Predictive theoretical and computational approaches for characterizing an engineered bioswale systems performance

Abstract

Runoff phenomena from urbanization exist as a leading cause of non-point source (NPS) pollution in receiving water bodies. Within island communities such as the archipelago of Hawaiʻi, discharging runoffs into the ocean severely affect hydrological, ecological, and anthropogenic environments. Low-impact development (LID) technologies provide environmentally friendly methods to treat polluted runoff flows with a reduced flow rate. Among the plethora of contemporary LID strategies, bioswales are considered as engineered, natural porous media used for the on-site retention of stormwater runoff and treatment of NPS pollution. Despite the global widespread use of bioswales since the early 1990s, current design guidances provide mostly empirical estimates of removal capabilities as opposed to scientific predictions based on underlying transport mechanisms. Thus, design optimization of bioswales at a fundamentally scientific level has prompted the current investigation to deal with the ubiquitous problem of stormwater management. Abstract In this dissertation, Chapter 1 provides the overview and underlying purpose of the research, which explores methods for understanding engineered bioswale phenomena. To achieve this goal, both theoretical and computational investigations were conducted in Chapter 2 through Chapter 4. Chapter 2 presents an original conceptual framework to quantify bioswale performance, which treats a bioswale as an engineering unit consisting of several conventional physico-chemical processes used in a water and wastewater treatment plant (WWWTP) with basic physical and chemical principles. A detailed conceptual model was created which described a bioswale as analogous to a conventional WWWTP process. This comprehensive conceptual model includes applicable fundamental equations to characterize transport phenomena and evaluate bioswale performance. Chapter 2 suggests innovative and original perspectives regarding computational fluid dynamics (CFD) as promising tools to compensate for existing deficiencies within conventional design approaches. Abstract In Chapter 3, CFD simulations were conducted so as to fundamentally investigate hydraulic and chemical transport phenomena within a bioswale system, as introduced in Chapter 2. In particular, coupled transport phenomena within a bioswale were studied using the open-source CFD software, OpenFOAM (www.openfoam.org), which was unprecedented in the theoretical bioswale literature. The unsteady behavior of momentum and mass transfer was investigated in a double-layered bioswale by seamlessly linking overland and infiltration flows at various time scales. To study the diffusive transport of a model pollutant, a new solver named interPhaseDiffusionFoam (available at https://github.com/enphysoft/interPhaseDiffusionFoam) was developed, which better mimics interfacial transport phenomena of dissolved non-volatile species at a water-air boundary. This investigation identified that heterogeneous infiltration patterns are originated in a strongly coupled manner by stormwater runoff velocity, reverse air flow, and the presence of the drain pipe. Overall, the 2D CFD simulations, used for multi-phase (water, air, and soil) transport phenomena, can be further applied to the structural designs of bioswales with specific geometric and hydraulic conditions throughout communities all around the world. Abstract Chapter 4 provides theoretical hydraulic-design perspectives regarding the characterization of the bioswale vegetation layer (BVL). The Chapter 4 study employed a meta-research approach (“research on research”) consisting of an original meta-theoretical development based on an in-depth literature review of well-accepted theories. The newly described design equation within this chapter links stormwater hydraulic properties (represented by Reynolds number Re) and bioswale geometry (characterized by a geometrical ratio, denoted as \eta) to predict the minimal bioswale length for effective performance within an emergent case. An innovative graphical method is developed so as to estimate the optimized length-to-width ratio of a respective bioswale. Moreover, this study found that a critical Reynolds number (\mathrm{Re}_{\mathrm{p,cr}}=10^{1.5}=31.623) exists as being universal and, hence, independent of a geometrical parameter ab. If the Reynolds number is higher than \mathrm{Re}_{\mathrm{p,cr}}, then the bioswale geometry solely determines \eta without being influenced by stormwater hydraulics. Cross-validation of the new meta-theory is conducted indirectly using experimental data available within the scientific literature. Thus, it is recommended based on chapter 4 findings that a safety factor of 3–5 be multiplied by the theoretical \eta (obtained using the graphical method) to ensure that the BVL zone provides effective hydraulic resistance to sufficiently decelerate incoming runoff flows within a surface BVL zone. This design theory, with the graphical method, is, to the best of my knowledge, the first approach that links the hydraulic characteristics of stormwater runoff and the geometric properties of a bioswale. Abstract Chapter 5 presents the final conclusions and suggestions for the future development of bioswale systems. The results of this dissertation revealed, for the first time, the relevant underlying physics of stormwater dispersal within a bioswale as an unsaturated porous media. The present bioswale study ultimately represents a further step towards designing ubiquitously robust stormwater treatment facilities so as to protect our precious land and natural waterways.