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Winter Design Storm Factor Determination for Airports (2012)

Chapter: Section 2 - Strategies for Selecting and Applying Winter Design Storm Factors

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Suggested Citation:"Section 2 - Strategies for Selecting and Applying Winter Design Storm Factors." National Academies of Sciences, Engineering, and Medicine. 2012. Winter Design Storm Factor Determination for Airports. Washington, DC: The National Academies Press. doi: 10.17226/22693.
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Suggested Citation:"Section 2 - Strategies for Selecting and Applying Winter Design Storm Factors." National Academies of Sciences, Engineering, and Medicine. 2012. Winter Design Storm Factor Determination for Airports. Washington, DC: The National Academies Press. doi: 10.17226/22693.
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Suggested Citation:"Section 2 - Strategies for Selecting and Applying Winter Design Storm Factors." National Academies of Sciences, Engineering, and Medicine. 2012. Winter Design Storm Factor Determination for Airports. Washington, DC: The National Academies Press. doi: 10.17226/22693.
×
Page 5
Page 6
Suggested Citation:"Section 2 - Strategies for Selecting and Applying Winter Design Storm Factors." National Academies of Sciences, Engineering, and Medicine. 2012. Winter Design Storm Factor Determination for Airports. Washington, DC: The National Academies Press. doi: 10.17226/22693.
×
Page 6
Page 7
Suggested Citation:"Section 2 - Strategies for Selecting and Applying Winter Design Storm Factors." National Academies of Sciences, Engineering, and Medicine. 2012. Winter Design Storm Factor Determination for Airports. Washington, DC: The National Academies Press. doi: 10.17226/22693.
×
Page 7
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Suggested Citation:"Section 2 - Strategies for Selecting and Applying Winter Design Storm Factors." National Academies of Sciences, Engineering, and Medicine. 2012. Winter Design Storm Factor Determination for Airports. Washington, DC: The National Academies Press. doi: 10.17226/22693.
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3 This section presents background information on key tech- nical and regulatory aspects of characterizing design events. 2.1 The Importance of Risk and Controlling Drivers The need to document the rationale in defining a winter design storm is driven by regulatory compliance require- ments (see Appendix A). Regulatory agencies will assume that the deicing runoff management system is sized and operated to meet permit and other requirements under all conditions up to the severity of the design event. The frequency of the design event will generally correspond to the expected fre- quency of not achieving permit requirements such as effluent limitations, capture percentages, and treatment efficiencies. Often, the first response when encountering permit exceed- ance or noncompliance is to compare the conditions under which the exceedance occurred with the winter design event conditions. This is especially the case where permit upset and bypass conditions (discussed in Appendix A, Section A.3) specifically reference winter design event characteristics. As a result, an intrinsic component in defining the design event is the expected frequency of meteorological and hydrological conditions that exceed it. Regulatory agencies implicitly or explicitly accept some frequency of exceedance of system capacity (and, by extension, permit compliance), and this frequency is critically important in defining the probabil- ity of a suitable design event. This concept is elaborated upon further in Section 3 in terms of level of service. At some point in the process of determining the appro- priate design storm event for constructing or implementing deicing controls, scientifically derived calculations must be compared with all of the regulatory standards that apply to the specific project to ensure compliance at an acceptable level of risk. Different drivers will apply in different cases. For example, if the Federal Aviation Administration (FAA) suggests that the 5-year, 24-hour precipitation event is appropriate for sizing storm water drainage conveyance, but the applicable Clean Water Act (CWA) permitting conven- tion is to not exceed numerical limits more often than once every 10 years, then necessary adjustments must be made to the overall design to ensure CWA permit compliance. (Permit writer’s guidance, often state-specific, commonly requires calculating allowable discharge concentrations that will not cause or contribute to a violation of receiving-water quality standards criteria during 7-consecutive-day low-flow conditions with a probability of occurring less than once in 10 years.) The opposite also could be true, in that the design event defined by drainage criteria may well exceed the regu- latory minimums based on pollutant controls. Hence, an understanding of all applicable regulatory drivers must be factored into the overall design storm event analysis. 2.2 Overview of Factors Used to Characterize a Storm Hydrologists and engineers commonly use seven distinct factors to define isolated storms: precipitation volume, precipi- tation intensity, precipitation temporal distribution, precipita- tion duration, temperature, probability, and inter-event period. 2.2.1 Precipitation Volume Precipitation volume refers to the total amount of precipita- tion that falls during a storm event. Some of the precipitation volume may fall as snow or sleet. Snow or sleet is recorded as precipitation but will not result in the same amount of runoff as the equivalent amount of rain. A minimum precipitation volume is often specified, such as 0.1 in., for an event to be officially recorded as a storm event. From an infrastructure design perspective, the volume of the winter design storm will have a direct impact on the size of the infrastructure needed to treat or store the runoff. This factor will also affect sizing of conveyance, but the conveyance sizing S e c t i o n 2 Strategies for Selecting and Applying Winter Design Storm Factors

