has a flange which fits into the corrugations of the chamber 20 as described above can be positioned within a corrugation at any point along the length of the chamber to provide a baffle or act as a weir in one such use the dome may have a cutout or through hole at an elevation alternatively with the kind of loose fit mentioned above there can be flow through the gap between the end dome and the chamber corrugation so the end dome functions like a weir if it is desired to prevent such flow appropriate sealant or gasketing can be employed using a dome as weir creates subchambers within the length of a chamber more than one dome may be positioned along the length of a chamber to create a multiplicity of subchambers the dome as weir is used to make the subchamber function as a reservoir and settlement basin thus water flowing along the length of the chamber will stagnate in velocity and desirable settling of entrained debris will be realized thus by strategic placement of dome weirs along the length of the chamber near the inlet end of a string of chambers a preferential region for settlement of heavier than water debris is created cleaning is made easier while the dome shaped end plate is preferred when a weir is desired in the generality of this aspect of the invention flat end plates may be used as weirs the chamber has another feature which is characterized by an approximate or exact elliptical curve this.