two-dimensional hydrodynamic modeling of the …

E-proceedings of the 38th IAHR World CongressSeptember 1-6, 2019, Panama City, Panama

doi:10.3850/38WC092019-1434

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TWO-DIMENSIONAL HYDRODYNAMIC MODELING OF THE ANCACHÍ RIVER (COLOMBIA), AND ITS IMPACT ON THE GEOTECHNICAL STABILITY OF THE

SHORES DUE TO VARIATIONS IN PORE PRESSURE

YESICA CAMACHO SALAZAR (1) COAUTHORS NELSON FELIPE MORENO CARDONA (2), CARLOS TORRES ROMERO (3)

(1, 2,3) Universidad Santo Tomas, Bogotá, Colombia,

[email protected]; [email protected]; [email protected]

ABSTRACT

This paper describes the two-dimensional modeling developed for the Ancachí River, located in the northwest of Colombia, this river leads its waters to the Pacific Ocean through an estuary zone located near the municipality of Nuquí. From many years ago a problem of erosion of riverbank has been seen that is affecting the communities located near the shore, even the landing track of the Reyes Murillo Airport. Some solutions have been done but not of them stopped the problem, so that is why there was a need to study the hydrodynamic of the estuary and its impact on the geotechnical stability of the shores. The 2D model was developed in Delft3D software, supported with field data as water levels, bathymetry, and topography and calibrated with velocities taken with ADCP (Acoustic Doppler Current Profiler). The results obtained were analyzed, and showed that tides have a strong control in the dynamics of the estuary, and that this variation of level daily is generating some erosion process in the shores.

Keywords: estuaries; hydrodynamic modeling; water level variation; geotechnical stability; pore pressure.

1 INTRODUCTION Estuary is the term used to describe a semi enclosed body of water where two environments converge, the

marine and the fluvial environment, and they contain combined characteristics of both such as tidal waves, riverine flows, topographies and coastlines (Zhang, Sun, Lin, & Huang, 2018), but at the same time their behavior and morphology is different because it mainly depends on what dynamic is predominant in the estuary, if its tide-dominated, wave-dominated or river-dominated, determined by three principal factors: wind, tide range and flow, and fluvial discharge.

As at least 60% of the most populated cities worldwide are located near o in an estuary (Bruner de Miranda, Andutta, Kjerfve, & Castro Filho, 2017), a need of understanding the dynamic that occurs in these zones emerges in recent years, in order to maximize their potential and preserve these ecosystems, because estuaries are known for the high nutrient transport rate, biological production and unique biodiversity (Stark, Smolders, Meire, & Temmerman, 2017). In addition to that, estuaries were not a focus of study until they began to develop a certain type of morphological and hydrodynamic response to climatic and anthropogenic factors, much more representative than years ago (Zhang et al., 2018), like climate change, rise of tides, use of estuaries as harbors, dredging, dam construction and deforestation of shoreline (Zhao et al., 2018), even some numerical models have been made to study and understand the dynamic of these zones, but especially in mid zones, not so much in tropical zones (Stark et al., 2017; Uncles, Stephens, & Harris, 2014; Xu & You, 2017). The main response to this factors that is calling the attention of the communities that live near estuaries and the scientific committee around the world is the erosive processes along the shore presented because of the dynamics of these zones (Daly, Miller, & Fox, 2015), and especially because of the composition of the shore’s soil, in most of the cases it is composed by mainly sand and less cohesive but fine-grained materials, this allows water, in its dynamic activity, to remove the solid particles and converts them into sediment.

2 PROJECT SITE The site of study is located in the municipality of Nuqui in the west of the department of Choco on the

Colombian Pacific coast, 120 kilometers far from Choco’s capital, Quibdo. To get there is necessary to access through airway from Quibdo or seaway from Buenaventura. Nuqui limits to the north with Bahia Solano, to the south with the municipality of Bajo Baudo, to the west with the Pacific Ocean, and to the east with the municipality of Alto Baudo. The municipal head is located near the shore of the Pacific Ocean and the mouth of the Ancachí River and the Nuquí River. The river selected to be studied is the Ancachí River.

The Ancachí River, is one of the many effluents that leads its waters to the Pacific Ocean, that according to the Land Management Scheme of Nuqui, is presenting a big erosion problem along the shoreline because of the influence of these two rivers, the deposited sediments have formed barriers of alluvial materials on the left bank of the river, near the mouth of the river to the sea. This reduces the capacity of the cause to transport the

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same volume of water, which forces the river to remove the materials deposited in the opposite margin and also in some seasonal activity it is observed that the river bed is presenting erosion due to rise in velocity presented because of the reduction of capacity in outflow. It is known that many attempts have been made to try to contain or stop the erosion process, such as placing soil-cement bags alongside the shore, this is a relatively low cost alternative but has not had the expected result so there was a need to build retaining walls in reinforced concrete in order to protect the shore better, because it was affecting places like the landing track of the Reyes Murillo airport that is located near the shore of the river but this alternative is expensive.