4will be equally sensitive to the intensity (Section 2.2.2) and assumptions regarding temporal distribution (Section 2.2.3) of the precipitation. 2.2.2 Precipitation Intensity Precipitation intensity refers to the volume of precipitation over time. A larger storm volume over a shorter period will be a more intense storm and have higher precipitation intensi- ties. Alternatively, that same storm volume spread out over a much longer period will be a much less intense storm and will have lower precipitation intensities. Peak intensities are usually critical factors in determining the conveyance system size, while volume is the more criti- cal factor for end-of-pipe treatment or storage device design. The goal of conveyance systems is to carry runoff away from other critical infrastructure (such as a tarmac) to a device for treatment, storage, or release into waters of the United States. If a conveyance system is designed for a low-intensity storm event, then the chances are much higher of the runoff sur- charging out of the piping and becoming a flooding problem. 2.2.3 Precipitation Temporal Distribution Precipitation temporal distribution describes the resulting shape or variation of intensity of a storm throughout the whole event. If the precipitation falls evenly throughout the storm, the resultant runoff will be different from if it rains gently for the first period and then is followed by a cloudburst. Several different distributions, such as the SCS Type II, developed by the Soil Conservation District (now Natural Resources Conservation Services) in the 1960s, are commonly used in sewer design. Understanding the typical temporal distribution of the region surrounding the airport will result in a design storm more representative of the locality and, therefore, in more efficient sizing of storm water facilities. Atlas 14 (NOAA, 2004–2011) presents temporal distributions characteristic of most localities in the United States. 2.2.4 Precipitation Duration Precipitation duration is the time elapsed between the first recorded precipitation and the last recorded precipitation in the storm event above a given threshold (for example, 0.1 in., as mentioned previously). Recognize that volume, intensity, distribution, and dura- tion are interrelated. A given volume in a given duration has a readily calculated average intensity (volume divided by duration). Different distributions can be applied within the parameters of the specified volume and duration, resulting in a variety of peak intensities. 2.2.5 Temperature Temperature affects the volume and state (liquid or frozen) of what falls to the ground but not necessarily the overall pre- cipitation volume (for example, 1 in. of rain may equal 10 in. of snow, but both equal 1 in. of equivalent precipitation). Temperature becomes an important consideration in the design and operation of storm water facilities. For example, if the storm event is freezing rain or snow and then there is a warm-up and melt off, all these different aspects of the storm event need to be understood when considering the size of the system and the conveyance of the runoff. 2.2.6 Probability The frequency or probability of a storm is defined by sta- tistical calculation of the probability of a given volume of pre- cipitation falling within a given duration. For the probability calculations to be meaningful, they must be calculated from a long series of reliable rainfall records representative of the location of interest. The statistical calculations will yield a table, or curve, defining the probability of a given rainfall intensity (inches per hour) occurring in a specific duration (hours). For a selected probability, the combination of precipitation depth and duration requires some distribution in time. A 1% storm is a storm that has a 1% probability of being exceeded in severity in any one year and is expected to be exceeded an average of once every 100 years. A 10% storm is a storm that will be exceeded in severity an average of once every 10 years, or alternately, has a 10% chance of being exceeded in any one year. A 50% storm is a storm that will be exceeded in severity an average of once every 2 years, or alternately, has a 50% chance of being exceeded in any one year. Choosing more frequent storms, such as a 50% storm, will result in smaller sizing of projects in design and a greater potential for those designs to flood if a larger storm does occur. Designing projects with a less frequent storm will result in larger projects (larger pipes, storage basins, etc.). 2.2.7 Inter-Event Period Inter-event period distinguishes one storm from another and is defined as the period between individual storm events. Appropriate inter-event periods vary with geographic (or more specifically, hydrometeorological) regions and can be typically between 3 and 24 hours. Understanding the inter-event period of the geography in question will aid in understanding the storage or conveyance time and size of the storm water facilities in question. For instance, short inter-event periods may mean that a facility needs to be sized large enough for several storms, since the inter-event period is short and the runoff cannot be conveyed away or treated that quickly before the next storm comes.

