Construction began Monday on a project at the Tujunga Spreading Grounds that is expected to double the amount of stormwater that can be captured at the facility to about 5 billion gallons per year. The $29-million project, to be completed in 2018, was funded by the city’s Department of Water and Power and designed by the county’s Department of Public Works. The project will increase the spreading grounds’ current capture and storage ability from 2.5 billion gallons, or 8,000 acre-feet of water, to 5 billion gallons or 16,000 acre-feet per year – enough water each year to supply 48,000 households in Los Angeles.