Low impact development case studies: Developers keeping the water clean
Session Chair: Sam Merrill, New England Environmental Finance Center, University of Southern Maine
Session abstracts:
Performance Evaluations for a Range of Stormwater LID, Conventional Structural, and Manufactured Treatment Devices in a Cold Climate
Robert M. Roseen, Thomas P. Ballestero, and James J. Houle
The UNH Stormwater Center, University of New Hampshire, Durham, NH
A suite of sixteen separate stormwater treatment best management practices (BMPs) were evaluated for a range of water quality performance and storm volume reduction. The study includes evaluation of 26 rainfall-runoff events between September 2004 and September 2006 representing a diverse array of hydrological and seasonal characteristics. Evaluated BMPs include conventional structural systems (swales, retention ponds), Low Impact Development (LID) designs (gravel wetland, bioretention, sand filter, tree filter, porous asphalt), and manufactured BMPs (filtration, sub-surface infiltration, and hydrodynamic separators). Contaminant event mean concentration (EMC), and water quality and water quantity performance efficiencies were evaluated for the range of storms. Prior research demonstrated that treatment performance of stormwater control measures varies widely in response to site-specific contaminant loading functions. For this reason, the devices were tested in parallel, with a single influent source providing uniform loading to all devices. Stormwater treatment BMPs were uniformly sized to target a rainfall-runoff depth equivalent to 90% of the annual volume of rainfall. Under the parallel and uniformly sized configuration, a normalized performance evaluation is possible because treatment devices of the same scale receive runoff from events of the same duration, intensity, peak flow, volume, antecedent dry period, and watershed loading. Runoff constituent analyses included total suspended solids (TSS), diesel range organics (DRO), dissolved inorganic nitrogen (DIN), zinc (Zn), and total phosphorus (TP). In addition, several water quality parameters (temperature, dissolved oxygen, pH, conductivity) were monitored as real-time data. Two years of performance data and real-time water quality parameters were use to evaluate seasonal variations between systems. Performance evaluations indicate that several LID designs have very high removal efficiencies of 80-100%. In contrast, conventional structural BMPs perform poorly or modestly for most measures. The manufactured systems tended to vary widely and were dependent upon the design and contaminant of interest.
Low Impact Development Strategies Illustrated Through New England Case Studies
Richard A. Claytor, Horsley Witten Group, Inc. Sandwich, MA
This session will review several basic strategies and tools to design better site development projects and will cover such topics as conservation design, reduced impervious cover, redevelopment strategies, natural-system stormwater management and establishment of riparian buffer areas. The applicability of these practices will be illustrated using several New England case studies.
Overcoming Local Obstacles to Implementing LID
Wendy Coffin, Woodard & Curran, Portland, ME
Low Impact Development (LID) strategies are being promoted at the state and federal level for new and redevelopment projects in order to maintain watershed hydrologic integrity and to reduce stormwater impacts. The basic tenets of the LID approach, maintaining pre-development hydrology, employing decentralized redundant stormwater management structures, maximizing time of concentration, and maximizing infiltration are very important to effective stormwater management. Unfortunately, some of these principles are counter to many long-standing engineering principles and standards.
To encourage and facilitate use of LID principles by developers and engineers, municipal ordinances, regulations, site design guidelines, and standards need to clearly allow their use. Traditional engineering design standards for curbing, parking lots, roads, sidewalks, and drainage utilities are often not conducive to LID implementation. In many municipalities, existing regulations, guidelines and design and construction standards conflict with, or greatly restrict the use of LID practices, thereby requiring use of the variance application process. The variance application process is time consuming and cumbersome to the developer and can be confusing and difficult for local planning boards and their staff to administer. In the end this can lead to continued reluctance to employ LID stormwater management practices.
This presentation will point out some of the typical contradictions between municipal design requirements and Low Impact Development designs and will provide an approach municipalities can take to overcome these obstacles with an overview of project case-studies. Important steps in this process will be highlighted. Additionally, the presentation will discuss typical concerns of municipal managers and the need for education and demonstration projects that reduce uncertainty related to LID implementation.
Jordan Cove Urban Watershed Project
Bruce Morton, Aqua Solutions, LLC., Marlborough, CT
The Jordan Cove Urban Watershed Section 319 National Monitoring Program Project is located in Waterford , Connecticut along the coast of Long Island Sound. The study began in 1995, and was designed to determine water quantity and quality benefits of using pollution prevention Best Management Practices (BMPs), now known as low impact development (LID) techniques, for managing stormwater runoff within a residential subdivision. Runoff monitoring took place over a 10 year timeframe, inclusive of pre-, during and post- construction. What made this project unique was not the use of specific LIDs for stormwater management, but rather the use of a multitude of LIDs in one location.
From the monitoring program, two major conclusions were drawn: 1) low impact development can maintain pre-development peak runoff and volume of runoff levels while generally not increasing pollutant export except for phosphorus and sediment; and, 2) traditional development increases runoff by two orders of magnitude while pollutant export also increases.
This presentation will summarize the findings of a real world study which dispels many apprehensions about the use of LIDs to control snowmelt and rainfall runoff events in an urban residential subdivision.