5 2.3 Multi-Storm Analyses Individual storms are anomalies in a continuous time series of variable, interrelated meteorological features. Storm water studies, and consequently storm water control engineering, simplify the variability into definitions more manageable for discrete storm events. For several purposes—particularly for facilities that must operate continuously both during and between storms or for which storage must work within the boundaries of fixed conveyance or treatment outlets— engineers elect to investigate the entire time series of interre- lated meteorological factors. The continuous analysis avoids a number of assumptions inherent in the definition of design storms (inter-event periods, storm event independence, joint probabilities of precipitation and temperature extremes, etc.). The analysis of continuous time series of numerous related meteorological factors can be extremely data intensive and hence is avoided except in those cases where the extra effort is necessary to refine conservative assumptions in the definition of design storms. Figure 2-1 illustrates the difficulty inherent in evaluating individual events within the continuum of multiple precipi- tation events. The dashed lines in the figure show the dura- tion (x-axis) of rainfall depth (y-axis) with the statistically projected recurrence interval (legend). The solid gray lines indicate the peak rainfall depths for the durations observed during the specific storms shown in the legend. Note that these storm curves are calculated by identifying the peak 1-hour period, the peak 2-hour period, and so forth, in the storm time series observed. The complexity increases expo- nentially with the number of variables (for example, temper- ature, dew point) considered. In those cases where assumptions surrounding the design storm may result in extremely large or complex facilities, the designer may choose to refine the facility design criteria through the use of continuous simulation using multiple- year time series of several interrelated meteorological time series parameters. The use of such continuous simulation approaches is beyond the scope of this guidance document but is recommended for further investigation in those partic- ular situations where continuous simulation may be required to refine the performance expectations of particular airport deicing facilities. 2.4 Applicability of Factors to Different Types of Deicing Projects The users of this guidance document will discover early in their project that many of the winter design storm fac- tors described in Section 2.2 will have some impact on their deicing project, whether their project requires implement- ing a simple block-and-pump collection system for a small regional airport or designing a centralized deicing facility at a major air carrier facility. What is also apparent is that dif- ferent winter design storm factors are more or less applicable to different types of deicing projects. Table 2-1 provides guidance regarding the factors to con- sider for different deicing project components. The list of types of deicing projects is based upon ACRP Report 14: Deicing Planning Guidelines and Practices for Stormwater Management Systems (CH2M HILL et al., 2009). The table provides the reader with an indication of the likely applicability of each From SEWRPC (2000) with storm data from maximum gage reported to Milwaukee Metropolitan Sewerage District. Figure 2-1. Depth-duration-frequency curves isolating design storms (gray) from continuous statistics (dashed).

