Laura Tiron Duţu
email: laura.dutu@geoecomar.ro.

Mireille Provansal
email: provansal@cerege.fr.

Jérôme Le Coz
email: jerome.lecoz@irstea.fr.

Florin Duţu

Contrasted sediment processes and morphological adjustments in three successive cutoff meanders of the Danube delta

Keywords: hydrosedimentary processes, river morphodynamics, cutoff meander, river training, Danube delta, acoustic Doppler current profiler (ADCP)

.

This paper selects three large cutoff meander reaches of the St. George branch (the Mahmudia, Dunavăţ de Sus, and Dunavăţ de Jos meanders noted here as M1, M2, and M3, respectively) as an example to analyse the human impact in the Danube River delta. The diversion of the flow induces strong modifications by acceleration of the fluxes through the artificial canals combined with dramatically enhanced deposition in the former meander where observed in two cases (M1 and M3) and slight modifications in M2. An exceptional flood that occurred in April 2006 offered a good opportunity for scanning different cross sections of the meander systems. Bathymetry, flow velocity, suspended-load concentration, and liquid and solid discharge data were acquired throughout several cross sections of both natural channels and artificial canals of the three cutoffs, using acoustic Doppler current profiler (ADCP) technology, in order to investigate the distribution of the flow and sediment and its impact on the hydrosedimentary processes in each channelized reach and adjacent former meander. Therefore, the results obtained during the 2006 flood were referred to a long-term evolution , analyzed by GIS technics.

Introduction

Morphological processes of the fluvial channel are controlled by liquid and solid fluxes through hydraulic forces exerted by the flow and sediment transport, erosion, and deposition, especially in the flood regime [START_REF] Bridge | Rivers and Floodplains: Forms, Processes, and Sedimentary Record[END_REF]. The geomorphologists particularly underlined the effectiveness of the bankfull discharge, renowned as the most morphogenic, i.e., the main long-term control factor of erosion or accumulation of sediments on river banks and bottom channel [START_REF] Kondolf | Tools in Fluvial Geomorphology[END_REF]. In this study, we use the definitions of bankfull proposed by [START_REF] Wolman | Magnitude and frequency of forces in geomorphic processes[END_REF] and [START_REF] Leopold | Fluvial Processes in Geomorphology[END_REF]: bankfull discharge is the stream flow that fills the main channel and begins to spill water into the active floodplain.

The bankfull stage approximates the stage at which the channel and the floodplain begin to be connected [START_REF] Rosgen | A classification of natural rivers[END_REF][START_REF] Knighton | Fluvial Forms and Processes[END_REF].

The climatic changes and the anthropogenic pressure in the last 100 years have transformed the majority of the river hydrosystems. Distributary network geometry is the most important factor controlling delta landforms [START_REF] Syvitski | Distributary channels and their impact on sediment dispersal[END_REF] and related hydrological, geological, and sedimentological processes [START_REF] Hood | Delta distributary dynamics in the Skagit River delta (Washington, USA): extending, testing, and applying avultion theory in a tidal system[END_REF]. Disturbances like river training operations, such as meander cutoff initiated for navigational or flood mitigation purposes, often lead to dramatic changes in the streamwise profiles [START_REF] Hooke | Changes in meander morphology[END_REF][START_REF] Kesel | Human modifications to the sediment regime of the lower Mississippi River flood plain[END_REF][START_REF] Kiss | Alterations of channel parameters in response to river regulation works since 1840 on the lower Tisza River (Hungary)[END_REF]. The evaluation of long-term effects of a meander cuttof project requires estimation of the long-term geomorphology of both the former meander and the man-made canal. The initial flow and sediment capacity will change with time as the hydraulic efficiency of the flow pathways within the former meander changes owing to the geomorphological evolution. Changes in the river bed associated with modified long-term sediment transport capacity may affect the stage-discharge relationship at the diversion site [START_REF] Letter | River Diversions and Shoaling[END_REF]. The impacts of a single, small-scale meander correction may have a minimal effect on the system, but the cumulative effects of a number of small diversions can become significant for the entire system. The paper presented here provides a case study of anthropic changes in the morphogenesis and hydrosedimentary dynamics of a set of rectified meanders. The main objective is estimating the intensity of the human activity impacts to river flow and sediment regime and to observe the different morphological responses. This paper expands to two downstream meanders the scope of a previous work about the flow and sediment processes during episodic flooding in the Mahmudia meander [START_REF] Jugaru Tiron | Flow and sediment processes in a cutoff meander of the Danube delta during episodic flooding[END_REF], to underline the contrasting processes and evolution between the three cutoffs, and to look for the relevant factors that explain different responses.

2.

Regional setting

Geographical context

With a mean annual water discharge of 1365 m 3 .s -1 and mean annual suspended sediment flux of 317 kg.s -1 , the St. George branch is the most southern branch of the Danube delta (5600 km 2 ), downstream of the main contributor of fresh water and sediment into the Black Sea, the Danube River. The St. George branch starts out from a bifurcation at Ceatal St.

George located 108 km upstream of the outlet to the Black Sea (Fig. 1) and carries about 24% of the water discharge and 21% of the suspended sediment discharge of the Danube [START_REF] Bondar | The Danube delta hydrologic database and modeling[END_REF]. The banks are bounded by a rich and continuous vegetal coverage (trees, poplars, and reeds). The St. George branch is highly sinuous and still morphologically dynamic as attested by migrating meanders [START_REF] Popa | Environment changes in the Danube delta caused by the hydrotechnical works on the St. George branch[END_REF][START_REF] Jugaru | Apports des Systèmes d'Information Géographiques à la perception des changements morphodynamiques (1970-2000) dans le delta du Danube[END_REF][START_REF] Tiron | Delta du Danube -Bras de St. George. Mobilité morphologique et dynamique hydrosédimentaire depuis 150 ans[END_REF] and by the fast evolution of its mouth [START_REF] Panin | The Danube delta. Geomorphology and Holocene evolution: a synthesis[END_REF].

The study area consists of three former meanders located in the middle part of the St.

George branch (Fig. 1): Mahmudia, Dunavăţ de Sus, and Dunavăţ de Jos meanders, named hereafter M1, M2, and M3, respectively. The M1 is the largest and the most complex meander in the St. George branch. It is located between km 84 and km 64 and results from the river impingement against the Mahmudia hills that deflected the St. George channel into a large meander loop named the Mahmudia-Uzlina meander bend [START_REF] Panin | The Danube delta. Geomorphology and Holocene evolution: a synthesis[END_REF]. Downstream, meanders M2 and M3 derive from one big fossil meander formed between 9000 and 7200 y BP and degraded over time [START_REF] Panin | Some aspects of fluvial and marine processes in Danube delta. Institutul de geologie si geofizica[END_REF]. The sediments of the submerged banks are fine and differently distributed according to the channel type: coarse silt to fine sand (between 20 and 135 μm) along the man-made canals, concave banks, and at the bifurcation/junctions between the natural/artificial channels and clay to fine silt (between 10 and 20 μm) along the former meanders [START_REF] Tiron | Dynamique sédimentaire dans un milieu deltaïque -Le Bras de St. George dans le delta du Danube[END_REF].

Control factors

Within the lower Danube, two barrages (Iron Gates I andII, built up in 1970 and1984, respectively) and the hydrotechnical regulation works along the Danube tributaries have dramatically decreased the sediment discharge at the Danube mouths (within 25-30%) [START_REF] Panin | Danube River sediment input and its interaction with the north-western Black Sea[END_REF][START_REF] Panin | The Danube delta. Geomorphology and Holocene evolution: a synthesis[END_REF].

Along the St. George distributary, the cutoff of the natural channels by navigational canals between 1984 and 1988 caused dramatic changes in the local distribution of river flow velocities, discharge, and sediment fluxes [START_REF] Ichim | Efectele barajelor in dinamica reliefului. Abordare geomorfologica[END_REF][START_REF] Popa | Environment changes in the Danube delta caused by the hydrotechnical works on the St. George branch[END_REF][START_REF] Panin | The Danube delta. Geomorphology and Holocene evolution: a synthesis[END_REF][START_REF] Tiron | Delta du Danube -Bras de St. George. Mobilité morphologique et dynamique hydrosédimentaire depuis 150 ans[END_REF]. Such river training works caused a change in the distribution of the solid as well as the liquid discharges in the whole delta, increasing fluxes through the St. George branch [START_REF] Popa | Environment changes in the Danube delta caused by the hydrotechnical works on the St. George branch[END_REF][START_REF] Panin | Danube River sediment input and its interaction with the north-western Black Sea[END_REF][START_REF] Panin | The Danube delta. Geomorphology and Holocene evolution: a synthesis[END_REF]. Locally, earth dikes (with 2-3 m in height) were built in the 1980s along the banks.

An experimental bathymetrycal survey between 1990 and 1996 [START_REF] Popa | Environment changes in the Danube delta caused by the hydrotechnical works on the St. George branch[END_REF] showed different initial responses of the three meanders to the cutoff straightening: vertical aggradations in M1 and M3 (maximum rate of 11 m and 9.6 m.y -1 , respectively) and no important influence of the artificial canal on M2.

In a short period of time (1970)(1971)(1972)(1973)(1974)(1975)(1976)(1977)(1978)(1979)(1980)(1981)(1982)(1983)(1984)(1985)(1986)(1987)(1988), the evolution of the study area was affected by two different impacts: at large and local scales. The reduction of sedimentary discharge, owed to the two most recent dams constructed upstream in the Danube basin (between 1970 and 1984), is overlapped by a shorter and spatially more differentiated evolution, owed to meander straightening (between 1984 and 1988). The two anthropogenic factors (damming and cutoffs) have significantly altered the flow and sediment dynamics of the river network.

The former meanders and the newly built channels evolve differently, as a result of the fundamental change of the water and sediment transport processes.

Hydraulic conditions during field observations

The measurements were made at the end of the maximum pick of a flood period that started at the beginning of April 2006 at high spring waters. During the measurements, the Danube flow at its entrance into the delta was 16,500 m 3 .s -1 , and St. George branch carried out ~ 4600 m 3 .s -1 . During the measurements (28-30 April 2006), the water flow discharge remained almost constant.

3.

Materials and methods

Hydroacoustic measurements (ADCP)

The recent application of ADCP (acoustic Doppler current profilers) to river discharge measurements has demonstrated their efficiency and reliability [START_REF] Yorke | Measuring river velocity and discharge with acoustic Doppler profilers[END_REF]. The data analyzed in this study were acquired with an ADCP Workhorse Sentinel 600 kHz (Teledyne RDI) mounted on a power boat [START_REF] Jugaru Tiron | Flow and sediment processes in a cutoff meander of the Danube delta during episodic flooding[END_REF].

