The Northern California coastal city of Arcata receives 45-plus inches of rain each year, and too much of that rainwater ends up in the city’s network of sewer pipes. In the older downtown section, some downspouts once emptied into sewer pipe. Consequently, during the wet season, an average of 5.9 mgd of water — sometimes more — flows into the sewer system.

Or it did until recently. City officials hope they have curtailed the infiltration. The first phase in overhauling the...