A method of manufacturing a water detention chamber by providing a polymer melt, injecting a CO2 blowing agent into the polymer melt, and injecting the polymer melt and CO2 blowing agent blend into a mold cavity. The mold cavity defines an arch-shaped corrugated chamber having upstanding ribs and a flange having an upper surface and a lower surface. The flange has one or more protrusions, preferably elongated members, at the first end of the chamber. The flange also has one or more mating apertures or cavities at the opposite end from the protrusions.