoing sediment flow reaches or exceeds 400 kg/s. SCENARIO 6: RISING FLOOD LEVELS - Flushing operations are not always carried out along the entire chain of dams starting from Beaumont Monteux; a time lag may be inserted between Beauvoir-St Hilaire and the other three reservoirs located downstream. Beauvoir and St Hilaire follow the flushing instruction of the dam located upstream in the chain and can hence be flushed while flood levels are rising, whereas the other reservoirs continue to follow the present instruction with flushing operations while water levels are falling. The sediment flow from the two reservoirs located upstream will hence transit towards Pizançon, which may be flushed if the hydrological conditions are in keeping with the present flushing instruction. If they are not, the flushing will be partial and will only concern the dams located upstream, and similarly will subsequently only concern the dams located downstream. The scenario results in the St Hilaire dam being opened frequently (14 times in 20 years). The simulations are conducted over 30 years. The last twenty years represent the period The first ten years are a running-in period. They are described by the chronologies characterising the period , for each scenario. For practical purposes, the fictitious period was attributed to them. At the end of this period, a state is obtained for each reservoir and for each scenario that will be used as a reference for observing variations in sediment volumes. The stream flow and sediment yield time series considered is interesting because it indicates that functioning remains regular over some fifteen years and includes a sequence that may be described as exceptional. VI.2 Results A number of lessons were learned from simulating variations in sediment volumes stored in the reservoirs, sediment loads and concentrations. For example, figure 7 illustrates variations in sediment volumes stored in Beauvoir reservoir for each scenario modelled. The simulations show that flushing is essential. Failure to carry out flushing leads to the formation of substantial SS stocks (a cumulative total in the order of several cubic hectometres) and, once the reservoirs reach saturation point, withdrawals are triggered by floods, simply through the increase in velocities. This increase is amplified in the reservoirs where additional drawdown of levels at the dams is necessary in order to limit risks of overflow at critical points. 1295

8 Among the alternative scenarios studied, the following general trends emerge: - The analysis of variations in sediment volumes in each reservoir from one scenario to another indicates that scenario 4 would be the best in terms of minimising the sediment volume stored in excess of the reference level of 01/01/ The flow of suspended solids leaving the downstream reservoir (Beaumont-Monteux) depends on the incoming flow upstream as well as on the fluctuating combination of deposition and erosion in each reservoir. This analysis, coupled with the analysis of concentrations for flushing operations common to the five scenarios, highlights the advantages of scenario 5. - Scenario 6 is of no particular interest since Pizançon reservoir does not have the capacity required to serve as a buffer reservoir. Its drawdown law results in frequent sediment withdrawals. To conclude, an analysis of the scenarios purely from the hydro-sedimentary point of view would rank them as follows, in order of preference: scenario 4 (frequent flushing operations), scenario 5 (drawdown), scenario 6 (upstream flushing during floods), scenario 3 (present). Volumes stored (millions m 3 ) 3.3 Scenario 01 (without flushing) Scenario 02 (without dams) Scenario 03 (Present) Scenario 04 (frequent flushing) Scenario 05 (drawdowns) 2.3 Scenario 06 (rising flood) Variation in sediment volumes stored at BEAUVOIR (Modelled) /80 01/82 01/84 01/86 01/88 01/90 01/92 01/94 01/96 01/98 01/00 01/02 01/04 01/06 01/08 01/10 Figure 7: Variation in sediment volumes stored at Beauvoir VII. CONCLUSION Through the creation of a hydro-sedimentary model on a large time and spatial scale, this innovative, ambitious study has provided valuable information regarding the management of flushing operations on the Lower Isère. It should be noted that model adjustment was satisfactory insofar as the differences between measurements and calculations were much smaller than one might have expected with this type of modelling (differences in the order of 50% are frequent). The results obtained with these models provided decisionmaking tools that will be useful for improving sediment management during flushing operations on the Lower Isere reservoirs. VIII REFERENCES AND CITATIONS [1] Krone, R. B., Flume studies of the transport of sediment in estuarial shoaling processes, Technical Report, Hydraulic Engineering Laboratory, University of California, Berkeley. [2] Partheniades, E., Erosion and Deposition of Cohesive Soils. Journal of the Hydraulics Division, Proceedings of the ASCE, vol 91, N HY1, pp