IAHS Scientific Assembly 2017, Port Elizabeth, South

IAHS Scientific Assembly 2017, Port Elizabeth, South Africa, 10–14 July 2017

1 GET,

UMR CNRS / IRD / UPS – UMR 5563 du CNRS, UMR234 de l’IRD, 31400 Toulouse, France 2 IMF - UCV Instituto de Mecánica de Fluidos, Caracas 1041-A, Venezuela. 3Centro de Sismología, Universidad de Oriente 6101 – Venezuela 4COEA, Instituto Venezolano de Investigaciones Científicas (IVIC), Caracas 1020-A, Venezuela; 5University of Napoli Federico II, 80138 Napoli, Italy

The Orinoco River is the third largest flow-discharge-river in the world with an annual mean water flow rate of 37,600 m3 s-1. Due to the presence of the Guiana shield on the right bank, the lower reach of the Orinoco presents a plan form characterized by alternance of contraction and expansion zones. Typical 1-1.5 km width narrow reaches are followed by 7-8 km wide reaches. A complex pattern of bed aggradation and degradation processes takes place during the hydrological cycle. Repeated surveys by an Acoustic Doppler Current Profiler (ADCP) were carried out in a channel (in expansion) in the Orinoco River, specifically over a fluvial island, representative of the lower Orinoco and nearby to the Ciudad Bolivar station dominated by sand waves and bars. For this purpose, temporal series of bathymetric cartography obtained by ADCP profiles combined with Differential Global Position System (DGPS) measurements (with dual-frequency), were used to recover the local displacement of bed forms in this island. The principal aims of this analysis were: (1) to understand the dynamics and evolution of sand waves and bars at this section and (2) to quantify the volume (erosion vs. accretion) of a channel bar with dunes by applying DEM of Difference (DoD) maps on time series of bathymetric data. This required a sampling with transects during the months of November 2016 and April 2017. Each bathymetric transect was measured twice (every 24 hours), and on the same trajectory obtained by a GPS receptor. The spatial analysis of ADCP data shows that a strategy of repeated surveys and flow field interpolation has the potential to simplify the acquisition of temporal series of bathymetries in slightly deep sections (~20m) with various flow conditions.

Table 1. Number and duration of representative SBES and ADCP surveys during three fieldworks at Orinoco River with associated flow conditions. Start

Number of surveys

Finish

Water level in cm

Rising waters 2016

7:20 May, 7 7:05 May, 8

12:12 May, 7 12:05 May, 8

783 800

20665 20994

Falling waters 2016

11:00 Nov, 11 8:00 Nov, 12 8:00 Nov, 13

17:00 Nov, 11 14:00 Nov, 12 14:00 Nov ,13

(ADCP + SBES) 9 10 3

1101 1098 1091

35866 35546 35016

10:00 Apr,21 8:00 Apr,22

17:00 Apr,21 17:00 Apr,22

(ADCP) 14 16

515 518

7119 9538

Fieldwork May-2016

Fieldwork Nov-2016

Discharge (m3 s -1)

Percent of Area of Interest with Detectable Change

NA

6.066,00 65.037,00 71.103,00 NA 94,96% ± Error Volume

379,77 109.807,20 110.186,97 109.427,43

368,12 ± 84,92 109.791,87 ± 910,52 110.159,99 ± 995,44

109.423,75 ± 914,47 ± Error Thic kness

Average Depth of Erosion (m) Average Depth of Deposition (m) Average Total Thickness of Difference (m) for Area of Interest Average Net Thickness Difference (m) for Area of Interest Average Total Thickness of Difference (m) for Area With Detectable Change Average Net Thickness Difference (m) for Area with Detectable Change

1400 1200

Net to Total Volume Ratio

1000 40000 800 30000

0,05 1,64

0,06 ± 0,01 1,69 ± 0,01

1,47

1,47 ± 0,01

1,46

1,46 ± 0,01

NA

1,55 ± 0,01

NA

1,54 ± 0,01

PERCENTAGES (BY VOLUME)

Percent Erosion Percent Deposition Percent Imbalance (depa rture from

50000

0,34% 99,66% 49,66%

0,33% 99,67% 49,67%

99,31%

99,33%

% Error

23,07% 0,83% 0,90% 0,84%

Q=73,938 m 3 s -1

gauge= 16.9 m

Q=33,138 m 3 s -1

gauge= 10.5 m

Q=1 1,362 m 3 s -1

gauge= 5.0 m

Q=4,2 58 m 3 s- 1

gauge= 2.38 m

% Error

23,07% 0,83% 0,90% 0,84% 0,90% 0,84%

equilibrium)

600

DoD=

DEM (Apr-2017) - DEM (Nov-2016)

400

10000

0 1/1/2016

Thresholded DoD Estimate:

7.753,00 67.123,00 NA 74.876

VERTICAL AVERAGES:

1600

20000

Raw

AREAL:

Total Area of Erosion (m²) Total Area of Deposition (m²) Total Area of Detectable Change (m²) Total Area of Interest (m²)

VOLUMETRIC:

gauge

60000

Table 2. Summary of DoD analysis Attribute

Total Volume of Erosion (m³) Total Volume of Deposition (m³) Total Volume of Difference (m³) Total Net Volume Difference (m³)

1800

Hydrograph 70000

DoD over the mid-channel bar using LODmin= 0.01

Fieldwork Apr-2017

80000

The ADCP surveys were used to evaluate flow dynamics associated with the erosion/deposition over a fluvial island in the reach downstream of Ciudad Bolivar town.

