New Search
Lower Amazon Floodplain Mapping Project Page 4 <br /> Pacific Water Resources, Inc. 6/16/04 <br /> • Digital and paper table of ERM / NGS reference locations <br /> • Structure sketches and photographs , <br /> • Mass points and breaklines data (i.e. DTM) <br /> • Digital ground surface (TIN) . <br /> • Digital base maps showing contours and stream structures (bridges, culverts, levees, and <br /> weirs),hydrography, roads, jurisdictions, aerial photography, etc. (as specified by FEMA) <br /> • FIS certification(s) for base map data, ground survey, and ground topographic model and `TIN' <br /> • Digital data submittal checklists <br /> • Metadata compliant with Federal Geographic Data Committee standards <br /> TASK 5. HYDROLOGIC ANALYSES <br /> This task will establish and briefly document the existing flood hydrographs and peak flows needed for the <br /> hydraulic analysis in Task 6. The Lower Amazon study area is extremely complex both hydrologically and <br /> hydraulically. Significant work has been performed by others over many decades to understand and alter area <br /> hydrology and hydraulics. The complexities of this area includes: a diversion of the main flow of Amazon <br /> Creek to the Fernridge Reservoir (named Amazon Channel on Figure 1), the original creek alignment (A <br /> Channel), a siphon structure on the Greenhill Road ditch to transport flow under the Diversion Channel, a <br /> structure on the A Channel which produces the flow split to the Diversion Channel, a structure at the <br /> confluence of the Greenhill tributary and the A Channel, existing uncertified levees downstream of Royal <br /> <br /> Avenue on the Amazon Channel, an existing wetland restoration project upstream from Royal Avenue (i.e. <br /> "1135 Levee" project mentioned earlier) and backwater effects from Fernridge Reservoir. Because of these <br /> complexities and the significant amount of storage available in the "1135 Levee" project, an unsteady flow <br /> hydraulic analysis will be conducted in Task 6. <br /> Existing XP-SWMM models created, documented and calibrated by URS as part of the City's Stormwater <br /> Basin Master Plan will be used to generate most of the hydrographs needed for the unsteady flow analysis <br /> described in Task 6. Figure 2 shows the aerial extent of the subbasin areas delineated by URS, which are <br /> contributing flows to the Lower Amazon study area. XP-SWMM models exist for: (1) the Amazon Basin <br /> (AM) including its tributary Willow Creek (WC), (2) Greenhill (GH) Basin and (3) the A3 Channel (A3) <br /> Basin. The Greenhill and A3 Channel subbasin areas are shown in Figure 2 as the portion of the City's <br /> Bethel-Danebo study area located east and northeast of the A Channel. <br /> <br /> New hydrologic models for a large area referred to as GH-000 in the City's Stormwater Basin Master Plan (i.e. <br /> Greenhill Road Ditch Drainage and Grosbeck Channel) and Dead Cow Creek (i.e. crosses SPRR east of <br /> Greenhill Road and west of Amazon Creek) will have to be developed as part of this task. These hydrologic <br /> models will consist of single subbasin areas since their objective is only to provide appropriate inflow <br /> hydrographs needed for the unsteady flow hydraulic analysis. <br /> The hydrologic analysis described in this task will develop the flood inflow hydrographs to the hydraulic <br /> model for the 1-and 0.2-percent annual chance storm events under existing conditions of urbanization <br /> throughout the Amazon and other tributary watersheds as defined by the City's Stormwater Basin Master <br /> Plans. The hydrologic analysis will develop the inflow hydrographs for two different design storm durations <br /> and a single historic rainfall event. The two different design storm durations will be 24 hours and 72 hours. <br /> The SCS Type lA 24-hour rainfall distribution and associated depth for the 1-percent annual chance rainfall <br /> established by the City's 5tormwater Basin Master Plan will be used in this project. The 72 hour storm will be <br /> based on work performed by MGS Engineering Consultants, "Development of <br /> Design Storms for the Portland, <br /> Oregon Area" dated July 9, 2001. <br /> Develop the 500-year 24-hour Rainfall Event. The City's Stormwater Master Plan did not address the 500- <br /> year (i.e. the 0.2-percent annual chance) flood event that FEMA requires. Working with information from the <br /> IDF Curve published in Volume I of the 1990 Eugene Areawide Drainage Master Plan, PWR will plot on <br /> probability paper the various 24-hour rainfall depths and extrapolate the 0.2-percent annual chance event. The <br /> <br />