Approaches

Data Sources

Data used in the hydrologic and hydraulic models are described in the table below.

Hydrologic Approach

A HEC-HMS Hydrologic model is built using two major models: a basin model and a meteorological model.

The basin model is a physical conceptualization of the watershed. Subbassins are created and fed into junctions (where the subbassins meet) which eventually continue to the watershed outlet. The necessary basin model parameters – basin and subbasin boundaries, areas, outlets, streams, transform and loss coefficients – are determined using GIS techniques.

The meteorological model simulates rainfall for the HMS model. To determine the 10-year, 50-year, 100-year, and 500-year flooding events the same return periods are referenced from NOAA’s ATLAS-14 for region-specific frequency rainfalls (1/100 frequency is a 100-year event). To calibrate the model, real rainfall data from Green Bay’s airport station was used in conjunction with real discharge data from USGS Gages during the same rain events.

The workflow (shown below) takes the input data and uses a collection of tools and techniques to build, run, and calibrate the hydrologic model.

The processes used in the meteorologic and basin models are based on Soil Conservation Services (SCS) methods. The curve number and impervious % were determined by cross-referencing spatial data with the TR-55 which associates land use and soil information to curve numbers and impervious %. The parameters were calculated for every pixel in the watershed and then the average value was calculated for each subbasin. The lag time took several parameters into account. The SCS methodology was used to calculate lag time. (Note: hydraulic length is the longest flow path in the subbasin).

The calibration process indicated the model was underperforming (not predicting enough discharge) and adjustments were made to the model inputs accordingly (a 5% increase to the Curve Number and a 10% decrease to the Lag Time). These adjustments agree with domain knowledge (the flow was infiltrating too much and not peaking at the control locations soon enough). The solution for the curve number, lag time, and imperivous % (the HEC-HMS inputs) is an approximation, and some calibration to the model statistics is expected. Unaccounted variables do exist in the model, such as baseflow (which is relatively low during large storm events).

After calibration, the model is run for the 1/10, 1/50, 1/100, and 1/500 frequency storm scenarios. Hydrographs of the discharge entering and leaving subbasins in the watershed are then used to characterize the peak discharge at different points along East River.

Hydraulic Approach

The hydraulic approach to characterizing flooding in the East River watershed was modeled in HEC-RAS.

Steam flow analysis is performed with discharge and downstream water level at a return period of 10-year, 50-year, 100-year, and 500-year. For example, a 100-year flood discharge means 1 percent annual chance of the discharge being equal or exceeded that number during any year. Although the return period represents the long term average period between floods of a specific magnitude, rare floods could occur at short intervals or even within the same year. For river discharge, we use the hydrology model HEC-HMS to compute flow discharge at the return periods of 10, 50, 100, 500 years. The discharge begins with small values at the upstream and gradually increases along the river due to the confluence of Bower Creek, Willow Creek, Baird Creek, and East River tributaries. For downstream water level, we use the water level data collected at NOAA station 9087079, which locates at the river mouth of the Fox River, to perform long-term analysis. Both peak-over-threshold (POT) and extreme value analysis (EVA) are used to fit the water levels with four distribution curves, including Normal, Log-normal, Gumbel, and Weibull distributions, and the results of water level with 10, 50, 100, and 500 recurrence intervals are averaged.

The raw HEC-RAS model of the East River is interpolated and geo-referenced along the river centerline. First, the raw cross-sections are interpolated with a maximum distance of 100 ft, resulting in a total number of 1335 cross-sections. Next, using the software GeoHecRas, we overlap maps of Google Earth and FEMA floodplain and draw the river centerline following the FEMA map. Then, we determine the start and end-points of the centerline, and the software can align the cross-sections along the centerline according to the interval distance of each cross-section. We further slide cross-sections along the centerline to ensure that bridges match their locations on the Google Earth map. Also, we rotate and move some cross-sections to make them have the same positions as historical FEMA records. The downstream cross-sections agree well with the FEMA map after alignment, but the upstream region has more discrepancies, which requires more manual movement.

The model is then set up with our inputs and boundary conditions.

We determine 16 cases by combining water levels and discharge data with different return periods. The smallest hazard event has a 10-year period of discharge and water level, while the worst event has a 500-year period for the two properties. Steady flow analysis is then performed based on 16 cases in the HEC-RAS model. Floodplains of the 16 steady flow cases are generated by combining the flood elevation results in the HEC-RAS model with DEMs of the Brown County. What is more, the floodplain is overlapped with land use data in ArcGIS, where the land use of Brown County are classified into nine general land use type. The flooding areas for each land use type are calculated under 16 cases.