Storm Sewer System - Laserfiche WebLink

New Search

• TECHNICAL MEMORANDUM July 13, 1995 Page 7 Intensity- Duration - Frequency Curves IDF curves are typically developed to reflect the characteristic rainfall conditions of the study area. Therefore, curves developed for other communities in varying locations will not be that useful as a reference for Medford. Sometimes, for large geographic areas, several IDF curves are required to accommodate diverse rainfall conditions. For example; the city of Portland uses two sets of IDF curves: one for east of the Willamette River and the other for the west side of the river. The use of a single or multiple IDF curves for the city of Medford will be discussed later in this technical memorandum. While the specific IDF curves used from city to city will differ, the frequency of the design storm is an important element to consider. The frequency or return period of a storm establishes the level of protection from flooding that a community is willing to accept. Table 3 -4 shows how other municipalities approach the design storm frequency issue. Table 3 -4 Design Storm Criteria for Other Jurisdictions City /County Design Storm Criteria City of Portland, All storm drainage designed to pass 10 -year storm without surcharging. All systems shall Oregon have capacity to pass the 25 -year design storm without damaging property or endangering human health or safety. Surcharging is allowed with 25 -year storm. Design duration is a 6- hour storm unless the 24 -hour storm would create a higher peak flow. City of Beaverton, Inlets, catch basins, and trash racks are to be designed for the 10 -year storm. Conveyance Oregon features such as pipes, ditches, and culverts shall be designed for the 25 -year storm. Improvements within the floodplain shall not produce inundation of existing buildings from the 100 -year event. Unified Sevyerage Inlets, catch basins, and similar appurtenances are designed for the 10 -year storm. The Agency of conveyance system (pipes, culverts, and channels) are to be designed for the 25 -year storm. Washington County, A supplemental design is used to demonstrate that the 100 -year storm shall not flood existing Oregon buildings. City of Lake Conveyance systems are designed for 10 -year storm. On -site detention is required to store Oswego, Oregon volume of stormwater that is difference of 50 -year storm on developed site and 10 -year storm on predeveloped property. City of Eugene, Design storm varies based upon drainage area and type of drainage improvement. For Oregon r drainage areas less than 40 acres; pipes -5 -year storm; open - channel and j culverts/bridges -10 -year storm, unless there is an exceptionally high cost differential between the 10 -year and 5 -year events. For 40 to 640 acres: open channel and pipes -10 -year storm; culverts/bridges under major arterials -25 -year design storm. For drainage areas greater than 640- acres, the 25 -year storm is used except for culverts/bridges under major arterials where the 50 -year storm is used. • J:\21 I9\1'ask3Wn31.eug DRAFT