During the field campaign, transverse ADCP profiles were systematically completed at relevant sections of the three studied meanders: around the upstream bifurcations (sectors A and A'), around the downstream confluences (sectors C, C', and C"), and along the cutoff meanders in the apex zones (profiles B, B', and B"). The marks 1, 2, and 3 describe the position of each profile in the sector: location on the natural single upstream channel (1), on the former meander (2), and on the artificial canal (3). In addition to the bottom-track reference used for ADCP velocity and path measurements, the data were continuously georeferenced by a vessel-mounted DGPS (ensemble pivot with mobile Trimble monofrequency ProXR). The bathymetry, water velocity, and discharge data provided by the four beams were handled by means of the WinRiver Teledyne RDI software, version 1.06.

Suspended sediment concentrations

The ADCP records the acoustic backscatter intensity (in dB) returned from scattering particles. This intensity is considered as an indicator of suspended-load concentration (SSC; [START_REF] Gartner | Estimating suspended solids concentrations from backscatter intensity measured by acoustic Doppler current profiler in San Francisco Bay, California[END_REF][START_REF] Dinehart | Repeated surveys by acoustic Doppler current profiler for flow and sediment dynamics in a tidal river[END_REF][START_REF] Kostaschuk | Measuring flow velocity and sediment transport with an acoustic Doppler current profiler[END_REF] and by calibration operations is assumed to be reliable enough for further assessment of the SSC distribution across sections and SSC flux estimates. The calibration methods for converting the backscatter intensity into suspended sediment concentration (SSC) require extensive sediment sampling operations.

Consequently, in several selected cross sections for this study, stationary ADCP in situ measurements were coupled with water sampling. Technical approaches about the water sampling and calibration methods were developed by [START_REF] Jugaru Tiron | Flow and sediment processes in a cutoff meander of the Danube delta during episodic flooding[END_REF]. The calibration and post-processing of ADCP data for SSC analysis were conducted using the Sediview software [START_REF] Land | Accoustic measurement of sediment flux in rivers and near-shore waters[END_REF].

Imagery analyses

Geographic information system (GIS) tools (MapInfo 6) were used to compare multitemporal topographical maps (between 1970 and 1984) and remote sensing data of Landsat 5 TM (1989) and Landsat 7 ETM+ (2000 and 2006) (Table 1). The TM and ETM+ data had 28, 15, and 20 m pixels. Further details of the map analyses, including estimation of rectification and digization error, have been previously described [START_REF] Elliott | Channel-pattern Adjustments and Geomorphic Characteristics of Elkhead Creek[END_REF][START_REF] Hood | Indirect environmental effects of dikes on estuarine tidal channels: thinking outside of the dike for habitat restoration and monitoring[END_REF][START_REF] Hood | A conceptual model of depositional, rather than erosional, tidal channel development in the rapidly prograding Skagit River delta (Washington, USA)[END_REF][START_REF] Hughes | Accuracy assessment of georectified aerial photographs: implications for measuring lateral channel movement in a GIS[END_REF]. The error sources (RMSE) include different spatial resolution of TM and ETM+ data, inaccuracies from manual delineation of banklines, water level differences, effects of vegetation, and other systematic errors. Even collectively considered, the errors are small relative to the magnitude of the overall planform changes observed in the delta [START_REF] Tiron | Delta du Danube -Bras de St. George. Mobilité morphologique et dynamique hydrosédimentaire depuis 150 ans[END_REF]. Banks-lines were defined by the abrupt transition from vegetated to unvegetated areas to determine the accretion and erosion areas because of the meander mobility in the period between different image acquisitions. A negative value was interpreted as erosion, a positive value as accretion, and a very low (close to zero) or zero value as stable areas.

Results

Flow and sediment processes at the reach scale

Authors like [START_REF] Shields | Sediment deposition in cutoff meander bends and implications for effective management[END_REF] (Table 2; Fig. 2) enhances here incision processes, while the liquid and solid fluxes through the former meander are greatly reduced, especially downstream of the first meander apex. In the former meander M1, down to very low values favourable to sediment deposition (0.8 to 0.1 m.s -1 between profiles A2 and C2). Around the apex zone (B), channel asymmetry produces the highest velocities near the concave bank (maximum of 1.2 m.s -1 ). The former main channel undergoes very visible infilling processes. These processes are attested by the high values of the three mentioned indicators: the ratio between the former channel length and the new channel length (4.4), the diversion angle (~ 70°), and the bed level difference between A1/A2 of 6.4 m and between A1/A3 of -4 m (Fig. 3).

Fig. 2. Distribution of local velocity magnitude within cross sections of the meanders M1, M2, and M3

Author-produced version of the article published in Geomorphology ( 2014

Table 2. Hydrodynamical and sedimentary parameters in investigated cross-sections

Fig. 3. The thalweg along the natural channel (A1-C1") and the artificial canal (A3-C3, A3'-C1'-C3").

M2 -Dunavăţ de Sus meander. The cutoff branch M2 takes over 85% of the water discharge. The sum of the liquid fluxes of the two branches A2' + A3' (2220 + 400 = 2620 m 3 .s -1 ) is lower but comparable with A1', upstream of the bifurcation (2650 m 3 .s -1 ). The water flow acceleration in the natural channel shows the presence of incision processes at the bifurcation sector (A1'-A2') certified by the high velocities values, the low bed level difference between the two sections (-2 m), the ratio between the former channel length and the new channel length (2.06), and by the value of the diversion angle (~ 10°). Downstream, along the artificial canal, the bed level difference is positive between A1' and A3' (2.1 m).

The ADCP image confirms the presence of the gravitationally related sedimentary flow (Fig. 4). Downstream, through the former meander, both water and sedimentary flows are diminished. The apex zone (B') is an incision active zone characterised by high velocities (between 1.2 and 2.2 m.s -1 in the central part), but even along the former meander the maximum velocity is high (1.6 m.s -1 in A2') (Table 2; Fig. 2). We may suppose that this zone provides the sediment supply for the next section.

M3 -Dunavăţ de Jos meander.

The water fluxes at the bifurcation of the M3 are distributed almost equally between the former meander (B" = 1047 m 3 .s -1 ) and the artificial canal (C3" = 1342 m 3 .s -1 ), with a slightly higher flux in the artificial canal (≈ 56% of total).

The almost equal distribution of the water flow between the two channels involves erosion processes localized in the first part of the former meander (between C1' and B"), but also in the apex zone (B") and a low sedimentation in the second downstream part (between B" and C2"). The high velocity values (between 1.2 and 1.6 m.s -1 ) enhance gravitationally related sedimentary flow with high concentration (~ 160 mg.l -1 ). In the former meander M3, the velocity is reduced in the apex zone (maximum of 1.3 m.s -1 in B") and also upstream of the confluence (maximum of 1.4 m.s -1 in C2"). The junction zone (C2"-C1") corresponds to a diminishing of the sediment concentrations (~ 110 mg.l -1 ). The ratio between the former channel length and the new channel length is 2.73, the diversion angle ~ 50°, and the bed level difference is positive between C1'/B" at 9 m and also between C1'/C3" at 6 m (Fig. 3).

Sedimentary concentrations patterns

The distribution of SS concentrations varies with every investigated section, depending on the local morphology. On the symmetrical sections A3, C3, and C3" (Fig. 4), the concentrations are found to be higher near the bed (> 180 mg.l -1 ) and the banks and lower (< 60 mg.l -1 ) near the free-surface and the central part of the channel.

Fig. 4. Distribution of local SSC within cross sections of the meanders M1, M2, and M3

The sections situated near the bifurcations (A1, A3, A1', A3', and C'), in the apex of the former meanders (B, B', and B") and close to the confluence (C1, C3, C1", and C3"), are characterized by the presence of SSC plumes issued from the resuspension of bed material (Fig. 4). The concentrations range between minimal values of 10-20 mg.l -1 and maximal values exceeding 180 mg.l -1 . Some vertical plumes with high SSC (A1, B, C1, A1', A2', A3', B', B", C3", C2", and C1") suggest that gravitationally related sedimentary flows are eroding sediments from the bottom layer. They are localized near the less steep slopes of convex banks (sections A1, B, and B') and also in deep sections (C1, C3, and C1").

Sediment budget

The total suspension fluxes through each investigated cross section (Table 2; Fig. 4) can be compared by drawing a balance (cf. Fig. 5) of the ways of transport and of their relationship to the hydrodynamics and morphology of the channel. ) and section A1 (338 kg.s -1 ) is 19 kg.s -1 . This relative variation, < 6%, is not significant in comparison with measurement uncertainties.

In the former meander M1, the sediment transit is weak and progressively decreases with higher by 17 kg.s -1 than the input A1' (328 kg.s -1 ). This low difference shows possible erosion in the bifurcation area, but it also could be explained by the potential measurement uncertainties. The retention of 34% of the sedimentary input in the former meander indicates a deposition area, localized precisely in the downstream sector.

In M3, the sum of output C2"+C3" (180 + 145 = 325 kg.s -1 ) is significantly greater than the input flux C1' (214 kg.s -1 ). This difference shows a sedimentary supply of 111 kg.s -1 from the bed or bank erosion between C1' and C3" or/and C1' and C2". The former meander receives a reduced solid flux with low SSC (81 mg.l -1 ); the SS concentrations are increasing on natural and artificial channels up to 166 (C2") and 108 mg.l -1 (C3"), which is additional evidence of the erosion processes located in the former meander or in the artificial canal.

The sedimentary fluxes are variable along the three studied reaches. In M1, for an input volume of ~ 18 x 10 3 m 3 /d (A1), the output is ~ 22 x 10 3 m 3 /d (C1). The sedimentary volume deposited within the former meander between A2 and C2 can be estimated at ~ 3.5 x 10 3 m 3 /d. The total erosion in M1 should therefore be explained by a sediment supply from erosion of the artificial channel around 7.5 x 10 3 m 3 /d. Five kilometres downstream at the entrance to M2 the sedimentary input volume is ~ 17 x 10 3 m 3 /d (A1'), which is < M1 output.

The volume of the deposited sediments within the former meanders M2 (between A2' and C1') and M3 (between C1' and C2") can be estimated to be, respectively, ≈ 6.22 x 10 3 and ≈ 3.5 x 10 3 m 3 /d. These results will be discussed later.

Overview of imagery analyses

Channel width evolution between 1970 and 2006

Channel widths are the distance from bank edge to the opposite bank edge, measured perpendicular to the channel thalwegs every kilometre streamwise. By superimposition of maps and satellite images, we can draw a first evolution diagram of the channel width between 1970 and 1984 and between 1989 and 2006, i.e., before and after the meanders rectification, respectively (Fig. 6).

Overall, the channel width decreases before the artificial works. Some sections are very active (A2, B, B', C1', and C1") with a maximum of -5.7 m.y -1 at km 84 on A2); and others are almost stable, close to the error limit.

After the cutoff works, the narrowing became more intensive. On M1 the retraction is moderate between km 86 and km 76 (-2.5 m.y -1 ), more important between km 75 and km 72 (-5.6 m.y -1 ) and lower downstream of km 71 (-0.1 and -0.5 m.y -1 ). Higher values (> -3 m.y -1 ) are localized in the apex zone at km 81 (B). Downstream, the narrowing of the channels of M2 and M3 is also generalised. The average evolution rate is slightly positive until 1990:

enlargement of the channel with a maximum of 1.8 m.y -1 at km 61 (on C1); and negative between 1990 and 2000: retraction of the channel with maximum of -5.1 m.y -1 .

