Project Methods
The main approach to this analysis was hydraulic modeling of the Yahara River utilizing HEC-RAS software.
HEC-RAS is a one-dimensional computer program that models the hydraulics of water flow through rivers and channels. Cross section shape changes, bends, as well as an appropriate manning n coefficient, are variables that are all used in the modeling. This software determined the water level profile between Lake Monona and Lake Waubesa, as well as the flow rate. These output profiles and plots were constructed using a variety of input data that came from normal lake and river conditions as well as simulating the conditions during the August 2018 flooding events.
​First, we calibrated our HEC-RAS model with a realistic manning n coefficient. To do this, we tested a range of manning n coefficients until the output flowrate matched the currently measured flows at the downstream section of the river.
Next, we constructed a delivery curve for current conditions by running the HEC-RAS model with various water surface levels at the upstream and downstream locations. Entering water surface levels at both the inlet and outlet of the river would allow us to generate a corresponding flow rate through the system. From here we checked the water surface profiles plots that were produced by HEC-RAS at each water surface level tested. Studying these plots we noticed a significant change in the profile at the bridge at Bridge Rd, as well as the railroad trestle, representing significant constriction points of interest for our study.
Once we identified these two constriction points, we tested the viability of multiple mitigation strategies at both locations to see if it would increase flow through the system.
Under choking conditions, flows pass through critical depths in the contracted channel reach. The specific energy of the flow should be consistent throughout the channel. The specific energy is dependant on water depth, velocity, and bottom elevation. The figure at the right demonstrates the choking process. A subcritical flow lacks the energy necessary to make it through a significant constriction. In order to reach that energy level, water upstream must back up and increase in depth. At the constriction, the water then drops to the critical depth. At both Bridge Rd and the railroad trestle, choking conditions were present. Choked flow specific energy plot from mudlaketrestle.weebly.com/choking-analysis
