Methodology
Objective
Carry out numerical simulations to investigate the influence of high frequency (~3 hour period) water level oscillations in Chequamegon Bay on typical flooding events upstream in the North Fish Creek basin, regarding flood amplitude, inundation area, timing, and period.
Study Site
Data Preparation
Data was obtained from United States Geological Survey (USGS) stations for North Fish Creek and in Chequamegon Bay. River discharge data was collected from a location about 12 miles upstream in North Fish Creek and. Chequamegon Bay water level stage was collected from a USGS station located at the Ashland Breakwater Lighthouse, which is about 4.5 miles from the study area downstream boundary. The data from this station was used as an approximation to the water level changes at the actual study area downstream boundary.
​
Terrain data for the North Fish Creek basin was obtained from National Oceanic and Atmospheric Administration (NOAA) Lidar data. This elevation data will be used to produce simulation models in HEC-RAS. Similarly, a projection file was obtained from NOAA Lidar data and will be used to fit the models to real-world coordinates.
Frequency Analysis
Several flood events and water level oscillation events were identified from the USGS Station data. These events were used to develop storm exceedance probablity models for both upstream boundary flood peak and downstream boundary maximum wave height of water level oscillations. Data for two hypothetical storms was extracted from the models:
20-year Storm:
Flood Peak = 127 cms
Max. Wave Height = 1.328 m
100-year Storm -
Flood Peak = 193 cms
Max. Wave Height = 1.638 m
Generation of Inputs
Upstream and downstream boundary conditions were generated for 20-year and 100-year storms. Using data from a typical 20 and 100-year storm, typical hydrographs were created to calculate a flooding time series. This time series will be used as the upstream boundary condition input for the HEC-RAS simulations. A sinusoidal water level oscillation time series was also calculated. This time series will be used as the downstream boundary condition input for the HEC-RAS simulations.
Run Simulations
All terrain, projection, and boundary condition input data was used in two simulations: 1-Dimensional and 2-Dimensional HEC-RAS models. Simulations for both storm events (20-year and 100-year) and with and without water level oscillations were modeled so that the influence can be analyzed.
​
1-Dimensional HEC-RAS Model:
​
For the 1D HEC-RAS model, the first step was to use the terrain and projection data to aid in outlining the Creek geometry. Creek centerlines, riverbank boundaries, flow paths, and cross-sections were traced. 1D HEC-RAS runs simulations based on cross-section data and is limited by the amount of cross-sections drawn. For a winding creek with a complex shape, such as North Fish Creek, the number of viable cross-sections is limited. Once the geometry of the Creek was established, an unsteady flow analysis was run using the upstream and downstream boundary conditions—calculated flow hydrograph and calculated stage hydrograph, respectively.
2-Dimensional HEC-RAS Model:
​
The 2D HEC-RAS model greatly improves the limitations associated with 1D. In 2D, a flow area is defined using the terrain and projection data. This negates the need for manual cross-section inputs and improves the resolution of the model. A 2D model is better-suited for modeling alluvial fans, like in the study area. Once the 2D flow area was established, an unsteady flow analysis was run using the same upstream and downstream boundary conditions as in the 1D model.
