As shoreline communities in the San Francisco Bay Area scramble to prepare for rising seas, they should also be mindful that protecting themselves could worsen flooding elsewhere. This is because seawalls can reflect and amplify tides. “Decisions in one location could affect hazards in another,” said environmental engineer Michelle Hummel, who began studying the bay while at UC Berkeley and is now at the University of Texas at Arlington. Her latest research reveals that building seawalls along relatively small sections of shore could raise water levels enough to have far reaching effects, even all the way across the bay. … But building seawalls in other places could, especially in the wide alluvial valleys where rivers flow into the bay.