FLOODING 86% of the land area in downtown Brockton, defined by the 40R Expansion District, is impervious. In addition, 40% of the city’s floodplains are developed (Arbeene et al). This means that when the city experiences a severe rain event or sudden snow melt, there is very little opportunity for the water to infiltrate into the ground. Instead, water pools in roads and basements, often causing significant property damage. In the period from 1978 to February 2014 the National Flood Insurance Program paid out $1,855,888 in flood insurance claims to residents in the city of Brockton. The city has 39 properties identified as repetitive loss properties having had two or more insurance claims over $1,000 in any rolling ten year period (Arbeene et al).
PERKINS PARK
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MBTA/BAT STATION
CITY HALL PLAZA
A small rain event can produce a substantial amount of surface runoff. In a 1 inch rain event, which is only 2% of Brockton’s average annual precipitation, over 3.1 million gallons of surface runoff is produced within downtown. That’s more than 4.5 Olympic sized swimming pools being filled in a single rain event. Now consider Brockton’s average annual precipitation, which is about 50 inches annually. That equates to more than 156 million gallons of water in a year running off of impervious surfaces. This is a substantial amount of untreated water that contributes to flooding and enters the Salisbury Brook and eventually the Salisbury River. It is also important to understand how valuable the remaining 14% of permeable surfaces are to downtown Brockton. If the remaining permeable and green surfaces were to be removed and replaced with impervious surfaces, such as asphalt or concrete, this would produce another 400,000 gallons of water in a 1 inch rain event within the downtown. Annually, these existing permeable surfaces in downtown may absorb 20 million gallons of water. Permeable surfaces and trees that can intercept, infiltrate, and filter water are invaluable to Brockton and can reduce flood damage and water quality impacts significantly. Therefore, it is critical for the city and other landowners to consider ways to treat and slow surface runoff and convey runoff from impervious surfaces to permeable surfaces where feasible.
Vacant Lots to Bioretention Gardens
TANNERY PARK SYCAMORE GROVE
SALISBURY PARK
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TARTAGLIA PARK
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What does a high coverage of paved surfaces mean for downtown Brockton?
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Major Bodies of Water Floodplains Downtown Brockton 40R District
100 YEAR FLOODPLAIN
Joan Nassauer at the University of Michigan collaborates with the City of Detroit to install and test bioretention gardens on vacant lots. These gardens fill basements of demolished homes with a bioretention soil mix and “are expected to achieve an average annual stormwater volume reduction of 300,000 gallons per site” (University of Michigan). The city of Brockton might consider implementing similar systems on sites that experience routine flooding and are unsuitable for commercial or residential development. This approach treats stormwater and increases upstream storage capacity. It may be appropriate for vacant lots in and around the floodplain.
BROCKTON URBAN GREEN SPACE PLAN EXISTING CONDITIONS AND ANALYSES
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