Fig. 6. Channel width evolution of the three meanders between 1970 and 2006 (the values in 1970 are taken equal to zero as the reference situation values)

Channel mobility between 1970 and 2006

An indicator of channel behaviour is the lateral shift, involving processes such as bank erosion, bend migration, and channel avulsion [START_REF] Leys | River channel planform change: software for historical analysis[END_REF]. At the entrance of M1, the channel between km 85 and km 79 is characterized by continuous aggradation on both banks; the channel width was strongly reduced. Sediment infilling is revealed by the creation of many internal islands along the former meander. The morphology of the confluence is influenced by the junction with the artificial canal with a progradation of the convex bank at km 64 (Fig. 7).

Downstream, sedimentation is the dominant process, located on both channel banks, along concave, convex, and straight reaches, with a visible preponderance on the left bank (spatial distribution of the flow energy diminished on the left bank). The junction with the artificial canals modifies the channel in the flow direction at km 54.5. Locally, some erosional areas are present (km 59, km 54.5, and km 52).

Fig. 7. Channel mobility of the three meanders between 1970 and 2006

The sinuosity of the entire study reach (from km 84 to km 49, measured along the natural channel) has increased from 2.43 (1970) to 2.52 (2006). For the same period, the sinuosity of M1 has increase from 4.26 to 4.33. M2 registered a slight sinuosity decrease (from 1.95 to 1.91), and M3 is the only former meander with stable sinuosity after the cutoff program (Table 3).

The contrasted behaviours of the three former meanders over the last four decades are summarized as follows: M1 -marked deposits (banks and islands) in the natural channel, with a retraction of the active width; M2 and M3 -still active dynamics (visible migration by accretion of the banks and slight erosion zones).

Table 3. Morphometric characteristics of the three meanders (where S = actual path length/shortest path length and diversion angle = angle between the former meander axis and the upstream single channel axis)

5.

Discussion

Former meanders are generally sediment storage locations (by progressive sedimentation of the channel bed and lateral banks, canals, and overflow). Nevertheless, each meander has a particular hydrosedimentary performance. In our case, there is no similarity between the three consecutive former meanders. The former meander M1 has the particularity of a large and complex cutoff, with two sinuous reaches located downstream of the bifurcation (which induces velocity decreases). At the bifurcation/confluence nodal points and in the artificial canals, a most intense hydrodynamic activity (high flow velocity) is present. These areas represent transit zones and possible sediment sources (by erosional processes) for the downstream areas. Therefore, in each case the artificial canals had a different impact on the meander's evolution, depending on several factors. A full picture of channel shape is composed through the combination of channel sinuosity for particular sections of the channel, channel meander wavelength, radius of curvature, channel width, and channel depth.

Author-produced version of the article published in Geomorphology ( 2014), Vol. 204, Pages 154-164 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2013.07.035

Long-term evolution

Between 1970 and 1984 (the period before the meander's rectification), the evolution of the three meanders is marked by a generally slow retraction, with local enlargement of the channel width.

After 1984, the meander's rectification introduces a rupture of the natural bends evolution, and the narrowing becomes the dominant process. After the meander's rectification, the retraction and the infilling processes are installed (fluvial energy diminished in the cutoffs). The issue of internal islands explains the aggradation of the channel bed.

The effects of the meander's rectification are observed as from the beginning of the work (1984), and they are amplified very fast. On M1, the channel narrowing is the dominant process after 1980 and expresses the progressive energy decrease from upstream to downstream. Between 1970 and 2006, only the sector from km 78 to km 81 of the M1 is still active; here, the progradation of the convex bank is important (between 2.9 and 3.25 m/y -1 , respectively). Downstream, along the former meander, the bank accretion clearly induces a sedimentary deficit to the M2 and M3.

The meanders M2 and M3 are characterised by enlargement of the channel until 1985.

After this date and more specifically after 1990, the tendency is the narrowing of the channel and the slight dynamic activity of the apex zones.

Globally, the three meanders present a general and comparable systematization, characterised by the predominance of the incision processes upstream of the apex zones, then infilling processes downstream of the apex zones, while the bifurcations and the junctions are very dynamic areas. However, the intensity and the magnitude of the hydrosedimentary processes seem to be different on each sector, resulting from some local factors described below.

Author-produced version of the article published in Geomorphology (2014), Vol. 204, Pages 154-164 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2013.07.035

Local control factors governing the distribution of flow energy

Physically conceptual models to predict the hydraulic and geomorphic conditions for an avulsion process were developed by many authors [START_REF] Mackey | Three-dimensional model of alluvial stratigraphy: theory and application[END_REF][START_REF] Slingerland | Necessary conditions for a meandering-river avultion[END_REF][START_REF] Barbe | Effects on dredging due to diversions from the lower Mississippi River[END_REF][START_REF] Winer | Mississippi River sediment diversions[END_REF][START_REF] Edmonds | Stability of delta distributary networks and their bifurcations[END_REF][START_REF] Letter | River Diversions and Shoaling[END_REF]. They showed that the avulsion depends on the main channel to former meander channel bed slopes ratio, the bed grain size, water surface elevation at the bifurcation areas, the diversion angle, etc. The results highlight the different ways of operation of the three former meanders: on M1, most of the liquid and sedimentary fluxes pass through the artificial canal; while on M2, the former meander is the principal water and sediment distributor. On M3, there is quasi-equality in the repartition of the liquid and sedimentary fluxes between the two channels. Obviously, the morphological impact of the rectification is variable depending on several local factors:

-The position of the study area along the St. George branch: there is a chaining in the processes development: for M1, the sediment infilling processes induce a sedimentary deficit for the downstream sector (M2 and M3), where the sedimentation started up later (in the 1990s). Here, situated upstream, M2 seems to be more active than M3.

-The diversion angle, or the angle between the main channel and the entrance to a diversion channel, is an important control on the division of the flow's momentum, and thus also on the ability of flow in the diversion to transport bed material [START_REF] Law | Dividing flow in an open channel[END_REF][START_REF] Hager | An approximate treatment of flow in branches and bends[END_REF]. Results from flume experiments reveal that discharge in the diversion is progressively reduced with increases in the diversion angle above 50° [START_REF] Vanoni | Sedimentation Engineering[END_REF].

The results show that the flow in a former meander with a diversion angle lower than 50° (M2 = 10° and M3 = 50°) has higher capacity to carry out sedimentary material, and thus the aggradation of bed material at the entrance and along the cutoff is mitigated (M3) or even absent (M2). Such former meander types are hydraulically connected to the main channel. On the other hand, flow in a cutoff with a high diversion angle (M1 = 70°) is less able to transmit sedimentary material, which thus promotes the development of sediment infilling processes at the entrance and along the cutoff.

-The L/l ratio (ratio between the length of the former meander and the artificial canal length L/l). The impact magnitude of the rectifications depends on the ratio between the lengths of the artificial canal and the former meander, which scales with the loss of flow energy in the former meander. The values range between 2.06 (M2) and 4.4 (M1) with an intermediary value of 2.73 (M3).

-The bed level difference at the bifurcation areas confirms the identified hydrosedimentary processes in the studied meanders. Negative values reflect the presence of local erosional processes and consequently enhancement of the liquid and sedimentary flows (A1-A3, A1'-A2'). On some reaches with positive values (A1-A2, A1'-A3'), the flow is reduced and infilling processes are located. On M3, the positive bed level deference on both channels (between C1'-B" and C1'-C3") has as a consequence the equality in the flows repartition, with a slight preponderance to the artificial canal where the bed level difference is lower.

-The sinuosity is an important morphological indicator to the channel evolution after the cutoff program. Flow volume and velocity, effects of channel curvatures on flows, sediment load and grain size, channel bank cohesiveness, and initial morphologies of channels (e.g., width and width-to-depth ratios) are the primary controlling factors of sinuosity evolution [START_REF] Schumm | The Fluvial System[END_REF][START_REF] Schumm | Evolution and response of the fluvial system, sedimentological implications[END_REF][START_REF] Schumm | Patterns of alluvial rivers[END_REF][START_REF] Miall | The Geology of Fluvial Deposits: Sedimentary Facies, Basin Analysis and Petroleum Geology[END_REF][START_REF] Ethridge | Fluvial seismic geomorphology: a view from the surface[END_REF]. For our sector, the increase of this index from 2.43 before the cutoff program (1970) to 2.52 in 2006 corresponds to a reduction of the energy of the river.

Separately, the three studied meanders have a slightly different evolution of sinuosity: M1 experienced an important increase of sinuosity (from 4.09 in 1970 to 4.4 in 2006). The sinuosity increase combined with the L/l ratio and with the high value of the diversion angle explains the former meander aggradation. M2 and M3 show slow variations of the sinuosity along this period: from 1.9 in 1970 to 1.84 in 2006 for M2, and from 2.28 in 1970 to 2.36 in 2006 for M3.

Event-scale morphodynamical analysis

The assessment of the sediment transport capacity during the 2006 flood are analyzed through the concept of the specific stream power [START_REF] Bagnold | An approach to the Sediment Transport Problem from General Physics[END_REF], indicating the stream's ability to adjust its channel morphology [START_REF] Biedenharn | Recent morphological evolution of the lower Mississippi River[END_REF], and the boundary shear stress, indicating the flow capacity to mobilize sediment from the bed as suspended-load or bedload.

ω = Ω /w (W.m -2 ) ( 1
)
where B is the bankfull channel width, and Ω is the stream power that writes:

Ω = ρ w g Q S (W.m -1 ) (2)
where the representative discharge Q [m 3 .s -1 ] is usually taken as the bankfull discharge Qbf.

In the present analysis, the discharge gauged by aDcp for each channel is considered as the representative discharge. As for bed shear stress estimates (see below), the energy slope S is approximated by the observed mean water slope.

The specific stream power is actually a complex index, especially when applied to meandering systems, where the water slope is influenced by the sinuosity and variations of the width of the channel [START_REF] Schmitt | La puissance fluviale: définition, intérêt et limites pour une typologie hydro-géomorphologique de rivière[END_REF]. Any change in the magnitude of flow, by cause of a diversion, has a direct impact on stream power. In the case of river diversions, as flow is diverted from the main channel, the stream power of the main channel is reduced, resulting in a reduction in the ability of the flow to do work at a given rate; i.e., a reduction in the ability of the main channel flow to transport sediment [START_REF] Letter | River Diversions and Shoaling[END_REF].

The results located high specific stream power values (above 30 W.m -2 ) in the artificial canal of M1 (A3 and C3; Table 2) and at the confluence (C3) because of low width and steep slope.