Previous studies in the zone or near to it were not found, and information about the river is limited, the information shown is product mainly of field work provided by ROTHEM INGENIERÍA SAS.

Figure 1. Erosion of riverbank at study site.

3 MATERIAL AND METHODS

3.1 Observational data One campaign of measuring was made during the month of July in 2018, was executed under conditions

of extreme level in the change of tides, that is, they were measured in the condition of low and high levels on the same day. Within the measurements were included: bathymetric, velocimetry measurements, characterization of bottom sediment and in suspension, overflights with drone, measurement of banks and materialization of GPS points.

Figure 2. Study site

3.2 Numerical model The two-dimensional hydrodynamic model was developed in Delft3D, which is a free 3D modeling suite

that integrates several modules for the simulation of water flow, sediment transport, waves, water quality, morphology and ecology applicable to cost analysis, rivers, lakes and estuaries. The development of the application was initiated by the company Deltares, a Dutch research institute for matters relating to water, soil and the subsurface. The governance equations that the program uses are the Shallow Water Equations for some scenarios like rivers of plain, bogs and flood plains.

3.2.1 Shallow Water Equations (SWE) Delft3D works with the Shallow Water Equations which are derived from the Navier-Stokes equations,

analyzing the case where the horizontal length scale is much larger than the vertical length scale. Under this condition, the law of conservation of the mass implies that the vertical velocity of the fluid is very small compared to the horizontal velocity. From the momentum equations it is said that the vertical pressure gradients are almost hydrostatic, and that the horizontal pressure gradients are due to the displacement of the pressure surface, which implies that the horizontal velocity field is constant throughout the depth of the fluid. Then, vertically integrating allows vertical velocity to be removed from the equations (Randall, 2006).

Kundu and Cohen (1990), make a representation of the equations of shallow waters or shallow waters in the following way:


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𝛿𝜂

𝛿𝑡+

𝛿

𝛿𝑥((𝑢(𝐻 + 𝜂)) +

𝛿

𝛿𝑦(𝑣(𝐻 + 𝜂))) = 0

[1]

𝛿𝑢

𝛿𝑡+ 𝑢

𝛿𝑢

𝛿𝑥+ 𝑣

𝛿𝑢

𝛿𝑦= −𝑔

𝛿𝜂

𝛿𝑥+ 𝑆𝑥(𝑥, 𝑦)

[2]

𝛿𝑣

𝛿𝑡+ 𝑢

𝛿𝑣

𝛿𝑥+ 𝑣

𝛿𝑣

𝛿𝑦= −𝑔

𝛿𝜂

𝛿𝑦+ 𝑆𝑦(𝑥, 𝑦)

[3]

Equation [1] corresponds to the conservation of the mass and equations [2] and [3] correspond to the conservation of momentum, where u is the velocity in (m / s) on the x axis, v is the velocity in the y axis, η is the disturbance of the free surface in (m), H is the average depth of water in (m) and g is the acceleration of gravity in (m / s2) (Oliveros, 2018)

3.2.2 Pre-process of the modeling 3.2.2.1 Grid

The grid was generated based on the topography points taken in field. Delft 3D uses finite difference method so the grid is structured, curvilinear and orthogonal, and offers many tools to smooth and refine it. The horizontal grid used is m=130 n=10.

Figure 3. Grid generation.

The quality of the grid is verified by the orthogonality aspect mainly. Delft3D measures this property

through the cosine of the angle, between the grid lines in - and - direction, the recommended values for this property are less than 0.02. This property is verified in our model showing that most of the values are less than 0.03, therefore the quality of the grid is great.

Figure 4. Orthogonality of the grid.

3.2.2.2 Initial conditions


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Parameter that indicates the conditions of the variables at the beginning of the calculations.(Aragón H, De Luna C, Vélez M, Fuentes M, & Rubio G, 2016). They are imposed along of all the computational domain in the first step of time. For this model the initial condition is a water level of 18 meters above sea level, this value was chosen for the stabilization of the model.

3.2.2.3 Boundary conditions They indicate the flow conditions at the limits of the physical system because they have a finite size

(Aragón H et al., 2016). These are placed on the borders of domain selected, especially in the borders where there is an income of mass to the system, either water or sediment, and which is subsequently transported along the longitude and width of the domain.

For the model developed at the in and out of the system, Dirichlet condition was used, water level boundaries were selected and time series for the forcing type, as the field data collected was mainly a water level in a time frame, therefore the boundary condition varies in time . The water level was taken in field from 10:15 am to 6:00 pm.

Figure 5. Variation of water level in field, July 28th, 2018.

With the values taken in field a projection of water level was made for the entire day, then a curve that fitted most of the points in the projection was generated through MATLAB, resulting in an equation that described the behavior of water level in a day, having now a value every second, as shown in the following figure:

Figure 6. Variation of water level in a day, generated in the numerical model.