The Orinoco River catchment with the location of the ~7 km study site reach shown with the red dotted line. The channel pattern is associated with an expansion zone, downstream of the measuring station.

3

(SBES) 9 9

Lowwaters 2017

The banks of a mid-channel bar (study area) are associated to highly erodible unconsolidated deposits (D50 =369.55 µm).

Mean discharge in m /s

Gauge (cm)

Survey Campaign

Wheaton JM. 2008

2.1 m

200

error assessment

0 10/4/2016

19/7/2016

Time

27/10/2016

4/2/2017

15/5/2017

INTERPOLATION

+

=

Nov-2016 - DEM

Flow direction

ADCP + DGPS

Apr-2017- DEM

Red color is associated with erosion, while Blue color with deposition

Rising waters (May-2016)

Falling waters (Nov-2016)

Flow direction

RTK- DGPS

Evolution of a mid-channel bar during falling waters

s This paper has outlined a data collection and processing methodology that enables measurements over a mid-channel bar, which is representative of one of the world's largest rivers, the Orinoco. Several bathymetric transect perpendicular to the flow were repeated every 24 hours, which allowed to understand the migration rates of sand waves and dunes in the study section. These comparisons made in relatively short periods of time facilitated the analysis of the kinematics of the sand dunes, in order to get a clearer idea about the evolution of these bed forms to different flow conditions. From the results of DoD analysis to quantify the deposition / erosion volume on this middle channel bar, it was possible to determine that during the time between the two surveys, a sedimentation process played a more important role than the erosion. Also, evaluating the vectors of mean velocity flow in both periods, a marked decrease in the speed of the river of 0.384 m/s to 0.156 m/s was observed. When the discharge began to decrease, part of the sediment load in suspension was subjected to deposition causing a vertical growth of the sediments on the bar, especially in the most downstream part of the island, where a sand dune develops in needle shape. One of the aspects that will have to be addressed in future works will be the sedimentary balance. For this, it is necessary to evaluate the same periods of time in different years to understand the morphological changes during the variations of the hydrological cycle. The next field campaigns will focus on the achievement of these objectives.

Acuña, G. (2017). VGM 17, el nuevo modelo geoidal LGFS-LUZ de ultra-altaresolución 30×30m para Venezuela y regiones vecinas. Notas de Geodesia Geométrica. Laboratorio de Geodesia Física y Satelital. Dpto. de Geodesia Superior. Escuela de Ingeniería Geodésica. Facultad de Ingeniería. La Universidad del Zulia. Dinehart, R., & Burau, J. (2005). Repeated surveys by acoustic Doppler current profiler for flow and sediment dynamics in a tidal river. Journal of Hydrology, 314, 1-21 López, J., & Perez-Hernandez, D. (1999). Some Morphological Aspects of the Orinoco River. In, IAHR Symposium on River, Coastal and Estuarine Morphodynamics. Genova. Italy. September (pp. 6-10) Parsons, D., Jackson, P., Czuba, J., Engel, F., Rhoads, B., Oberg, K., Best, J., Mueller, D., Johnson, K., & Riley, J. (2013). Velocity Mapping Toolbox (VMT): a processing and visualization suite for moving‐vessel ADCP measurements. Earth Surface Processes and Landforms, 38, 1244-1260 Wheaton, J.M., Brasington, J., Darby, S.E., & Sear, D.A. (2010). Accounting for uncertainty in DEMs from repeat topographic surveys: improved sediment budgets. Earth Surface Processes and Landforms, 35, 136-156 Williams, R. (2012). DEMs of difference. Geomorphological Techniques, 2

The quality of the DEMs obtained with the implementation of this combined methodology of ADCP transects and RTK-DGPS measurements allowed to obtain high-detail bathymetric models, where it is possible to observe the sand-wave patterns in the bottom of the island. This offers considerable potential to advance our understanding of the dynamics of fluvial processes in the Orinoco, as it is possible to design a series of bathymetric surveys sufficiently detailed and low cost to apply correlation techniques. In this way, it will be possible to accurately and rapidly quantify the velocities of displacement of these bed forms.

Yepez, S., Laraque, A., Martinez, J.M., De Sa, J., Carrera, J.M., Gallay, M., & Lopez, J.L. (2016). Retrieval of suspended sediment concentrations using LANDSAT-8 OLI data in the Orinoco River (Venezuela). In, Journée Thématique du Programme National de Télédétection Spatiale: Apport des missions Sentinel- Copernicus à l'observation de la Terre. CNES, Paris