An injection molded plastic corrugated leaching or storm water chamber has a peak corrugations segment with a flattened and thickened web. The web is the portion of the corrugation which runs from the peak to the floor of the adjacent valley. Preferably, flattened web segments are at the apexes of adjoining corrugations, to create an increased thickness band region, running lengthwise along the chamber from an injection nozzle location or sprue. The flattened web segments enable better melted plastic flow during molding, without increasing nesting height.