Five Hundred Gallons from One Roof: Where a Colonia Downspout Sends the Rain

The Rutgers Water Resources Program calculates that in a one-inch rain an 800-square-foot roof sheds 500 gallons, and a single downspout may take every drop of it if the gutter drains that way. A rain barrel of ordinary size holds about 50 gallons, so it is full after the first tenth of an inch. The Newark airport gauge, the NOAA normals station this site uses, averages 46.6 inches of precipitation a year. Scaled by plain arithmetic, not measured, that comes to something like 23,000 gallons a year across one small roof. Nearly all of it leaves through a few inches of gutter and one pipe.

The EPA's guidance on redirecting downspouts says many of them empty onto pavement, which sends the water to the street. Others are tied into underground pipes, which can run to either the storm drain or the sanitary sewer. The agency advises thinking through where the water will go, so that it causes neither property damage nor unsafe conditions. The page adds a caveat: not every site is a candidate for disconnecting a downspout. The page also says it is no longer updated. A pipe that vanishes into the ground at the foot of a downspout is worth a question to someone who knows the system.

Woodbridge Township's resale and refinance sheet lists what its sanitary sewer ordinance prohibits discharging: basement floor drains (which have to be cemented over), sump pumps, yard and driveway drains, and missing, loose or cracked sewer trap caps. It never mentions roof leaders, downspouts or gutters, so it says nothing either way about a downspout tied into a pipe. The state's 2018 construction notice counts exterior gutters and leaders as ordinary maintenance work, with repair, replacement or new installation all included, and says code compliance still applies. The township's Building Department answers at 732-602-6003 and Public Works at 732-738-1311.

Woodbridge's description of its floodplains puts river flooding in the township's northwest along the South Branch Rahway River and Pumpkin Patch Brook, with the Colonia section as the example it names. The brook runs north toward Clark, and the document says suburban development increased runoff and caused long-standing overbank flooding. Pumpkin Patch Brook discharges to Robinsons Branch. Near Jordan Road, just upstream of the Inman Avenue crossing, the ground is about 66.5 feet and FEMA's 100-year flood elevation is 66 feet; the document says some homes with basements face increased flood-damage risk. The same document notes severe erosion and lost channel vegetation along the South Branch in Merrill Park.

The township's floodplain page lists areas of special flood hazard in four zone types, AE, A, Coastal A and VE, and says the flooding is both tidal and fluvial, meaning river-driven. Under New Jersey's Inland Flood Protection Rule, adopted in July 2023, the design flood elevation raises the mapped river flood elevation by three feet. The same page says a new flood policy typically takes about 30 days to take effect. The state insurance department's storm sheet agrees: flood insurance is not effective until 30 days after it is sold. Neither document places a particular address in a zone; the township's Division of Engineering, at 732-602-6057, handles flood zone questions.

The National Weather Service's review of the remnants of Ida lists, for Middlesex County, flooding on U.S. 9 near the Edison Bridge, flooding that closed Interstate 287 near Exit 3 in northwest Edison, and multiple water rescues across the county. It carries no rainfall total for Woodbridge, and Colonia's stretch of the South Branch is not named. On a night like that, the roof's share of the water is small, and it is also the share an owner can direct.