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TECHNICAL MEMORANDUM <br /> July 13, 1995 <br /> Page 8 <br /> Table 3 -4 Design Storm Criteria for Other Jurisdictions (continued) <br /> City /County Design Storm Criteria <br /> City of Salem, Catch basins, gutters, and connector, pipes are designed for the 5 -year storm. Laterals, trunk <br /> Oregon lines and drainage ways not shown within a floodplain on the Federal Insurance Rate Maps <br /> (FIRM) are designed for the 10 -year storm. Facilities within a FIRM floodplain are designed <br /> for the 100 -year storm. <br /> Spokane County, A 10 -year design storm is used for off -site runoff. A 50 -year storm is used where off -site <br /> Washington runoff contributes to a drainage facility. <br /> King County, A 25 -year design storm is used for pipes, culverts, drainage ditches, and channels with <br /> Washington 0.5 -foot of freeboard required for pipes, drainage ditches, and channels. Pipes and culverts <br /> must be designed to pass the 100 -year storm under surcharged conditions. Drainage ditches <br /> and channels must carry the 100 -year storm without overtopping. <br /> Douglas County, The minor drainage system shall be designed for the 5 -year storm and minimize disruption to <br /> Colorado the urban environment. The major drainage system shall be designed for the 100 -year event <br /> and minimize health and safety hazards. A 2 -year storm may be used for design if approved <br /> by County Engineer. <br /> City of Lakewood, Storm frequency is based upon land use: residential (low density) and parks-2 year <br /> Colorado residential (high density), light commercial, office, and industry-5 -year. A 100 -year storm is <br /> used for design to protect against loss of life and serious, substantial property damage. <br /> On -site versus Off -site Detention/Retention <br /> Most communities accommodate both on -site and off -site detention /retention facilities, either <br /> as naturally occurring drainage system features (e.g., lakes, wetlands, and floodplains) or as <br /> man -made components (e.g., parking lot ponding, low -lying recreational areas, landscaped <br /> open ponds, detention ponds, roof storage, and oversized underground pipes). Urbanization <br /> and development typically increase the impermeable surface, due .to added paved and roof <br /> areas, and thus increase rainfall runoff. The function of detention facilities is to temporarily <br /> store runoff and release water at a rate that is equal to or less than the hydraulic capacity of <br /> the downstream drainage system. The function of retention facilities is to store runoff, <br /> without release, for evaporation, infiltration, or reuse, such as for irrigation. Table 3 -5 lists <br /> the on -site detention provisions for several jurisdictions. <br /> InfiltrationTystems <br /> Infiltration systems are generally not appropriate for the city of Medford because of the <br /> relatively impermeable nature of local soils. Though some areas have permeable surface <br /> sands and gravels, most of the area is underlain by impenetrable surfaces such as bedrock, <br /> clay, and hardpan. Also, when rainfall is most frequent in the winter months, the <br /> groundwater table is typically high and the surface soils are saturated. All of these conditions <br /> 1:\21 I4\Task3Vm31.eug DRAFT <br />