Energy gradient and geomorphological processes along a river influenced by neotectonics (the Saône river, France)

Geodinamica

Acta

(Paris) 1999, 12, 1, l-10

Energy gradient and geomorphological processes along a river influenced by neotectonics (the SaGneriver, France) Laurent a UMR 5600 N environnement, b DCpartement de geographic, universite (Received

Astrade”.

Jean-Paul

ville, soci& >>, 18, rue Chevreul, 69007 Paris-IV-Sorbonne, 1, rue Victor-Cousin,

1 October

1997;

I energy

gradient

I stream

power

apports SaGne

27 April

France

1998)

skdimentaires I France

I gradient

d’knergie

I puissance

I

1. Introduction

/ SaBne

d’knergie et processus gt5omorphologiRCsumC - Gradient ques sur un tours d’eau soumis a la nCotectonique SaSne, France). L’objectif de cet article est de mettre en

accepted

Lyon, France 75230 Paris,

et par la paleodynamique fluviale holoctne. Une etude de terrain a Cte realisee lors des trues de decembre-janvier 19931994 (2375 m3 s-‘) et de janvier-fevrier 1995 (1826 m3 s-l). Une sectorisation fine, utilisant les puissances brute et specifique ainsi que les forces tractrices, a ett realisee sur plus de 400 km grace a l’utilisation de profils en travers et longitudinaux d’espacement kilometrique. Cette sectorisation, confrontee aux resultats de I’analyse de terrain, montre que la mesure de l’energie est bien correlee avec les caracteristiques du fonctionnement hydro-morphologique. 0 Elsevier, Paris

- The purpose of this study is to link the sediment transit and the flood plain storage of the SaGne to hydromorphological characteristics of the hydrosystem, which exemplifies a new approach to sediment dynamics. The study of suspended sediment concentration in terms of temporal evolution, together with sediment deposition in terms of spatial variability, is a way to record the longitudinal evolution of the sediment load, which expresses the available energy gradient from upstream to downstream in hydrosystem. The SaBne river is a 480-km-long Rhone tributary, with an oceanic pluvial regime, and an average yearly discharge of 440 m 5-l at Lyons. The watercourse is characterised by very gentle slopes controlled by the neotectonics of the Bresse trough and by Holocene fluvial dynamics. Sediments were sampled during the December 1993-January 1994 flood (2 375 m3 se’) and the 1995 January-February flood (1 826 m3 s-l). A fine partition into homogeneous sectors, using stream power as well as shear stress, has been realized on a 400 km reach using longitudinal and cross-sections at one kilometre intervals. This partition, compared with the results of the field sampling, shows that the amount of energy is closely connected to the hydromorphological characteristics of the river. 0 Elsevier, Paris

Abstract

sediment load river / France

Bravardb

(la

relation le transit et le stockage des sediments d’un tours d’eau, la SaBne, et les caracteres hydro-morphologiques de l’hydrosyst&me, approche encore peu appliqute a la dynamique des stdiments. L’Ctude de l’evolution temporelle des concentrations des mat&es en suspension et de la variabilite spatiale des apports stdimentaires par debordement est un moyen d’enregistrer l’evolution longitudinale de la charge ; cette evolution traduit les variations amont-aval de l’energie disponible dans l’hydrosysteme. La SaGne est un affluent du RhBne long de 480 km, a regime pluvial oceanique, dont le module est de 440 m3 SK’ a Lyon. Le tours est caracterise par la faiblesse des pentes moyennes controlees par la neotectonique du fosse de Bresse -l-

This study aims to define the processes involved in the construction of an alluvial plain through the effects of river overflow in the context of recent regional tectonic activity. With flood recurrence on an annual or biannual basis, highly dynamic processes contribute to the development of the flood plain. This development seems well represented in the Allen classification [I], that differentiates flood plains developed through lateral accretion from those with vertical sedimentation. The SaBne river is a good example of the latter category. Firstly, the Early Holocene sedimentation is well marked by the superposition of sedimentary levels clearly dated by means of archeological artefacts, secondly the channel has kept a relative plane stability since the Late Glacial period [2, 31. Allen [1] showed that vertical accretion depends on internal factors, such as suspended sediment sizes, total load characteristics, channel migration rates and water overflow velocities, as well as external factors like movement of base level through subsidence, which is particularly demonstrated in the case of the SaGne. In this kind of flood plain, the process for sediment recovery by spillage flows is very restricted with in compari-

