Material Transport Analysis

Material Transport Analysis

A hydrodynamic model can be used to understand the movement of currents in a water body. These currents influence how the materials move around and how long they would stay in the bay before they are flushed out into the sea. A practical application of the model is assessing how materials like pollutants would disperse over Manila Bay. Several numerical studies on the transport of materials and marine debris are associated with hydrodynamic models. Project e-SMART uses a DELFT3D hydrodynamic model to characterize flows in Manila Bay and to analyze material transport within the bay.

The dispersion of pollutants in Manila Bay can be illustrated using a material transport model. Most of the freshwater influx to the bay comes from Pampanga River (around 50%) and Pasig River (about 20%). With this, it is good to investigate: what if pollutants were dumped in Manila Bay, specifically at the mouths of Pasig River and Pampanga River?

The simulations showed that materials travel across the bay in patterns that vary according to when (time of the year) and where (location of river outlet) the materials were released. Their movement closely follows the current circulation patterns which are dependent on the bay’s hydrodynamics (see https://bit.ly/MBHydrodynamics).

Based on the material transport model, majority of the materials dumped at the Pasig River’s outlet during dry season disperse to the north along the coasts of Bulacan and Pampanga, and remain across Manila Bay for 6 months before they are flushed out. At the first two weeks after release, materials behave in a counter-clockwise manner, which agrees with the hydrodynamics model result during dry season. Most particles exit the bay through the channel between Corregidor Island and Bataan because of a strong westward current pulling the materials towards this channel (evident in T + 12 days). Six months after release, some particles still remain within the bay which totally flushes out after about two more months. During wet season, materials originating from Pasig River travels south and readily disperse towards Manila Bay’s center. On the first weeks of dispersion, the clockwise movement of materials is evident.

If the source location were at the Pampanga River’s outlet, during dry season, materials traverse southward to Bataan and east towards Pampanga coastlines before it disperses across the center of Manila Bay after 30 days. However, during wet season, majority of the materials from Pampanga move towards the east until it reaches the mouth of Pasig River before it scatters across the bay. This means that most of the pollutants or sediments coming from Pampanga will affect the coasts of Metro Manila. Similarly, materials stay longer during the dry season when river discharges from surrounding watersheds are lower and flushing out currents are weaker than during the wet season.

In both scenarios and in both seasons, it only takes a month for particles to disperse fully across Manila Bay. Moreover, during the dry season, it takes 7 days longer for materials to be flushed out of the bay when they are released at the Pampanga River outlet than when they are released at the Pasig River outlet. Similarly, during the wet season, it takes a longer time (44 days longer) to flush out materials coming from the Pampanga River than from the Pasig River.

The knowledge of the fate and transport of materials coming from different sources and at different seasons provide vital information that can aid in streamlining solution interventions where the movement of pollutants is an important consideration. Based from the particle tracking analysis, it was found that the flushing potential of Manila Bay was highest during the period when the season changes from dry to wet.

As previously discussed, materials travel across Manila Bay differently depending on the location of the source and timing of the season. The movement of materials is largely affected by the dominant circulation pattern exhibited during a certain season, which are caused by several factors such as wind, density, tides, and river discharges.

In this scenario, a single particle which can represent a PET bottle, was released at 6 different river outlet locations during wet season (release time set on July 1, 2018). These river outlet locations were selected because of high urban density within their watersheds or because its location is critical due to the bay’s morphology. Based on the simulation, particle travels differently and leaves the bay at varying times as well. Interestingly, a similar track pattern can be observed when a particle originates at the outlets of Pasig, Pampanga, and Balanga Rivers. In these scenarios, the particle traverses in a clockwise circular path for some amount of time, before it eventually flushes out of the bay. The regular fluctuation of tides or the rhythmic ebbing and flooding of currents cause particles to remain in the bay longer. Sometimes, a particle can be driven out only when the seasons shift from wet to dry, in which the dominant wind direction and current changes. The clockwise track of the particle also agrees with the circulation pattern during wet season simulated by the hydrodynamics model (see https://bit.ly/MBHydrodynamics). Other particles may exit through the south of Corregidor Island just like with the particle released at the mouth of Meycauayan River, primarily because there is enough flushing out current at that time to drive it off Manila Bay as it nears the exit.

The fate and transport of the PET bottle as described above is a general representation of material movement as a result of the particle tracking analysis conducted.

The material transport model simulates transport processes by means of particle tracking method using the results from the 3D hydrodynamics model. The tracks are followed in three dimensions over time, whereby a dynamic concentration distribution is obtained by calculating the mass of particles in the model grid cells. In the simulation, 10,000 particles were instantaneously released at the mouth of Pasig River and Pampanga River on July 1, 2018 for the wet season, and on January 1, 2018 for the dry season. It used the results from the hydrodynamic model which is represented by three layers of water while vertical dispersion was computed by depth averaged algebraic method. The particle tracking model used results from the hydrodynamic model with truncated three-layer setup into a single-layer.

The hydrodynamic simulation had a warm-up period at the start before the materials or pollutants were introduced. The hydrodynamic model was set up using primary and secondary data (see https://bit.ly/MBHydrodynamics). Included in this set-up were daily river discharges coming from watersheds draining to Manila Bay. These daily discharges were generated from long-term hydrologic simulations.

The simulation for tracking single particles originating from different locations is dependent on the timing of release. The results shown in this post were based from particles released on July 1, 2018, assumed to represent wet season particle movement and distribution.

Material trapping (drag) due to the presence of natural (mangroves) or artificial (fish pens/cages) obstructions were not taken into consideration in the present model set-up.

The hydrodynamic model for the particle tracking model was calibrated using water level and satellite data for the years 2017 to 2019 (see https://bit.ly/MBHydrodynamics).