In the artificial canals of M2 and M3, ω values are quite low (from 3.9 W.m -2 in A1' to 6.9

W.m -2 in C3"). In the former meanders, stream power values are logically much lower because of the lower slope and longer length (A2 to C2 and A2' to C1': below 3.5 W.m -2 ; C1' to C2": below 1 W.m -2 ).

The mean bed shear stress (τ 0 ) corresponds to the unit tractive force exerted on the bed:

τ 0 = ρgRS (N.m -2 ) ( 3
)
where ρ is the fluid density (1000 kg.m -3 for sediment-free water), g is the gravitational acceleration (9.81 m.s -2 ), R is the hydraulic radius [m], S is the water energy slope [m.m -1 ] (i.e., S = 0.056 m.km -1 in the former meander of M1 (A2-C2), S = 0.024 m.km -1 in the former meander of M2 and M3 (A2'-C2"), and S = 0.174 m.km -1 in the artificial canal of M1 (A1-C1), S = 0.042 m.km -1 in the artificial canal of M2 and M3 (A1'-C1")).

The values of τ 0 are generally in good agreement with the stream power values. The shear stress is significantly higher in the artificial canals but with different intensity from one meander to another (A3-C3 above 22 N.m -2 in M1 and between 6.8 and 10.2 N.m -2 in M2 and M3) (Fig. 8).

In the former meanders, shear stress values are dramatically lower (below 2.7 N.m -2 in A2, B", and C2" and between 4.1 and 4.9 N.m -2 in B, C2, A2', and C1). The apex of M1 and M2

(at sections B and B') corresponds to the most active reaches in the former meanders. The sedimentary dynamics are dramatically reduced in the former meander of M3. Globally, bed shear stress values indicate high erosion capacity in the artificial canal and lower and decreasing erosion capacity in the former meanders (deposition reaches) but with some evidenced differences between the three consecutive meanders. The results offer a more continuous view of the variability in the erosion capacity along natural channels and artificial canals. This analysis of potential erosion/deposition reaches is coherent with the values of mean cross sections averaged SSC (Fig. 4) as well as the SS flux analysis supported by the box model (Fig. 5). The decrease in mean SSC up to bifurcations (A2, A2') confirms that sediment deposition mainly occurs in the first half of the former meanders of M1 and M2. In the artificial canals of M1 and M3, the increase in SSC confirms the existence of erosion processes already evidenced as upwelling, sediment-laden plumes originating from the bed. These vertical sediment plumes are associated with high values of τ 0 for sections A1, A3, C1, C3, C1", and C3", which allows for the resuspension of the bed material.

6.

Conclusions

The evaluation of the meanders evolution after a cutoff program requires an understanding of the critical processes that affect the hydrodynamics and sediment transport within the river system. The contrasted flow and sediment processes in three successive cutoff meanders of the Danube delta were investigated using acoustic and GIS techniques The first artificial canal (of M2) is less active than the second one (of M3). Thus, the overall sediment budget, in fact influenced by the artificial canals, has a tendency to erosion in the upstream system and deposition in the downstream system (energy diminished). All these results confirmed the importance of the artificial canals on the geomorphic evolution of the

Fig. 1 .

 1 Fig. 1. Location of study reach within the St. George Branch, Danube delta and the ADCP profile position

  ,[START_REF] Constantine | Controls on the alluviation of oxbow lakes by bed-material load along the Sacramento River, California[END_REF][START_REF] Le Coz | Morphodynamics of the exit of a cut-off meander: experimental findings from field and laboratory studies[END_REF], analysed the morphological adjustments of cutoff meanders. They suggested the importance of (i) the channel length (or slope) ratio, (ii) the diversion angle (and shape of the bifurcation edge), and (iii) the bed level difference between both channels. Keeping in mind these indicators (which are linked to the distribution of water and sediment fluxes in the channels), we may draw a specific characterisation of the study reaches, based on the observations acquired during the 2006 flood: M1-Mahmudia meander. At the nodal point of bifurcation (A1), the water flux balance is conservative. The water discharge A2+A3 (510 + 2436 = 2946 m 3 .s -1 ) is roughly equal to the discharge through A1 (2905 m 3 .s -1 ). The cutoff channel of M1 receives only 18% of the upstream flow. Then, the water discharge slightly decreases along the former meander, from 510 (A2) to 406 m 3 .s -1 (C2), possibly because of observed outflows to the floodplain and toward small channels starting from the left bank (to the Uzlina lake). The water flow acceleration in the artificial canal (from 1.8 m.s -1 upstream of the bifurcation to 2.3 m.s -1 )

Fig. 5 .

 5 Fig. 5. A box model for SS fluxes, liquid discharge, and SSC by section through the study area and the balance of the sedimentary transit for each meander

  distance, from 73 (A2) to 8 kg.s -1 (C2), indicating an obvious siltation in the main channel (aggradation of the river bed and banks, development of islands rapidly fixed by the riparian forest). The flow and sediment flux issuing from the meander lose 104 and 65 kg.s -1 in comparison to the flow entering the meander. The high SSC values appear again downstream of the confluence (C1).The SSC values confirm this evolution: the SSC increases between A3 and C3 from 100 to 130 mg.l -1 (indicator of erosion) and decreases between A2 and C2 from 140 to 20 mg.l -1 (indicator of deposition). The high value of SSC in A2 (140 mg.l -1 ) can be explained by the active bed dynamics at the bifurcation area.The flow decrease (≈ 590 m 3 .s -1 ) between sections C1 and A1' is by reason of an export toward the Murighiol canal in the right bank. Downstream, at the entrance in M2, the artificial canal is taking only ~ 15% from the total sedimentary flux. The remaining flux (~ 85%) passes through the former meander M2. At the bifurcation (C1'), there is a quasiequality in the distribution of the fluxes between the artificial canal (C3") and the former meander (B"). But the sum of the sedimentary fluxes A2'+A3' (300 + 45 = 345 kg.s -1 ) is

Fig. 8 .

 8 Fig. 8. Mean bed shear stress from estimates with Eq. (3)

(

  acoustic Doppler profiling and map/image analyses). The results revealed at the episodic and long-term temporal scales by the combination of these two methods were consistent and complimentary. In the context of an important local human impact as a result of the meander's rectifications in the 1980's the GIS interpretation offered a solid basis for ADCP data interpretation. At the same time, the ADCP and water sampling survey performed during the 2006 flood offered actual data on the distribution of water and suspended-load fluxes through natural and man-made channels. As a consequence of the modified flow pattern, after the cutoff program, the three studied meanders exhibited different responses: on M1, the water flow acceleration in the artificial canal enhanced incision processes, while the liquid and solid fluxes through the former meander were greatly reduced (with sediment deposition processes), especially downstream of the first meander apex. Downstream, meanders M2 and M3 are characterized by the predominance of erosion upstream of the apex and sedimentation downstream, until the confluence, where the dynamics are reactivated. But the combination of all indicators underlines the disparity of operation of both meanders. It finds its explanation first in the unequal distribution of flow between the former meanders and the two artificial channels, which maintains the overall larger dynamics of the former meander M2.

  Author-produced version of the article published in Geomorphology(2014), Vol. 204, Pages 154-164 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2013.07.035 three meanders. The unequal repartition of fluxes is in relation with a series of geomorphological control factors: channel length ratio, the diversion angle, sinuosity or bed level difference within each subsystem. Additionally, the predominance of fine and cohesive sediments with low slope and the vegetal coverage of the banks correspond to the moderate dynamics of a river and explain the relative stability of the former meanders after the cut-off program.Globally, the hydrodynamic conclusions are in good agreement with the GIS interpretations. The study reach has different responses to the implementation of human works, depending on several local factors analysed in this paper.

Table 1 . List of the maps and satellite images used for the analysis of the three meanders

 1

	Topographical	Source	Date	Map	Projection Pixel dimension	RMSE
	maps			scale		(m)	(m)
	L-35-107-C	Topographical Military	1970-1972	1:50000	Gauss -	4	4
	L-35-107-D	Direction, Romania			Krügger
	L-35-107-C-b	Topographical Military	1980-1984	1:25000	Gauss -	2	4
	L-35-107-C-d	Direction, Romania			Krügger
	L-35-107-D-c
	Image type	Date	Pixel	Canals	RMSE
		dimension (m)		(%)
	Landsat 5 TM	20.08.1989	28	2, 3 and 4	9.3
	Landsat 7 ETM+	06.07.2000	15	8	5
	Landsat 7 ETM+	13.08.2006	20	2, 3 and 4	6.6
	4.

  ), Vol. 204, Pages 154-164 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2013.07.035

	Sector Section KM Width (m)	Maximum depth (m)	Maximum velocity (m.s -1 )	Mean velocity (m.s -1 )	Area (m 2 )	Liquid discharge (m 3 .s -1 )	Flux (kg.s -1 )	Mean SSC (mg.l -1 )	ω (W.m -2 )	τ 0 (N.m -2 )
	MAHMUDIA MEANDER (M1)
		A1	84.5 362	16.3	1.8	0.9	3312	2905	338		14.1	15.5
	A	A2	84.0 99	9.9	1.3	0.8	650	510	73		2.2	2.7
		A3	84.3 100	20.8	2.2	1.4	1790	2436	246		34.4	24.1
	B	B	81.0 90	15.5	1.2	0.7	938	642	70		3.3	4.9
		C1	63.6 183	16.9	2.5	1.2	2708	3241	422		27.2	22.0
	C	C2	64.3 347	13.2	0.5	0.1	2626	406	8		0.4	4.1
		C3	64.5 145	22.2	2.3	1.2	2143	2655	336		29.8	23.4
	DUNAVAT DE SUS MEANDER (M2)
		A1'	58.4 282	14.5	1.7	1.0	2621	2650	328		3.9	6.8
	A'	A2'	58.0 246	16.5	1.6	0.9	2332	2220	300		2.1	4.4
		A3'	57.8 80	12.4	1.2	0.7	566	400	45		2.1	5.1
	B'	B'	56.5 207	22.5	2.1	0.6	3200	2080	255		2.4	5.3
	C'	C1'	54.3 183	21.1	2.1	1.1	2451	2640	214		3.4	4.8
	DUNAVAT DE JOS MEANDER (M3)
	B"	B"	51.7 173	12.1	1.3	0.6	1710	1047	172		0.9	2.2
		C1"	49.2 175	24.6	2.2	0.9	2662	2394	273		5.6	10.2
	C"	C2"	49.7 336	11.3	1.5	0.6	1774	1083	180		0.7	2.7
		C3"	49.8 80	15.1	1.2	1.3	1024	1342	145		6.9	6.8

Author-produced version of the article published in Geomorphology (2014), Vol. 204, Pages 154-164 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2013.07.035


Laura Jugaru Tiron

Jérôme Le Coz
email: jerome.lecoz@cemagref.fr.

Mireille Provansal
email: provansal@cerege.fr.

Nicolae Panin
email: panin@geoecomar.ro.

Guillaume Raccasi
email: raccasi@cerege.fr.

Guillaume Dramais
email: guillaume.dramais@cemagref.fr.

