Geospatial datasets and hydraulic models for the Grand River, Red Cedar River and Sycamore Creek near Lansing, Michigan (ver. 1.1, February 2023)

Hydrologic and hydraulic analyses were done for selected reaches of the Grand River, Red Cedar River and Sycamore Creek near Lansing, Michigan. To update and expand a portion of the Federal Emergency Management Agency detailed Flood Insurance Study, the U.S. Geological Survey (USGS) and the City of Lansing initiated a cooperative study. The study comprised a 3.2-mile reach of the Grand River, a 30.2-mile reach of the Red Cedar River, and a 12.0-mile reach of Sycamore Creek. Historical streamflow data from multiple streamgages, Grand River at Lansing, MI. (USGS station number 04113000), Red Cedar River at East Lansing, MI. (USGS station number 04112500), Red Cedar River near Williamston, MI. (USGS station number 04111379), and Sycamore Creek at Holt Road near Holt, MI. (USGS station number 04112850) along with regional regression equations were used to estimate instantaneous peak streamflows for floods with 10-, 4-, 2-, 1-, 0.2-percent, and 1-percent plus annual exceedance probabilities. The 1-percent plus flood elevation is defined by the Federal Emergency Management Agency as a flood elevation derived by using streamflows that include the average predictive error for the regression equation streamflow calculation for the Flood Risk project. This error is then added to the 1-percent annual exceedance probability flood streamflow to calculate the 1-percent plus streamflow. The annual exceedance probability streamflows were then used in a Hydrologic Engineering Center-River Analysis System step-backwater model to determine water-surface elevation profiles and flood-inundation boundaries for the 10-, 4-, 2-, 1-, 0.2-percent, and 1-percent plus annual exceedance probability floods, and a regulatory floodway, along a selected reach of each stream. Each hydraulic model was calibrated to the current stage-streamflow relations at each streamgage. Flood-inundation boundaries for the 1- and 0.2-percent annual exceedance probability floods and a regulatory floodway were mapped for each stream.

针对密歇根州兰辛附近的格兰德河（Grand River）、红雪松河（Red Cedar River）以及梧桐树溪（Sycamore Creek）的选定河段开展了水文与水力学分析。为更新并扩充美国联邦紧急事务管理局（Federal Emergency Management Agency）详细洪水保险研究的部分内容，美国地质调查局（U.S. Geological Survey, USGS）与兰辛市联合启动了一项合作研究。本次研究覆盖格兰德河3.2英里河段、红雪松河30.2英里河段以及梧桐树溪12.0英里河段。研究团队利用多个水文测站的历史河道流量数据——包括密歇根州兰辛市格兰德河水文站（美国地质调查局站号：04113000）、东兰辛市红雪松河水文站（站号：04112500）、威廉斯顿附近红雪松河水文站（站号：04111379）以及霍尔特附近霍尔特路梧桐树溪水文站（站号：04112850）——结合区域回归方程，估算了年超越概率分别为10%、4%、2%、1%、0.2%以及1%+的洪水瞬时峰值流量。美国联邦紧急事务管理局将1%+洪水水位定义为：基于河道流量计算得到的洪水水位，该流量包含了本次洪水风险项目中回归方程流量计算的平均预测误差；将该误差叠加至年超越概率1%的洪水流量，即可得到1%+洪水流量。随后，研究团队将各年超越概率对应的河道流量输入至水文工程中心河道分析系统（Hydrologic Engineering Center-River Analysis System）逐段回水模型中，以计算各河道选定河段内年超越概率分别为10%、4%、2%、1%、0.2%以及1%+的洪水水面线分布与洪水淹没范围，同时涵盖法定行洪通道范围。每一套水力学模型均以各水文测站当前的水位-流量关系进行率定。研究团队为每条河道绘制了年超越概率1%、0.2%的洪水以及法定行洪通道的淹没范围图。