GNICIED TCEJORP EPYT ngiseD mrotS srotcaF Regulatory/Environm ental Factors N PDES/SPDES Perm it/Environm ental O pera�onal Factors Peak Departure Rates Airfield U � liza�on/Flow Rate Physical Factors Storage Volum e Restric� on Deicing Runoff Collec�on System Restric� on Land Constraints Clim atological Factors Geographic Region Hydrologic Factors Precipita�on Volum e Precipita�on Intensity Precipita�on Tem poral Distribu� on Precipita�on Dura� on Tem perature Probability Inter-Event Period Precipita�on Type tfarcriA gnicieD ecruoS noitcudeR derarfnI gnicied ygolonhcet ○ ● ● ● ● ● ○ ● ○ — — — ● ○ ○ gnicieD ffonuR noitcelloC/tnemniatnoC seitilicaF norpA noitcelloc metsys ● — — ● ● — ● ● ● ● ● — ○ ● ● locylG noitcelloc selcihev ● ● ● ○ ○ — ● ○ ○ ○ ○ ○ ○ ○ ○ kcolB - dna - pmup metsys ● — — ● ● — ● ○ ● ● ○ ● ● ● ● dleifriA eganiard tiforter/ngised/gninnalp ● — — ● ● ● ● ● ● ● ● — ○ ● ● dezilartneC gnicied ytilicaf ● ● ● ● ● ● ● ● ● ● ● — ○ ● ● recieD - nedal wons tnemeganam ● — ○ ○ — ● ● ● — — ○ ○ ○ ○ ○ gnicieD ffonuR metsyS stnenopmoC elbatroP sknat ● ○ ○ ● ○ ● ○ ● ● ○ ● ● ● ● — raludoM sknat ● — — ● ○ ● ○ ● ● ○ ● ○ ● ● — sdnoP ● — — ● ○ ● ○ ● ● ○ ● ○ ● ● — tnenamreP sknat ● — — ● ○ ● — ● ● ○ ● ○ ● ● — launaM dna detamotua noisrevid sevlav ● — — ○ ○ — — ○ ○ ○ ○ ○ ○ ○ — laeR - emit gnirotinom ygolonhcet ● — — — — — — — — ○ — ○ — ○ — Table 2-1. Matrix to determine applicability of design storm factors of various design project types.

G N I C I E D T C E J O R P E P Y T n g i s e D m r o t S s r o t c a F Regulatory/Environm ental Factors N PDES/SPDES Perm it/Environm ental O perational Factors Peak Departure Rates Airfield U ti lization/Flow Rate Physical Factors Storage Volum e Restricti on Deicing Runoff Collection System Restricti on Land Constraints Clim atological Factors Geographic Region Hydrologic Factors Precipitation Volum e Precipitation Intensity Precipitation Tem poral Distributi on Precipitation Durati on Tem perature Probability Inter - event Period Precipitation Type h c t a C n i s a b s e v l a v / s t r e s n i ● — — ○ ○ — — — ○ ○ ○ — ○ — — g n i c i e D f f o n u R g n i l c y c e R / t n e m t a e r T W T O P e g r a h c s i d ● — — ● — ○ — ○ — ○ — — — ○ — c i b o r e a n A d e z i d i u l f d e b r o t c a e r ● — — ● — ● — ○ — — — ○ ○ ● — g n i t a c o r p i c e R e c a f r u s b u s t n e m t a e r t ● — — ● — ● ● ○ — — — ○ ○ ● — g n i v o M - d e b r o t c a e r o i b t n e m t a e r t m e t s y s ● — — ● — ● — — — — — ○ ○ ● — g n i c n e u q e S h c t a b r o t c a e r ● — — ● — ● — — — — — ○ ○ ● — l a r u t a N t n e m t a e r t m e t s y s ● — — ● — ● ● ○ — — — ○ ○ ● — e n a r b m e M n o i t a r t l i f ● — — ● — ● — — — — — ○ ○ ● — l o c y l G y r e v o c e r ) g n i l c y c e r ( ● — — ○ — ○ — ○ ○ ○ ○ ○ ○ ○ — D N E G E L y r e V e l b a c i l p p a ● t a h w e m o S e l b a c i l p p a ○ t o N e l b a c i l p p a —

