Computational Methods in Low Impact Development Stormwater Control Measures

The goal of low impact development (LID) is to restore the so-called natural hydrology or an equivalent condition and reduce pollutant loads from new and existing developed areas as much as possible. Low impact development practices are broadly categorized as Conservation Design, GSI controls, and Structural Practice stormwater controls. Low impact development approaches integrate stormwater control measures (SCMs) to manage runoff in a manner that approximates the temporal and spatial distribution of a particular site's reference conditions. This chapter discusses the fundamentals of small-scale hydrologic and hydraulic processes at work in LID SCMs. It examines how complex processes are recognized in terms of interactions between the basic mechanisms in various SCMs. Computational approaches are needed to characterize the processes of infiltration, interception/evapotranspiration (ET), surface conveyance and detention to evaluate the performance of LID SCMs. This chapter briefly describes each of these processes in detail, as well as the computational approaches used to parameterize them in LID studies.

Computational approaches needed to characterize the processes of infiltration, interception/evapotranspiration, and surface conveyance and detention were integrated into hydrologic and hydraulic (HH) models only in the last two decades, specifically to simulate the impacts of low impact development (LID) stormwater control measures (SCMs) at the site and watershed scale. This chapter begins with the history and an overview of HH models and their various applications in LID computations and modeling. It broadly describes the types of models available, namely lumped, quasi-distributed, or fully distributed models, as well as the types of interrelated decisions that modelers need to make to construct the models used in LID computations, such as those associated with the level of subcatchment aggregation, the level of conduit skeletonization, and the procedure used for parameter specification. The chapter concludes with a description of some of the challenges faced by modelers while incorporating LID SCMs into the HH models and the wide range of variables that complicate LID computations.

A variety of modeling software selections exist that might be more appropriate for a particular application in low impact development (LID) computations. Becoming familiar with a range of modeling tools and the theory behind them is essential in selecting the appropriate tool for the job and for enabling the modeler to use the software as an extension of their engineering judgment. This chapter includes summaries of seven commonly used modeling software programs: HydroCAD; EPA's Storm Water Management Model (SWMM), INFO-SWMM and XP-SWMM; MIKE+; WinSLAMM (Source Loading and Management Model); Western Washington Hydrology Model (WWHM); MGS-Flood; PCSWMM; and OpenHydro. Each summary includes an overview and tables highlighting the computational capabilities related to representing LID practices. With the exception of EPA-SWMM, which is open source and freely available, all these models are proprietary and sold commercially.

Since many hydrologic and hydraulic processes in the low impact development (LID) systems have been parameterized and incorporated into software, much less is known regarding the accuracy of these conceptualizations vis-à-vis actual site performance. This chapter provides a brief overview of the state of the art, highlighting the vast capabilities of a few of the models commonly used at present, as well as the ways to improve accuracy of these computations through field monitoring. It focuses on approaches to representing LID stormwater control measures (SCMs) in watershed models. A framework is presented for modeling media flow responses in filtering SCMs such as green roofs, permeable pavement and bioretention cells with underdrains, along with variants such as tree planters and sand filters. The chapter also describes the advantages of using controlled outlets in filtering systems and of how these systems can improve LID performance.