Date of Award

Spring 2019

Project Type

Thesis

Program or Major

Civil Engineering

Degree Name

Master of Science

First Advisor

Thomas P. Ballestero

Second Advisor

James J. Houle

Third Advisor

John M. Davis

Abstract

Increased stormwater runoff due to the construction of impervious surfaces is a major issue in urban environments, causing combined sewer overflows, erosion in natural waterways, and damage to infrastructure. Subsurface gravel filter (SGF) system, a type of Green Stormwater Infrastructure (GSI), can effectively reduce stormwater runoff volumes and peak flows by infiltrating runoff. Current GSI design guidelines require that these systems be statically sized to store the 24-hour storm depth equaled or not exceeded approximately 90% of the days with rainfall. Across the United States, this design depth is roughly equal to 2.5 centimeters (1 inch) of rainfall. This sizing technique does not account for the dynamics of system performance such as horizontal infiltration through the sides of the systems, unsaturated soil conditions, or the dynamic nature of runoff generation. By neglecting these factors, subsurface infiltration systems may end up being oversized for desired runoff reduction objectives. For this study, the hydrologic performance of SGF systems was evaluated through a combination of monitoring data and computer modeling.

Monitoring data was collected for two SGFs in Dover, NH which are statically designed, according to NH stormwater regulations, to capture and treat the runoff from 1-inch of rainfall. One system is located under Grove St and was found to infiltrate substantial volumes of runoff even though the soils surrounding the system were found to have a relatively low hydraulic conductivity (i.e. <0.5 inches per hour). On average, over the 1-year monitoring period, the Grove St SGF infiltrated 84% of the runoff it collected. The second system, which was located under the parking lot of the Seacoast Kettlebell workout center, did not meet design expectation as it infiltrated negligible volumes of runoff during each storm event. The extremely low hydraulic conductivity of the soils at the Kettlebell site, effects of high groundwater level, and close proximately of the system to Berry Brook appear to have severely limited infiltration.

Analysis of the systems with three computer-based infiltration models, including an unsaturated flow model, a Green-Ampt model, and a unit-gradient, saturated flow model, showed that system performance was highly dependent on horizontal infiltration. The unsaturated properties of soils appeared to have only minor effects on total infiltration volumes due to the rapid transition from unsaturated to saturated flow conditions. Statistical analysis of the model results for the Grove St SGF showed that the unit-gradient model was the most accurate of the three models. Together, monitoring and modeling results confirm that subsurface gravel filters and other infiltration-type GSI could be more accurately sized to meet runoff reduction objectives if horizontal and vertical infiltration are accounted for by incorporating the unit-gradient model into system design techniques.

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