Construction of a storm drain system has begun on the former Tower Park site at 17th and G streets in Modesto, where affordable-housing developers American Baptist Homes of the West and Satellite Affordable Housing Associates are converting the property into a 50-unit apartment complex for low-income seniors. The storm drain work, being done by Allen Waggoner Construction of Ripon, is expected to continue all week. Officials have said the housing project will cost $13.7 million, and the complex’s funding includes federal money administered by Modesto and state tax credits. Modesto picked the two developers to turn the former park into affordable housing. Officials have said construction should take about a year.