Flood Inundation Mapping Data For Johnson Creek near Sycamore, Oregon

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.本数据集特征的核心数据源为美国地质调查局（U.S. Geological Survey）发布的2017-5024号科学调查报告：《俄勒冈州梧桐溪附近约翰逊溪洪水淹没制图数据》。HEC-RAS水力学模型的模拟范围为俄勒冈州波特兰市SE 174大道上游至约翰逊溪与威拉米特河汇流处的12.9英里河段。本模型所采用的部分水力学参数取自美国联邦紧急事务管理署（Federal Emergency Management Agency）2010年发布的《洪水保险研究报告》，覆盖俄勒冈州波特兰市、姆尔特诺默县、克拉克默斯县与华盛顿县，为三卷本第一册，发布日期为2010年11月26日。本项目所用的数字高程模型（Digital Elevation Model, DEM）由2015年获取的激光雷达（LiDAR）数据生成，数据提供方为俄勒冈州地质与矿产工业部。行业通行标准通常会从数字高程模型中剔除桥面高程数据，因此本数据集中可能会将实际未被淹没的桥面标记为淹没状态。在评估洪水期桥梁的通行可用性时，需结合实际场景进行判断：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可靠的参考信息。本次洪水图层的生成采用了两种模型方案：第一种为稳态模型方案，采用非恒定流模拟，将最大流量持续保持数日，以通过非恒定流框架实现稳态模型的模拟效果。该稳态模型方案生成的不确定性区域包含于sycor_breach.shp形状文件中。第二种为非稳态模型方案，同样采用非恒定流模拟，基于2015年12月年度峰值洪水的水文过程线（涨水与退水段）完整还原径流全过程。该非稳态模型方案生成的洪水范围多边形包含于sycor.shp形状文件中，同时配套生成了水深栅格数据，其结果为美国地质调查局水文监测站14211500对应流量下洪水淹没范围的最优估算结果。