An assembly for receiving and diffusing stormwater in underground installations includes a horizontally elongated plastic chamber having an arched cross-sectional shape with an upwardly directed apex having an inlet portal. A sediment-accumulating compartment is positioned below the chamber in vertical alignment with the portal. A rigid riser pipe of circular cylindrical configuration penetrates the portal and extends vertically between a bottom extremity located within the chamber and a top extremity positioned substantially at ground level in association with a grated street-level stormwater receiving structure.