Ordinance No. 20513 - Laserfiche WebLink

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Figure 7. Water Gauge Location. � 7 We were fortunate to have a NOAA Weather Service river gauge onsite at the inlet structure to the EWEB steam plant (Figures 7 & 8). The gauge provides Willamette River level data from 1990 to the present, which allows accurate calculation of a 20+ year flood recurrence interval. The floodplain is divided into 4 sections by flood recurrence intervals of 2 years, 5 years, 10 years, and 20+ years. The 2 -year flood is generally synonymous with the bankfull river level and Ordinary High Water Mark (OHWM). The Environmental Protection Agency describes bankfull as "originally used to describe the incipient elevation on the bank where flooding begins." The Code of Federal Regulations defines the OHWM as follows: "The term ordinary high water mark means that line on the shore established by the fluctuations of water and indicated by physical characteristics such as a clear, natural line impressed on the bank, shelving, changes in the character of soil, destruction of terrestrial vegetation, the presence of litter and debris, or other appropriate means that consider the characteristics of the surrounding areas. " The recurrence interval for this 2 -year floodplain was determined by listing the highest water level for each of the last 20 years and then ranking the water elevations in descending order. These elevations were averaged to find the flood elevation with a 50% probability of occurrence in any year. It is important to note that throughout a given year the river level is normally below the OHWM or 2 -year flood elevation. Similarly, the Ordinary Low Water Mark (OLWM) is determined by ranking the lowest river level for each of the 20 years of data then averaging for the normal low water level. This OLWM is the average baseflow of the river resulting from snowmelt and precipitation that infiltrates into the soil and eventually moves through the soil to the stream channel as groundwater. The recurrence interval for floodplains above OHWM are calculated from the 20 years of annual high water elevations by determining the probability of occurrence as 20% for a 5 -year flood, 10% for a 10 -year flood, and 1% for a 20+ year flood. (Figures 9, 10, and 11). 10 Figure 8_ Water Gauge_