Abstract: A trap for mounting in a catch basin having a wall with a cylindrical opening provided therethrough. The trap, which is designed to impede floating pollutants from leaving the catch basin, includes a front plate member with an outlet opening, a rear plate member with an inlet opening, and a connecting member extending between and connecting the front plate member and the rear plate member. The trap is arranged so that it can be mounted to the wall of the catch basin by inserting it through the cylindrical opening. A further aspect provides a trap having a front plate member with an outlet opening and a rear plate member with an inlet opening, and connecting means for mounting the plate members on the wall of the catch basin. Also provided is a process for constructing a trap in a side wall of a catch basin, comprising steps of boring a cylindrical opening through the side wall with a coring machine and securing a front plate member and a rear plate member to the side wall.