# A first approach to modelling fine-sediment dynamics in the Santa Cruz River estuary, southern Patagonia

Lucas Bindelli, Federico Haspert, Mariano Re, Lucas Hernández, Leandro Kazmierski

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Lucas Bindelli, Federico Haspert, Mariano Re, Lucas Hernández, Leandro Kazmierski. A first approach to modelling fine-sediment dynamics in the Santa Cruz River estuary, southern Patagonia. Jean-Yves Reynaud; Eric Armynot du Châtelet. Short Papers from the 19th French Congress of Sedimentology, Lille, 84, Association des Sédimentologistes Français, pp.5-8, 2025, 2-907205-83-8. 10.70665/NJHI3616. hal-05005167

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# A FIRST APPROACH TO MODELLING FINE-SEDIMENT DYNAMICS IN THE SANTA CRUZ'S RIVER ESTUARY, SOUTHERN PATAGONIA

Lucas BINDELLI¹,², Federico HASPERT¹, Mariano RE¹, Lucas HERNÁNDEZ¹ and Leandro KAZMIERSKI¹

¹Subgerencia Laboratorio de Hidráulica, Instituto Nacional del Agua (INA), Argentina.

²Facultad de Ingeniería, Universidad de Buenos Aires (FIUBA).

Corresponding author: lucasbindelli@gmail.com

**Abstract:** A Hydrodynamic numerical model coupled with a Particle Tracking python Library was generated, with the objective of describing particle plume dynamics. A general comparison of modelled plume dynamics with fine-sediment plume dynamics obtained from SENTINEL-2 satellite images was made. Results show a geostrophic-like movement for particles going outside the estuary into the sea at ebb tide, with most of them re-entering at flood tide, and some leaving the estuary's influence zone towards North-East. The comparison with satellite images shows good agreement, though further details should be put into particles concentration and location for a better representation.

**Keywords:** Numerical modelling, Fine-Sediment plume Dynamics, Hypertidal, Estuaries, Argentina.

## 1 INTRODUCTION

Santa Cruz's River Estuary (SCRE), located in Santa Cruz Province, Argentina, presents an area of 141 km², receiving river discharges of both the Santa Cruz and Chico River, with mean values of 715 m³/s and 25 m³/s respectively (Bindelli, 2018) and containing visible amounts of clay sediment. It also presents a semi-diurnal and macro-tidal regime that can reach a tidal range of up to 12 m. As stated in Dinápoli et al. (2024), it is part of a region of astronomical dominance, with river discharges and wind having minor influence in hydrodynamics. The sediment plume can be observed in satellite images, presenting a clear tidal influence due to the estuary's high tidal range, and presumably some seasonal influence as well, due to the river's seasonal regime. As for the fine sediment characterization, samples were taken from the estuary as part of an Environmental Assessment for engineering works on the area, presenting Kaolinite as the main component, followed by some presence of quartz (Carol et al., 2017).

In this work, a coupled hydrodynamic and particle tracking model is used to describe the macro-tidal dynamics of the SCRE. Then, as a first approach to modelling the fine-sediment dynamics, a comparison of these results to SENTINEL 2 satellite images is made.

## 2 MATERIALS AND METHODS

The modelling strategy was based on a system of nested models (Fig. 1) to achieve greater discretization/precision/accuracy inside the estuary, using module FLOW of Delft3D software (Deltares, The Netherlands).

<figure><img src="image_2.png" /><figcaption>Figure 1 - Nested model strategy and domains. SCRE: Santa Cruz's River Estuary.</figcaption></figure>

For this, first a regional model of the Argentinian Sea (named Rank 0) was created, with an extension able to properly simulate tidal dynamics in the Continental Shelf and forced by 14 tidal components obtained from
the global model FES2014b. Then, a nested model was built (Rank 1), using as open boundary conditions the results generated by the Rank 0 model, increasing grid resolution at a 1:5 ratio to preserve an acceptable precision, without disproportionally increasing computational time. This nesting technique was applied two more times, creating a Rank 2 and finally a Rank 3 model, accurately propagating tidal information from open seas towards the estuary's mouth (Bindelli et al., 2023). Ranks 0 to 2 were modelled in 2D, while for Rank 3 a 3D representation was made, using a grid resolution of 100 m and a sigma model composed of 20 layers with varying distribution to accurately represent bottom friction effects. The bathymetry was generated by combining different sources. GEBCO (General Bathymetric Chart of the Oceans) global data with 30" resolution was used for the deeper parts, and for the shallower parts, bathymetric charts provided by the Hydrographic National Service (SHN), coastal lines obtained from LANDSAT satellite images post-processing and bathymetric surveys of specific parts of the estuary were used.

