In order to calculate flow fields behind the trash rack and the head loss caused by trash-barriering, the waterweed lump congregated in front of the trash rack was simplified as watertight entity that has the same shape and the same size of the waterweed lump. We adopted ANSYS CFX software and VOF method in numerical simulation of the flow fields of trash-barriering, calculated several schemes, and analyzed the influences of blockage. The results show that: the water level difference and the head loss of numerical simulation are consistent with results of experiment. Because of tiny water permeability of the waterweed lump in front of the trash rack, there are nuances between the flow fields behind the trash rack of numerical simulation and experiment. The specific gravity of the waterweeds is less than that of water and the waterweeds block the up part of the trash rack, which makes the flow velocity through the down unblocked part of the trash rack increase rapidly. As a result, the velocity behind the trash rack increases in the lower area, and decreases, even the backflow appears in the upper area. With the increase of the blockage ratio, the turbulence scale behind the trash rack increases. When the blockage ratio increases to 0.7, the velocity uniformity already decreases to -1.57. The head loss of trash-barriering increases when the blockage ratio and the velocity in front of the trash rack increase. For certain velocity in front of the trash rack, when the blockage ratio reaches 0.6 similar to 0.7, the head loss would increase rapidly.