Philippe Dussouillez
email: dussouillez@cerege.fr.

Flow and sediment processes in a cutoff meander of the Danube Delta during episodic flooding

Keywords: river morphodynamics, sediment fluxes, acoustic Doppler current profiler (aDcp), former meander, Danube Delta, St

This article analyzes the water and suspended solid fluxes through a straightened meander of the southern branch of the Danube Delta (the St. George branch) during episodic flooding. The Mahmudia study site corresponds to a vast natural meander which was cut off in 1984-1988 by an artificial canal opened to shipping. The meander correction accelerated fluxes through the artificial canal and dramatically enhanced deposition in the former meander. After his formation, the cutoff meander acted as sediment storage locations, essentially removing channel and point bar sediments from the active sediment budget of the main channel. Increases in slope and stream power in reaches upstream and downstream have also occurred, but to a lesser degree.

During the one-hundred-year recurrent flood in April 2006, bathymetry, flow velocity and discharge data were acquired across several sections of both natural and artificial channels with an acoustic Doppler current profiler (aDcp Workhorse Sentinel 600 kHz, Teledyne RDI) in order to investigate the distribution of the flow and sediment and his impact on sedimentation in a channelized reach and its adjacent cutoff. The contrasting hydro-sedimentary processes at work in both channels and bifurcation/confluence nodal points are analyzed from the measured flux distribution, morphological profiles and velocity and concentration patterns. In the cutoff, a diminishing of the intensity of the flow velocity (c. 50%) and of the SSC was observed correlated with the aggradation of the river bed. In the bifurcation/confluence nodal points and in the artificial canal were observed the most intensive hydrodynamic activity (high flow velocity, SSC concentration, degradation of the river bad). Both the event-scale and long-term morphological trends of the alluvial system are discussed analyzing the boundary shear stress and SSC variability. Excess boundary shear stress in the sub-reaches directly affected by cutoffs resulted in scour that increased downstream bed material load. These high sediment loads play a key role in driving morphological adjustments towards equilibrium in the cutoff channel.

The approach followed in this paper combines detailed episodic in-situ aDcp measurements and robust numerical 1D modeling in order to provide a practical comprehension of the relevant

Introduction

Large river deltas are complex alluvial systems of outstanding social, ecological, and economical importance. Most of them are deeply influenced by human activity: dam construction [START_REF] Stanley | Nile delta: recent geological evolution and human impact[END_REF], [START_REF] Stanley | Nile delta: extreme case of sediment entrapment on a delta plain and consequent coastal land loss[END_REF] (Nile River); [START_REF] Panin | Dinamica sedimentelor pe bratele Dunarii in delta. Modificarile determinate de amenajarile hidrotehnice[END_REF] and [START_REF] Jugaru | Apports des Systèmes d'Information Géographiques à la perception des changements morphodynamiques (1970-2000) dans le delta du Danube[END_REF] (Danube River)); [START_REF] Batalla | Reservoir-induced hydrological changes in the Ebro River basin (NE Spain)[END_REF] (Ebro River); meander cutoff [START_REF] Biedenharn | Recent morphological evolution of the lower Mississippi River[END_REF], [START_REF] Kesel | Human modifications to the sediment regime of the Lower Mississippi River flood plain[END_REF], [START_REF] Harmar | Morphological chamges of the Lower Mississippi River: geomorphological response to engineering intervention[END_REF] (Mississippi River), [START_REF] Popa | Environment changes in the Danube Delta caused by the hydrotechnical works on the St. George branch[END_REF] (Danube River), [START_REF] Kiss | Alterations of channel parameters in response to river regulation works since 1840 on the Lower Tisza River (Hungary)[END_REF] (Tisza River)). Generally speaking, channel mobility is found to be reduced effect of the damming, in association with a decrease in sediment inputs [START_REF] Hooke | Changes in Meander Morphology[END_REF][START_REF] Stanley | Nile delta: extreme case of sediment entrapment on a delta plain and consequent coastal land loss[END_REF][START_REF] Stanley | Nile delta: recent geological evolution and human impact[END_REF][START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF]. Moreover, river training operations (such as construction levees and meander cutoff) initiated for navigational and flood defense purposes often lead to dramatic changes in the streamwise profiles [START_REF] Lane | The Effect of Cutting Off Bends in Rivers[END_REF][START_REF] Keller | Development of alluvial stream channels[END_REF][START_REF] Hooke | Changes in Meander Morphology[END_REF][START_REF] Kesel | Human modifications to the sediment regime of the Lower Mississippi River flood plain[END_REF][START_REF] Kiss | Alterations of channel parameters in response to river regulation works since 1840 on the Lower Tisza River (Hungary)[END_REF]. Meander correction affects both the hydraulic and morphodynamical behavior of the modified branches that sedimentation occurs in time, while newly built canals usually experience degradation [START_REF] Ichim | Efectele barajelor in dinamica reliefului[END_REF].

Main climatic parameters like mean annual temperature and annual precipitation play only a relatively minor role in determining the processes of erosion and deposition in rivers but the intensity of precipitation and its seasonal distribution are much more important [START_REF] Vandenberghe | Climate forcing of fluvial system development: an evolution of ideas[END_REF]. Extreme events, at a recurrence interval of years or tens of years leave the largest imprints in sediment production.

This paper reports and analyzes new data on the hydrological and sedimentary processes at work during a morphogenic flood in a large modified meander of the St. George branch, the southern branch of the Danube Delta. The 100-year recurrent flood that occurred in 2006 offered an exceptional opportunity for scanning different cross sections of the Mahmudia River system by means of the emerging Doppler profiler (aDcp) technology in order to analyze the impact on sedimentation and dynamic processes in the study area. Such experimental data, coupled with one-dimensional hydrodynamical modeling, contribute to the understanding of morphological consequences of the rectification.

In a first section, the work context is presented in regard to the river system, the Mahmudia meander and the hydrological conditions during the field observation campaign. The next section is dedicated to experimental and numerical methodology, i.e., hydroacoustic measurements, water sampling procedures, and the one-dimensional hydrodynamical model set up for further analysis of the experimental data. Then a third section gathers the results relative to the water and sediment fluxes through the whole system, the streamwise variability of morphological and hydrodynamical parameters, and local hydrosedimentary processes. In the last section, morphodynamical trends and sediment balances are discussed at the local, meander, and delta scales.

Regional setting

Fluvial background and anthropogenic forcing

The Danube is a major river with a drainage basin of 817 000 km 2 [START_REF] Ielenicz | Atlas Geografic General[END_REF], a total length of 2860 km, and a total mean annual discharge of 6500 m 3 .s -1 . The present suspended sediment discharge in the Black Sea is about 25-30 × 10 6 t.y -1 [START_REF] Panin | Danube river sediment input and its interaction with the Northwestern Black Sea[END_REF]. The Danube is the main contributor of fresh water and sediment into the Black Sea, through the largest deltaic system in Europe. The Danube Delta (5600 km 2 ) consists of three branches. Unlike the northern branch, which is situated along the Ukrainian border and has mostly remained natural, the other two branches have been fitted for navigation to a large extent through meander correction (Fig. 1A).

Fig. 1. (A) General map of the Danube Delta. (B) Mahmudia former meander with the aDcp profiles position

While the Danube sediment discharge decreased in the last century mainly due to damming (Iron Gates I and Iron Gates II in 1970's), the water discharge increased [START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF]. Also the 25-30% sediment discharge reduction was not as extreme as in the case of the Nile (98%, [START_REF] Sestini | Implications of Climatic Changes for the Nile Delta[END_REF] or Ebro (95%, [START_REF] Jimenez | Medium-Term Coastal Response at the Ebro Delta, Spain[END_REF]. The increase in water discharge can be attributed to climatic changes, and partly after damming to marsh reduction. Major rivers draining the Carpathian and Balkan mountains are dammed, and discharge is therefore insignificant [START_REF] Giosan | Longshore Sediment Transport Pattern Along Romanian Danube Delta Coast[END_REF]. The relatively small reduction in the sediment discharge after damming, was due to increased erosion of the river bottom and islets in the lower course of the river [START_REF] Mihailescu | Lower Danube Recent Alluvia Sediments and Sedimentary Factors[END_REF], and to meander cut-offs in 1990's, along the distributaries in delta area [START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF].

The most southern branch, the St. George branch, starts out from a bifurcation 108 km upstream of the outlet to the Black Sea. It carries 23% of the Danube water discharge and 21% of the Danube sediment discharge [START_REF] Bondar | The Danube Delta hydrologic database and modeling[END_REF]. The width of the distributary varies from 150 to 550 m, and the water depth varies between 3 and 27 m under the local low water mark [START_REF] Bondar | The Danube Delta hydrologic database and modeling[END_REF]. The St. George channel is highly sinuous and still morphologically dynamic, as attested by migrating meanders [START_REF] Jugaru | Apports des Systèmes d'Information Géographiques à la perception des changements morphodynamiques (1970-2000) dans le delta du Danube[END_REF] and by the fast evolution of its mouth: the progradation of a deltaic lobe and the displacements of the barrier-like Sakhalin Island during the twentieth century [START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF].

In the last decades, a number of changes in the St. George branch behavior in response to anthropogenic forcing have been observed. Following the building of the Iron Gates I dam and the hydrotechnical regulation works along the Danube tributaries, the suspended sediment discharge decreased by c. 10% at Isaccea station (KM 54, upstream of the delta head). The construction of the Iron Gates II dam, at Ostrovul Mare (KM 846), induced a catastrophic decrease in the suspended sediment discharge: the sediment load measured at Isaccea dropped by an additional 50% [START_REF] Panin | Danube river sediment input and its interaction with the Northwestern Black Sea[END_REF]. During the years 1984-1988, six free meanders were modified in order to improve navigation. The total length of the St. George branch was diminished to 32 km from the previous 108 km. The man-made intersecting channels were initially 7-8 m deep and 75-100 m wide [START_REF] Popa | Environment changes in the Danube Delta caused by the hydrotechnical works on the St. George branch[END_REF]. Such river training works caused a change in the distribution of the solid as well as the streamflow in the whole delta, enhancing fluxes through the St. George branch into the sea.

The Mahmudia experimental site

The former Mahmudia free meander is located in the middle part of the St. George branch, between KM 84 and KM 64 (Fig. 1A). This meander results from the river impingement against the Mahmudia hills that deflected the St. George channel into a large meander loop named the Mahmudia-Uzlina meander bend [START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF]. This is the largest and the most complex meander in the St. George branch. Its wavelength is 5.1 km, the radius of curvature is 3.93 km and the degree of sinuosity is 4.1.

