Date of Award

Spring 2021

Project Type

Thesis

Program or Major

Civil Engineering

Degree Name

Master of Science

First Advisor

Thomas Ballestero

Second Advisor

James Houle

Third Advisor

Alison Watts

Abstract

This research examined the effectiveness of GSI and other BMPs to control urban flooding for extreme precipitation events and compared the impacts of increasing impervious cover with the impacts of increasing rainfall intensity caused by climate change. The City of Dover has spent the last decade implementing best management practices in the 185-acre Berry Brook watershed to combat stream pollution and flooding caused by urbanization. Improvements to the watershed included building additional headwater wetland area, daylighting and restoring 1,100 feet of stream, and redirecting stormwater to GSIs, thereby reducing the effective impervious cover from 30% to 10%. Four PCSWMM models of the Berry Brook watershed were developed for the analysis: a pre-implementation model, a model of the pre-implementation watershed set to 15% IC, a model of the pre-implementation watershed set to 0% IC, and a model of the watershed after BMP implementation. The four models were used to examine the effects of GSI implementation, changing impervious cover, and climate change on urban watershed hydrology for the 2-year, 10-year, 50-year, and 100-year extreme precipitation events. The effectiveness of GSI and other BMPs to control urban flooding caused by extreme precipitation events was tested by comparing the peak flows, time to peak flows, runoff depth, and total storm flow volume. A long-term rainfall-runoff simulation from 2001 to 2011 was also done for the watershed with and without GSI. It was found that BMP implementation caused an median decrease in extreme peak flow of 7%, an increase in the time to peak flow of 3 minutes, a decrease in the runoff depth of 29%, and a decrease in the total storm flow volume of 30%. GSI impact was more prevalent in short duration extreme precipitation events than in long duration events. In the 10-year analysis, annual maximum flow decreased 8%. The infiltration of rainfall increased by 17% and the stormwater runoff decreased by 40%. This showed implementing GSI in an urban watershed will reduce flooding caused by extreme precipitation events but not eliminate it. For common storms of about no more than 1.3 inches, it was found that GSI reduced peak flows by a median of 68%. Increasing IC in the watershed was shown to have a much more dramatic effect than the increase in rainfall caused by climate change. Impact was still more prevalent in short duration extreme precipitation events than in long duration events. The difference between the BMP-managed watershed under future climate change conditions and the traditionally managed watershed under current day conditions was minimal, implying BMP implementation will keep flooding from getting any worse as the climate shifts, but by itself, GSI will not eliminate urban flooding.

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