The formation of slug flow in horizontal pipes governs the characteristics of geyser events in storm sewers. In this work, horizontal pipe is decoupled from the vertical pipe, and the newly formed boundary is subjected to sinusoidal disturbance to investigate the response of the gas-liquid interface. The role of pressure disturbance is greatly influenced by the initial pressure and velocity conditions, and the pipe's diameter and length. Using the non-dimensional momentum equation, we identified two significant non-dimensional parameters, coefficient of pressure gradient and coefficient of shear stress rate as a function of the operating conditions and dimension of the system. These parameters directly influence the pressure gradient and viscous effects in the momentum equations and thereby the formation of slugs. This hypothesis is analyzed using 2D numerical simulations in Ansys Fluent for various operating conditions (pressure head 3 times to 50 times pipe diameter) and pipe lengths (10 to 1000 times pipe diameter). Since we are interested in the qualitative analysis of slugs distribution and not the exact shape and slugs' volume fraction details, 2D simulation methodology is adopted. The two non-dimensional parameters justified the numerical results for the dramatic events of slug flow and large wave formations. It is seen that, large values of the coefficient of pressure gradient enhances the possibility of slug formations, whereas the magnitude of the shear stress rate coefficient governs the scale of influence along the length of the pipe. These findings could provide insights on the mitigation of unwanted slug formation encountered in multiphase flow systems.