A diffusion model for low reynolds number flows through a constructed stormwater wetland

Proceedings of the Seventh International Conference on Hydroscience and Engineering, Philadelphia, PA, September 2006. http://hdl.handle.net/1860/732The dynamics of diffusion in laminar flow constructed stormwater wetlands are presently not fully understood. A field experiment conducted at Villanova University in 2006 compared field diffusion coefficients with those of the laboratory experiments of Nepf et al. (1997) and Serra et al. (2004); all of these studies examined the effect of plant density on diffusion coefficients. The field conditions at Villanova included several additional factors that were not present in the laboratory experiments: non-uniform flow, plant debris and additional bed shear stress. The results of this study show that these field factors significantly affect the diffusion coefficients and that a new model is needed to predict the diffusion coefficients in field conditions

Hydrostatic and non-hydrostatic internal wave models

Numerical models have become an indispensable tool for ocean and inland flow modeling. Such models typically use the hydrostatic approximation based on the argument that their horizontal length scales are longer than the vertical length scales. There are a wide variety of physical processes in oceans and inland water systems, and many of these processes are adequately modeled with the hydrostatic approximation. However, internal waves contribute to the physics that influence mixing in a density stratified system and have been previously shown to be non-hydrostatic. The neglect of non-hydrostatic pressure in a hydrostatic model is problematic since non-hydrostatic pressure plays a significant role in internal wave evolution balancing nonlinear wave steepening. Where non-hydrostatic pressure is neglected in a model, the governing equations are missing a piece of the physics that control the internal wave evolution, so it should not be surprising that the evolution may be poorly predicted. Despite the knowledge that the non-hydrostatic pressure is necessary for correctly modeling the physics of a steepening internal wave, the high computational cost of solving the nonhydrostatic pressure has limited its use in large-scale systems. Furthermore, the errors associated with hydrostatic modeling of internal waves have not been quantified. This dissertation quantifies the differences between hydrostatic and non-hydrostatic simulations of internal wave evolution and develops a method to a priori determine regions with non-hydrostatic behavior. In quantifying the errors and differences between the two models this research provides the characteristics of model error with grid refinement. Additionally, it is shown that hydrostatic models may develop high wavenumber “soliton-like” features that are purely a construct of model error, but may seem to mimic physical behaviors of the non-hydrostatic system. Finally, it is shown that regions of significant non-hydrostatic pressure gradients can be identified from a hydrostatic model. This latter finding is a building block towards coupling local nonhydrostatic solutions with global hydrostatic solutions for more efficient computational methods. The work presented here provides the foundations for future non-hydrostatic model development and application.Civil, Architectural, and Environmental Engineerin

Hydrostatic and non-hydrostatic internal wave models

Center for Research in Water Resource

Evaluating the Risk-Based Performance of Bioinfiltration Facilities under Climate Change Scenarios

Many communities throughout the world are utilizing green infrastructure practices to mitigate the projected impacts of climate change. While some areas of the world are anticipating droughts, other areas are preparing for an increased flood risk, due to changes in precipitation volume and intensity. Cities rely on practices such as bioinfiltration to sustainably capture stormwater runoff and provide resilience against climate change. As cities aim to increase resilience and decrease climate-change-associated risks, a greater understanding of these risks is needed. A risk-based approach was used to evaluate bioinfiltration design and performance. Climate projections from the Couple Model Intercomparison Project Phase 5 were used to create near-term (2020–2049) and long-term (2050–2079) climate datasets for Philadelphia, Pennsylvania, using two representative concentration pathways (RCPs 2.6 and 8.5). Both near-term and long-term climate models demonstrated increased precipitation and daily temperatures, similar to other areas in the U.S. Northeast, Midwest, Great Plains, and Alaska. Climate data were used to model bioinfiltration practices using continuous simulation hydrologic models. Overflow events and cumulative risk increased from bioinfiltration sites when compared to the baseline scenario (1970–1999). This study demonstrates how to apply a risk-based approach to bioinfiltration design using climate projections and provides recommendations to increase resilience in bioinfiltration design

Methodology to simulate unsaturated zone hydrology in Storm Water Management Model (SWMM) for green infrastructure design and evaluation.

Hydrologic models such as the USEPA Stormwater Management Model (SWMM) are commonly used to assess the design and performance of green infrastructure (GI). To accurately represent GI performance models used in design need to be able to address both the hydrology/hydraulics of the catchment and the GI unsaturated (vadose) zone hydrology. While hydrologic models, such as SWMM, address the need for catchment hydrology/hydraulics, they often simplify the unsaturated zone hydrology. This paper presents a methodology utilizing existing components of SWMM to represent unsaturated zone hydrology in an accessible format that does not require adjustments to the SWMM source code. The methodology simulated the unsaturated soil water movement by considering flow caused by differences of soil matric head and flow caused by gravity between soil layers with finite depth/length. The flow flux related to the soil matric head is a function of soil water diffusivity (D) and the soil moisture gradient, where D can be represented by a pump curve in SWMM. The flow flux related to gravity was controlled by unsaturated hydraulic conductivity (K) only and was also simulated by a pump. The methodology was compared to another variably saturated model, HYDRUS, with theoretical soils (with single layers of sand, loam, silt, and clay, as well as dual-layer scenarios). Field data was used to compare the methodology to HYDRUS and the SWMM LID (Low Impact Development) module. In all comparisons the presented methodology and HYDRUS delivered similar results for the vadose zone response to a storm event, while the LID module of SWMM exhibited slower water movement. The results showed that under natural conditions, the approximation of the presented methodology yielded satisfactory results to simulate flow through the unsaturated vadose zone

Feasibility of using an energy balance to measure evapotranspiration in green stormwater infrastructure.

Effective green stormwater infrastructure (GSI) design requires comprehensive quantification of the volume of water that can be treated or removed over a given time period. It is recognized that evapotranspiration (ET) can be a substantial pathway for stormwater volume reduction in bioretention systems. However, measuring ET is often difficult and expensive, such as with lysimeters or a mass balance approach. This research focused on a new technique for quantifying ET in bioretention systems by exploring an approach using thermal imaging to calculate ET by measuring the flux of energy at the canopy surface. This thermal imaging approach was compared to ET measurements given by a traditional mass balance approach. The experimental setup had three benchtop scale vegetated lysimeters planted with Switchgrass. Time lapse thermal images of the Switchgrass plants were taken at 10 second intervals and paired with meteorological data. The data were used in an energy balance to estimate the mass of water lost from the lysimeter plant/soil system. That mass was compared to the change in weight measured by weighing the lysimeter before and after the data collection period. For comparison, reference ET was also calculated for the vegetated systems using three common reference ET equations. The uncalibrated energy balance equation developed here estimated an averaged ET over 12 data collection days within 1 mm of the mass balance measured ET. These findings demonstrate the feasibility of using a thermal image energy balance technique to estimate ET