L. Astrade, J.-P. Bravard son to the deposition process (vertical accretion) so that the net result of sedimentation is highly positive. Along the Saone, upstream to downstream transformations are subjected to existing parameters due to longitudinal variations in the flow conditions linked to the increase in discharge and slope modifications. The aim of this study is to highlight the influence of hydromorphological factors in the SaBne hydrosystem, disclosed by the longitudinal sectorisation, and thanks to several determining variable data, on the sediment storage and transit. The flood plain of this Rhone tributary, of which the lower slope is controlled by the Holocene fluvial neotectonic and palaeodynamic, represents a huge storage environment characterised by very long floods and very low velocities. Temporal evolution of suspended sediment concentration at a station, at the time of an average winter flood, has been compared with the discharge evolution and also with the flood plain flooding. In other respects, spatial variability of sediment deposition associated with two major floods is explained by upstream to downstream variations of the energy involved in the hydrosystem. The approach proposed in this paper is based on the concept of availability of potential energy in the minor channel as a determining factor in longitudinal sediment transfer. This energy can be described by the specific stream power and shear stress for the bankfull channel capacity (Qb), as well as by the speed of flow calculated for a loyear recurrence (Q,a) which leads to high overflow and sedimentation.

2. Description

of the studied

fluvial

section

The Saone flows for 480 km between Viomenil (Vosges) and Lyons, meeting the Rhone (kilometric point or Kp 0). It drains a basin of 30 060 km* (figure I). The Couzon station, near Lyons (Kp 17) has a 442 m3 s-’ yearly average discharge including a specific flow of 14.8 L s-‘/km*. The Saone flow, characterised by an oceanic pluvial influence, is also subject to Mediterranean effects downstream, added to a slight nival component through the Doubs river which rises in the Jura mountains [4]. The average yearly hydrogram shows high levels in winter with occasional very extended long-lasting floods (Qta = 2 400 m” s-l at Couzon), and an outstanding minimum in August. The Saone river is particularly known for its very gentle longitudinal slope (figure 2A). The average grade of which is calculated as between 10 and 85% of the total length since the mouth is raised 0.00015 m/m [5]. The largest slope variation occurs in the section 200 km upstream (slope: 0.0009 m/m) while stream gradient is subjected to an extreme flatness downstream of the Doubs river junction (Kp 167; slope: 0.00006 m/m). These elements (longitudinal variation of the slope and alluvial plain width) explain the old practice used in navigation to distinguish several “Saones”. The Upper Saline up to Gray (Kp 286) the Small SaGne from Gray -2-

to the mouth of the Doubs (Kp 167), and the Large Sache from Verdun-sur-le-Doubs to Lyons. Today, this division is systematically used by the government department and in all kinds of studies on the river. The critical discharge for downstream floods is 1400 m3 S-’ at Couzon. This value, which is reached almost every year, is less than the theoretical bankfull channel capacity situated around 1.5 years (1 670 m3 s-‘) (figure 3). A typical flood associated with normal precipitation takes 6 to 8 days to develop but, most of the time, the river is subject to successive swellings, which make the recession period longer. Following persistent rains, the flood period may last several weeks. Some historic floods are well documented, for example in 1840 (4 300 m3 s-l), 1955 (2 800 m3 s-l), 1981, 1982 and 1983 (with rates of 2 580, 2 300 and 2 530 m3 s-‘, respectively) (figure 3). The Saone river specificity is due to this significant flood plain which is the result of floods; this represents a determining element for the quality of the environment but can also constrict riparian communities. Because of the quantity of problems and developments induced, the Saone river is now considered as a model for developments [6]. The 140 km linear spit built along the low flow channel since the 1840 disastrous flood today protects a 16 900 ha area of agricultural land. Moreover, in nearly three centuries, the SaGne river has been transformed into a navigable waterway 372 km in length of which 314 km is river. This is due to the concentration of flow using partial narrowing method, the division into 25 levels using dams (figure 2B) and also channel digging by means of several dredgings. The low average slope, its steepening from Anse (Kp 35) and large scale flooding are then the main factors representing Saone river dynamics when discharge is high. The geological history of the basin is linked to those points, and also to the tectonic effects together with fluvial adjusting. The SaBne basin occupies a great part of the Bressan rift created since the Oligocene period and quickly invaded by either sea and lacustrine waters. The present morphology is partly due to Pliocene and Quaternary activity, because the rift was the point of convergence for fluvial and glacial flows. The filling and emptying of the rift during the PlioPleistocene interval are directly linked to the only south exsurgence, sometimes closed, which induced the base level for flows, sedimentation and Bressan erosion [7, 81. Classically, current long profile references can be explained by the persistence of subsidence tectonics during the Holocene, particularly between Verdun-surle-Doubs and Villefranche-sur-SaGne [9, lo]. Recent studies of the dynamics of aggradation since the Late Glacial period can therefore complete the interpretations of the higher slope between Anse and Lyons. The downstream section is the result of accumulation profile realized by a right bank tributary of the SaBne river. Since the final Dryas, in the lower SaGne valley, a gravelly alluvial floor has been built up by the Azergues river in continuation of the lower plain. This accumula-