For calibration, a combination of surveyed water level and velocity data was employed. Water levels were measured near the estuary's mouth for 3 days, and 8 velocity profiles were measured with an ADCP (Acoustic Doppler Current Profiler) at different points in the estuary and in different times of the tidal wave, for a period of 10 to 25 minutes each. Results range between 0.75 to 2 meters per second with constant direction during each measurement. The calibration parameters were Manning's coefficient (0.025) and horizontal eddy viscosity (540 square meters per second) showing good agreement with all the data (Fig. 2).

<figure><img src="image_3.png" /><figcaption>Figure 2 – Comparison of levels and velocities between measured and modelled data.</figcaption></figure>

Once the hydrodynamic model was calibrated, a coupling with a particle tracking module was made, using a python library named OceanParcels. This library is a Lagrangian simulator that takes as input velocity fields generated by the Delft3D model. Originally created for ocean circulation, it can be used for multiple applications, such as active or passive particle tracking of water, plankton, plastic and fish<fnref n="1" />. In this case, it was used for representing clay (Kaolinite) coming from the Santa Cruz River into the estuary and its interaction with the tidal dynamics.

This work being a first approach to modelling fine sediment plume dynamics, some hypotheses were made to simplify the analysis:

- River discharge is minimal compared to tidal flow (see Tab. 1 in Results), therefore only a fixed discharge of 715 m³/s from the Santa Cruz River is included. This implies that particle movement is mainly due to tidal dynamics and that the influence of the river discharge is of second order, disregarding the river's cyclic behavior. The analysis is then performed by comparing stages of low and high tide rather than comparing times of the year.

- For this stage, only depth-averaged velocity fields were used to perform particle tracking, neglecting vertical displacements, since no threshold depth was established on satellite images for the visibility of the fine sediment plume.

- There is no diffusion process considered in the simulations, since the estuary is well mixed (no stratification), which leads to a homogeneous distribution of sediment in the estuary.

- Since the main kind of particles present in the estuary are Kaolinite (Carol et al., 2017), which is an insoluble and stable clay material, the modelled particles are presumed to be inert (non-reactive with the surrounding environment).

<footnote n="1">¹ https://oceanparcels.org/</footnote>
- Since simulations are short in morphological scale, no morphological changes are included, and only fixed amounts of particles are modelled for a better understanding of particle behavior in this time span.

The simulation period was three months, to allow the comparison of satellite images of fine sediment plumes at different tidal levels. Also, different kinds of simulations were undertaken: On the one hand, simulations were made to obtain characteristic parameters of the overall movements, like particle's residence time inside the estuary and the length of the trajectory for neap and spring tides. This was made by placing small groups of particles at the interface between the Santa Cruz River and the estuary. On the other hand, a homogeneous distribution of particles along the entire estuary's surface was laid, to get the equivalent of the sediment's plume dynamics.

Finally, the results obtained were compared to SENTINEL 2 images. At this stage, only schematic comparisons were made, aiming at capturing the fine sediment plume's dynamic and not a detailed characterization of the concentration of sediment in the estuary.

## 3 RESULTS AND DISCUSSION

Regarding Delft3D-FLOW simulations, Table 1 shows the variation of the mean discharge of Santa Cruz River, which denotes a seasonal regime, but if this is compared to the tidal flow rate that enters or exists the estuary at each cycle (Tab. 2), river discharge accounts for 2% to 8% of total flow, with mean value of 4%. Also, the maximum ratios encountered only happen at specific and brief periods of time, which is when river discharge is at its maximum (during March) and astronomical tide is on a neap cycle. This validates the hypothesis that river discharge is of secondary importance to modelling fine-sediment plume dynamics and can be replaced by mean discharge on a first approach. These calculations were made using measured data at Charles Fuhr Hydrometric Station (located in Santa Cruz River, upward from the estuary), and tidal flow was obtained directly from the model by computing flow rates at the mouth of the estuary.

