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18 A Handbook for Addressing Water Resource Issues Affecting Airport Development Planning Summary of Fact Sheet 3: Storm Water Quantity and Floodplains Many of the most common airport development projects can affect the quantities of storm water discharged from the airport site. Those storm water discharges can in turn affect the propensity for regional flooding and runoff-related impacts. Airports are frequently located near water bodies and as such, airport projects can affect, and be affected, by floodplains. Regulation of storm water quantities associated with development projects has occurred pri- marily at municipal, regional, and state levels to date. Storm water quantity regulations are typ- ically designed to protect downstream infrastructure and minimize flooding during large, infrequent storm events. The regulations often include requirements allowing little, if any, change in the runoff quantities before and after development projects. A series of federal orders and guidelines, plus some state regional requirements have been enacted and developed to pro- tect floodplains. The key elements of the regulatory framework include developments within a floodplain, increases to flood elevations, and changes to runoff inside the floodplain. Airport development projects that involve construction of new impervious surfaces (e.g., pavement, buildings) in previously undeveloped areas are typically the types of projects most affected by storm water quantity and floodplain regulations. Projects where one type of imper- vious surface is replaced by another typically are not affected significantly. Project planning and design considerations typically associated with storm water quantity and floodplain regulation compliance include the following: Does the project extend into the 100-year floodplain? Will the project's unavoidable location result in additional flooding or endanger human life? What are regulatory requirements governing flow rates and volumes from new developments? Is there an existing storm water runoff and transport model to characterize discharges? Will any storm water be diverted to a new discharge location and new watershed? Will storm water detention be required to meet regulatory requirements, how much space will it require, and where will it be located? Will storm water detention cause issues with wildlife attractants? Will storm water piping and pumping be required to convey storm water to detention struc- tures, treatment facilities, or new discharge locations? The process for assessing core regulatory requirements, determining the measures needed for compliance, and reducing the effects of managing storm water quantity and floodplain issues on the development project are presented in Fact Sheet 3 in Appendix B. A summary of the guid- ance for reducing those project impacts follows. Guidance for Reducing Project Impacts from Managing Issues Associated with Storm Water Quantity and Floodplains Water Resource Inventory Create an inventory for storm water data that includes maps of airport areas with flooding issues and capacity limitations. Understand the source of the floodplain boundaries obtained from federal agencies and include mapping of floodplain boundaries in the inventory. The boundaries in Flood Insurance Rate Map (FIRM) documents may not be accurate on smaller project scales.

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Handbook Overview 19 Modeling, Calculations, Coordinate with regulators to determine approved and Analyses modeling programs, calculation protocols, and level of detail that may be acceptable. Identify the appropriate level of detail to collect in sup- port of modeling and analyses and provide a reasonable estimate of storm water discharges (e.g., select a minimum pipe size or drainage basin size to be modeled). Conceptual Planning Consider development of a storm water master plan or integrating storm water master planning into airport master planning. Master plans may be useful for considering the storm water needs for long-term or phased development, in addi- tion to widespread development. Control Siting Consider potential floodplain impacts associated with long-term development as part of storm water master planning process. Identify and characterize potential sites for controls in the Conceptual Planning Phase. Avoid impacts inside the floodplain, where possible. Design Implementation Build infiltration and temporary storage into devel- opment project design where possible to reduce extent of necessary end-of-pipe controls. Avoid project-by-project designs of controls that do not consider effects to overall airport storm water drainage. Consider the need for modifications to controls to minimize attraction of hazardous wildlife. Construction of Controls Coordinate design and construction of controls with design and construction of the develop- ment project to avoid development project delays from unfinished work. Scheduling and Sequencing Reduce management and consultant costs with well-planned and coordinated schedules for development project and storm water quantity assessments. Stakeholder Coordination Coordinate with local agencies, property owners, and regulators to assist with identifying core reg- ulatory requirements, interests, and data on local flooding issues.