Energy

gradient

along

the SaBne

river

VOSGES

ms-3

(/’‘, 1000 ‘s

2000

* 3000’ ’ 4000*

HVDSO~TI~~W

:

13OOto1420m ) 1 OOOto1300m 70010 1 OOOm 400 lo 700 m 160 lo 4OOm 10%

w

w P

20%

30x

40%

Sampling of sediment deposition Sampling of suspended sediments Water level recorder

\

Figure

1. The SaBne river basin. A. Hypsometry B. Gumbel law of each water level recorder.

Figure 1. Le bassin B. Relation

of the basin

de la SaBne. A. Hypsometrie du bassin de Gumbel aux stations de jaugeage.

and sampling et sites

-3-

sites

de prelevements

of sediments de sediments

deposition fins

and suspended et de mat&es

sediments. en suspension.

L. Astrade,

J.-P.

Bravard

180 mNGF 400

170

I

160

350

B-

150

300

q

00

upper

300

350

250

200

loo

150

50

SatMe& f

small Sahe

-b

250

W Q U-J

large Sa6ne 4

200

150

t 150

100

river long profiles. A. Water level longitudinal profile from slope averages. C. Water level calculated for ten-year-recurrence

Viomtnil flood

5w

450

350

400

300

250

200

Figure 2. Saone Lyons with 1983 floods.

* KP 0

50

to Lyons. B. Thalweg profile from Corre to [16] and water levels for the 1840, 1955 and

Figure 2. Profils Lyon 1955

en long de la Saone. A. Profil en long du plan d’eau de ViomCnil a Lyon. B. Profil en long du thalweg de Corre a avec pentes moyennes par secteurs. C. Cotes du plan d’eau dtcennal calcult [16] et totes relevees pour les trues de 1840, et 1983.

mhtximirm

disclkrae

‘$r$aximum discharge of : jakifeb. 1995 fhod : I(susp;ended pafliculate m&tars) ’ oveflowing

1

Figure 3. Gumbel floods

considered

Figure 3. Relation des trues

utilisees

law in Couzon for sampling.

water

de Gumbel a la station pour les prelevements.

level

oL&?@d

af kp 31’

5

10

Recurrence

(years)

2

recorder

de Couzon

(Kp (Pk

17) with

maximum

17) avec les debits

-4-

50

discharges maximum

of major des trues

floods

de reference

of the SaBne de la SaGne

river

and of

et de celles

Energy gradient along the SaBneriver tion then induces a fossilization under several metres of sediment of the high slope long profile of the Sa6ne river during the Allerod after a controlled incision by the Rhone from the Boiling. The other effect was to create a dam at the confluence point between the Azergues and SaGne rivers, which can explain, together with tectonics, the fine sedimentation thickness observed in the upstream flood plain, the low slope of the Large SaBne and also the width of the flood plain [2, 31.

