What if Hurricane Harvey Happened Here? – How Boston Wet Weather Scenarios are used for Planning Flood Emergency Response

Development of wet weather scenarios are required for planning purposes to evaluate and determine flood emergency response for the Boston Water and Sewer Commission (BWSC), Massachusetts. While ‘Hurricane Harvey-scale’ rainfall is unlikely in Boston, hurricanes and other types of extreme storm events have produced flooding and inundation within the BWSC storm and sanitary sewer system. Planning for an extreme storm events requires representative wet weather scenarios for evaluation of potential flooding and to develop appropriate emergency response planning. Extreme event rainfall will be used to model flow and inundation experienced within the BWSC sewer and storm drain systems. The results of this project will identify impacts of major storm events for evacuation, adaptation, and resiliency planning.

Historical storm classification was performed to understand the range of temporal and spatial rainfall distributions expected from tropical storms, convective, Nor’easters, and Frontal storms as they occur in the Boston area. Representative hyetograph shape, duration and intensity, and motion were developed for each of these major storm types. Scenarios include prior extreme wet weather events producing up to the 500-year storm depth. For example, a rain event will be modeled with durations and associated depths for a range of typical durations, ranging from 24, 48, and 72-hour periods. Developed storm event data for each classification represents within-class extreme events and high-resolution gauge adjusted radar rainfall for an actual thunderstorm event. Storms recommended for use in modeling have typical hyetograph shape, storm motion with temporally varying depths representative of the local climate.

Historical rainfall hyetographs were analyzed using long-term NOAA National Weather Service (NWS) gauge precipitation. The largest storm depth, 303 mm in 40 hours, occurred on August 18 – 20, 1955, produced by a tropical storm that generated significant precipitation due to its slow forward motion. This storm created flooding in several New England states including Massachusetts and the Boston area. The second greatest event produced 8.34 inches in 71 hours on May 13 – 15, 2006, caused by a frontal event with onshore wind flow associated with a nearly stalled low-pressure system anchored over the Great Lakes. This storm produced significant rainfall, flooding, and sewage releases impacting the Merrimack River. Other historically relevant storms with significant flooding impacts, occurred on June 12 – 14, 1998, September 10, 1999, and September 15 – 17, 1999. The first two were generated by frontal systems while the latter was associated with remnants of a tropical system, Hurricane Floyd. Representative hyetograph shape was determined from 70 years of hourly precipitation data recorded by the NWS Boston Logan International gauge, comprising 3,100 storm event periods. Appropriate storm depths were determined for each storm classification as illustrated in Figure 1 below. Several historically extreme events are indicated on the plot, with the filled square representing enveloping depth-duration for the Tropical storms shown here. Radar analysis of storm motion revealed typical direction and speed associated with each storm type. Storm motion was then used to create dynamic storms with associated characteristics for modeling inundation and impacts associated with each wet weather scenario.

Based on analysis of historical rainfall measurements, from both rain gauge and weather radar, realistic spatial and temporal distribution of rainfall are characterized for representative storm classifications: 1) Airmass, 2) Nor’easters, 3) Frontal storms, and 4) Tropical storms. The spatial and temporal characteristics for each storm classification was determined for the BWSC service area. Seven storms composed of four storm classifications, Airmass, Nor’easter, Tropical, and Frontal, and three historically significant storm events are recommended as wet weather scenarios for use in flood inundation modeling and planning. This presentation will focus on the wet weather scenario development, which is complete. Storm motion, hyetograph shape, precipitation frequency depths, duration, and intensity of the storm types in the wet weather scenarios has been completed for use in modeling impacts. Because the flood inundation modeling and emergency planning portion of the project is a work in progress at this time, updates on the modeling of inundation and emergency planning will be provided at the time of presentation.

Figure 1 Precipitation frequency depth-duration for historical Tropical storms (1 in = 25.4 mm)