A restrictor plate assembly is disclosed. The assembly is adapted for being positioned within a catch basin throat so that the throat extends rearward of the assembly. The assembly has a longitudinally extending restrictor plate and a restrictor plate securing system, which includes a first clamp arm, pivotally positioned against the restrictor plate, for engaging a first throat surface of the catch basin; a second clamp arm pivotally positioned against the restrictor plate, for engaging a second throat surface of the catch basin, the second throat surface opposing the first throat surface; and an urging member which simultaneously urges the first and second clamp arms against the first and second throat surfaces, respectively.