3.2.3 Running and calibration of the model The model was run in the module Delft3D-FLOW, taking into account the grid quality and variation of

the time step, several models were run until it was stable and showed no errors in the running program. The simulation time used was one day, the day the data was taken. The time step used according to the grid refinement was 0.1 min. For the model it was used some hydrodynamic constants like gravity and water density, with values of 9.81m/s2 and 1027 kg/m3 (density of saltwater) respectively.

For the calibration of the model, the velocities taken in field with ADCP were used by comparing this results with the velocities in the model in the exact time step the ADCP was taken in field and in the same spot, then adjusting the Manning roughness, until the showed similar results. Six gauges were made along the river,


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as shown in the following, but only three were calibrated. Gauge one is located upstream (right side of the picture), gauge two is located downstream (left side of the picture), and gauge three is located near the landing track.

Figure 7. Gauges made in field with ADCP.

Figure 8. Gauge 1, ADCP results.

Total Q Width Total Area Boat Vel. Flow Vel.

m³/s m m² m/s m/s

9,926 31.61 35.20 0.242 0.311



m³/s m m² m/s m/s

3,053 29.42 48.44 0.104 0.091


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m³/s m m² m/s m/s

14,341 33.50 61.62 0.137 0.228


A scenario was run for each gauge in order to calibrate the Manning roughness in the exact time the flow velocity was taken. The results of the calibration are shown in Table 1.

Table 1. Manning roughness and flow velocities compared for each gauge.

Gauge Manning

Roughness

Average flow velocity in model

[m/s]

Flow velocity in field [m/s]

1 0.150 0.091 0.091

2 0.050 0.315 0.311

3 0.071 0.228 0.228

The velocities taken in each gauge are similar, comparing also with gauge 4, 5 and 6, oscillating between 0.15 m/s and 0.36 m/s. The velocity in gauge 1 is lower compared to the other values so it is assumed that could be a mistake in gauging. Therefore, the Manning roughness chosen is an average between the calibrated values of gauge 1 and 2, then the value is 0.0605.

4 RESULTS AND ANALYSIS The water level variation is essential to determine if an estuary is controlled by tides or not, it is not the only

way, but is the simplest. The water level shows some rise and fall during a day, like a tide oscillation, with two high levels and two low levels in a day, in intervals of approximately 6 hours between a high and low level and 12 hours between the high or low levels. This shows that the dynamic of the estuary zone is tidally-dominated.

Also, there is a difference between upstream and downstream water level, this is because the hydraulic gradient of the river, and shows the sea inlets and river outlets, this means, when the sea rises the water starts to enter the estuary zone and water level of the river rises gradually as sea level falls until it reaches the highest level and falls gradually causing the sea level rise.

Figure 12. Water level along the river rising in upstream region, river outlet.


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Figure 13. Water level along the river in static level.

Figure 14. Water level along the river rising in downstream region, sea inlet.

5 DISCUSION The initial hypothesis is that this estuary is more influenced by tides than by river discharge because of the

low velocities shown with the ADCP, an average of 0.25m/s, also because of the low suspended sediment obtained in field. This influence can be seen in the results shown by the model in the water level variation plot (Figure 11, 12, 13), and because of this variation of water level, the shores may present a problem of erosion that could be piping or sapping erosion which also depends on the material of the shore, in this case a silty sand and the matrix suction values.

The influence of water level fluctuations in erosion of the shoreline, has been studied in riverbanks and mainly seasonal variation (Chen, Hsieh, & Yang, 2017; Liang, Jaksa, Ostendorf, & Kuo, 2015), not so much in tropical estuary zones, where the variation is tends to be the same in the year and have a high influence of tides, that produces variation of levels during the day, but the erosion process is likely to be the same just that in this case is produced in a short period of time, the water level rise and saturates the soil and when it falls there might be an outflow or exfiltration that removes the soil grains away, forming some internal cavities, causing debilitation and possible forming of tension cracks that will lead into the instability of the shore.

6 CONCLUSIONS The dynamic process of the Ancachí River were studied through field data recollected and the numerical

two-dimensional (2D) model. Field measurements and numerical model of the water level show that the dynamic this estuary zone is mainly controlled by tides, the inlets and outlets of water in a day. In addition to that, velocities taken in field were relatively low and suspended sediments were almost unseen that this reinforces the initial hypothesis that this estuary is tidally-dominated.

This constant variation of the water level could be the main cause of the instability of the shores, eventually there is a need of analyzing the relation of this variation and the suction process in the soil, taking into account the moisture retention characteristics and the variation of the saturation front.

ACKNOWLEDGEMENTS The author would like to thank teachers and coauthors Nelson Felipe Moreno Cardona and Carlos

Torres Romero from the Santo Tomas University who helped to make this research possible, and Jose Oliveros Acosta from the Javeriana University for all the knowledge in hydrodynamic modelling. Special thanks to ROTHEM INGENIERIA SAS for providing all the field work used in this research.


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