Ordinance No. 20513 - Laserfiche WebLink

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Portland Office of Transportation lists the lower Willamette flood levels through downtown Portland as 30.5 feet in 1964 and 31.8 feet in 1996, indicating that the 1996 event was the largest flood event in the 60 years after hydrologic manipulation from the dams. The river gauge data on the EWEB Riverfront shows the flood level beginning to top out above the 10 -year flood level of 20 feet above 390 feet datum. The 20 -year flood level is only 0.6 feet above the 10 -year flood because the water storage in the valley through Alton Baker Park across the river provides significant volume capacity at and above this elevation. Therefore, an increase in flood recurrence interval from 20 -year to 100 -year flooding should cause little increase in water elevation due to the large volume of storage in Alton Baker Park . 6 Aquatic and Riparian Functions The principal aquatic and riparian functions calculated for the floodplain were water storage volume, primary production area, and habitat support extent. These in turn form the basis of riparian and aquatic habitat, and associated biodiversity. The presence of water and vegetation are the basis of a riparian wetland area, which provides numerous secondary functions such as sediment stabilization, thermoregulation, and nutrient removal. These three functions thus help represent a wide array of ecological functions and ecosystem services performed by riparian areas. The following descriptions of three aquatic functions are developed from the Hydrogeomorphic (HGM) Assessment of Oregon Wetland and Riparian Sites for the Willamette Valley Ecoregion Riverine Impounding and Slope/Flat Subclasses'. Water Storage Volume: "The capacity of a wetland or riparian area to store or delay the down - slope movement of surface water for long or short periods, and in doing so to potentially influence the height, timing, duration, and frequency of inundation in downstream or downslope areas." Primary Production Area: The amount of riparian vegetation provides a measure of the area available for primary production, defined as the "capacity of a wetland or riparian area to use sunlight to create particulate organic matter (or wood), leaves and detritus through photosynthesis, which forms the basis of the food web and maintains water quality." Habitat Support Extent: The capacity of a wetland or riparian site to support the life requirements of most aquatic- dependent species native to temperate regions. Habitat support for riparian species includes resident and anadromous fish, invertebrates, amphibians and turtles, waterbirds, mammals and songbirds. Both the linear extent and width of riparian habitat constitute key metrics. These three aquatic functions were derived from the floodplain maps by simple quantifying measurements, which do not consider the quality of the site. Water storage volume was calculated by assessing bankline slope and flood area to determine the additional volume provided in each floodplain level as cubic feet. This measurement gives a general view of the floodplain capacity to store and delay down slope movement of water. Primary production area was calculated as the area of each floodplain level in square feet and acreage to give an overview of the potential growth zone for riparian vegetation. Habitat support extent was calculated as the linear feet of river bankline at each level and the maximum and minimum width of each floodplain to assess the dimensions available for riparian species use. 6 We note that the 100 -year flood level on this property according to FEMA maps is 417 -418'_ Other sources have indicated likelihood that the actual flood elevation would be closer to 422'_ All proposals made in the master plan are thus concepts that need to be investigated and engineered- 7 Can be obtained at http: / /oregon.gov /DSL /WETLAND /hgm__guidebook_shtml using the FormsjPublications link 13