The cutoff of the natural channel by a navigational canal since 1984-1988 (Fig. 1B) caused dramatic changes in the distribution of river flow velocities, discharge, and sediment fluxes [START_REF] Ichim | Efectele barajelor in dinamica reliefului[END_REF][START_REF] Popa | Environment changes in the Danube Delta caused by the hydrotechnical works on the St. George branch[END_REF][START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF]. As a consequence, a number of morphological responses occurred during the period 1990-2000 when the former meander experienced aggradation up to 11 m (i.e. 1.8 m.y -1 ) [START_REF] Popa | Environment changes in the Danube Delta caused by the hydrotechnical works on the St. George branch[END_REF] and decrease in width (an average of -2.7 m.y -1 , with a maximum of -7.5 m.y -1 (about 33%) at KM 81).

Sediment infilling is manifested by the issue of the island located at KM 74. The island developed from the left bank and blocked the entrance of the Gârla Filatului canal; another island appeared at KM 65 in 1990 and increased in area twofold during 1990-2000 [START_REF] Jugaru | Apports des Systèmes d'Information Géographiques à la perception des changements morphodynamiques (1970-2000) dans le delta du Danube[END_REF].

Hydraulic conditions during the field observations (100-year recurrent flood of 2006)

Along the Lower Danube section, the flooding events occur when the water discharge are over 10000 m 3 .s -1 . A statistical analysis of the data set for 160 years , concerning the mean annual water discharge of the Danube River, show that, at the delta apex, 89 flooding events were recorded, of which one flooding event with the largest ever recorded mean annual water discharge of 16000 m 3 .s -1 in 1897 (also the largest daily water discharge of 20940 m 3 .s -1 [START_REF] Bondar | Studiul privind consecintele asupra mediului, precum si cele economice si sociale in Rezervatia Biosferei "Delta Dunarii" Romania, determinate de constructia canalului Bastroe din Rezervatia Biosferei Dunarea-Ucraina[END_REF].

The period during which the measurements were made (28-30 April, 2006) corresponds to a 100year recurrent flood that started at the beginning of April 2006 during the high spring waters and lasted for a period of several months. During the measurements, the flow rate of the Danube was 16500 m 3 .s -1 at its entrance into the delta at Ceatal Izmail (Fig. 1A) and 4600 m 3 .s -1 at the bifurcation of the St. George branch at Ceatal St. George (Fig. 1A). During the three days when measurements took place, the water flow discharge remained almost constant (Fig. 2).

Materials and methods

Hydroacoustic measurements (aDcp)

The application of aDcp (acoustic Doppler current profilers) to river discharge measurements has recently gained considerable popularity because of their efficiency and reliability (for a review see, e.g., [START_REF] Yorke | Measuring river velocity and discharge with acoustic Doppler profilers[END_REF]. Based on the signal processing of the echoes of ultrasonic pulses, the deployment of an aDcp mounted on a boat yields the river channel bathymetry and the tree-dimensional velocity field from which the total discharge can be computed. The frequency of ultrasonic pulses operated by aDcp directly affects the measuring range (increasing with lower frequency) and the uncertainty associated with flow velocity measurements (also increasing with lower frequency). In this study case, with flow depth up to 20 m, a 600 kHz aDcp was a good practical compromise in order to avoid range limitation while obtaining acceptable flow and SSC uncertainty with accurate enough spatial resolution. The data analyzed in this study were acquired with an aDcp Workhorse Sentinel 600 kHz (Teledyne RDI) mounted on a powered boat. During the field campaign, transverse aDcp profiles were systematically completed at relevant sections of the Mahmudia River system (Fig. 1B): around the upstream bifurcation (sector A), around the downstream confluence (sector E), along the cutoff meander (sectors B-C-D), and along the straight outlet reach downstream of the confluence (sector F). In addition to the bottom-track reference used for aDcp velocity and path measurements, the data were continuously georeferenced by a vessel-mounted DGPS (ensemble pivot with mobile Trimble monofrequency ProXR). The bin size was set to 50 cm, and the blank distance (i.e., the unmeasured top layer height) was 80 cm.

Suspended sediment concentrations

For each bin and each beam, the aDcp records the acoustic backscatter intensity as the acoustic energy (in dB) returned from scattering particles. This intensity is an accepted indicator of suspended-load concentration (see, e.g., [START_REF] Reichel | Suspended sediment monitoring in a fluvial environment: advantages and limitations applying an acoustic Doppler current profiler[END_REF][START_REF] Gartner | Estimating suspended solids concentrations from backscatter intensity measured by acoustic Doppler current profiler in San Francisco Bay, California[END_REF][START_REF] Dinehart | Repeated surveys by acoustic Doppler current profiler for flow and sediment dynamics in a tidal river[END_REF][START_REF] Kostaschuk | Measuring flow velocity and sediment transport with an acoustic Doppler current profiler[END_REF], although it depends on the properties of the water (temperature, absorption), of the acoustic device itself (frequency, measuring mode, hardware), as well as of the suspended particles (granulometry, concentration, mechanical properties).

Consequently, calibration methods for converting backscatter intensity into suspended sediment concentration (SSC) require extensive sediment sampling operations [START_REF] Rotaru | ADcp measurements of suspended sediment fluxes in Banat rivers, Romania. Balwois[END_REF]. Here, the state-of-the-art, pragmatic SediView methodology [START_REF] Land | Acoustic measurement of sediment fluxes in rivers and nearshore waters[END_REF] was followed: a manual and iterative calibration of hydroacoustic parameters against SSC samples was achieved using the SediView software dedicated to the processing of Teledyne RDI aDcp data. Once calibrated, hydroacoustic measurements potentially offer the convenient, nonintrusive, synoptical surveying of bathymetry, water velocity, and SSC, i.e., all required information to assess suspended sediment fluxes across a river section.

Consequently, stationary aDcp in situ measurements were coupled with water sampling with a 5 L horizontal Niskin-type bottle. In several selected cross sections, coupled aDcp and sample data were acquired in three separate verticals at depths of 3, 6, 9, and 12 m. The sampled SSC (expressed in mg.l -1 ) was measured in the laboratory by means of a HACH DR/2000 digital spectrophotometer (using the 630 method). Thus samples were used to calibrate aDcp concentration profiles. The resulting calibration data cloud (Fig. 3) shows that an acceptable agreement between backscatter data and SSC was reached, even if deviations up to 30% appear for two samples. Because of technical restrictions, the number of samples is not high enough to fully understand the reasons for such residuals. However, as errors do not show any clear trend with the sampling depth nor the SSC value, this calibration operation was assumed to be reliable enough for further assessment of the SSC distribution across sections and SSC flux estimates.

Fig. 3. Results of the hydroacoustic calibration of aDcp backscatter data against SSC sampling

One-dimensional hydrodynamical model (MAGE)

A one-dimensional hydrodynamical model was set up using the MAGE code (Cemagref).

MAGE solves the unsteady one-dimensional shallow water equations in looped channel networks, i.e. including diffluences and confluences. Computational cross-sections every 100 m were built from bathymetry profiles extracted from the aDcp transects. Assuming constant water slope, the aDcp bathymetry profiles were vertically shifted to fit the 1.25 m water fall that occurred between sections A1 and F2. This water fall was computed from observed flood levels and topography data extracted from topographical maps of the Danube Delta (scale 1:25 000).

As no data in the floodplain were available, only the main channel flow was simulated. The downstream and upstream boundary conditions are the observed water level and a constant discharge of 3000 m 3 .s -1 , respectively. A homogeneous Manning coefficient of 0.06 leads to a simulated water fall of 1.20 m between A1 and F2. This quite high flow resistance value likely involves the effect of sinuosity and overbank flow processes on energy losses.

Results

Overview of water and suspended sediment fluxes

By comparing the total suspension fluxes through each section (Fig. 4), we can draw a first balance of the ways of transport and of their relationship to the hydrodynamics and morphology of the channel. Most of the sedimentary flux (≈ 73%) passes through the artificial canal (between A3-E3). In the former meander, the sediment transit is weak and progressively decreases with the distance (Fig. 4). The high SSC values reappear downstream the confluence (E1-F2). This heterogeneity and its spatial variations enable us to design a box model of the transport of sediments for the investigated sector.

At the nodal point of bifurcation, the water as well as suspended sediment flux balances are conservative. The water discharge A2+A3 (510 + 2436 = 2946 m 3 .s -1 ) exceeds the discharge through A1 (2905 m 3 .s -1 ), while the loss of sediments between cross sections A2+A3 (73 + 246 = 319 kg.s -1 ) and section A1 (338 kg.s -1 ) is 19 kg.s -1 , i.e., a relative variation lower than 6% (not significant in comparison with measurement uncertainties).

The water discharges and the sediment fluxes progressively decrease along the cutoff meander from 510 m 3 .s -1 (A2) to 406 m 3 .s -1 (E2) and from 73 kg.s -1 (A2) to 8 kg.s -1 (E2), respectively, indicating an obvious siltation in the main channel and in the floodplain as well. Sediment flux "losses" are significant between sections B/C1 (70/27 kg.s -1 ) and D/E2 (22/8 kg.s -1 ) only. All in all, the flow and sediment flux issuing from the meander lost 104 m 3 .s -1 and 65 kg.s -1 in comparison to the flow entering the meander. These gross values suggest a complex balance between local supplies and losses toward the most important canal in the left bank, the Uzlina canal (the mean values of the water (m 3 .s -1 ) and sediments (kg.s -1 ) discharges entering the Uzlina lake through the canal are 16.4 m 3 .s -1 (or 1.5%) and 3.0 kg.s -1 (or 1%), values for the period 1981-1989).

Fig. 4. A box model for SS fluxes, streamflow, and SSC by section through the Mahmudia river network

In the artificial canal, the water discharge balance is rather conservative (+ 9%), whereas the output sediment flux increases by 90 kg.s -1 (+ 36.5%) between sections A3 and E3. Hence, the canal potentially is an area with significant erosion.

The confluence is a complex sector because profile E1 exhibits a sedimentary flow (422 kg.s -1 ) larger than the total sum of the potential supplies (E2+E3 = 344 kg.s -1 ), while the water discharge balance is almost conservative. The flow decrease between sections E1, F1 and F2 (3241 m 3 .s -1 to 2514 m 3 .s -1 and 2476 m 3 .s -1 ) is due to a export of ≈ 700 m 3 .s -1 toward the Murighiol canal in the right bank. The decrease in the sedimentary transit (422 kg.s -1 to 236 kg.s - 1 ) between profiles E1 and F2 can be explained by a progressive decrease in energy but also by the overflowing that causes a progressive decrease in discharge and concentration, from the sedimentation in the floodplain.

Morphological profiles

Raw bathymetry data extracted from the aDcp measurements are not georeferenced, and the elevations are relative to the local free-surface level. For further morphological analysis, the cross-sectional profiles were georeferenced by DGPS tracking; the free-surface stage was estimated according to the 1.25 m water fall observed between sections A1 and F2. Hence, the margin of error in bed elevation estimates typically is 2-3%. Because of unmeasured areas on the sides of channels, the total cross-sectional width typically is underestimated by 2-5%.

