Assessment of low impact development for managing stormwater with changing precipitation due to climate change

Abstract

Evidence suggests that built environments will need to function under climatic conditions different from the recent past. Observed warming temperatures and changes in precipitation already suggest that historical observations and personal experience may not be reliable guides to future conditions. Yet, uncertainties remain about future climatic conditions, particularly at local and regional scales where land use planning decisions are made. Faced with this uncertainty, responding to climate change will require identifying key vulnerabilities of the built environment and developing adaptive strategies for reducing the risk of harmful impacts. One area of potential vulnerability is stormwater management. Increased precipitation due to climate change could exacerbate the impairment of surface waters due to increases in stormwater runoff. This study considers the potential effectiveness of low impact development, specifically compact development with decreased impervious cover, for reducing stormwater impacts on surface water under changing precipitation patterns. The study location is a redevelopment project south of Boston, MA, USA. A simple stormwater model, SGWATER, is used to assess the sensitivity of stormwater runoff and pollutant loads to changes in impervious cover, precipitation volume, and event intensity. Simulation results suggest that when expressed on a constant percent basis, stormwater runoff is most sensitive to changes in site impervious cover, followed by changes in precipitation volume and event intensity. The study illustrates, in a simple but quantitative way, the potential benefits of a common low impact development practice for increasing the resilience of communities to changing precipitation patterns.

Introduction

Stormwater runoff from roads, rooftops, parking lots, and other impervious cover in urban and suburban environments is a well known cause of stream degradation, commonly referred to as urban stream syndrome. Common impacts of stormwater runoff include increased flooding, channel instability, water quality impairment, and disruption of aquatic habitats (NRC, 2008, Paul and Meyer, 2001, Roy et al., 2005, Walsh et al., 2005). Stream degradation has made the protection and restoration of aquatic habitats through comprehensive stormwater management an important sustainability goal of communities. Typically, stormwater management practices are designed to meet performance standards based on historical climate conditions. In the coming decades, however, the built environment, including stormwater management systems, may need to meet performance expectations under climatic conditions different from those in recent history (IPCC, 2007, Karl et al., 2009, Milly et al., 2008, USGCRP, 2009).

During the past century much of the U.S. has experienced warming temperatures and changes in the amount, intensity, and form of precipitation (Douglas and Fairbank, 2011, Groisman et al., 2005, IPCC, 2007, USGCRP, 2009). Climate modeling experiments suggest these trends are likely to continue or accelerate throughout the 21st century, although uncertainty remains regarding future precipitation, particularly at spatial scales important to local and regional planning (IPCC, 2007, USGCRP, 2009). The implications of projected climate change for communities will be determined, in large part, by the design, operation, and maintenance of the built environment. Adapting the built environment, including stormwater management infrastructure, will involve assessing vulnerabilities and implementing strategies to offset or reduce negative social, economic, and ecological effects associated with climate change (Smit & Wandel, 2006). Given the inherent uncertainty of the problem, successful adaptation strategies will likely need to encompass practices and decisions to reduce vulnerabilities across a wide range of plausible future climatic conditions (LaGro, 2008).

Faced with uncertainty about future climate change, and given constraints on available resources, communities may choose to pursue no-regrets strategies – actions that are beneficial in addressing current stormwater management needs regardless of whether or how climate may change in the future (Means, Laugier, Daw, Kaatz, & Waage, 2010). Many communities across the U.S. are now implementing smart growth and low impact development (LID) practices to meet stormwater management goals (e.g., see Benedict and McMahon, 2006, NRC, 2008, USEPA, 2011a). A general aim of LID is increased onsite retention and infiltration of stormwater runoff. Common practices include higher density development, installation of green infrastructure such as pervious pavement and grass swales, preservation of natural lands, re-use of already developed lands (e.g., see LeFevre et al., 2010, USEPA, 2011b), and other approaches for reducing impervious cover (Arnold and Gibbons, 1996, Brabec, 2009, Schueler, 1994, Sutherland, 1995). LID may also provide for assimilating stormwater pollutants and increasing infiltration are well documented (e.g., see Bedan and Clausen, 2009, NRC, 2008, USEPA, 2011b).

LID may also provide increased resilience to future climate change, but little is known about LID performance in this context. Studies have explored the potential implications of climate change for stormwater management infrastructure, including LID. Gill, Handley, Ennos, and Pauleit (2007) found LID to be effective in moderating potential climate change impacts such as extreme temperatures and increased surface runoff.

Rosenberg et al. (2009) assessed stormwater sensitivity to climate change in the Puget Sound region of Washington State using the Hydrologic Simulation Program-Fortran (HSPF) model (Bicknell, Imhoff, Kittle, Thomas, and Donigian, 2005).

Results were inconclusive, but suggest that increased runoff volumes may require modification to current stormwater management. Stormwater Management Model (SWMM) (Rossman, 2010) simulations of an urban site in British Columbia, Canada, using modified Intensity-Duration-Frequency (IDF) curves extrapolated to represent projected future storm event intensities for 2020 and 2050, suggested that site infrastructure would need minimal upgrades to account for the larger runoff volumes, but that future increases in stormwater runoff could still result in impaired stream health (Denault, Millar, & Lence, 2006). Waters, Watt, Marsalek, and Anderson (2003) completed SWMM simulations to identify the vulnerabilities of a stormwater management system in Ontario, Canada by increasing the design event intensity by 15%. The study found the system pipe size inadequate in some areas, but peak discharge could potentially be offset with disconnected downspouts and increased storage (Waters et al., 2003).

An improved understanding of LID in the context of climate change adaptation can help inform stormwater management decisions to reduce the risk of harmful future impacts. In this paper we assess the potential effectiveness of one common element of LID, compact development with reduced impervious cover, for decreasing stormwater runoff and pollutant loads under conditions of changing precipitation. A simple stormwater model, SG WATER (Smart Growth Water Assessment Tool for Estimating Runoff), is used to evaluate runoff and pollutants under a range of hypothetical climate change and land use scenarios for a redevelopment project in Massachusetts (USEPA, 2002a, USEPA, 2002b). The goal is to illustrate, in a simple but quantitative way, the sensitivity of stormwater runoff and pollutant loads from alternative land use scenarios across a range of potential future changes in precipitation. This knowledge can help build our understanding of how to adapt stormwater management practices and build climate resilient communities.