<table><caption>Table 1 - Monthly Mean Discharge of Santa Cruz River.</caption><thead><tr><th>Month</th><th>1</th><th>2</th><th>3</th><th>4</th><th>5</th><th>6</th><th>7</th><th>8</th><th>9</th><th>10</th><th>11</th><th>12</th></tr></thead><tbody><tr><th>Discharge [m³/s]</th><td>840</td><td>1090</td><td>1277</td><td>1155</td><td>925</td><td>690</td><td>510</td><td>384</td><td>306</td><td>331</td><td>444</td><td>604</td></tr></tbody></table>

<table><caption>Table 2 - Tidal Flow Characteristics of SCRE obtained by Delft3D-FLOW.</caption><thead><tr><th>Maximum Tidal Flow Rate [Qmax]</th><th>Mean Tidal Flow Rate [Qmean]</th><th>Minimum Tidal Flow Rate [Qmax]<sup>min</sup></th><th>Mean River Discharge [Qmod]</th><th>[Qmod] / [Qmax]<sup>min</sup></th></tr></thead><tbody><tr><td>111,090 m³/s</td><td>37,902 m³/s</td><td>16,038 m³/s</td><td>715 m³/s</td><td>4%</td></tr></tbody></table>

Regarding the OceanParcels simulations, for the case of small groups of particles located at the interface between the river mouth and the estuary, particles follow elongated circulations, with, average length of particle trajectory per cycle of 25 kilometers for spring tides and 15 kilometers for neap tides. Also, the average permanence time of particles was between 2 and 3 weeks for non-stranded particles, although a more thorough analysis should be performed, since sample size was not statistically robust.

For the case of homogeneous particle distribution simulations, a circulation pattern can be observed: ebb tides create deflection towards the South-West when particles leave the estuary and flow tides create deflection towards the North-East for these outer particles. Most of these enter back into the estuary, but some of them leave the estuary's hydrodynamic influence zone and travel to the North-East along the coast until they leave the domain (Fig. 3a, b). This behavior is probably due to geostrophic circulation.

Another interesting pattern observed for longer period simulations is that stranded particles (or with very little movement) in the estuary's margins are present all along the coast. This behavior could have a role in the formation of the tidal flats of the estuary (Fig. 3c).
<figure><img src="image_4.png" /><figcaption>Figure 3 – Modelled particle dynamics: a) Ebb tide, b) Flood tide, c) Stranded particles at the end of the simulation.</figcaption></figure>

Finally, comparison of these results to satellite images (Fig. 4) shows similarities between both fine sediment (satellite images) and particle (modelled) plumes, extending outside the estuary and leaning towards the South-West at ebb tides. The modelled plume presents a smaller extension though, possibly due to the original position of particles.

<figure><img src="image_5.png" /><figcaption>Figure 4 – Modelled particle plume (left) and fine sediment plume captured by satellite image (right).</figcaption></figure>

## 4 CONCLUSION

Even though simplifications have been made, results show a good agreement for the overall dynamics, representing the shape and distribution of the particles in a similar way to that observed for fine sediment in the satellite images. On the other hand, the limitations present in the current analysis should be further investigated: The modelling domain should be extended towards the ocean to capture the entirety of the sediment's plume; sensitivity analysis should be made for the inclusion of diffusion processes, as well as considering a variable river discharge. Also, some calibration should be made in terms of particle concentration and turbidity threshold in the visible range of satellite images. Possible application of these modelling tools could be to establish a deposition rate of sediments for a better assessment of the sediment accretion process in the tidal flats in the geomorphological short term.

## 5 ACKNOWLEDGMENT

The authors would like to express appreciation for the support of CNRS by means of the International Research Project "Hypertide", enabling the presentation of this work at the 19th French Congress on Sedimentology. They also thank Emmanuel Poizot for his careful review.

## 6 REFERENCES

Bindelli, L., 2018. Aprovechamiento hidrocinético en el estuario del río Santa Cruz. Trabajo Final de Ingeniería Civil. Facultad de Ingeniería, Universidad de Buenos Aires.

Bindelli, L., Re, M., Kazimierski, L.D., 2023. Hydrokinetic Energy Potential in Argentine Estuaries with High Tidal Ranges. IEEE Latin American Transactions, 21, 1049-1055.

Carol, E., Spalletti, L.A., Schwarz, E., 2017. Determinación de mineralogía por difracción de rayos X de sólidos suspendidos en muestras del río Santa Cruz. Centro de Investigaciones Geológicas, UNLP.

Dinápoli, M., Simionato, C.G., 2024. Study of the tidal dynamics in the Southwestern Atlantic Continental Shelf based on data assimilation. Ocean Modelling, 188, 102332.