Regarding the natural former meander, the mean talweg slope (A2-E2) is very low (0.089 m.km - 1 ). The upstream reach (A1-A2, downstream of the bifurcation) corresponds to an important reduction of the depth (Fig. 5). The talweg line remains located on the right bank (which was concave before the cutoff) and varies from -16 m (A1) to -9 m (A2) in depth (i.e., a counterslope of -8.75 m.km -1 ); in the left bank between B and C1 (the convex bank of the sinuosity), another counter-sloping area attests to an accumulation of sediments. Downstream, the talweg depth increases locally in the concave bank of the apex (-15.5 m on B, -10.6 m on C2). Close to the confluence upstream, profile E2 shows a deepening toward the right bank (-13.2 m). It could be attributed either to the original concave pool of the bend or to the water swirls coming out from the canal; between E2 and E1 a steep talweg slope (22.9 m.km -1 ) is observed.

The entrance in the artificial canal (A1-A3) corresponds to an increase in depth (from -16 to -0.8 m, or a local talweg slope of 7.4 m.km -1 ). Between A3-E3, the slope of the canal decreases (0.3 m.km -1 ). However, the confluence (E3-E1) is characterized by a counter-slope of -8.5 m.km -1 , which may correspond to a sedimentary deposition caused by the junction of both flows.

In the natural channel downstream of the confluence, the talweg goes from -18.8 m (F1) to -11.3 m (F2) in depth, with a counter-slope of -8 m.km -1 , which likely corresponds to deposition between the two sections.

Fig. 5. The talweg and water stage profiles along the natural channel (A1-F2) and the artificial canal (A3-E3)

The observed morphology clearly records the impact of the artificial canal on the whole alluvial system. The natural channel adapts to the flow decrease, but its response varies streamwise because of sediment infilling. On the contrary, the artificial canal is characterized by an increase in depth that creates a well-calibrated shape suitable for maintaining high energy. The intersections of the cutoff channel with the artificial canal at the bifurcation and at the confluence are accumulation sectors with counter-slopes, either in relation to flow/velocity decrease (A2) or to the junction of contrasting flows (E1).

Flow and sediment processes

The analysis of the aDcp velocity data provides evidence as to the highly heterogeneous hydrodynamical conditions in the different reaches of the Mahmudia alluvial system (Table 1).

Upstream of the bifurcation, the flow (2905 m 3 .s -1 ) is homogeneously distributed across the approach section A1 (Fig. 6A). The average velocity is 0.9 m.s. -1 . Then the bifurcation splits the flow into two hydrodynamically contrasted branches: the main flow (2436 m 3 .s -1 or 82%) is captured by the artificial canal (A3) where velocity increases (an average of 1.4 m.s -1 ). The former meander (A2) receives only 18% of the upstream flow (510 m 3 .s -1 ), with slightly reduced velocities (an average of 0.8 m.s -1 ). Farther downstream, the former meander is characterized by a progressive decrease in average velocity (0.7 to 0.1 m.s -1 from B to E2).

Fig. 6. aDcp depth-averaged velocitys and river-bank contours within sectors of (A) bifurcation and (B) confluence. Channel contours were digitized from a satellite image dating from June 2000 for different hydrological conditions than during the measurement period

The flow rate also decreases (from 642 to 406 m 3 .s -1 from B to E2) because of a waste of water toward small channels starting from the left bank. Around the first apex (B), the channel dissymmetry produces highest velocities near the concave bank of the meander. In C1 and C2

sections, the velocities are homogeneously distributed again, as the channel is much less sinuous.

At the confluence (Fig. 6B), the E2 profile is still characterized by low and homogeneous velocities (average 0.1 m.s -1 ) and by a low discharge (406 m 3 .s -1 ). It sharply differs from the upstream profile of the artificial canal (E3) where a high discharge arrives (2655 m 3 .s -1 ) with high velocity (average 1.2 m.s -1 ); E3 is an asymmetrical profile (vertical fast curls close to the right bank between 1.8 and 2.4 m.s -1 , lower velocities toward the left bank and close to the bottom, ranging from 0.8 to 1.5 m.s -1 ). Downstream of the confluence (E1), the flow is concentrated in the central part of the channel. This velocity asymmetry stems from the sharp contrast between input flows from both upstream channels. The resulting total discharge is 3241

m 3 .s -1 .
Downstream of the confluence, the influence of the fast waters coming out from the artificial canal is rapidly diminished. The velocities progressively increase from 0.9 m.s -1 in F1 to 1.1 m.s - 1 in F2. However, a nucleus of higher velocity (between 1.1 and 1.4 m.s -1 ) persists on the F1 right bank, determined by the water supply of the artificial canal, while the velocity distribution remains uniform for F2 (Fig. 8).

Fig. 7. Cross section averaged velocity: aDcp measurements and simulation results

Fig. 8. Distribution of local velocity magnitude within cross-sections of the Mahmudia river network (A1/A3/B/E1/E3/F1)

The aDcp discharge and velocity measurements allow the evaluation of the one-dimensional numerical simulation performance (MAGE). The simulated distribution of the total discharge (3000 m 3 .s -1 ) between the artificial cutoff canal (2520 m 3 .s -1 ) and the former meander (480 m 3 .s - 1) is in acceptable agreement with the discharges measured by aDcp (Table 1). Simulated mean velocities also closely follow the measured trends (Fig. 7).

However, results in the upstream part of the former meander suggest that some extra investigation of sections would have been necessary there in order to describe the channel more accurately: measurements are lacking to decide whether the simulated velocity profile is correct or some hydraulic controls are missing. In the last two sections (F1 and F2), downstream simulated velocities are biased high because of significant overbank flow: flow rate in the main channel is 3000 m 3 .s -1 in the model, whereas about 500 m 3 .s -1 are missing in the aDcp discharges.

In spite of these limitations, the simulation results are reliable enough for further interpretation of the hydrodynamical behavior of the system.

Local hydrosedimentary processes

The distribution of local velocities and SS concentrations varies with every investigated section, depending on the local morphology. In particular, the sections situated near the bifurcation (A1/A3) in the apex of the former meander (B) and those close to the confluence (E1, E3, and F1) are characterized by high local velocities (from 1.2 to 2.5 m.s -1 ) and by the presence of gravitationally related sedimentary flows associated with SSC plumes (Figs. 8 and9). The concentrations range between minimal values of 10-20 mg.l -1 and maximal values exceeding 180 mg.l -1 .

For the symmetrical sections A3, E3, E1, and F1, the concentrations are classically higher near the bed and the banks and lower near the free-surface and the central part of the channel. Section E1 situated upstream of the confluence records flows with very contrasting concentrations.

Going into the details of three-dimensional velocity and concentration data, some vertical plumes with high SSC suggest that gravitationally related sedimentary flows are eroding sediments from the bottom layer. Such hydrosedimentary patterns particularly appear near the less steep slopes of convex banks (sections A1, B, and F1) and also in deep and calibrated sections (E3 and E1). Author-produced version of the article published in Geomorphology ( 2009), vol 106, 3-4, 186-197 The original publication is available at http://www.sciencedirect.com/ DOI: 10. 1016/j.geomorph.2008.10.016 It also illustrates the fact that between two aDcp cross sections some hydraulic control sections can be missing for the one-dimensional numerical model.

Fig. 9. Distribution of local SSC within cross sections of the Mahmudia River network (A1/A3/B/E1/E3/F1)

The following discussion will focus on potential coherence between our results and the other available data concerning the geomorphological evolution of the studied area. The deposition/erosion reaches suggested by boundary shear stress and SSC variability as well as the morphological evolution will be analyzed. A balance of the total SSC flux through the St.

George branch is drawn.

Author-produced version of the article published in Geomorphology (2009), vol 106, 3-4, 186-197 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2008.10.016

Discussion

Event-scale morphodynamical analysis

Local morphodynamical processes during the 2006 flood are analyzed through two geomorphological parameters: the specific stream power indicating the stream ability to adjust its channel morphology [START_REF] Biedenharn | Recent morphological evolution of the lower Mississippi River[END_REF]; and the boundary shear stress indicating the flow capacity to mobilize sediment from the bed as suspended-load or bedload.

Morphodynamical activity is generally assessed by the specific stream power (ω) as an indicator for the river transport capacity [START_REF] Bagnold | An Approach to the Sediment Transport Problem from General Physics[END_REF]:

ω = Ω / B [W.m -2 ] ( 1
)
where B is the bankful channel width and Ω is the stream power that writes:

Ω = ρ g Q S [W.m -1 ] (2)
where the representative discharge Q [m 3 .s -1 ] is usually taken as the bankful discharge Qbf. In the present analysis, the discharge gauged by aDcp for each channel is taken as the representative discharge. As for bed shear stress estimates (see below), the energy slope S is

Author-produced version of the article published in Geomorphology ( 2009), vol 106, 3-4, 186-197 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2008.10.016 approximated by the observed mean water slope. The specific stream power is actually a complex index, especially when applied to meander systems, where the water slope is influenced by the sinuosity and variations of the width of the channel [START_REF] Schmitt | La puissance fluviale: définition, intérêt et limites pour une typologie hydro-géomorphologique de rivière[END_REF].

Table 2 -Values of ω and τ 0 in the Mahmudia meander cross-sections

Sectors with high specific stream power (above 29 W.m -2 ) are to be found in the artificial canal (A3, E3, Table 2) because of low width and steep slope. In the former meander, stream power values are logically much lower because of the lower slope and longer length (A2/E2: below 4 W.m -2 ). In the upstream (A1) and downstream (E1/F2) single channels, ω values are quite high (from 14.1 W.m -2 in A1 to 27.2 W.m -2 in E1 and around 20 W.m -2 in F1/F2).

The mean bed shear stress (τ 0 ) corresponds to the unit tractive force exerted on the bed:

τ 0 = ρ g R S [N.m -2 ] ( 3
)
where ρ is the fluid density (1000 kg.m -3 for sediment-free water), g is the gravitational acceleration (9.81 m.s -2 ), R is the the hydraulic radius [m], S is the water energy slope [m.m -1 ] (i.e., S = 0.056 m.km -1 in the former meander (A2-E2) and S = 0.174 m.km -1 in the main channel (A1/F2)).

Experimental estimates of τ 0 are generally in good agreement with the MAGE simulation results (Fig. 11). The numerical results offer a more continuous view of the variability in the erosion capacity along both channels. However, the dramatic increase of simulated bed shear stress values in the very downstream part of the reach (F2) seems unrealistic because of the ignored overbank flow effects.

Compared with the value in the approach section A1, the shear stress is significantly higher in the artificial canal (A3/E3) and in the downstream channel (E1/F1). In the former meander, shear stress values are dramatically lower (A2/E2). The apex at section B corresponds to the most Author-produced version of the article published in Geomorphology (2009), vol 106, 3-4, 186-197 The original publication is available at http://www.sciencedirect.com/ DOI: 10. 1016/j.geomorph.2008.10.016 This analysis of potential erosion/deposition reaches is coherent with the values of mean cross sections averaged SSC (Fig. 12) as well as the SS flux analysis supported by the box model (Fig. 4).