3. Methods In order to show the influence of slope variations on the storage and transit of suspended sediments, we studied the evolution of suspended sediment concentration during a typical flood. Spatial variability in sediment deposition granulometry was also investigated in the low flow channel and flood plain interface, following a succession of winter floods. Samples of suspended sediments from the 1995 January-February flood (maximum discharge 1 826 m3 s-‘) were collected from the Trevoux footbridge (Kp 31, figure IA), downstream of the last tributary, the Azergues river, and also at the outlet of the huge flood plain. We collected two samples of surface water every three hours each ten metres away from the low flow channel centre line, during the eleven days of rising waters until the maximum flood flow was reached. Afterwards, sampling was carried out once a day while the waters were rising for the eight other floods which occurred in 1995 (42 samples). Results calculate every mineral element caught by a 0.45 micron membrane (AFNOR standard). The sampling of sediment deposition near the low flow channel was systematically recorded (up to 35 m from the bank) on 16 convex bank areas, characterised by low slope and non-wooded areas (figure IA), after the December 1993-January 1994 flood, which had a maximum discharge of 2 375 m3 s-’ (NB: due to technical constraints, suspended sediment samples could not be coupled with bank samples). The granulometry of the 99 samples obtained was studied, using a TAII Coulter counter. Data processing was done using the CM graph method, also called Passega method [ 111, which consists of linking two parameters of cumulative granulometric curve (M median together with C coarse fraction superior to 99 %) to deposit and transport ways [ 121). Studies carried out on the upper Rhone and Arve rivers showed a changing CM image from one part of a waterway to another depending on data representing slope and flow [ 13, 141; high values for C and M are associated with high turbulence unable to throw coarse particles outside the low flow channel. Thus every section can be linked to a CM image which reflects local flow conditions. The Passega image was applied in conjonction with C and M values taken from this image for all different sampling areas along the Sabne river. This -5-

method was adopted to explain longitudinal variations over different river sections. The second part of this study was to define the hydromorphological functioning of the SaSne river in order to identify longitudinal heterogeneity of the hydrosystem which can induce a longitudinal variation in the sediments described. We applied the principle of spatial variability analysis regarding the energy available. The method consists of studying the longitudinal aspects of several morphological values from Corre (Kp 407) to Lyons: sinuosity, thalweg slope, low flow channel and flood plain length. Hydraulic values have been added: the speed concerning 10 years recurrence flow and stream power (W): W (W/m) = r.g. Q,,.S (1) r representing the water density (1 000 kg/m3), g corresponding to gravitational acceleration (9.8 m/s’), Qt, bankfull channel capacity S, the average slope of water line (in m/m); the specific stream power (w): w (W/m2) = W/l (2) 1 is the length; and shear stress (7): z (N/m2) = r.g.Rh.Se (3) Rh is the hydraulic ray and Se the energy line slope. These values have been calculated for cross-sections of the low flow channel and flood plain at one kilometre intervals [4]. The values of lo-year-recurrence discharge (Q,e) and of bankfull channel capacity (Qb) were applied to every cross-section, considered in the case of rivers like Saone as belonging to a 1.5-year-recurrence [ 151, which is very close to reality in this case. These flows cfigure 1B) were evaluated using data from eight water level recorders (by adjusting a series of maximum annual floods between 1924 and 1983 with regard to the Gumbel law). From Corre to the Doubs confluence, available data given by stations were completed by an intermediary collection point taking into account the main tributaries. Downstream, we used the linear interpolation method between measurement points to calculate the flows, in order to better integrate the flood removal capacity. From Kp 0 to 211, the water level for Q,a was calculated for each cross-section following 1955 and 1983 floods [ 161. Upstream we used the linear interpolation method from Kp 211 to Ray-sur-Saone (Kp 325), then from Ray-sur-Saone to Cendrecourt (Kp 392). Without any measurement, we returned the values for bankfull channel capacity, this choice could be acceptable due to the weakness of slopes and because the aim was to point out discontinuities. Calculation of the Saone flooding area removal capacity (c) was carried out in 27 places, for lo-year-recurrence discharge and intermediary catchment areas surfaces (S) [ 171: c = Q,o/So.75 (4) Low c values can be explained by an increase in flow which is not proportional to the increase of drained surface downstream. This raises two different removal areas, including c index values between 1 to 1.I: the Small SaBne from KP 300 to Kp 170 and the Large SaBne from KP 120 to Kp 15 (figure 4).

