ORCID Identifier(s)

0000000313420939

Graduation Semester and Year

Summer 2025

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Civil Engineering

Department

Civil Engineering

First Advisor

Jessica Abbie Eisma

Abstract

Intense flooding, driven by inadequate stormwater infrastructure, rapid urbanization, and climate change, poses a significant threat to urban areas. This problem is particularly severe in coastal cities like Houston, where climate change contributes to high-intensity storms and sea-level rise. Consequently, vulnerable communities such as Kashmere Gardens and Trinity/Houston Gardens suffer disproportionately from flooding, a challenge exacerbated by aging gray infrastructure and limited drainage capacity. In response, Green Stormwater Infrastructure (GSI), a nature-based approach, is increasingly recognized as a vital tool for mitigating these hazards, especially where conventional systems are insufficient.

This study employs a multi-stage approach to evaluate the effectiveness of GSI strategies in Houston. Initially, the EPA National Stormwater Calculator (SWC) was used as a screening tool to assess strategies like permeable pavement, rain gardens, green roofs, and rain barrels based on their runoff reduction potential and community interest, which allowed for the mapping of suitable implementation sites.

Subsequently, a high-resolution integrated 1D-2D Hydrologic & Hydraulic (H&H) model, StormWise, was used to conduct a more detailed analysis. The model assessed the watershed-scale performance of both individual and combined GSI strategies (including green roofs, permeable pavements, rain gardens, and street planters). These evaluations were conducted under a wide range of scenarios, including three implementation levels (25%, 50%, and 100%), various design storms (2- to 500-year return periods with 1-, 3-, and 7-day durations), and both current and future conditions reflecting projected climate and land use changes for 2050 and 2100.

The results reveal several key contributions to advancing the understanding and application of GSI for urban flood mitigation.

First, the study introduces a framework that links spatial planning of GSI with detailed hydrologic performance evaluation using geospatial data, empirical methods, and high-resolution 2D H&H modeling. This represents the first effort to simulate GSI in a 2D H&H environment that does not include native 2D GSI package (StormWise). Results show that GSI shows good performance (9-37% and 12-31% reduction in peak runoff and runoff volume at the watershed outlet) for runoff management under small to moderate storm events. Effectiveness of GSI diminishes under extreme events, confirming the need for complementary flood mitigation measures. Among the strategies examined, permeable pavement, green roofs, and rain gardens emerged as the most effective strategies in this watershed. While permeable pavement and green roofs are more effective in runoff volume reduction, rain gardens are particularly effective in peak runoff reduction, especially when one unit area of strategies are assessed.

Second, the results highlight that GSI performance and benefits do not scale proportionally with implementation levels. Full (100%) implementation substantially reduces surface runoff, peak flow, rainfall excess, and enhances initial abstraction and groundwater recharge at the watershed scale. However, partial adoption (50% and 25%) produced complex and sometimes opposite outcomes such as increased rainfall excess and decreased groundwater recharge. These patterns indicate the presence of thresholds and the importance of connectivity between practices. System-wide benefits of GSI, particularly those related to infiltration and groundwater recharge, only emerge once a critical level of implementation is reached. In addition, the degree of connectivity between different GSI elements affect their overall performance, with greater interconnection leading to more substantial hydrologic improvements.

Analysis across multiple spatial scales (watershed, study area, and polygon) further demonstrates that although GSI has limited influence on watershed-scale hydrology, it produces pronounced benefits within its immediate implementation areas, especially in polygons with high GSI density. This finding suggests that concentrated, high-density installations may be more effective than widely dispersed practices, though further studies in other contexts are needed to confirm this pattern. Longer-duration simulations also revealed diminishing returns under extended rainfall events, emphasizing the limits of GSI when used in isolation.

Third, by modeling both land use and climate change scenarios, the study demonstrates that climate change causes a stronger influence on runoff generation than land use change alone, and that the combined effects are non-additive. Without GSI, all future land use and climate change scenarios consistently increased runoff and peak flows. With GSI, meaningful reductions (17-21% in runoff volume and 14-20% in peak runoff) were observed across all scenarios, with the largest relative benefits under combined land use and climate change conditions. These findings underscore the role of GSI as an adaptation measure that can offset some of the adverse effects of climate change, even though its absolute effectiveness diminishes under more extreme futures.

All in all, this work, methodologically, provides a transferable approach for embedding GSI in 2D H&H models. Scientifically, it identifies threshold and connectivity effects that shape GSI outcomes and demonstrates how GSI interacts with land use and climate change in a non-linear manner. Practically, it reinforces that while GSI alone cannot address extreme flooding, it is an effective element of integrated stormwater management strategies aimed at building resilience in vulnerable urban communities.

Keywords

Green Stormwater Infrastructure (GSI), Low Impact Development (LID), Stormwater Management, Stormwater Management Model (SWMM), Hydrologic and Hydraulic Modeling, StormWise, ICPR, Land Use Change, Climate Chane, Watershed Assessment

Disciplines

Civil Engineering | Environmental Engineering | Hydraulic Engineering | Other Civil and Environmental Engineering | Urban, Community and Regional Planning

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