The decrease in mean SSC up to section C1 (50 mg.l -1 ) confirms that sediment deposition mainly occurs in the first half of the former meander. In the artificial canal, the increase in SSC between A3 and E3 (from 100 to 130 mg.l -1 ) confirms the existence of erosion processes already evidenced as upwelling sediment-laden plumes originating from the bed. These vertical sediment plumes are associated with high values of τ 0 for sections A1, B, E1, E3, and F1, which allows for the resuspension of the bed material.

Long-term morphodynamical analysis

The measurements performed in April 2006 represent an opportunity for relating the multiannual morphological balances to the sedimentary flows occurring during a morphogenic flood event.

Slope adjustments are often the dominant morphological response when alluvial channels are cut off that facilitated the accretion of the former meanders [START_REF] Biedenharn | Recent morphological evolution of the lower Mississippi River[END_REF]. [START_REF] Shields | Environmental aspects of clearing and snagging[END_REF] explained that the accretion of the alluvial plug (i.e. the terrestrial unit formed at the mouth of the former channel) are controlled by the geometry of the former channel, particularly by the ratio between the former channel length and the new channel length, and the upstream angle between them. Most authors suggest that the older the cut-off channel, and the closer it is to the main channel, the thicker the overbank deposits will be (Piegay et al (2000)). [START_REF] Lane | The Effect of Cutting Off Bends in Rivers[END_REF] distinguished two phases: 1. immediate response occurring a short time after the cutoff is implemented; 2. subsequent response taking place gradually and over a very long period.

For the Mahmudia cutoff meander, the first phase was monitored by [START_REF] Popa | Environment changes in the Danube Delta caused by the hydrotechnical works on the St. George branch[END_REF] who carried out a sequence of bathymetric measurements during a period of six years immediately after the building of the artificial canal. The measurements were made with a portable computerized system (Biotron S.A Company, Bucharest) that performed simultaneous measurements of the flow velocity and depth. The velocity was measured with an electromagnetic currentmeter, while depth was measured by a differential pressure sensor. The measurements were carried out every year in September or October, when the Danube discharge usually reaches the lowest values.

The bathymetry profiles were situated at the bifurcation of the Mahmudia meander, close to our three aDcp profiles (A1/A2/A3). (1990)(1991)(1992)(1993)(1994)(1995)(1996) in the Mahmudia modified system [START_REF] Popa | Environment changes in the Danube Delta caused by the hydrotechnical works on the St. George branch[END_REF]

Fig. 13. River-bed evolution

and maximal monthly discharges between 1990 and 2004 at Ceatal St. George gauging station

The monitoring of the bifurcation cross sections (Fig. 13) shows that the channel morphology adjusted suddenly during the studied period with the intense plugging of the former meander inlet (aggradation rates up to 11 m, i.e., 1.8 m.y -1 ) and the intense scouring of the artificial canal (degradation rates up to 13 m, i.e., 2.1 m.y -1 ). Dramatic deposition in the former meander even caused the development of islands rapidly fixed by the riparian forest. In addition, [START_REF] Jugaru | Apports des Systèmes d'Information Géographiques à la perception des changements morphodynamiques (1970-2000) dans le delta du Danube[END_REF] showed that the meander mobility in the horizontal plane has been fixed since the 1990.

The comparison of bathymetry profiles measured by Popa (1990Popa ( -1996) ) and our measurements of 2006 makes sense as the hydrologic regime was similar between 1990 and 2000 (homogeneity of the maximal monthly discharges, cf. Fig. 13). The comparison shows that during the last few years the morphodynamical processes have dropped in intensity: the sedimentation rate in the former meander was only 0.1 m.y -1 during 1996-2006, whereas the erosion rate in the artificial canal was 0.2 m.y -1 . Such a contrast in evolution rates is much more significant than potential errors stemming from the different measuring procedures followed by [START_REF] Popa | Environment changes in the Danube Delta caused by the hydrotechnical works on the St. George branch[END_REF] and us. This clearly indicates that after a fast relaxing evolution, the morphology of the alluvial system has reached its new dynamic equilibrium.

Sediment balance in St. George branch

The SSC concentrations found in the investigated sections during the flood (10-180 mg.l -1 ) are quite low in comparison with data recorded in the world major rivers. [START_REF] Meybeck | Global variability of daily total suspended solids and their fluxes in rivers[END_REF] reported the following values for daily SSC: 849 mg.l -1 for the Mississippi River (USA), 962 mg.l -1 for the Mekong River (Thailand), 8240 mg.l -1 for the Rio Grande River (USA), and even 15 760 mg.l -1 for the Colorado River (USA). A study on the Rhône River during a flood period indicates high SSC values of 3689 ± 0.05 mg.l -1 (Antonelli et al., in press).

The volume of the deposited sediments within the former meander between the A2-E2 sections can be estimated to be 0.0035 x 10 6 m 3 .d -1 , for a discharge water of 4600 m 3 .s -1 (at Ceatal St.

George).

An estimation of the suspended solid discharge at the mouth of the St. George branch and to compare it with the previous estimates is calculated. [START_REF] Panin | Danube river sediment input and its interaction with the Northwestern Black Sea[END_REF] and [START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF] calculated the amount of solid fluxes for the period following the building of the dam (1983)(1984)(1985)(1986)(1987)(1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000). They estimated the suspended sediment output at the mouth of the St. George branch to be 24.2% of the total Danube output and to lie between 3.8 x 10 6 and 6 x 10 6 m 3 .y -1 . For the same period and the same station, Bondar (unpublished data) mentioned a value of 2.99 x 10 6 m 3 .y -1 .

For the three days during our aDcp measurements, we estimated the solid volume going out of the Mahmudia system to be between 0.043 x 10 6 and 0.068 x 10 6 m 3 . These values are higher than the data reported by [START_REF] Panin | Danube river sediment input and its interaction with the Northwestern Black Sea[END_REF] and [START_REF] Panin | The Danube Delta. Geomorphology and Holocene evolution: a synthesis[END_REF] (between 0.012-0.016 x 10 6 m 3 .d -1 ). Deviations between estimates are significant, but unfortunately the same order of magnitude as the expected errors associated with the estimating method each author used.

Further research could deal with sediment storage in the former meanders located downstream.

The St. George branch loses alluvial material load along its course by progressive sedimentation and laterally through channels, canals, crevasses, and overflow. The storage amounts to ≈ 0.005 x 10 6 m 3 .d -1 (≈ 43%, mean value for the years 1981-1989), according to [START_REF] Bondar | The Danube Delta hydrologic database and modeling[END_REF]. This high value can be explained by the discontinuity in sediment transport capacity from training works achieved in that specific period of time, as studied at the Mahmudia site in this paper.

Conclusion

This paper reports the flow and sedimentary analysis of a cutoff meander in the Danube Delta, the Mahmudia cutoff meander system. The aDcp and water sampling survey performed during the 2006 100-year recurrent flood offered data on the distribution of water and suspended-load fluxes through both natural and man-made channels. Since the meander correction, morphological trends were assessed through the bed shear stress estimates and through the available morphological monitoring. The impact of the 2006 100-year recurrent flood is observed in the velocity and SSC distributions on the investigated cross-sections. The water flow acceleration in the artificial canal (from 1.8 m 3 .s -1 upstream the bifurcation to 2.3 m 3 .s -1 ) enhances incision processes in the artificial canal (gravitationally related sedimentary flow), while the liquid and solid fluxes through the former meander are greatly reduced (from 338 kg.s - 1 upstream the bifurcation to 73 kg.s -1 even 8 kg.s -1 , with sediment deposition processes), especially downstream of the first meander apex. The former main channel undergoes very

Fig. 2 .

 2 Fig. 2. Danube daily discharges at the bifurcation of the St. George branch (Ceatal St. George gauging station)

  They may be related to the development of the bedforms.Popa et al. (unpublished report) and[START_REF] Panin | Dinamica sedimentelor pe bratele Dunarii in delta. Modificarile determinate de amenajarile hidrotehnice[END_REF] reported the existence of dunes with heights ranging from 0.5 to 2 m and a wavelength of 20-40 m; they can be entirely mobilized during extreme floods like the 2006 great flood. A longitudinal aDcp profile of 2.4 km was acquired during the 2006 campaign along the former meander between KM 83.5 and KM 81.5 (fig.10). These bathymetry data show the structure of high dunes ranging from 0.5 to 2 m and from 4 to 13 m in waveheight, respectively.

Fig. 10 .

 10 Fig. 10. Longitudinal talweg profile in the former meander between KM 83.5 and KM 81.5

Fig. 11 -Fig. 12 .

 1112 Fig. 11 -Mean bed shear stress from 1D simulation and estimates with Eq. 3

Table 1 -Hydrodynamical and sedimentary parameters in investigated cross-sections

 1 Author-produced version of the article published in Geomorphology(2009), vol 106, 3-4, 186-197 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2008.10.016

	Section	A	B	C	D	E

  reach in the former meander. Thus, simulated bed shear stress values indicate high erosion capacity in the artificial canal and lower and decreasing erosion capacity in the former meander (deposition reaches).

			A		B	C		D		E		F
	Sections
		A1	A2	A3	B	C1	C2	D	E1	E2	E3	F1	F2
	ω [W.m -2 ]	14.1	2.2	34.4	3.3	0.6	0.8	0.6	27.2	0.4	29.8	19.7	18.6
	τ 0 [N.m -2 ]	15.5	2.7	24.1	4.9	1.9	2.6	2.8	22.0	4.1	23.4	21.1	16.4

active

Author-produced version of the article published in Geomorphology(2009), vol 106, 3-4, 186-197 The original publication is available at http://www.sciencedirect.com/ DOI: 10.1016/j.geomorph.2008.10.016

visible infilling processes (observed through the development of islands fixed by the riparian forest) correlated with the decrease in hydraulic energy. Since 1996, the morphology of the alluvial system seems to have nearly reached a new state of dynamical equilibrium (diminishing of the sedimentation rate in the former meander (0.1 m.y -1 ) and of the erosion rate in the artificial canal (0.2 m.y -1 )).

Similar measurements for other hydrological events and even for other deltaic meanders would be valuable to assess the generality of our conclusions. In particular, knowing how the observed processes scale with the multiannual flow rate in the St. George branch would be useful. Our process-oriented observations can contribute to further comprehensive studies with extended time and space scales in order to establish a sediment balance for the whole St. George branch or the whole Danube Delta.

Acknowledgements

This paper was accomplished within a bilateral Brancusi "Fonctionnement des systèmes hydrosédimentaires de deux zones deltaïques: le Rhône et le Danube", project holding between GeoEcoMar (Bucharest, Romania) and CEREGE (Aix-en-Provence, France) and within a National Project, "The Black Sea: Biotical, paleoecological and paleoclimatic evolution since the last 7000 years" (PNCDI II IDEI, no. 364/2007).

The authors thank the team of Geomorphology from CEREGE for their technical and logistic support and for their participation in the campaign for data acquisition. Many thanks as well to the Cemagref (Lyon, France) for logistic support and data processing.