L. Astrade, J.-P. Bravard

Cp 2

140 120 loo 80

B 5 60

BlfJ1

E

I

40

A-

Figure 4. A. Specific stream power of the SaBne river from Corre to Lyons. B. Removal capacity of SaBne river flooding (the greater c is than 1 the less the discharge increases proportionally to the basin surface and the more the floodplain has an important role in removal). C. Granulometry of sediments deposited by the 1993 December-1994 January flood. Figure 4. A. Puissance spCcifique de la SaBne de Corre a Lyon. B. CapacitC d’CcrCtement du champ d’inondation de 1, moins le dCbit augmente C. GranulomCtrie des sCdiments

4. Variability and available

of sediment energy

proportionnellement deposCs par la true

& la surface du bassin de dCcembre 1993-janvier

et plus 1994.

les

plaines

jouent

un

(plus c’est proche rBle

d’tcr&tement).

The maximum diameter of particles (C) observed for sediment deposition ranged between 150 and 300 pm, with a median (M) between 25 and 140 pm. On a CM image this distribution indicates mainly a high homogeneity and a low flow turbulence due to the fact that sediments sampled nearness of the low flow channel are issued from a uniform suspended transport (section RS of Passega image) (figure 6). Those values are very different from those obtained in the flood plain of the Rhone which is the centre of deposits issued from graduated suspensions, elements representing high turbulence in the low flow channel and its margins. Nevertheless, the inter-site variability of M and C values of the flood plain sediments cause differences in the capacity to put fine grain sizes in suspension figure 4C). The Saane river near Mdcon plain presents the lowest C and M values (35 pm and 150 ,um) even though the section located downstream of Anse shows coarser granulometry (respectively 240 pm and 90 pm).

load

Suspended sediment concentrations show a great homogeneity between both sampling series studied, which includes flood concentration and discharge (figure 5A). A maximum value of 70 mg L-’ was observed in the rising water period situated exactly six days and eighteen hours prior to maximum discharge. We were able to synchronize maximum values of the first overflow which occurred at the sampling point (1 400 m3 s-‘) with the theoretical bankfull channel capacity (1 700 m3 SK’) calculated at the Couzon station (14 km downstream). This result induces the fact that the more efficient discharges for sediment transportation, as well as for their deposit in the flood plain, are nearer to bankfull capacity than extreme events. Samplings made on the other eight 1995 floods also confirmed the determinant role of the first winter floods because concentrations were about two times higher than durin the January-February flood (maximum of 127 mg L- f for a 1 900 m3 s-’ flood). Data obtained on other hydrosystems using the Meybeck diagram [IS], indicate a relatively low suspended sediment concentration in the downstream part of the SaGne river (figure 5B).

By studying longitudinal variations of stream velocity and hydraulic forces, we quantified the distribution of the SaGne river energy. For example, the stream power is inversely reflected by the loss of energy in the waterway, and the sediment drive is indicated by the shear stress. These variables are mainly controlled by the water slope during high-water periods, which is how-6-

Energy

70

along the

gradient

SaBne

river

mg.l-1 -

bankiull channel capacity calculated at Couzon (Kp 17)

Overflows on lower banks

, m3s4

20 -I 1100

1200

1300

1400

1500

1600

1700

A : Little 1965 B : South 1969

1800

1900

Colorado, Cameron, (US Geol. Survey) Saskatchewan, Lemsford, (Env. Canada)

C : Fraser, Hope, 1969 (Env. Canada) D : MBkong, Mukdahan, 1962 (Hydr. Ann. MBkong) E : Peace River, Peace 1974 (Env. Canada)

River,

F : RhBne, Scex (Meybeck, 1970 & 1972) G : St-Lawrence, (Env. Canada)

La Salle

SAONE (l/s/km*)

Figure 5. Suspended sediment concentration in TrCvoux (Kp 31). A. Relationship between discharge and suspended sediment concentration for the 1995 January-February flood. B. Evolution of suspended sediment concentration for several rivers [IS]; position of the 1995 January-February SaBne flood and of the total floods for 1995. Figure 5. Concentration de! mat&es en suspension B TrCvoux (Pk 3 I). A. Rapport entre le dCbit et la concentration pour la true de janvier-fkvrier 1995. B. Evolutions comparCes des concentrations pour des rivikres de rtgimes varits [ 181 ; position de la true de la SaBne de janvier-fkvrier 1995 et de l’ensemble des trues de 1995.

