DC is using decades-old rain predictions to plan local infrastructure projects, though more recent data is available

by Marlene Berlin

Starting this winter, DC Water is set to undertake the rehabilitation of the century-old Soapstone Valley sewer system. The goal is to keep the sewer pipes whole and harden them against erosion and other severe weather impacts for decades to come.

The design of infrastructure projects such as these is determined in part by the A14, the National Oceanic and Atmospheric Administration’s atlas of precipitation frequency predictions. This is a bible for infrastructure projects all over the country. And it is severely out of date.

The Washington Post reported in April that the NOAA predictions for some localities were based on 60-year-old statistics. In some places, 100-year storms were occurring twice as frequently as projected. (A 100-year storm, statistically speaking, has a one percent chance of dropping a certain amount of rainfall in one day, in a given year.) Yet many states and municipalities require infrastructure designers to use A14 data for projects intended to last another 50 years or more.

The District’s A14 was updated in 2006, using 40-year-old data. And this, too, is out of date due to rapid changes in the frequency of intense rain storms. I heard this from David Conrad, a neighbor in Chevy Chase who is one of the volunteer “weed whackers” at Broad Branch Stream. He is also an expert on water resources management, and he recently studied the impact of federal programs on floodplains for the National Association of Flood Plain Managers.

This association sent a letter to the House Subcommittee on Environment on April 20th, calling on Congress to fund regular updates of the A14, given climate change and the need to invest in upgrading our infrastructure.

Some localities have taken it upon themselves to invest in better rainfall estimates. And the District does have access to more accurate predictions. Phetmano Phannavong of Atkins Global, a former floodplain manager at the DC Department of Energy and the Environment, told me that DOEE has done a lot of work in making predictions based on climate change modeling. He sent me this DOEE-funded climate change adaptation plan from 2013, which includes this precipitation table.

In comparing the DC data, the A14 defines a 100-year storm as 8.33 inches of rain falling within 24 hours. The DOEE projections change over time, with a 100-year, 24-hour storm at 10.5 inches for 2015-2034, 10.3 inches for 2035-2054 and 14 inches in the 2080s.

The Forest Hills Connection asked DC Water about the rain predictions it is using for the Soapstone sewer rehabilitation project. Emmanuel Briggs, the director of community relations, responded by sending an attachment of NOAA’s 2006 A14. When we pressed DC Water about the accuracy of these figures, William Elledge, a senior design manager, relayed in an email that the agency is working with DOEE. In addition, he said they are “protecting their assets” in the stream from 100-year storm events using the following measures:

“…exposed manholes will be protected by armoring and imbricated rock walls. Exposed pipes will be buried beneath riffle grade control material to a minimal depth of 12 inches. Where sewer pipe crossing protection is required, the stream bed will be raised, providing 12 inches of cover above the top of the sewer pipe to reduce debris impacts on the pipes and scour on their downstream ends, and ensure long-term stability. In addition to these designed protections, the area will be inspected periodically to evaluate integrity of the constructed protection features.”

Burak Kaynak, the DC Water manager for this project, later clarified that this is based on A14 predictions, not DOEE-modeled rain projections.

Forest Hills Connection has also asked DOEE to comment on which rain predictions DC should be using for its infrastructure projects, and will provide an update once the agency provides its response.