8deicing storm factor to those types of deicing projects. Appli- cability ratings were categorized as very applicable, somewhat applicable, and not applicable, defined as follows: • Very applicable. The design storm factor is directly related to the deicing project type and needs to be considered when developing the project (e.g., precipitation volume directly affects the design of drainage infrastructure for apron col- lection systems, block-and-pump systems, or centralized deicing facilities). • Somewhat applicable. The deicing storm factor could have some impact on the deicing project type and may need to be considered in the project design process [e.g., precipitation volume may have an impact on the discharge of deicer-laden runoff to a publicly owned treatment works (POTW)]. • Not applicable. There is most likely no interrelationship between the winter design storm factor and the deicing project type (e.g., precipitation duration has no impact on the handling of aircraft deicing materials). For example, the design storm for sizing a storage facility and the design storm for assessing the ecological impact of deicing discharges would likely be described using different factors. For sizing a storage facility, the design storm factors could include hydrologic factors such as rainfall intensity, inter-event period, and storm recurrence interval, or physi- cal factors such as existing storage capacity, piping infrastruc- ture, discharge rates, and land availability. Storm factors for determining the ecological impact of deicing discharges could include environmental factors such as NPDES/SPDES (State Pollutant Discharge Elimination System) permit requirements as well as hydrologic and receiving-water factors. Another example is the consideration of the frequency of the storm used to design a typical apron collection system to divert deicer-laden runoff to a storage tank versus design- ing an on-site treatment system for airport deicing runoff. Airfield drainage conveyance systems are typically sized for a 5- or 10-year design storm frequency, in accordance with FAA’s Advisory Circular (AC) 150/5320-5C, “Surface Drain- age Design” (2006). However, for a particular deicing project such as a centralized deicing pad, the design storm frequency is likely much less than the 5-year storm when historical weather records are taken into consideration. Unlike warm-weather precipitation events, peak storms and intensities during deicing operations seldom correlate to periods when the full hydraulic capacity of the drainage sys- tem is needed. Therefore, consideration should be given to a lesser storm frequency, one that is suitable more for the typi- cal winter storm used for sizing deicer-laden runoff diver- sion piping. This piping would be sized relative to the most likely design event that is supported by historical weather records evaluated for the deicing season. For example, analy- sis of the weather data should begin with isolating the period during which deicing activities normally take place (October through April, for instance). The number of precipitation events to be considered can then be further narrowed down by analyzing only those precipitation events that occur when the outside temperature is 38°F and falling or staying steady, because deicing activities related to precipitation events seldom occur above 38°F. Another example is where an airport has stringent effluent limits in its NPDES permit that may require aggressive col- lection and treatment to maintain compliance. In this case, the NPDES permit limits could drive the system designer to size the runoff collection pipes for a larger storm recurrence interval than what might otherwise be required for managing wintertime storm water. In some cases, even the relationship of the same winter design storm factors to different airport types will produce drastic differences in recommendations for deicing project components. One example of this can be seen in the type of precipitation that an airport experiences as a result of its geography. Airports in warmer geographies may be subject to deicing activities only when a freezing rain or light snow event occurs, while airports in northern geographies could base their deicing collection on their winter season being loosely defined as October through April. The deicing proj- ect solution to meet the particular drivers at each of these airports will be very different. An airport in a warmer climate may need to implement only a glycol recovery vehicle (GRV) or a simple block-and-pump system followed by a flushing to collect deicing runoff, whereas a snowbelt airport may be required to have a centralized deicing facility equipped with a sophisticated deicing collection system, storage tanks, and treatment systems.

Next: Section 3 - Decision Support Tool »
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TRB’s Airport Cooperative Research Program (ACRP) Report 81: Winter Design Storm Factor Determination for Airports identifies the relevant factors in defining a winter design storm for use in sizing airport deicing runoff management systems and components.

The guidebook also provides a decision support tool for identifying an appropriate winter design storm for an airport-specific project; a review of regulations as they pertain to deicing runoff; and suggestions for target levels of service, including the acceptable level of risk of the designed system not meeting performance standards.

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