ever determined by the flood plain and thalweg longitudinal profiles (figure 2B). The thalweg exhibits a horizontal slope until Mscon (0.00001 m/m from Kp 167 to Kp 81), and shifts thereby toward Anse where slopes become similar to that of the upstream sectors (0.00021 m/m). As a consequence of this morphology, the decennial flooding water slope is 0.00024 m/m between Anse and Lyons, compare to 0.00005 m/m upstream @gure 2C). Average speed calculated for Qlo is 0.60 m s-‘, also indicating high longitudinal discontinuities. Even though speed is higher due to narrowing between the -7-

SaBne and Daubs levees (0.8 m s-’ between Kp 167 and Kp 142), the average speed for Qlo is only 0.4 m s-’ from Chalon-sur-SaGne (Kp 142) to Anse. It then increases uniformly with slope steepness and flood plain narrowing to seven time faster (1 to 2.7 m s-‘) and recovers values close to those at Lyons (from Kp 21). PardC 1191 considered the waterflow as “very low”, pointing out that for major floods speeds reached 1.5 m s-’ at Chalon-sur-Saane, M&on and Villefranchesur-Sa8ne. However these measurements are really exceptional, corresponding to stations in the middle of urban centres which are protected by narrow levees,

L. Astrade,

J.-P.

h&dim 20

30

40

50

00

Bravard

M 80

200mhs

100

Figure 6. CM image of sediment deposition of the 1993 December-1994 January SaBne flood. Samplings are grouped by site (Kp). Arrows represent the evolution of the uniform suspended transport in the hydrosystem. Figure 6. Image CM des stdiments dtposCs par la true de la SaBne de dtcembre 1993-janvier 1994. Les Cchantillons sent regroup& par site (Pk).

Les

fll?ches

reprcsentent

I’tvolution

de la suspension

confirming the slowness of SaGne river. By comparison, the average speed of the Rhone at Kp 142 (Chautagne) reaches 2.84 n-? s-l for a 1 144 m3 s-’ discharge [20]. Figures on specific stream powers confirm the longitudinal heterogeneity of the SaGne river (figure 4A). In the Upper SaBne (average of 9 W mm2), the incidence of the Doubs river floods and levees is determinant upstream and downstream of the confluence. A clear opposition exists between the low stream power section from Chalon-sur-SaBne to Anse (3.1 W m-‘) and the downstream section, which has a stream power nine times higher (27.7 W mm2) due to the increase of slope and decrease of the bed length. Compared to rivers in the Alps and the Massif Central piemonts, the specific stream power is about 100 to 150 W me2 for the Ardeche river and about 66 to 144 W rn-’ for the Dr6me river. These values were calculated with a recurrence of bankfull channel capacity inferior to that of the SaGne river

uniforme

de l’amont

vers

I’aval

de l’hydrosysttme.

Nevertheless, for the latter section, observations allow us to divide it in two parts on each side of Anse, forming a Flat Large SaBne and a Downstream Large SaBne. The reach centred on Verdun-sur-le-Doubs (70 km) is highly influenced by the Doubs river confluence as well as by embanking. The Flat Large SaBne (105 km between Chalon-sur-SaGne and Anse) is the most representative section of the river due to the weakness of values describing energy and flood plain area, particularly upstream of Thoissey (Kp 62), where the first signs of the last 35 km of the watercourse steepness begin. The Downstream Large SaBne is considered as a typical reach of the overall hydrosystem. Unlike the Flat Large SaBne, although situated immediately downstream of it, this reach has high stream velocity and at the same time the highest stream power, in addition to the fact that it is situated in a place where the watercourse and the plain are extensively used and managed [22].

1211.

