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1 This guidebook was prepared to provide standardized guidance for airports and their consultants to use in defining appropriate design storm event conditions for sizing deic- ing runoff control systems and associated components. This guidance is intended both to facilitate efficiency in the design process and help airports communicate the rationale for siz- ing their systems to funding authorities, regulatory agencies, and other interested stakeholders. 1.1 Background Designers of systems managing airport deicing runoff must size system components to ensure adequate collection, convey- ance, storage, and treatment capacity during peak winter events. Undersizing collection and drainage components can result in unacceptably frequent flooding, overflows, or bypasses of stor- age and treatment units, and exceedances of National Pollutant Discharge Elimination System (NPDES) permit limitations. Oversizing these components may avoid or at least reduce the occurrence of these conditions but at unnecessary cost. The challenge of appropriate sizing is addressed by describ- ing a set of meteorological, operational, environmental, and other conditions that define the upper bounds of what the sys- tem is designed to contain. In this research, the term design storm event is used to characterize this set of conditions. Design storm event characteristics can take the form of meteorologi- cal (snowfall, freezing precipitation, temperature), operational (total deicer usage), environmental (receiving-water quality, flow, or temperature), or performance (storage capacity) fac- tors. Although these conditions are centered on winter storms, the design conditions often encompass periods when runoff processes continue beyond the time of the actual winter storm. There are several aspects to describing the design storm event: 1. Selecting the right factors to characterize the event. Each airport project will present a unique set of circumstances and demands that require a specific set of factors to describe critical conditions. Design storm event factors relate to both winter storm conditions and how those conditions drive deicing operations, the generation of runoff from those operations, and the environmental responses to that runoff. 2. Defining the acceptable frequency of occurrence of criti- cal values for those factors. The second consideration is specifying how often event conditions may be exceeded. The issue often becomes defining what is acceptable to the regulatory agency or agencies whose requirements and cri- teria are involved. There is no standard definition of this, and acceptability may hinge on many factors and diver- gent considerations, some of which may change over time. The answer ultimately depends on negotiations involving regulatory policy and considerations of cost versus capac- ity and incremental benefit. As discussed in Appendix A, this highlights the need for including regulatory factors in the definition of a design storm event. 3. Estimating the expected frequency of design event con- ditions in the context of evaluating design alternatives. Determining the expected frequency for which design conditions will be exceeded for a given design alternative involves the calculation of a recurrence interval. For deicing projects, this presents a complex problem because it requires consideration of storm events of varying duration and the weather conditions that precede and follow the storm and determine the volume and rate runoff. Thus, a statistical approach is needed that describes the probability of criti- cal sequences of events (for example, warm rain following heavy snowfall) extending over variable periods of time. In the absence of standardized guidance, airports have addressed the deicing design event issue in various waysâ some of them successfully, others less so. This guidebook was prepared to provide standardized guidance for the selection of deicing design storm events or conditions for deicing run- off management systems. S e c t i o n 1 Introduction
1.2 Overview of a Winter Storm The term winter design storm refers to a specific meteo- rological condition for which winter facilities (for example, deicing facilities) are designed. The concept of a design storm is used extensively in the design of storm water facilities. A winter design storm captures the unique seasonal factors affecting facility performance and objectives. Winter design storms necessarily address the fact that precipitation may fall as either a liquid or a solid (snow), and that snow may accumulate and run off later. Airport deicing facility designs, which are concentrated on aprons and deicing pads, must handle winter flows primarily in liquid form since the deic- ing rapidly converts snow and ice to liquid. Airport runway designs, on the other hand, necessarily address the accumu- lated snow, whose runoff characteristics are further compli- cated by the spatially varied accumulations in plowed snow piles. This current study focuses on design storms in the liquid phase since the primary interest is design storms for deicing runoff controls. Conventional storm water facilities are commonly designed for two purposes: first, to rapidly convey storm water runoff away from busy areas, and second, to trap excess sediment and attached pollutants. Consequently, design storms for storm water controls focus on (1) meteorological factors contrib- uting to peak flow rates that govern conveyance sizing and (2) first-flush storm volumes, which typically contain most of the sediment load. The characteristics of the storm water design storm (frequency, depth, and duration) are often specified by locally applicable drainage regulations in order to reduce the risk of flooding. Winter design storms are similarly used to size peak-flow- related conveyance facilities but also to size whole systems that include treatment facilities and equalization storage facilities. Sizing of the latter involves determining optimum storage requirements to reduce peak flows to the cost-effective treat- ment rate. Winter storm water facilities also differ from con- ventional storm water controls in that the peak flow rates are a complex function of precipitation intensity and form (snow or liquid), and the treatment must deal with dissolved pollut- ants (principally deicers). Storm water design storms are most commonly specified in terms of rainfall event volume falling in a given amount of time (for example, 1 in. of rainfall in a 24-hour period). That volume-duration pair can be associated with a recur- rence probability using available depth-duration-frequency evaluations, such as those available in Atlas 14: Precipitation- Frequency Atlas of the United States (NOAA, 2004â2011). Designers often assign a distribution (the fraction of the storm volume that falls at each increment of the storm dura- tion) to better define the temporal runoff pattern that must be handled in the conveyance, storage, or treatment facilities. These common intensity-duration-frequency (IDF) evalu- ations and distribution functions are proven and appropriate for sizing deicing system components that are dependent on peak flow rates (for example, pipes and treatment) but inap- propriate for sizing equalization storage where runoff from multiple storms may need to be stored prior to being treated. The U.S. Environmental Protection Agency (EPA) combined sewer overflow control policy (EPA, 1994) advises designers to address the conveyance/storage/treatment balance using con- tinuous simulation of long-term precipitation time series rather than attempting to characterize the probability of inter-event dewatering periods. Several airports (Dayton International, Washington Dulles, OâHare, Detroit Metropolitan, Portland International) have applied continuous simulation of the long- term wintertime precipitation time series and found doing so to be the most effective means of evaluating precipitation, runoff, storage, treatment, and discharge interactions. Thus, different approaches are needed to meet the requirements of the two dif- ferent design situations: deicing system components where the timing and magnitude of flows correspond closely with pre- cipitation, and system components (or even whole systems) where the timing and magnitude of flows occur as functions of sequences of individual winter storms and associated weather phenomena (for example, melt-off events). 1.3 Purpose and Objectives This guidebook provides standardized guidance for air- ports and their consultants to use in understanding the tech- nical and regulatory issues in defining design conditions and developing an appropriate design storm event for their spe- cific requirements. This guidance is intended to both facili- tate effectiveness and efficiency in the design process and help individual airports communicate the rationale for sizing their systems to stakeholders and environmental regulatory agen- cies that may have limited experience with deicing systems. 1.4 Structure of the Guidebook The guidebook is structured as a series of four sections plus supporting information. Section 1 introduces the topic and objectives of the guidebook. Section 2 provides important background information required to understand the regula- tory context of defining a winter design event, the various factors that should be considered in defining a design storm, and the concept of a multiday event. After this background, Section 3 describes a decision support tool that embodies a structured process for identifying an appropriate winter design storm. Section 4 presents five case studies that illus- trate how the decision tool captures design decisions that air- ports have faced in different deicing management program situations. Supporting information is provided in the form of the references and Appendix A. 2
