Methods
River Geometry
The river geometry was obtained from John Reimer at the Dane County Land and Water Resources Department. The data was in HEC-RAS form with various cross sections along the river. This data was originally in the form of x,y,z coordinates, and was processed in GeoHEC-RAS to obtain the cross sections.
HEC-RAS
There were three HEC-RAS models created: a model representing pre-dredging conditions, a model representing 50 feet of dredging, and a model representing bank to bank dredging. The three model geometries for a single cross section can be seen in the images to the right. All dredging was assumed to be two feet in depth, following the current dredging plan. The two dredging options were created by lowering the necessary length on the bottom of each cross section by two feet. All dredging was done between banks. Dredging was done as close to the middle of the river as possible for the 50 foot model.
After developing the geometry files in HEC-RAS, the flow and boundary conditions were set. The boundary condition used was the downstream rating curve of the river which was generated from the USGS gaging site just downstream of Lake Waubesa. There is no gaging site on this length of the Yahara River, so it was assumed that the flow into Lake Waubesa was equal to the flow out of Lake Waubesa, and that the surface of lake has the same elevation at all points. The rating curve was adjusted to account for the difference in elevation of the USGS site and the downstream cross section of the river.
Flood Frequency Analysis
A flood frequency analysis was done to find typical flow values at this USGS gauge station. This was done by predicting flow values corresponding to specific return periods or probabilities of the historical flow rates.
To conduct the flood frequency analysis, we analyzed the entire historical data of the USGS Gauge station at our site. We took the annual peak flow from 1930 all the way to 2018.
Once we collected the annual peak flow values, we conducted a statistical analysis using the concepts of exceedance probabilities and taking the CDF of the Gumbel distribution of our data. From this data, we then plotted the peak streamflow against the return period to create the flood frequency curve.
No Dredging
50 ft Dredging
Bank to Bank Dredging
Equations used in probability calculations
CDF of Gumbel Distribution
