Flood Inundation Extents for Flows of 800 to 3,080 cfs at Gage 14211500, Johnson Creek near Sycamore, Oregon (sycor.shp)

The basis for these features is U.S. Geological Survey Scientific Investigation Report 2017-5024 Flood Inundation Mapping Data for Johnson Creek near Sycamore, Oregon. The domain of the HEC-RAS hydraulic model is a 12.9 mile reach of Johnson Creek from just upstream of SE 174th Avenue in Portland, Oregon to its confluence with the Willamette River. Some of the hydraulics used in the model were taken from Federal Emergency Management Agency, 2010, Flood Insurance Study, City of Portland, Oregon, Multnomah, Clackamas and Washington Counties, Volume 1 of 3, November 26, 2010. The Digital Elevation Model (DEM) utilized for the project was developed from LiDAR data flown in 2015 and provided by the Oregon Department of Geology and Mineral Industries. Bridge decks are generally removed from DEMs as standard practice. Therefore, these features may be shown as inundated when they are not. Judgement should be used when estimating the usefulness of a bridge during flood flow. Comparing the bridge to the surrounding ground can be more informative in this respect than simply looking at the bridge itself. Two model plans were used in the creation of the flood layers. The first is a stable model plan using unsteady flow in which the maximum streamflow is held in place for a long period of time (a number of days) in order to replicate a steady model using an unsteady plan. The stable model plan produced the areas of uncertainty contained in the sycor_breach.shp shapefile. The second is an unstable model plan that uses unsteady flow in which the full hydrograph (rising and falling limb) is represented based on the hydrograph shape of the December 2015 peak annual flood. The unstable model plan produced the flood extent polygons contained in the sycor.shp shapefile and the depth rasters and represents the best estimate of flood inundation for the given streamflow at U.S. Geological Survey streamgage 14211500.

本数据集所涉要素的基础数据源自美国地质调查局（U.S. Geological Survey, USGS）2017-5024号科学调查报告《俄勒冈州西卡莫尔附近约翰逊溪洪水淹没制图数据》。本次使用的HEC-RAS水力学模型的模拟河段总长12.9英里，范围起自俄勒冈州波特兰市SE 174大道上游不远处，终至约翰逊溪与威拉米特河的汇流处。模型所采用的部分水力参数取自联邦紧急事务管理局（Federal Emergency Management Agency, FEMA）2010年11月26日发布的《洪水保险研究报告》，该报告覆盖俄勒冈州波特兰市、穆尔特诺马县、克拉克马斯县及华盛顿县，为全系列3卷中的第1卷。本项目所使用的数字高程模型（Digital Elevation Model, DEM）由2015年采集的激光雷达（Light Detection and Ranging, LiDAR）数据生成，数据由俄勒冈州地质与矿产工业局提供。行业标准惯例通常会从DEM数据中剔除桥面高程，因此部分桥面在本数据集中可能被标记为受洪水淹没，但实际未必如此。在评估洪水径流期间桥梁的可用性时，需结合实际情况进行判断；相较于仅观察桥梁本身，将桥梁与周边地面高程进行对比，可获得更具参考价值的信息。本次洪水图层的生成采用了两种模型方案：第一种为恒定流模拟方案（采用非恒定流算法实现），即将最大径流量维持较长时长（数天），以通过非恒定流方案近似实现恒定流模拟。该恒定流模拟方案生成的不确定性区域存储于sycor_breach.shp形状文件中。第二种为非恒定流非稳定模拟方案：基于2015年12月年度最高洪水的水文过程线形态，完整还原涨水与退水的完整过程。该非稳定模型方案生成的洪水范围多边形存储于sycor.shp形状文件中，同时生成了深度栅格数据，其结果为美国地质调查局水文测站14211500对应径流量下洪水淹没范围的最优估算结果。