Methods

Creating Cross Sections

Bathymetry Data

Used bathymetry data from the Dane County Department of Natural Resources survey from 2024. A total of 7,618 points were surveyed from Lake Kegonsa to Highway B.

RAS Mapper

Used RAS Mapper to interpolate the data points and generate a projected channel. Then, thirty-five profiles were cut across the channel to generate the cross sections.

Civil 3D

Used Civil 3D to measure the distance downstream between cross sections. The model used 3,399.4 feet of river length.

HEC RAS Model Set-Up

Cross Sections

Thirty-five cross sections were entered into the HEC-RAS Geometry Editor. Cross-section profiles were derived using RAS Mapper, and bank elevations were identified using Google Earth.

Substrate Conditions

Used Yahara riverbed composition to create a weighted Manning’s coefficient to reflect the majority sandy, but also gravel and muck, bottom conditions of the river.

Steady State Flow Analysis

Upstream Boundary

The upstream boundary was set using known water surface elevations from Lake Kegonsa. Since the dam at the lake’s outlet is open during winter, the model used the average lake elevation for the months of December through March from 2011–2024. This resulted in an upstream boundary elevation of 842.8 feet.

Downstream Boundary

The downstream boundary was determined using a rating curve developed from a monitoring station downstream, which measured stage and discharge. The curve incorporated the maximum, minimum, and average values for each winter month to set appropriate boundary conditions.

HEC RAS Ice Cover Variations

Ice Cover Thickness and Roughness

Three ice thicknesses were selected to represent common ice uses. Thin, medium, and thick ice were defined as 0.33 feet for walking, 1 foot for snowmobiling, and 2 feet for multiple trucks and ice shanties. As ice thickness increases, surface roughness also increases and is represented in the model by higher Manning’s roughness coefficients as shown in the graph.

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