# The Travel Time of Floatable Litter of Different Densities Influenced by River Flow Velocity
## 5 Conclusions  
This study was carried out to assess the travel time for different densities of floatable litter influenced by river flow velocity. This study provides basic information on FL migration behavior through the evaluation of FL travel time in rivers. This research also discovered two other main aspects: (i) the evaluated density of selected objects representing common FL found in rivers; and (ii) the identified correlation between FL density and its travel time.  
The knowledge in identifying the effect of FL density on its travel time as an approach to understanding the FL migration behavior is very useful for future reference and studies of factors influencing litter transport and migration in rivers. Reliable data of FL travel time with different densities were crucial for planning and environmental assessment for river management. In addition, it is beneficial in outlining a methodology for riverine litter study that is low-cost, and simple in a short time. Therefore, the fulfilments of the two main specific research objectives have been systematically concluded below for a better understanding of the output.  
Based on the first objective of this study, the travel time for FL with different densities influenced by river flow rate conditions was determined. The obtained results revealed that cloth has the highest density, \(\rho\) = 1154.44 kg/m\({}^{3}\), while FL with the lowest density is food polystyrene, \(\rho\) = 4.76 kg/m\({}^{3}\). A total of 65% of the FL density is less than 200 kg/m\({}^{3}\), whereas 69% of it is plastic-type litter. The result shows that FL commonly found in rivers is plastic-type with mostly similar density values. The density of an FL highly depends on its mass-to-volume ratio. Thus, information on FL density is important instead of considering its mass, volume, and size separately. Moreover, density plays a key role in determining both the floating and sinking of an FL which later affects its migration and resident time in the river.  
River flow velocity was determined manually by using the general velocity equation and extrapolation of the float method as suggested by [11]. The measurement recorded the highest river flow velocity as Condition A, v = 0.230 m/s, followed by Condition B, v = 0.167 m/s, and Condition C, v = 0.144 m/s. The river flow velocity measured was higher when there was precipitation in the area of Sg. Berkelah, Pahang. Thus, the values of flow velocity comply with the suggestion of previous studies about factors affecting river flow velocity, which is that the more water is added into rivers via tributaries and streams, the higher the flow velocity will be.  
As the outcome of the third research objective, the relationship between FL density and its travel time has been identified. The correlation between FL density and its travel time is not significant. The change in FL density does not influence the change in FL travel time in rivers. However, travel time might be influenced by FL with intermediate- to high-density, \(\rho\) > 1000 kg/m\({}^{3}\). In conclusion, density is not a factor affecting the travel time of horizontally migrated litter, but only for vertical transport of litter. Furthermore, FL tends to migrate horizontally because it floats on the river water. The travel time of FL is not influenced by its low density, \(\rho\) < 1000 kg/m\({}^{3}\), but by other factors that give more influence on FL travel time than the density factor. The factors could be the effect of wind resistance on the high-floating litter in the water; the effect of wind force on the FL migrating downstream causing it to move faster and further; turbulent flow in the river; water entrance into FL which increases its density; and that density factor is more significant toward vertical transport of FL.