Magnetohydrodynamics (MHD) is usually used in energy harvesting. Application of MHD power generators can be seen at sea, chimneys, and industry for large-scale energy harvesting. MHD produces an electromotive force by using a fluid conductor, such as plasma gas, liquid metal, or saltwater, to generate a magnetic field (e.m.f.). The electromagnetic field will be captured using an electrode made of aluminium, copper, or another conducting metal. For this project, the MHD system has been seen to have the potential to become a sensor for detecting water flow inside underground drains. This MHD sensor could help to prevent floods from occurring. Many factors has been found and most commonly that the flood incident occurred due to the overflow of rivers and clogged underground drains. Therefore, flood prevention measures must be taken immediately. This work contains a study of the MHD system and MHD generator with different number of magnets and output power as a signal. It can be found that the more magnet used give a better signal in output power as well as a sign of a good drainage flow in underground drains.