# The Travel Time of Floatable Litter of Different Densities Influenced by River Flow Velocity
## 4 Discussion  
The measurement of mass, volume, and size of FL is crucial to obtain the FL density value. The relationship between these parameters explains the density values. For example, FL2 has a higher density compared to FL1, while FL2 has a smaller mass and volume compared to FL1. This is because an FL with a high mass value is not necessarily going to be high-density, especially when it has a small mass-to-volume ratio. The relationship between density, mass, and volume shows how density measures the mass-to-volume ratio of an FL, with a higher ratio indicating more density [22]. The density value is higher when  
Figure 8: FL travel time vs. FL density in all conditions. the fraction value is larger. The fraction is larger when the first number (the numerator) is greater than the second number (the denominator). This shows that knowing the density of FL will explain the FL migration in rivers more compared to only considering the FL size and mass separately. Density value is important in identifying a variety of substances and objects [22]. Density offers a convenient means of obtaining the mass of an FL from its volume and vice versa. Moreover, for floatable objects, the small difference in density value affects the submergence level of objects in the river and their buoyancy. The majority of FL selected in this study are plastic. This is because about 80% of riverine and marine litter is plastic [23; 24]. Polyethylene is the world's most common plastic. Polyethylene plastic is categorized into high-density (HDPE), low-density (LDPE), and linear low-density plastic. HDPE and LDPE plastics are more common. The average density of HDPE and LDPE plastics are 941-965 kg/m\({}^{3}\) and 910-925 kg/m\({}^{3}\), respectively; both categories are in the low-density class, \(<\)1000 kg/m\({}^{3}\)  
Moreover, the lower density of litter results in a lower sedimentation rate compared to litter with a higher density that tends to sink more easily. The low sedimentation rate of litter causes it to be transported further with a higher possibility to get into the marine environment. Thus, plastic-type FL is transported further in rivers and ends up in the ocean due to its low sedimentation rate. Examples of common plastic-type FL found in the ocean are foam, plastic bags, cups, bottles, and packing materials [25]. The river flow velocity for each condition is different when they are measured in different weather conditions (i.e., the number of hours after precipitation and no precipitation). According to [26], it explains that higher precipitation is expected to cause higher average river flow velocity in some areas. While river flow velocity increases as the volume of water increase through the input of river water from streams and tributaries [27]. The significant difference between FL travel time in each condition shows that the different values of flow velocity result in the different value data sets of FL travel time. The higher river flow velocity results in a shorter FL travel time. Flow rate determines the fluxes of pollutants in rivers [6; 7; 8]. River flow rate affects river flow velocity [27]. There is no influence from density on the travel time of FL; this is because the density factor affects the vertical transport of FL more than horizontal transport in rivers. However, there seems to be a trade-off between vertical and horizontal transport of litter in which, during the low influence of vertical transport, the FL migration behavior is more influenced by horizontal transport. The density factor has no significant effect on FL horizontal transport; instead, other factors (e.g., wind, shape, turbulence) can influence the FL travel time more. Moreover, the density values of FL evaluated are very low which causes a tendency for horizontal transport to happen more. According to [4], density is one of the litter characteristics that play a key role in vertical transport.