Analytical Probabilistic Models for Evaluating the Hydrologic Performance of Structural Low Impact Development Practices

Abstract

<p>Low Impact Development (LID) practices have been increasingly used to mitigate the adverse impacts of urbanization. Reliable methods are in need to provide hydrologic performance assessment of different types of LID practices. The purpose of this thesis is to develop a set of analytical models which can be used to assist the planning and design of commonly used structural LID practices such as green roofs, rain gardens, bioretention and permeable pavement systems.</p> <p>The analytical LID models are derived on the basis of exponential probability density functions (PDF) of local rainfall characteristics and mathematical representations of the hydraulic and hydrologic processes occurring in association with the operation of LID practices. Exponential PDFs are found to provide good fits to the histograms of rainfall characteristics of five cities located in different climatic zones. The mathematical representations are all physically based and most of the input parameters used in these representations are the same as those required in commonly used numerical models.</p> <p>The overall reliability of the analytical LID models are tested by comparing the results from these analytical models with results determined from long-term continuous simulations, in addition to that the accuracy of the analytical model for green roofs is also verified against observations from a real case study. The long-term rainfall data from the five cities and a variety of LID practice design configurations are used in the comparisons. The relative differences between the results calculated using the analytical LID models and the results determined from corresponding SWMM simulations are all less than 10%.</p> <p>The Howard’s conservative assumption is adopted in the development of the analytical models for rain gardens and permeable pavement systems. This assumption results in conservative estimations of the stormwater management performances of these LID practices. Instead of adopting the Howard’s conservative assumption, an approximate expected value of the surface depression water content of a bioretention system at the end of a random rainfall event [denoted as ] is derived and used in the development of the analytical model for bioretention systems. The use of is proven to be advantageous over the use of the Howard’s conservative assumption.</p> <p>The analytical LID models are comprised of closed-form mathematical expressions. The application of them can be easy and efficient as illustrated in the application examples. For a specific location of interest, with a goodness-of-fit examination of the exponential PDFs to local rainfall data and verification of the accuracy of the analytical LID models, these models can be used as a convenient planning, design, and management tool for LID practices.</p>