According to some authors [ 10, 23, 241, the typical form of the longitudinal profile is the result of downthrow tectonic movements and regional subsidence variations. The Bresse Fosse subsidence would lead to a tilting movement of the long profile sustained throughout the Quaternary added to a relative survey of the south of Bresse with regard to the north. The latter interpretations favour combining this hypothesis together with the ones on fluvial control [3]. The sector of confluence with the Azergues river seems to be a limit point between the Sa6ne river upstream of Anse, which undergoes subsidence and damming effects from the alluvial fan of the Azergues river, and the SaBne river downstream. These elements are above all influenced by the complex role of long profiles fluctuations of the Rhone and Azergues rivers under hydroclimatic control.

Finally, using the global shear stress calculated from depth and slope indications, we find local effects as well as two distinct sections, from Chalon-sur-Saane to Anse in one part (2.8 N m-*) and the downstream section in second part (20.6 N m-*).

5. Conclusion By combining morphological and hydrological variables, we have highlighted longitudinal discontinuities of the SaBne river hydrosystem (figure 7). The principal divisions of the river are situated at Heuilley-sur-SaBne (Kp 255), Verdun-sur-le-Doubs (Kp 167) and Anse (Kp 35), confirming traditional partitioning of the river into an Upper SaBne, Small SaBne and Large SaBne. -8-

Energy gradient along the SaBne river weighted

variables A.-

I I IJ St-JEAN-de-LOSNE \

I

I.

,I

Kp MO-Icp62

I

m

-

I

THOISSEY I I -1 : ANSE \ N;“VILLE S/Sk \ LYON-VAISE , I / I ’ -’

-+

I@7-l(po I

Figure 7. Homogeneous sections of the SaGne river from Corre to Lyons based on upstream to downstream evolution of the determinant variables. In order to compare variables, values have been weighted from 0 (lowest value obtained for all variables) to 1 (highest value). Figure 7. Les troncons homogenes de la SaBne de Corre a Lyon, definis sur ia base de l’evolution amont-aval de variables-cl&.. Afin de comparer les variables entre elles, les valeurs ont CtC ponderees de 0 (valeur la plus faible obtenue pour chaque variable) a 1 (valeur la plus ClevCe).

The weakness and longitudinal variations of the SaGne river slope have direct effects on suspended transport conditions, particularly along the Flat Large SaBne. The average width of the decennial flood plain reaches 2400 m on up to a 150-km-long stream section between -9-

Heuilley-sur-SaBne and MBcon, with many sectors longer than 3500 m. Potential flood areas could thus reach 260 000 ha, corresponding to 9% of the basin. Flooding areas of the SaGne river show two major removal zones near the confluence with the Doubs river;

L. Astrade, J.-P. Bravard one from

Seurre to Saint-Jean-de-Losne situated on the from Chalon-sur-Saane to Mgcon, which mainly removes the Doubs river flooding (figure 4B). The loss of energy seems to have a determinant role in the suspended charge transit and in the storage of deposited sediment in the flood plain. Flood water decantation can easily explain part of the weakness of sediment concentrations observed in the downstream part of the plain, in TrCvoux. Low energy, unable to produce significant turbulence in the overflow area explains granulometry lower than that of the upstream section of the SaBne river and especially of the Downstream Large SaBne which undergoes higher energy effects (figure 4). Small

SaBne, and the other

The sectorisation of the Sabne hydrosystem, together with field data, shows a high correlation between available energy variability at each section and variability of the sediment load. The main conclusion of this study is that the slowness of the SaGne river in flood, the exceptional term of overflowing, the low concentrations of suspended sediment downstream together with fine granulometry of bank deposits have a good correlation with the energy of the fluvial system. The complex geological history of the basin added to the river development seem to be the principle factors in the Sabne flood dynamics and sedimentary budget due to their effects on the long profile and on the flood plain characteristics.

Acknowledgements This study is part of the Ph.D. thesis defended in Paris in 1996 by the principal authors and supervised by Prof. J.P. Bravard. The authors are particularly grateful to the Agence de l’eau Rh8ne-MCditerranCe-Corse which kindly provided topographic data, to the Laboratoire de GCographie Physique of the Jean Moulin-Lyon III University and also the Compagnie Nationale du Rh6ne which supplied the equipment requested for sediment analysis. Blandine Astrade provided the English version of the text.

References [I] Allen J.R.L., A review of the origin and characteristics recent alluvial sediments, Sedimentology 5 (1965) 89-191.

of

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